diff --git a/AGENTS.md b/AGENTS.md
index f7abe9d..ed8fec1 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -32,8 +32,8 @@ the tracked file (`docs/architecture/overview.md`).
 
 The settings window (`SemiPlot.UI/Settings`) is the viewer's only writer of the section folders. It
 edits only keys that already exist, each in the file that owns it, and only the keys the operator
-changed; it creates no file, and it edits no key of `opc/` yet. The startup-failure window offers it only
-when both sections it edits, `app/` and `connection/`, read and carry every key it edits. The
+changed; it creates no file. The startup-failure window offers it only when all three sections it edits,
+`app/`, `connection/` and `opc/`, read and carry every key it edits. The
 production loaders validate a staged copy before any file is replaced, and every change takes effect at the
 next start except the theme, which every running process applies live
 (`docs/architecture/overview.md#the-live-theme`).
diff --git a/SemiPlot/SemiPlot.Tests.Unit/UI/Localization/ResourcesTests.cs b/SemiPlot/SemiPlot.Tests.Unit/UI/Localization/ResourcesTests.cs
index 28c37ac..fc292c2 100644
--- a/SemiPlot/SemiPlot.Tests.Unit/UI/Localization/ResourcesTests.cs
+++ b/SemiPlot/SemiPlot.Tests.Unit/UI/Localization/ResourcesTests.cs
@@ -79,6 +79,7 @@ public void CompositeFormats_KeepTheirPlaceholder()
 	{
 		Resources.StatusLayerFormat.Should().Contain("{0}");
 		Resources.SettingsFieldRequired.Should().Contain("{0}");
+		Resources.SettingsKeyAbsent.Should().Contain("{0}").And.Contain("{1}");
 		Resources.ExportMessageStartUnreadable.Should().Contain("{0}");
 		Resources.ExportMessageEndUnreadable.Should().Contain("{0}");
 		Resources.ExportPensHeader.Should().Contain("{0}").And.Contain("{1}");
diff --git a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveSandbox.cs b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveSandbox.cs
index 00a5038..61c084f 100644
--- a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveSandbox.cs
+++ b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveSandbox.cs
@@ -5,6 +5,7 @@
 using Microsoft.Extensions.Logging.Abstractions;
 
 using SemiPlot.Core.Configuration;
+using SemiPlot.DataSource.OpcUa.Configuration;
 using SemiPlot.DataSource.Postgres.Configuration;
 using SemiPlot.UI.Settings;
 using SemiPlot.UI.Startup;
@@ -25,6 +26,7 @@ public SettingsSaveSandbox()
 		ConfigDirectory = _temporary.Path;
 		AppDirectory = Path.Combine(ConfigDirectory, StartupSequence.SettingsDirectoryName);
 		ConnectionDirectory = Path.Combine(ConfigDirectory, StartupProbe.ConnectionDirectoryName);
+		OpcDirectory = Path.Combine(ConfigDirectory, StartupProbe.OpcDirectoryName);
 		ShippedConfiguration.CopyTo(ConfigDirectory, "secret");
 	}
 
@@ -34,6 +36,8 @@ public SettingsSaveSandbox()
 
 	public string ConnectionDirectory { get; }
 
+	public string OpcDirectory { get; }
+
 	public string ConnectionFile => Path.Combine(ConnectionDirectory, ConnectionFileName);
 
 	public void Dispose()
@@ -61,7 +65,13 @@ public static TError SingleError<TError>(Result result)
 
 	public string DirectoryOf(ConfigurationSectionName section)
 	{
-		return section == ConfigurationSectionName.App ? AppDirectory : ConnectionDirectory;
+		return section switch
+		{
+			ConfigurationSectionName.App => AppDirectory,
+			ConfigurationSectionName.Connection => ConnectionDirectory,
+			ConfigurationSectionName.Opc => OpcDirectory,
+			_ => throw new ArgumentOutOfRangeException(nameof(section), section, null)
+		};
 	}
 
 	public Result Save(IReadOnlyDictionary<ConfigurationSectionName, IReadOnlyDictionary<string, string>> edits)
@@ -78,6 +88,15 @@ public PostgresConnectionSettings LoadConnection()
 		return loaded.Value;
 	}
 
+	public OpcUaConnectionSettings LoadOpc()
+	{
+		var loaded = OpcUaConnectionLoader.Load(OpcDirectory);
+
+		loaded.IsSuccess.Should().BeTrue(Describe(loaded));
+
+		return loaded.Value;
+	}
+
 	public Dictionary<string, byte[]> Snapshot()
 	{
 		return Directory.GetFiles(ConfigDirectory, "*", SearchOption.AllDirectories)
diff --git a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveTests.cs b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveTests.cs
index 6c7982c..807e5ca 100644
--- a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveTests.cs
+++ b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveTests.cs
@@ -7,6 +7,7 @@
 
 using SemiPlot.Core.Configuration;
 using SemiPlot.Core.Data.Errors;
+using SemiPlot.DataSource.OpcUa.Configuration;
 using SemiPlot.DataSource.Postgres.Configuration;
 using SemiPlot.UI.Messages;
 using SemiPlot.UI.Settings;
@@ -38,6 +39,8 @@ public void Dispose()
 	[InlineData(ConfigurationSectionName.Connection, "port", "70000", ConnectionFileProblem.OutOfRange)]
 	[InlineData(ConfigurationSectionName.Connection, "host", "scada-01", ConnectionFileProblem.HostNotIPv4)]
 	[InlineData(ConfigurationSectionName.Connection, "password", "", ConnectionFileProblem.MissingField)]
+	[InlineData(ConfigurationSectionName.Opc, "port", "70000", ConnectionFileProblem.OutOfRange)]
+	[InlineData(ConfigurationSectionName.Opc, "host", "scada-01", ConnectionFileProblem.HostNotIPv4)]
 	public void AnInvalidValueIsRefusedAndNoFileChanges(
 		ConfigurationSectionName section,
 		string key,
@@ -58,6 +61,7 @@ public void AnInvalidValueIsRefusedAndNoFileChanges(
 		problem.Should().Be(expectedProblem);
 		(error as AppSettingsError)?.Key.Should().Be(key);
 		(error as ConnectionFileError)?.Fields.Should().Equal(key);
+		(error as ConnectionFileError)?.Section.Should().Be(section);
 		_sandbox.ShouldMatch(before);
 	}
 
@@ -253,6 +257,7 @@ public void AnUnmodifiedShippedCopyWithOnlyThePasswordFilledPromotes()
 	[Theory]
 	[InlineData(ConfigurationSectionName.App, "locale", "de")]
 	[InlineData(ConfigurationSectionName.Connection, "port", "70000")]
+	[InlineData(ConfigurationSectionName.Opc, "port", "70000")]
 	public void ARefusedSaveNamesTheOperatorsFolderAndNeverTheStagingOne(
 		ConfigurationSectionName section,
 		string key,
@@ -271,8 +276,53 @@ public void ARefusedSaveNamesTheOperatorsFolderAndNeverTheStagingOne(
 		};
 		path.Should().Be(realDirectory);
 		(error as ConnectionFileError)?.Fields.Should().Equal(key);
+		(error as ConnectionFileError)?.Section.Should().Be(section);
 		var detail = ArchiveFailureMapper.Map(error).Detail;
 		detail.Should().Contain(realDirectory).And.NotContain(SettingsSave.StagingPrefix);
 	}
 
+	[Fact]
+	public void AnOpcEditReachesTheOpcFileAndLeavesTheOtherSectionsByteIdentical()
+	{
+		var before = _sandbox.Snapshot();
+
+		var result = _sandbox.Save(Edit(ConfigurationSectionName.Opc, ("port", "55875")));
+
+		result.IsSuccess.Should().BeTrue(Describe(result));
+		var opc = _sandbox.LoadOpc();
+		opc.Host.Should().Be("127.0.0.1");
+		opc.Port.Should().Be(55875);
+		var after = _sandbox.Snapshot();
+		after.Keys.Should().BeEquivalentTo(before.Keys, "the save creates no file");
+		var otherSections = before.Keys
+			.Where(file => Path.GetDirectoryName(file)
+				is StartupSequence.SettingsDirectoryName or StartupProbe.ConnectionDirectoryName)
+			.ToList();
+		otherSections.Should().HaveCount(2, "the shipped set carries one app file and one connection file");
+
+		foreach (var file in otherSections)
+		{
+			after[file].Should().Equal(before[file], file);
+		}
+
+		_sandbox.StagingDirectories().Should().BeEmpty();
+	}
+
+	[Fact]
+	public void TheCertificateStoreUnderTheOpcSectionSurvivesASaveUntouched()
+	{
+		var certificates = Path.Combine(
+			_sandbox.OpcDirectory, OpcUaConnectionLoader.CertificateStoreDirectoryName, "own", "certs");
+		Directory.CreateDirectory(certificates);
+		var certificate = Path.Combine(certificates, "semiplot.der");
+		byte[] content = [0x30, 0x82, 0x01, 0x0a];
+		File.WriteAllBytes(certificate, content);
+
+		var result = _sandbox.Save(Edit(ConfigurationSectionName.Opc, ("port", "55875")));
+
+		result.IsSuccess.Should().BeTrue(Describe(result));
+		_sandbox.LoadOpc().Port.Should().Be(55875);
+		File.ReadAllBytes(certificate).Should().Equal(content);
+		_sandbox.StagingDirectories().Should().BeEmpty();
+	}
 }
diff --git a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelStand.cs b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelStand.cs
index 8bb73e6..7742882 100644
--- a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelStand.cs
+++ b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelStand.cs
@@ -13,12 +13,9 @@ public static SettingsViewModel Build(
 		MessagePanelViewModel messagePanel,
 		Action? restartApplication = null)
 	{
-		var (app, connection) = SettingsSave.ReadOwned(configDirectory);
-
 		return new SettingsViewModel(
 			configDirectory,
-			app,
-			connection,
+			SettingsSave.ReadOwned(configDirectory),
 			messagePanel,
 			NullLogger<SettingsViewModel>.Instance,
 			restartApplication ?? (() => { }));
diff --git a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelTests.cs b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelTests.cs
index 967c605..703a62b 100644
--- a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelTests.cs
+++ b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelTests.cs
@@ -7,6 +7,8 @@
 
 using ReactiveUI;
 
+using SemiPlot.Core.Configuration;
+using SemiPlot.DataSource.OpcUa.Configuration;
 using SemiPlot.DataSource.Postgres.Configuration;
 using SemiPlot.UI.Localization;
 using SemiPlot.UI.Messages;
@@ -29,12 +31,15 @@ public sealed class SettingsViewModelTests : IDisposable
 
 	private readonly string _connectionDirectory;
 
+	private readonly string _opcDirectory;
+
 	private readonly MessagePanelViewModel _messagePanel = new();
 
 	public SettingsViewModelTests()
 	{
 		_appDirectory = Path.Combine(_configDirectory, StartupSequence.SettingsDirectoryName);
 		_connectionDirectory = Path.Combine(_configDirectory, StartupProbe.ConnectionDirectoryName);
+		_opcDirectory = Path.Combine(_configDirectory, StartupProbe.OpcDirectoryName);
 		ShippedConfiguration.CopyTo(_configDirectory);
 	}
 
@@ -52,6 +57,7 @@ public void ADirectoryPopulatesEveryField()
 			_connectionDirectory,
 			"connection.yaml",
 			"host: 10.20.30.40\nport: 5433\ndatabase: archive\nuser: viewer\npassword: secret\n");
+		WriteFile(_opcDirectory, "opc.yaml", "host: 10.20.30.50\nport: 55875\n");
 
 		using var viewModel = Build();
 
@@ -62,6 +68,8 @@ public void ADirectoryPopulatesEveryField()
 		viewModel.Database.Should().Be("archive");
 		viewModel.User.Should().Be("viewer");
 		viewModel.Password.Should().Be("secret");
+		viewModel.OpcHost.Should().Be("10.20.30.50");
+		viewModel.OpcPort.Should().Be(55875);
 		_messagePanel.Entries.Should().BeEmpty();
 	}
 
@@ -77,6 +85,8 @@ public void TheShippedSetPopulatesEveryFieldAndLeavesThePasswordEmpty()
 		viewModel.Database.Should().Be("semiplot");
 		viewModel.User.Should().Be("semiplot");
 		viewModel.Password.Should().BeEmpty();
+		viewModel.OpcHost.Should().Be("127.0.0.1");
+		viewModel.OpcPort.Should().Be(49875);
 		_messagePanel.Entries.Should().BeEmpty();
 	}
 
@@ -142,28 +152,37 @@ public void ASectionWithADuplicateKeyBlocksTheSaveAndNamesTheSection()
 			Resources.FormatSettingsSectionUnreadable(Resources.SettingsInterfaceHeader));
 	}
 
-	[AvaloniaTheory]
-	[InlineData(true, false)]
-	[InlineData(false, true)]
-	public void AMissingSectionFolderIsReportedAndBlocksTheSaveWithAStatedReason(bool appAbsent, bool connectionAbsent)
+	[AvaloniaFact]
+	public void AnUnreadableOpcSectionIsReportedOnceAndBlocksTheSave()
 	{
-		if (appAbsent)
-		{
-			Directory.Delete(_appDirectory, recursive: true);
-		}
+		WriteFile(_opcDirectory, "opc.yaml", "host: [unclosed\n");
 
-		if (connectionAbsent)
-		{
-			Directory.Delete(_connectionDirectory, recursive: true);
-		}
+		using var viewModel = Build();
+		viewModel.Password = "secret";
+
+		_messagePanel.Entries.Should().ContainSingle().Which.View.Detail.Should().Contain(_opcDirectory);
+		viewModel.OpcHost.Should().BeEmpty();
+		viewModel.OpcPort.Should().BeNull();
+		viewModel.Host.Should().Be("127.0.0.1");
+		CanSave(viewModel).Should().BeFalse();
+		viewModel.ValidationMessage.Should().Be(Resources.FormatSettingsSectionUnreadable(Resources.SettingsOpcHeader));
+	}
+
+	[AvaloniaTheory]
+	[InlineData(ConfigurationSectionName.App)]
+	[InlineData(ConfigurationSectionName.Connection)]
+	[InlineData(ConfigurationSectionName.Opc)]
+	public void AMissingSectionFolderIsReportedAndBlocksTheSaveWithAStatedReason(ConfigurationSectionName section)
+	{
+		var (directory, _, header) = SectionOf(section);
+		Directory.Delete(directory, recursive: true);
 
 		using var viewModel = Build();
 		viewModel.Password = "secret";
 
 		_messagePanel.Entries.Should().ContainSingle();
 		CanSave(viewModel).Should().BeFalse();
-		viewModel.ValidationMessage.Should().Be(Resources.FormatSettingsSectionUnreadable(
-			appAbsent ? Resources.SettingsInterfaceHeader : Resources.SettingsConnectionHeader));
+		viewModel.ValidationMessage.Should().Be(Resources.FormatSettingsSectionUnreadable(header));
 	}
 
 	[AvaloniaFact]
@@ -176,7 +195,8 @@ public void AnAppFileWithoutALocaleRefusesTheSaveAndNamesTheAbsentKey()
 		viewModel.Password = "secret";
 
 		CanSave(viewModel).Should().BeFalse();
-		viewModel.ValidationMessage.Should().Be(Resources.FormatSettingsKeyAbsent(AppSettingsLoader.LocaleKey));
+		viewModel.ValidationMessage.Should().Be(
+			Resources.FormatSettingsKeyAbsent(Resources.SettingsInterfaceHeader, AppSettingsLoader.LocaleKey));
 	}
 
 	[AvaloniaFact]
@@ -191,7 +211,63 @@ public void AConnectionFileWithoutAPortRefusesTheSaveAndNamesTheAbsentKey()
 		viewModel.Port = 5432;
 
 		CanSave(viewModel).Should().BeFalse();
-		viewModel.ValidationMessage.Should().Be(Resources.FormatSettingsKeyAbsent(PostgresConnectionLoader.PortKey));
+		viewModel.ValidationMessage.Should().Be(
+			Resources.FormatSettingsKeyAbsent(Resources.SettingsConnectionHeader, PostgresConnectionLoader.PortKey));
+	}
+
+	[AvaloniaFact]
+	public void AnOpcFileWithoutAHostRefusesTheSaveAndNamesTheSectionAndTheKey()
+	{
+		WriteFile(_opcDirectory, "opc.yaml", "port: 49875\n");
+
+		using var viewModel = Build();
+		viewModel.Password = "secret";
+
+		CanSave(viewModel).Should().BeFalse();
+		viewModel.ValidationMessage.Should().Be(
+			Resources.FormatSettingsKeyAbsent(Resources.SettingsOpcHeader, OpcUaConnectionLoader.HostKey));
+		_messagePanel.Entries.Should().BeEmpty();
+	}
+
+	[AvaloniaTheory]
+	[InlineData(ConfigurationSectionName.App, ConfigurationSectionName.Connection)]
+	[InlineData(ConfigurationSectionName.Connection, ConfigurationSectionName.Opc)]
+	public void OfTwoUnreadableSectionsTheMessageNamesTheEarlier(
+		ConfigurationSectionName earlier, ConfigurationSectionName later)
+	{
+		var (earlierDirectory, earlierFile, header) = SectionOf(earlier);
+		var (laterDirectory, laterFile, _) = SectionOf(later);
+		WriteFile(earlierDirectory, earlierFile, "host: [unclosed\n");
+		WriteFile(laterDirectory, laterFile, "host: [unclosed\n");
+
+		using var viewModel = Build();
+
+		viewModel.ValidationMessage.Should().Be(Resources.FormatSettingsSectionUnreadable(header));
+	}
+
+	[AvaloniaTheory]
+	[InlineData(ConfigurationSectionName.App, ConfigurationSectionName.Connection)]
+	[InlineData(ConfigurationSectionName.Connection, ConfigurationSectionName.Opc)]
+	public void OfTwoSectionsLackingAKeyTheMessageNamesTheEarlier(
+		ConfigurationSectionName earlier, ConfigurationSectionName later)
+	{
+		var (header, key) = RewriteWithoutOneEditedKey(earlier);
+		RewriteWithoutOneEditedKey(later);
+
+		using var viewModel = Build();
+
+		viewModel.ValidationMessage.Should().Be(Resources.FormatSettingsKeyAbsent(header, key));
+	}
+
+	[AvaloniaFact]
+	public void AnUnreadableSectionIsNamedBeforeAKeyAnEarlierSectionLacks()
+	{
+		RewriteWithoutOneEditedKey(ConfigurationSectionName.Connection);
+		WriteFile(_opcDirectory, "opc.yaml", "host: [unclosed\n");
+
+		using var viewModel = Build();
+
+		viewModel.ValidationMessage.Should().Be(Resources.FormatSettingsSectionUnreadable(Resources.SettingsOpcHeader));
 	}
 
 	[AvaloniaFact]
@@ -214,6 +290,7 @@ public async Task ChangingOnlyTheTheme_LeavesTheConnectionFileByteIdenticalAndSe
 	{
 		ShippedConfiguration.FillPassword(_configDirectory, "secret");
 		var before = File.ReadAllBytes(Path.Combine(_connectionDirectory, "connection.yaml"));
+		var opcBefore = File.ReadAllBytes(Path.Combine(_opcDirectory, "opc.yaml"));
 		using var viewModel = Build();
 		viewModel.SelectedTheme = viewModel.Themes.Single(choice => choice.Token == "dark");
 
@@ -223,6 +300,7 @@ public async Task ChangingOnlyTheTheme_LeavesTheConnectionFileByteIdenticalAndSe
 		viewModel.IsRestartPending.Should().BeFalse("every process applies a saved theme live");
 		AppSettingsLoader.Load(_appDirectory).Value.Theme.Should().Be(AppThemeVariant.Dark);
 		File.ReadAllBytes(Path.Combine(_connectionDirectory, "connection.yaml")).Should().Equal(before);
+		File.ReadAllBytes(Path.Combine(_opcDirectory, "opc.yaml")).Should().Equal(opcBefore);
 	}
 
 	[AvaloniaFact]
@@ -265,6 +343,23 @@ public async Task AConnectionSaveSetsTheRestartNotice()
 		viewModel.IsRestartPending.Should().BeTrue();
 	}
 
+	[AvaloniaFact]
+	public async Task AnOpcSaveSetsTheRestartNotice()
+	{
+		ShippedConfiguration.FillPassword(_configDirectory, "secret");
+		var connectionBefore = File.ReadAllBytes(Path.Combine(_connectionDirectory, "connection.yaml"));
+		using var viewModel = Build();
+		viewModel.OpcPort = 55875;
+
+		await viewModel.SaveCommand.Execute();
+
+		_messagePanel.Entries.Should().BeEmpty();
+		viewModel.IsRestartPending.Should().BeTrue("the OPC UA session reads its address at the start");
+		viewModel.RestartNotice.Should().Be(Resources.SettingsRestartNotice);
+		OpcUaConnectionLoader.Load(_opcDirectory).Value.Port.Should().Be(55875);
+		File.ReadAllBytes(Path.Combine(_connectionDirectory, "connection.yaml")).Should().Equal(connectionBefore);
+	}
+
 	[AvaloniaFact]
 	public async Task RestartNow_RunsTheHandedRestartOnlyWhileTheNoticeShows()
 	{
@@ -307,6 +402,8 @@ public async Task EveryFieldTheWindowEditsReachesTheFileTheLoadersRead()
 		viewModel.Database = "archive";
 		viewModel.User = "viewer";
 		viewModel.Password = "secret";
+		viewModel.OpcHost = "10.20.30.50";
+		viewModel.OpcPort = 55875;
 
 		await viewModel.SaveCommand.Execute();
 
@@ -320,6 +417,9 @@ public async Task EveryFieldTheWindowEditsReachesTheFileTheLoadersRead()
 		connection.Database.Should().Be("archive");
 		connection.Username.Should().Be("viewer");
 		connection.Password.Should().Be("secret");
+		var opc = OpcUaConnectionLoader.Load(_opcDirectory).Value;
+		opc.Host.Should().Be("10.20.30.50");
+		opc.Port.Should().Be(55875);
 	}
 
 	[AvaloniaFact]
@@ -360,6 +460,7 @@ public async Task ASaveSendsOnlyTheChangedKeyOfASection()
 	[InlineData(nameof(SettingsViewModel.Database), nameof(SettingsViewModel.IsDatabaseValid))]
 	[InlineData(nameof(SettingsViewModel.User), nameof(SettingsViewModel.IsUserValid))]
 	[InlineData(nameof(SettingsViewModel.Password), nameof(SettingsViewModel.IsPasswordValid))]
+	[InlineData(nameof(SettingsViewModel.OpcHost), nameof(SettingsViewModel.IsOpcHostValid))]
 	public void TheSaveCannotExecuteWhileATextFieldIsBlank(string field, string validity)
 	{
 		using var viewModel = Build();
@@ -388,6 +489,24 @@ public void TheSaveCannotExecuteWhileThePortIsNotAWholeNumberInRange(double? por
 		CanSave(viewModel).Should().BeFalse();
 	}
 
+	[AvaloniaTheory]
+	[InlineData(null)]
+	[InlineData(0d)]
+	[InlineData(65536d)]
+	[InlineData(49875.5)]
+	public void TheSaveCannotExecuteWhileTheOpcPortIsNotAWholeNumberInRange(double? port)
+	{
+		using var viewModel = Build();
+		viewModel.Password = "secret";
+		CanSave(viewModel).Should().BeTrue();
+
+		viewModel.OpcPort = (decimal?)port;
+
+		viewModel.IsOpcPortValid.Should().BeFalse();
+		viewModel.IsPortValid.Should().BeTrue("the archive port is a field of its own");
+		CanSave(viewModel).Should().BeFalse();
+	}
+
 	[AvaloniaTheory]
 	[InlineData("port: not-a-number")]
 	[InlineData("port: 70000")]
@@ -421,6 +540,38 @@ public void AHostNameInTheFileLoadsIntoAnInvalidHostField()
 		_messagePanel.Entries.Should().BeEmpty();
 	}
 
+	[AvaloniaTheory]
+	[InlineData("port: not-a-number")]
+	[InlineData("port: 70000")]
+	public void AnOpcPortTheRuleRejectsLoadsAsAnEmptyField(string port)
+	{
+		WriteFile(_opcDirectory, "opc.yaml", $"host: 127.0.0.1\n{port}\n");
+
+		using var viewModel = Build();
+		viewModel.Password = "secret";
+
+		viewModel.OpcPort.Should().BeNull();
+		viewModel.IsOpcPortValid.Should().BeFalse();
+		CanSave(viewModel).Should().BeFalse();
+		viewModel.OpcHost.Should().Be("127.0.0.1");
+		_messagePanel.Entries.Should().BeEmpty();
+	}
+
+	[AvaloniaFact]
+	public void AHostNameInTheOpcFileLoadsIntoAnInvalidOpcHostField()
+	{
+		WriteFile(_opcDirectory, "opc.yaml", "host: scada-01\nport: 49875\n");
+
+		using var viewModel = Build();
+		viewModel.Password = "secret";
+
+		viewModel.OpcHost.Should().Be("scada-01");
+		viewModel.IsOpcHostValid.Should().BeFalse();
+		CanSave(viewModel).Should().BeFalse();
+		viewModel.IsHostValid.Should().BeTrue("the archive host is a field of its own");
+		_messagePanel.Entries.Should().BeEmpty();
+	}
+
 	[AvaloniaTheory]
 	[InlineData(nameof(SettingsViewModel.SelectedLanguage))]
 	[InlineData(nameof(SettingsViewModel.SelectedTheme))]
@@ -472,6 +623,12 @@ public void TheValidationMessageNamesTheFirstInvalidFieldInFormOrder()
 		viewModel.ValidationMessage.Should().Be(
 			Resources.FormatSettingsFieldRequired(Resources.SettingsPasswordLabel), "the shipped password is empty");
 
+		viewModel.OpcHost = "scada-01";
+		viewModel.OpcPort = null;
+
+		viewModel.ValidationMessage.Should().Be(
+			Resources.FormatSettingsFieldRequired(Resources.SettingsPasswordLabel), "the OPC UA block follows the archive");
+
 		viewModel.Database = string.Empty;
 		viewModel.Host = "scada-01";
 
@@ -487,20 +644,31 @@ public void TheValidationMessageNamesTheFirstInvalidFieldInFormOrder()
 
 		viewModel.Password = "secret";
 
+		viewModel.ValidationMessage.Should().Be(Resources.SettingsOpcHostInvalid);
+
+		viewModel.OpcHost = "127.0.0.1";
+
+		viewModel.ValidationMessage.Should().Be(Resources.SettingsOpcPortInvalid);
+
+		viewModel.OpcPort = 49875;
+
 		viewModel.ValidationMessage.Should().BeEmpty();
 	}
 
-	[AvaloniaFact]
-	public void TheValidationMessageNotifiesWithTheFieldItReads()
+	[AvaloniaTheory]
+	[InlineData(nameof(SettingsViewModel.Port), nameof(Resources.SettingsPortInvalid))]
+	[InlineData(nameof(SettingsViewModel.OpcPort), nameof(Resources.SettingsOpcPortInvalid))]
+	public void TheValidationMessageNotifiesWithTheFieldItReads(string field, string message)
 	{
 		using var viewModel = Build();
+		viewModel.Password = "secret";
 		var notified = new List<string?>();
 		viewModel.PropertyChanged += (_, e) => notified.Add(e.PropertyName);
 
-		viewModel.Port = null;
+		typeof(SettingsViewModel).GetProperty(field)!.SetValue(viewModel, null);
 
 		notified.Should().Contain(nameof(SettingsViewModel.ValidationMessage));
-		viewModel.ValidationMessage.Should().Be(Resources.SettingsPortInvalid);
+		viewModel.ValidationMessage.Should().Be(Resources.ResourceManager.GetString(message, Resources.Culture));
 	}
 
 	private SettingsViewModel Build(Action? restartApplication = null)
@@ -528,6 +696,35 @@ private static bool LatestOf<TInput, TOutput>(ReactiveCommand<TInput, TOutput> c
 		}
 	}
 
+	private (string Directory, string File, string Header) SectionOf(ConfigurationSectionName section)
+	{
+		return section switch
+		{
+			ConfigurationSectionName.App => (_appDirectory, "app.yaml", Resources.SettingsInterfaceHeader),
+			ConfigurationSectionName.Connection =>
+				(_connectionDirectory, "connection.yaml", Resources.SettingsConnectionHeader),
+			ConfigurationSectionName.Opc => (_opcDirectory, "opc.yaml", Resources.SettingsOpcHeader),
+			_ => throw new ArgumentOutOfRangeException(nameof(section), section, null)
+		};
+	}
+
+	private (string Header, string Key) RewriteWithoutOneEditedKey(ConfigurationSectionName section)
+	{
+		var (directory, file, header) = SectionOf(section);
+		var (content, key) = section switch
+		{
+			ConfigurationSectionName.App => ("theme: light\n", AppSettingsLoader.LocaleKey),
+			ConfigurationSectionName.Connection => (
+				"host: 127.0.0.1\ndatabase: semiplot\nuser: semiplot\npassword: secret\n",
+				PostgresConnectionLoader.PortKey),
+			ConfigurationSectionName.Opc => ("port: 49875\n", OpcUaConnectionLoader.HostKey),
+			_ => throw new ArgumentOutOfRangeException(nameof(section), section, null)
+		};
+		WriteFile(directory, file, content);
+
+		return (header, key);
+	}
+
 	private static void WriteFile(string directory, string name, string content)
 	{
 		File.WriteAllText(Path.Combine(directory, name), content);
diff --git a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewTests.cs b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewTests.cs
index 2923cd4..65feead 100644
--- a/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewTests.cs
+++ b/SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewTests.cs
@@ -32,8 +32,6 @@ namespace SemiPlot.Tests.Unit.UI.Settings;
 [Trait("Category", "Unit")]
 public sealed class SettingsViewTests : IDisposable
 {
-	private const string InvalidClass = "invalid";
-
 	private readonly TemporaryDirectory _configDirectory = new("semiplot-settings-view");
 
 	private readonly string _appDirectory;
@@ -69,6 +67,9 @@ public void TheRealisedDialog_ShowsTheLoadedValues()
 			Named<TextBox>(dialog, "SettingsDatabase").Text.Should().Be("semiplot");
 			Named<TextBox>(dialog, "SettingsUser").Text.Should().Be("semiplot");
 			Named<TextBox>(dialog, "SettingsPassword").Text.Should().Be("secret");
+			Named<TextBox>(dialog, "SettingsOpcHost").Text.Should().Be("127.0.0.1");
+			Named<NumericUpDown>(dialog, "SettingsOpcPort").Value.Should().Be(49875);
+			Named<NumericUpDown>(dialog, "SettingsOpcPort").Text.Should().Be("49875");
 			Named<TextBlock>(dialog, "SettingsValidationMessage").Text.Should().BeEmpty();
 			InvalidFields(dialog).Should().BeEmpty();
 		}
@@ -262,6 +263,55 @@ public void TheDialog_KeepsItsSizeAndItsButtonsWhenAFieldTurnsInvalid()
 		}
 	}
 
+	[AvaloniaFact]
+	public void TheDialog_KeepsItsSizeAndItsButtonsWhenAnOpcFieldTurnsInvalid()
+	{
+		using var viewModel = Build();
+		var dialog = Realise(new SettingsDialog { DataContext = viewModel });
+		try
+		{
+			var opcHost = Named<TextBox>(dialog, "SettingsOpcHost");
+			var save = Named<Button>(dialog, "SettingsSaveButton");
+			var validBounds = dialog.Bounds;
+			var validSaveBounds = save.Bounds;
+
+			HeadlessInput.Clear(dialog, opcHost);
+			HeadlessInput.Type(dialog, opcHost, "scada-01");
+
+			viewModel.ValidationMessage.Should().NotBeEmpty();
+			dialog.Bounds.Should().Be(validBounds, "a message never resizes the form");
+			save.Bounds.Should().Be(validSaveBounds, "a message never moves the buttons");
+		}
+		finally
+		{
+			dialog.Close();
+		}
+	}
+
+	[AvaloniaFact]
+	public void AnInvalidOpcHost_NamesTheOpcFieldOnTheMessageLine()
+	{
+		using var viewModel = Build();
+		var dialog = Realise(new SettingsDialog { DataContext = viewModel });
+		try
+		{
+			var opcHost = Named<TextBox>(dialog, "SettingsOpcHost");
+			var save = Named<Button>(dialog, "SettingsSaveButton");
+
+			HeadlessInput.Clear(dialog, opcHost);
+			HeadlessInput.Type(dialog, opcHost, "scada-01");
+
+			Named<TextBlock>(dialog, "SettingsValidationMessage").Text.Should().Be(Resources.SettingsOpcHostInvalid);
+			opcHost.Classes.Should().Contain(InvalidClass);
+			Named<TextBox>(dialog, "SettingsHost").Classes.Should().NotContain(InvalidClass);
+			save.IsEffectivelyEnabled.Should().BeFalse("an OPC UA host must be an IP address");
+		}
+		finally
+		{
+			dialog.Close();
+		}
+	}
+
 	// Semi paints the focused border from its own style: docs/architecture/ui-theme.md#a-form-never-resizes-on-validation
 	[AvaloniaFact]
 	public void AnInvalidField_PaintsItsBorderWithTheErrorBrushFocusedOrNot()
diff --git a/SemiPlot/SemiPlot.Tests.Unit/UI/Startup/StartupFailureWindowTests.cs b/SemiPlot/SemiPlot.Tests.Unit/UI/Startup/StartupFailureWindowTests.cs
index 6c49695..c69ccdb 100644
--- a/SemiPlot/SemiPlot.Tests.Unit/UI/Startup/StartupFailureWindowTests.cs
+++ b/SemiPlot/SemiPlot.Tests.Unit/UI/Startup/StartupFailureWindowTests.cs
@@ -12,9 +12,11 @@
 
 using Microsoft.Extensions.Logging.Abstractions;
 
+using SemiPlot.DataSource.OpcUa.Configuration;
 using SemiPlot.DataSource.Postgres.Configuration;
 using SemiPlot.Tests.Unit.UI.Settings;
 using SemiPlot.UI;
+using SemiPlot.UI.Localization;
 using SemiPlot.UI.MainWindow;
 using SemiPlot.UI.Messages;
 using SemiPlot.UI.Settings;
@@ -91,28 +93,46 @@ public void TheSettingsButton_IsHiddenWithoutAConfigurationDirectory()
 
 	[AvaloniaTheory]
 	[InlineData("missing-directory")]
-	[InlineData("no-files-in-a-section")]
 	[InlineData("unreadable-file")]
 	[InlineData("key-absent")]
+	[InlineData("no-files-in-a-section")]
+	[InlineData("opc-missing-directory")]
+	[InlineData("opc-no-files")]
+	[InlineData("opc-unreadable-file")]
+	[InlineData("opc-key-absent")]
 	public void TheSettingsButton_IsHiddenWhenASectionCannotSave(string fault)
 	{
 		using var configDirectory = ShippedConfiguration.CopyToTemporaryDirectory();
+		var appFile = Path.Combine(configDirectory, StartupSequence.SettingsDirectoryName, "app.yaml");
+		var opcDirectory = Path.Combine(configDirectory, StartupProbe.OpcDirectoryName);
 		switch (fault)
 		{
 			case "missing-directory":
 				Directory.Delete(configDirectory, recursive: true);
 				break;
+			case "unreadable-file":
+				File.WriteAllText(appFile, "locale: [unclosed\n");
+				break;
+			case "key-absent":
+				File.WriteAllText(appFile, "theme: light\n");
+				break;
 			case "no-files-in-a-section":
 				File.Delete(Path.Combine(configDirectory, StartupProbe.ConnectionDirectoryName, "connection.yaml"));
 				break;
-			case "key-absent":
-				var appWithoutLocale = Path.Combine(configDirectory, StartupSequence.SettingsDirectoryName, "app.yaml");
-				File.WriteAllText(appWithoutLocale, "theme: light\n");
+			case "opc-missing-directory":
+				Directory.Delete(opcDirectory, recursive: true);
 				break;
-			default:
-				var appFile = Path.Combine(configDirectory, StartupSequence.SettingsDirectoryName, "app.yaml");
-				File.WriteAllText(appFile, "locale: [unclosed\n");
+			case "opc-no-files":
+				File.Delete(Path.Combine(opcDirectory, "opc.yaml"));
+				break;
+			case "opc-unreadable-file":
+				File.WriteAllText(Path.Combine(opcDirectory, "opc.yaml"), "host: [unclosed\n");
+				break;
+			case "opc-key-absent":
+				File.WriteAllText(Path.Combine(opcDirectory, "opc.yaml"), "host: 127.0.0.1\n");
 				break;
+			default:
+				throw new ArgumentOutOfRangeException(nameof(fault), fault, null);
 		}
 
 		using var viewModel = NewFailureViewModel(configDirectory);
@@ -132,6 +152,19 @@ public void TheSettingsButton_IsShownOverAnEmptyPassword()
 		ButtonIsVisible(viewModel, "StartupFailureSettings").Should().BeTrue();
 	}
 
+	[AvaloniaFact]
+	public void TheSettingsButton_IsShownOverAnOpcValueTheLoaderRefuses()
+	{
+		using var configDirectory = ShippedConfiguration.CopyToTemporaryDirectory();
+		var opcFile = Path.Combine(configDirectory, StartupProbe.OpcDirectoryName, "opc.yaml");
+		File.WriteAllText(opcFile, "host: 127.0.0.1\nport: 70000\n");
+
+		using var viewModel = NewFailureViewModel(configDirectory);
+
+		viewModel.OffersSettings.Should().BeTrue("the dialog opens the refused port as an invalid field");
+		ButtonIsVisible(viewModel, "StartupFailureSettings").Should().BeTrue();
+	}
+
 	[AvaloniaFact]
 	public void TheExitButton_ClosesTheWindow()
 	{
@@ -238,6 +271,52 @@ public async Task TheSettingsDialog_OverAnEmptyPassword_WritesItIntoTheExistingC
 		window.Close();
 	}
 
+	[AvaloniaFact]
+	public async Task TheSettingsDialog_OverARefusedOpcPort_WritesTheFixedPortIntoTheOpcFile()
+	{
+		using var configDirectory = ShippedConfiguration.CopyToTemporaryDirectory();
+		ShippedConfiguration.FillPassword(configDirectory, "secret");
+		var opcDirectory = Path.Combine(configDirectory, StartupProbe.OpcDirectoryName);
+		File.WriteAllText(Path.Combine(opcDirectory, "opc.yaml"), "host: 127.0.0.1\nport: 70000\n");
+		var connectionFile = Path.Combine(configDirectory, StartupProbe.ConnectionDirectoryName, "connection.yaml");
+		var appFile = Path.Combine(configDirectory, StartupSequence.SettingsDirectoryName, "app.yaml");
+		var connectionBefore = File.ReadAllBytes(connectionFile);
+		var appBefore = File.ReadAllBytes(appFile);
+		using var viewModel = NewFailureViewModel(configDirectory);
+		var window = new StartupFailureWindow { DataContext = viewModel };
+		ViewProbe.Realise(window);
+		HeadlessInput.Click(window, ViewProbe.Named<Button>(window, "StartupFailureSettings"));
+		await HeadlessWait.Until(() => window.OwnedWindows.OfType<SettingsDialog>().Any());
+		var dialog = window.OwnedWindows.OfType<SettingsDialog>().Single();
+		var settings = dialog.DataContext.Should().BeOfType<SettingsViewModel>().Which;
+		var save = ViewProbe.Named<Button>(dialog, "SettingsSaveButton");
+		var opcPort = ViewProbe.Named<NumericUpDown>(dialog, "SettingsOpcPort");
+		var opcPortText = ViewProbe.Descendant<TextBox>(opcPort, "PART_TextBox");
+		var message = ViewProbe.Named<TextBlock>(dialog, "SettingsValidationMessage");
+
+		opcPort.Value.Should().BeNull("the loader refuses port 70000");
+		opcPortText.Text.Should().BeNullOrEmpty();
+		opcPort.Classes.Should().Contain(ViewProbe.InvalidClass);
+		message.Text.Should().Be(Resources.SettingsOpcPortInvalid);
+		save.IsEffectivelyEnabled.Should().BeFalse();
+		HeadlessInput.Type(dialog, opcPortText, "49875");
+
+		opcPort.Classes.Should().NotContain(ViewProbe.InvalidClass);
+		message.Text.Should().BeEmpty();
+		save.IsEffectivelyEnabled.Should().BeTrue(settings.ValidationMessage);
+		HeadlessInput.Click(dialog, save);
+		await HeadlessWait.Until(() => settings.IsRestartPending);
+
+		OpcUaConnectionLoader.Load(opcDirectory).Value.Port.Should().Be(49875);
+		File.ReadAllBytes(connectionFile).Should().Equal(connectionBefore);
+		File.ReadAllBytes(appFile).Should().Equal(appBefore);
+		viewModel.MessagePanel.Entries.Should().BeEmpty();
+
+		dialog.Close();
+		Dispatcher.UIThread.RunJobs();
+		window.Close();
+	}
+
 	[AvaloniaFact]
 	public void TheRestartButton_IsPresentOnlyWithLaunchOptions()
 	{
diff --git a/SemiPlot/SemiPlot.Tests.Unit/UI/ViewProbe.cs b/SemiPlot/SemiPlot.Tests.Unit/UI/ViewProbe.cs
index ee17577..17b33c1 100644
--- a/SemiPlot/SemiPlot.Tests.Unit/UI/ViewProbe.cs
+++ b/SemiPlot/SemiPlot.Tests.Unit/UI/ViewProbe.cs
@@ -7,6 +7,8 @@ namespace SemiPlot.Tests.Unit.UI;
 
 internal static class ViewProbe
 {
+	public const string InvalidClass = "invalid";
+
 	public static TWindow Realise<TWindow>(TWindow window)
 		where TWindow : Window
 	{
diff --git a/SemiPlot/SemiPlot.UI/Localization/Resources.resx b/SemiPlot/SemiPlot.UI/Localization/Resources.resx
index 2ed3a4c..b6b13db 100644
--- a/SemiPlot/SemiPlot.UI/Localization/Resources.resx
+++ b/SemiPlot/SemiPlot.UI/Localization/Resources.resx
@@ -648,6 +648,9 @@
   <data name="SettingsPasswordLabel" xml:space="preserve">
     <value>Password</value>
   </data>
+  <data name="SettingsOpcHeader" xml:space="preserve">
+    <value>OPC UA connection</value>
+  </data>
   <data name="SettingsRestartNotice" xml:space="preserve">
     <value>Saved. Takes effect after a restart.</value>
   </data>
@@ -669,6 +672,12 @@
   <data name="SettingsPortInvalid" xml:space="preserve">
     <value>Port: a whole number 1–65535</value>
   </data>
+  <data name="SettingsOpcHostInvalid" xml:space="preserve">
+    <value>OPC UA host: expected an IP address such as 127.0.0.1</value>
+  </data>
+  <data name="SettingsOpcPortInvalid" xml:space="preserve">
+    <value>OPC UA port: a whole number 1–65535</value>
+  </data>
   <data name="SettingsFieldRequired" xml:space="preserve">
     <value>{0}: required</value>
   </data>
@@ -676,7 +685,7 @@
     <value>{0}: a file cannot be read. The message panel names it.</value>
   </data>
   <data name="SettingsKeyAbsent" xml:space="preserve">
-    <value>Key '{0}': no file of its section carries it. Add it to a file by hand.</value>
+    <value>{0}: no file carries the key '{1}'. Add it to a file by hand.</value>
   </data>
   <data name="PenFormNameRequired" xml:space="preserve">
     <value>Name: required</value>
diff --git a/SemiPlot/SemiPlot.UI/Localization/Resources.ru.resx b/SemiPlot/SemiPlot.UI/Localization/Resources.ru.resx
index 107e5dc..86b7767 100644
--- a/SemiPlot/SemiPlot.UI/Localization/Resources.ru.resx
+++ b/SemiPlot/SemiPlot.UI/Localization/Resources.ru.resx
@@ -648,6 +648,9 @@
   <data name="SettingsPasswordLabel" xml:space="preserve">
     <value>Пароль</value>
   </data>
+  <data name="SettingsOpcHeader" xml:space="preserve">
+    <value>Подключение к серверу OPC UA</value>
+  </data>
   <data name="SettingsRestartNotice" xml:space="preserve">
     <value>Сохранено. Вступит в силу после перезапуска.</value>
   </data>
@@ -669,6 +672,12 @@
   <data name="SettingsPortInvalid" xml:space="preserve">
     <value>Порт: целое число 1–65535</value>
   </data>
+  <data name="SettingsOpcHostInvalid" xml:space="preserve">
+    <value>Сервер OPC UA: ожидается IP-адрес вида 127.0.0.1</value>
+  </data>
+  <data name="SettingsOpcPortInvalid" xml:space="preserve">
+    <value>Порт OPC UA: целое число 1–65535</value>
+  </data>
   <data name="SettingsFieldRequired" xml:space="preserve">
     <value>{0}: обязательное поле</value>
   </data>
@@ -676,7 +685,7 @@
     <value>{0}: файл не читается. Его имя в панели сообщений.</value>
   </data>
   <data name="SettingsKeyAbsent" xml:space="preserve">
-    <value>Ключ '{0}': ни один файл раздела его не содержит. Добавьте его в файл вручную.</value>
+    <value>{0}: ни один файл не содержит ключ '{1}'. Добавьте его в файл вручную.</value>
   </data>
   <data name="PenFormNameRequired" xml:space="preserve">
     <value>Имя: обязательное поле</value>
diff --git a/SemiPlot/SemiPlot.UI/Settings/SettingsDialog.axaml b/SemiPlot/SemiPlot.UI/Settings/SettingsDialog.axaml
index dedbbe6..5f2ca4e 100644
--- a/SemiPlot/SemiPlot.UI/Settings/SettingsDialog.axaml
+++ b/SemiPlot/SemiPlot.UI/Settings/SettingsDialog.axaml
@@ -5,7 +5,7 @@
         xmlns:local="clr-namespace:SemiPlot.UI.Settings"
         xmlns:text="clr-namespace:SemiPlot.UI.Localization"
         mc:Ignorable="d"
-        d:DesignHeight="560"
+        d:DesignHeight="575"
         x:Class="SemiPlot.UI.Settings.SettingsDialog"
         x:DataType="local:SettingsViewModel"
         Title="{x:Static text:Resources.SettingsTitle}"
@@ -28,7 +28,7 @@
 
         <Grid Grid.Row="0"
               ColumnDefinitions="Auto,*"
-              RowDefinitions="Auto,Auto,Auto,Auto,Auto,Auto,Auto,Auto,Auto"
+              RowDefinitions="Auto,Auto,Auto,Auto,Auto,Auto,Auto,Auto,Auto,Auto,Auto,Auto"
               ColumnSpacing="14"
               RowSpacing="10">
 
@@ -131,6 +131,40 @@
                      Classes.invalid="{Binding !IsPasswordValid}"
                      Text="{Binding Password}" />
 
+            <TextBlock Grid.Row="9"
+                       Grid.ColumnSpan="2"
+                       Margin="0,10,0,0"
+                       Text="{x:Static text:Resources.SettingsOpcHeader}"
+                       FontWeight="Bold" />
+
+            <TextBlock Grid.Row="10"
+                       Grid.Column="0"
+                       VerticalAlignment="Center"
+                       Text="{x:Static text:Resources.SettingsHostLabel}" />
+
+            <TextBox Grid.Row="10"
+                     Grid.Column="1"
+                     Name="SettingsOpcHost"
+                     Classes.invalid="{Binding !IsOpcHostValid}"
+                     Text="{Binding OpcHost}" />
+
+            <TextBlock Grid.Row="11"
+                       Grid.Column="0"
+                       VerticalAlignment="Center"
+                       Text="{x:Static text:Resources.SettingsPortLabel}" />
+
+            <NumericUpDown Grid.Row="11"
+                           Grid.Column="1"
+                           Name="SettingsOpcPort"
+                           HorizontalAlignment="Stretch"
+                           Classes.invalid="{Binding !IsOpcPortValid}"
+                           Minimum="{x:Static local:SettingsViewModel.LowestPort}"
+                           Maximum="{x:Static local:SettingsViewModel.HighestPort}"
+                           Increment="1"
+                           FormatString="0"
+                           ParsingNumberStyle="Integer"
+                           Value="{Binding OpcPort}" />
+
         </Grid>
 
         <Grid Grid.Row="1"
diff --git a/SemiPlot/SemiPlot.UI/Settings/SettingsSave.cs b/SemiPlot/SemiPlot.UI/Settings/SettingsSave.cs
index 9722cfa..579a9b3 100644
--- a/SemiPlot/SemiPlot.UI/Settings/SettingsSave.cs
+++ b/SemiPlot/SemiPlot.UI/Settings/SettingsSave.cs
@@ -6,11 +6,16 @@
 
 using SemiPlot.Core.Configuration;
 using SemiPlot.Core.Data.Errors;
+using SemiPlot.DataSource.OpcUa.Configuration;
 using SemiPlot.DataSource.Postgres.Configuration;
 using SemiPlot.UI.Startup;
 
 namespace SemiPlot.UI.Settings;
 
+internal sealed record SectionRead(ConfigurationSectionName Section, Result<OwnedSection> Owned);
+
+internal sealed record AbsentKey(ConfigurationSectionName Section, string Key);
+
 /// <summary>
 /// Reads the section folders the settings window shows, and writes the operator's edits back once the production
 /// loaders accept a staged copy. A section's failure names its real folder, not the staged copy; nothing escapes as
@@ -34,32 +39,32 @@ public static class SettingsSave
 		new(
 			ConfigurationSectionName.Connection,
 			StartupProbe.ConnectionDirectoryName,
-			directory => PostgresConnectionLoader.Load(directory).ToResult())
+			directory => PostgresConnectionLoader.Load(directory).ToResult()),
+		new(
+			ConfigurationSectionName.Opc,
+			StartupProbe.OpcDirectoryName,
+			directory => OpcUaConnectionLoader.Load(directory).ToResult())
 	];
 
-	/// <summary>Both sections as the files carry them, never through the typed loaders.</summary>
-	internal static (Result<OwnedSection> App, Result<OwnedSection> Connection) ReadOwned(string configDirectory)
-	{
-		return (Read(ConfigurationSectionName.App), Read(ConfigurationSectionName.Connection));
-
-		Result<OwnedSection> Read(ConfigurationSectionName section)
-		{
-			return ConfigurationSection.ReadOwned(SectionDirectory(configDirectory, section), section);
-		}
-	}
-
-	/// <summary>The first key the dialog edits that no file of its section carries, else null.</summary>
-	internal static string? FirstAbsentKey(OwnedSection app, OwnedSection connection)
+	/// <summary>Each section the dialog edits in form order, as its files carry it, not through the typed loaders.</summary>
+	internal static IReadOnlyList<SectionRead> ReadOwned(string configDirectory)
 	{
-		var appKey = SettingsViewModel.AppKeys.FirstOrDefault(edited => !app.Owners.ContainsKey(edited.Key))?.Key;
-
-		return appKey
-			?? SettingsViewModel.ConnectionKeys.FirstOrDefault(edited => !connection.Owners.ContainsKey(edited.Key))?.Key;
+		return
+		[
+			.. _sections.Select(loader => new SectionRead(
+				loader.Section,
+				ConfigurationSection.ReadOwned(Path.Combine(configDirectory, loader.DirectoryName), loader.Section)))
+		];
 	}
 
-	private static string SectionDirectory(string configDirectory, ConfigurationSectionName section)
+	internal static AbsentKey? FirstAbsentKey(IReadOnlyList<SectionRead> sections)
 	{
-		return Path.Combine(configDirectory, _sections.Single(loader => loader.Section == section).DirectoryName);
+		return sections
+			.Where(read => read.Owned.IsSuccess)
+			.SelectMany(read => SettingsViewModel.KeysOf(read.Section)
+				.Where(edited => !read.Owned.Value.Owners.ContainsKey(edited.Key))
+				.Select(edited => new AbsentKey(read.Section, edited.Key)))
+			.FirstOrDefault();
 	}
 
 	public static Result Save(
diff --git a/SemiPlot/SemiPlot.UI/Settings/SettingsViewModel.cs b/SemiPlot/SemiPlot.UI/Settings/SettingsViewModel.cs
index 8be7191..af0c576 100644
--- a/SemiPlot/SemiPlot.UI/Settings/SettingsViewModel.cs
+++ b/SemiPlot/SemiPlot.UI/Settings/SettingsViewModel.cs
@@ -2,13 +2,12 @@
 using System.Reactive;
 using System.Reactive.Linq;
 
-using FluentResults;
-
 using Microsoft.Extensions.Logging;
 
 using ReactiveUI;
 
 using SemiPlot.Core.Configuration;
+using SemiPlot.DataSource.OpcUa.Configuration;
 using SemiPlot.DataSource.Postgres.Configuration;
 using SemiPlot.UI.Localization;
 using SemiPlot.UI.Messages;
@@ -34,11 +33,11 @@ public sealed class SettingsViewModel : ReactiveObject, IDisposable
 	private readonly string _sectionRefusal;
 	private Dictionary<string, string> _loadedApp;
 	private Dictionary<string, string> _loadedConnection;
+	private Dictionary<string, string> _loadedOpc;
 
-	public SettingsViewModel(
+	internal SettingsViewModel(
 		string configDirectory,
-		Result<OwnedSection> app,
-		Result<OwnedSection> connection,
+		IReadOnlyList<SectionRead> sections,
 		MessagePanelViewModel messagePanel,
 		ILogger<SettingsViewModel> logger,
 		Action restartApplication)
@@ -50,40 +49,27 @@ public SettingsViewModel(
 		Languages = [.. SettingsVocabulary.Languages.Select(ChoiceOf)];
 		Themes = [.. SettingsVocabulary.Themes.Select(ChoiceOf)];
 
-		var appValues = ValuesOf(app);
-		var connectionValues = ValuesOf(connection);
-		_sectionRefusal = RefusalOf(app, connection);
+		var appValues = ValuesOf(sections, ConfigurationSectionName.App);
+		var connectionValues = ValuesOf(sections, ConfigurationSectionName.Connection);
+		var opcValues = ValuesOf(sections, ConfigurationSectionName.Opc);
+		_sectionRefusal = RefusalOf(sections);
 
 		SelectedLanguage = Match(Languages, appValues.GetValueOrDefault(AppSettingsLoader.LocaleKey));
 		SelectedTheme = Match(Themes, appValues.GetValueOrDefault(AppSettingsLoader.ThemeKey));
 		Host = connectionValues.GetValueOrDefault(PostgresConnectionLoader.HostKey, string.Empty);
-		Port = PortOf(connectionValues);
+		Port = PortOf(connectionValues.GetValueOrDefault(PostgresConnectionLoader.PortKey));
 		Database = connectionValues.GetValueOrDefault(PostgresConnectionLoader.DatabaseKey, string.Empty);
 		User = connectionValues.GetValueOrDefault(PostgresConnectionLoader.UserKey, string.Empty);
 		Password = connectionValues.GetValueOrDefault(PostgresConnectionLoader.PasswordKey, string.Empty);
+		OpcHost = opcValues.GetValueOrDefault(OpcUaConnectionLoader.HostKey, string.Empty);
+		OpcPort = PortOf(opcValues.GetValueOrDefault(OpcUaConnectionLoader.PortKey));
 
-		_loadedApp = SpelledAsInTheFile(CurrentApp(), appValues);
-		_loadedConnection = SpelledAsInTheFile(CurrentConnection(), connectionValues);
-
-		var canWriteSections = _sectionRefusal.Length == 0;
-		var canSave = this.WhenAnyValue(
-			vm => vm.SelectedLanguage,
-			vm => vm.SelectedTheme,
-			vm => vm.IsHostValid,
-			vm => vm.IsPortValid,
-			vm => vm.IsDatabaseValid,
-			vm => vm.IsUserValid,
-			vm => vm.IsPasswordValid,
-			(language, theme, host, port, database, user, password) =>
-				canWriteSections
-				&& language is not null
-				&& theme is not null
-				&& host
-				&& port
-				&& database
-				&& user
-				&& password);
+		_loadedApp = SpelledAsInTheFile(Current(AppKeys), appValues);
+		_loadedConnection = SpelledAsInTheFile(Current(ConnectionKeys), connectionValues);
+		_loadedOpc = SpelledAsInTheFile(Current(OpcKeys), opcValues);
 
+		var canSave = this.WhenAnyValue(vm => vm.ValidationMessage)
+			.Select(message => message.Length == 0);
 		SaveCommand = ReactiveCommand.CreateFromTask(SaveAsync, canSave);
 		var canRestartNow = this.WhenAnyValue(vm => vm.IsRestartPending)
 			.CombineLatest(SaveCommand.IsExecuting, (pending, saving) => pending && !saving);
@@ -107,6 +93,12 @@ public SettingsViewModel(
 		new(PostgresConnectionLoader.PasswordKey, vm => vm.Password)
 	];
 
+	internal static IReadOnlyList<EditedKey> OpcKeys { get; } =
+	[
+		new(OpcUaConnectionLoader.HostKey, vm => vm.OpcHost),
+		new(OpcUaConnectionLoader.PortKey, vm => TextOf(vm.OpcPort))
+	];
+
 	public static decimal LowestPort => NetworkAddress.LowestPort;
 
 	public static decimal HighestPort => NetworkAddress.HighestPort;
@@ -192,6 +184,29 @@ public string Password
 		}
 	}
 
+	public string OpcHost
+	{
+		get;
+		set
+		{
+			this.RaiseAndSetIfChanged(ref field, value);
+			this.RaisePropertyChanged(nameof(IsOpcHostValid));
+			this.RaisePropertyChanged(nameof(ValidationMessage));
+		}
+	}
+
+	/// <summary>Null while the field holds no whole number in range, which keeps the save disabled.</summary>
+	public decimal? OpcPort
+	{
+		get;
+		set
+		{
+			this.RaiseAndSetIfChanged(ref field, value);
+			this.RaisePropertyChanged(nameof(IsOpcPortValid));
+			this.RaisePropertyChanged(nameof(ValidationMessage));
+		}
+	}
+
 	public bool IsHostValid => NetworkAddress.IsIPv4(Host);
 
 	public bool IsPortValid => IsPort(Port);
@@ -202,6 +217,10 @@ public string Password
 
 	public bool IsPasswordValid => !string.IsNullOrWhiteSpace(Password);
 
+	public bool IsOpcHostValid => NetworkAddress.IsIPv4(OpcHost);
+
+	public bool IsOpcPortValid => IsPort(OpcPort);
+
 	/// <summary>
 	/// The section a file cannot be read from or a key it lacks, else the rule the first invalid field breaks in form
 	/// order; empty while the form can save.
@@ -233,12 +252,11 @@ public static async Task<SettingsViewModel> OpenAsync(
 		ILoggerFactory loggerFactory,
 		Action restartApplication)
 	{
-		var (app, connection) = await Task.Run(() => SettingsSave.ReadOwned(configDirectory));
+		var sections = await Task.Run(() => SettingsSave.ReadOwned(configDirectory));
 
 		return new SettingsViewModel(
 			configDirectory,
-			app,
-			connection,
+			sections,
 			messagePanel,
 			loggerFactory.CreateLogger<SettingsViewModel>(),
 			restartApplication);
@@ -270,14 +288,17 @@ internal void FollowAppliedTheme(AppThemeVariant theme)
 
 	private async Task SaveAsync()
 	{
-		var app = CurrentApp();
-		var connection = CurrentConnection();
+		var app = Current(AppKeys);
+		var connection = Current(ConnectionKeys);
+		var opc = Current(OpcKeys);
 		var appEdits = EditsOf(app, _loadedApp);
 		var connectionEdits = EditsOf(connection, _loadedConnection);
+		var opcEdits = EditsOf(opc, _loadedOpc);
 		var edits = new Dictionary<ConfigurationSectionName, IReadOnlyDictionary<string, string>>
 		{
 			[ConfigurationSectionName.App] = appEdits,
-			[ConfigurationSectionName.Connection] = connectionEdits
+			[ConfigurationSectionName.Connection] = connectionEdits,
+			[ConfigurationSectionName.Opc] = opcEdits
 		};
 
 		var saved = await Task.Run(() => SettingsSave.Save(_configDirectory, edits, _logger));
@@ -291,8 +312,11 @@ private async Task SaveAsync()
 
 		_loadedApp = app;
 		_loadedConnection = connection;
+		_loadedOpc = opc;
 
-		if (appEdits.Keys.Any(key => key != AppSettingsLoader.ThemeKey) || connectionEdits.Count > 0)
+		if (appEdits.Keys.Any(key => key != AppSettingsLoader.ThemeKey)
+			|| connectionEdits.Count > 0
+			|| opcEdits.Count > 0)
 		{
 			IsRestartPending = true;
 		}
@@ -339,19 +363,17 @@ private string FirstBrokenRule()
 			return Resources.FormatSettingsFieldRequired(Resources.SettingsUserLabel);
 		}
 
-		return IsPasswordValid
-			? string.Empty
-			: Resources.FormatSettingsFieldRequired(Resources.SettingsPasswordLabel);
-	}
+		if (!IsPasswordValid)
+		{
+			return Resources.FormatSettingsFieldRequired(Resources.SettingsPasswordLabel);
+		}
 
-	private Dictionary<string, string> CurrentApp()
-	{
-		return Current(AppKeys);
-	}
+		if (!IsOpcHostValid)
+		{
+			return Resources.SettingsOpcHostInvalid;
+		}
 
-	private Dictionary<string, string> CurrentConnection()
-	{
-		return Current(ConnectionKeys);
+		return IsOpcPortValid ? string.Empty : Resources.SettingsOpcPortInvalid;
 	}
 
 	private Dictionary<string, string> Current(IReadOnlyList<EditedKey> keys)
@@ -386,39 +408,59 @@ private static Dictionary<string, string> EditsOf(
 			.ToDictionary(StringComparer.Ordinal);
 	}
 
-	private static string RefusalOf(Result<OwnedSection> app, Result<OwnedSection> connection)
+	private static string RefusalOf(IReadOnlyList<SectionRead> sections)
 	{
-		if (app.IsFailed)
+		if (sections.FirstOrDefault(read => read.Owned.IsFailed) is { } unreadable)
 		{
-			return Resources.FormatSettingsSectionUnreadable(Resources.SettingsInterfaceHeader);
+			return Resources.FormatSettingsSectionUnreadable(HeaderOf(unreadable.Section));
 		}
 
-		if (connection.IsFailed)
-		{
-			return Resources.FormatSettingsSectionUnreadable(Resources.SettingsConnectionHeader);
-		}
+		return SettingsSave.FirstAbsentKey(sections) is { } absent
+			? Resources.FormatSettingsKeyAbsent(HeaderOf(absent.Section), absent.Key)
+			: string.Empty;
+	}
 
-		var absentKey = SettingsSave.FirstAbsentKey(app.Value, connection.Value);
+	private static string HeaderOf(ConfigurationSectionName section)
+	{
+		return section switch
+		{
+			ConfigurationSectionName.App => Resources.SettingsInterfaceHeader,
+			ConfigurationSectionName.Connection => Resources.SettingsConnectionHeader,
+			ConfigurationSectionName.Opc => Resources.SettingsOpcHeader,
+			_ => throw new ArgumentOutOfRangeException(nameof(section), section, null)
+		};
+	}
 
-		return absentKey is null ? string.Empty : Resources.FormatSettingsKeyAbsent(absentKey);
+	internal static IReadOnlyList<EditedKey> KeysOf(ConfigurationSectionName section)
+	{
+		return section switch
+		{
+			ConfigurationSectionName.App => AppKeys,
+			ConfigurationSectionName.Connection => ConnectionKeys,
+			ConfigurationSectionName.Opc => OpcKeys,
+			_ => throw new ArgumentOutOfRangeException(nameof(section), section, null)
+		};
 	}
 
-	private IReadOnlyDictionary<string, string> ValuesOf(Result<OwnedSection> section)
+	private IReadOnlyDictionary<string, string> ValuesOf(
+		IReadOnlyList<SectionRead> sections, ConfigurationSectionName section)
 	{
-		if (section.IsFailed)
+		var owned = sections.Single(read => read.Section == section).Owned;
+
+		if (owned.IsFailed)
 		{
-			_messagePanel.ReportFailure(section, _logger);
+			_messagePanel.ReportFailure(owned, _logger);
 
 			return new Dictionary<string, string>();
 		}
 
-		return section.Value.Values;
+		return owned.Value.Values;
 	}
 
-	private static decimal? PortOf(IReadOnlyDictionary<string, string> values)
+	private static decimal? PortOf(string? text)
 	{
 		var parsed = int.TryParse(
-			values.GetValueOrDefault(PostgresConnectionLoader.PortKey),
+			text,
 			NumberStyles.None,
 			CultureInfo.InvariantCulture,
 			out var number);
diff --git a/SemiPlot/SemiPlot.UI/Startup/StartupFailureViewModel.cs b/SemiPlot/SemiPlot.UI/Startup/StartupFailureViewModel.cs
index d1f973c..8d3552c 100644
--- a/SemiPlot/SemiPlot.UI/Startup/StartupFailureViewModel.cs
+++ b/SemiPlot/SemiPlot.UI/Startup/StartupFailureViewModel.cs
@@ -59,8 +59,8 @@ public StartupFailureViewModel(
 	public MessagePanelViewModel MessagePanel { get; }
 
 	/// <summary>
-	/// True only when both configuration sections read as the files carry them; the dialog edits existing keys and
-	/// creates no file, so any other state leaves the failure text alone to instruct the operator.
+	/// True only when every configuration section the dialog edits reads as the files carry it; the dialog edits
+	/// existing keys and creates no file, so any other state leaves the failure text alone to instruct the operator.
 	/// </summary>
 	public bool OffersSettings { get; }
 
@@ -98,9 +98,9 @@ public void RestartApplication()
 
 	private static bool EverySavedKeyPresent(string configDirectory)
 	{
-		var (app, connection) = SettingsSave.ReadOwned(configDirectory);
+		var sections = SettingsSave.ReadOwned(configDirectory);
 
-		return app.IsSuccess && connection.IsSuccess && SettingsSave.FirstAbsentKey(app.Value, connection.Value) is null;
+		return sections.All(read => read.Owned.IsSuccess) && SettingsSave.FirstAbsentKey(sections) is null;
 	}
 
 	private void Restart(InstanceLauncher launcher)
diff --git a/docs/architecture/charting.md b/docs/architecture/charting.md
index ce613ae..c008f22 100644
--- a/docs/architecture/charting.md
+++ b/docs/architecture/charting.md
@@ -645,8 +645,8 @@ models, backed by renderer-agnostic models in `SemiPlot.Core`. Responsibilities:
   and the chart's navigation, and writes the fault and recovery entries (`data-integration.md`).
 - `MainWindow/AboutDialog` + `AboutInfo` — the modal naming the product, the assembly version and the
   configuration directory this run read.
-- `Settings/SettingsDialog` + `SettingsViewModel` — the modal that edits the `app/` and `connection/`
-  section folders, opened from Edit -> Settings (`overview.md#the-settings-window`).
+- `Settings/SettingsDialog` + `SettingsViewModel` - the modal that edits the `app/`, `connection/` and
+  `opc/` section folders, opened from Edit -> Settings (`overview.md#the-settings-window`).
 - `PenEditor/PenEditorWindow` + `PenEditorViewModel` (+ `PenFormViewModel`, `PenGroupsViewModel`, the
   row and membership VMs, over `Bridge/ArchiveCallQueue`) - the modal that edits the pen catalogue through
   `IPenCatalogueEditor`, opened from Edit -> Pens and groups (`overview.md#the-pen-and-group-editor`).
diff --git a/docs/architecture/data-integration.md b/docs/architecture/data-integration.md
index 94c6a82..a207012 100644
--- a/docs/architecture/data-integration.md
+++ b/docs/architecture/data-integration.md
@@ -1771,8 +1771,8 @@ included, `Unparseable`, each a `ConnectionFileError` of section
 The certificate store root is `<config-dir>/opc/pki`, the folder the four directory stores of the client
 configuration sit under (Realtime, The OPC UA live source). The section is read top-level only, so a
 `.yaml` file under `pki` is never parsed (`OpcUaConnectionLoaderTests.AYamlFileUnderThePkiFolderIsNotRead`).
-The settings window does not edit `opc/`; an operator edits the file by hand, and the change takes effect
-at the next start.
+The settings window edits `host` and `port` of `opc/` under the rules it applies to every section
+(`overview.md#the-settings-window`), and a saved change takes effect at the next start.
 
 ## Startup
 
@@ -1818,7 +1818,7 @@ and `Run_WithAnOpcPortOfZero_FailsOnThatSectionBeforeAnyRead` pin the failing st
 
 The container, the pens and the extent cross the boundary in a `StartupData` record, so `TrendWindow.Build` awaits nothing. `Program.Main` passes the settings, the `StartupData` or the first `IError`, and the parsed `StartupOptions` to `App.RunStarted(AppSettings?, StartupData, StartupOptions)` on
 success and to `App.RunFailed(AppSettings?, IError, StartupOptions?)` on failure, the configuration
-directory reaching the settings window and the keys reaching `InstanceLauncher` on both paths (the failure window offers Settings only when both sections read and every
+directory reaching the settings window and the keys reaching `InstanceLauncher` on both paths (the failure window offers Settings only when all three sections read and every
 key the dialog edits is present; `overview.md#another-instance`): on success `App` builds one
 `TrendWindow` (`overview.md#one-window-per-process`); on failure `App` maps the error through
 `ArchiveFailureMapper` and opens `Startup/StartupFailureWindow`, which names what broke and what to do
diff --git a/docs/architecture/overview.md b/docs/architecture/overview.md
index 95f0c9e..5edfe3d 100644
--- a/docs/architecture/overview.md
+++ b/docs/architecture/overview.md
@@ -59,7 +59,7 @@ Constraint: **$0 budget** — only free/OSS components.
 |     +-- AppStatusBar        archive and OPC UA state, layer |
 |     +-- PenEditorWindow     pens and groups dialog          |
 |     +-- CsvExportWindow     range to CSV file dialog        |
-|     +-- SettingsDialog      app and connection settings     |
+|     +-- SettingsDialog      app, connection, OPC UA fields  |
 |     +-- AboutDialog         version and config folder       |
 |   ViewModels (ReactiveUI) ◄── TrendCoordinator (Rx hub)     |
 |                           ◄── PenCatalogueSync (5 s reads)  |
@@ -203,7 +203,7 @@ A failed start opens `Startup/StartupFailureWindow` instead, over its own `Start
 failure's title, detail and remedy, a message panel of its own, and the Settings, Restart, About and Exit
 buttons. The view model builds and disposes that panel; the window disposes the view model when it
 closes. Settings and Restart are present when the launch keys parsed, which is when the view model has an
-`InstanceLauncher`; Settings also needs both sections to read and carry every key it edits
+`InstanceLauncher`; Settings also needs every section it edits to read and carry every key it edits
 (`#the-settings-window`). Restart starts a copy and closes the window only once the copy has started.
 `App` stores the panel of the window the process shows, the container's in `ConfigureStarted` on a start and
 the failure window's in `CreateMainWindow` on a failed start, and `App.ResolveMessagePanel` returns that
@@ -429,7 +429,7 @@ one method: nothing else may resolve a service through a static.
 
 `Edit` -> `Settings` opens `Settings/SettingsDialog`, the viewer's only writer of the section folders. It
 edits `locale` and `theme` in `app/`, and `host`, `port`, `database`, `user` and `password` in
-`connection/`. It does not edit `opc/` yet; the operator edits `opc/opc.yaml` by hand. A key the window
+`connection/`, and `host` and `port` in `opc/`, in an OPC UA block after the archive block. A key the window
 does not show, such as `schema`, stays in its file and survives every rewrite. A saved `theme` applies live in every running process (`#the-live-theme`), so
 a save that changed only the theme shows no notice. The dialog shows a restart notice after a save that
 changed any other key, and that change takes effect at the next start. A save that also changed the theme
@@ -449,23 +449,31 @@ Each connection field takes only what its loader accepts. `host` is a text box c
 blank. A field that breaks its rule takes the error border, the message line names the first such
 field, and the save stays disabled (`ui-theme.md#a-form-never-resizes-on-validation`). A file value the
 rule refuses opens as an invalid field: a host name stays as text in a field with the error border, and
-a port that is not a whole number in range opens empty.
+a port that is not a whole number in range opens empty. The OPC UA `host` and `port` follow the same
+rules, the ones `OpcUaConnectionLoader` applies through `ConnectionFile`, and each has a message of its
+own (`SettingsOpcHostInvalid`, `SettingsOpcPortInvalid`), because the message line is not under the field.
 
 The window reads the files, not the typed settings. `SettingsSave.ReadOwned` runs
-`ConfigurationSection.ReadOwned` over both section folders, which returns each section's scalar values
+`ConfigurationSection.ReadOwned` over the three section folders, which returns each section's scalar values
 as text and the file that owns each key, and the view model fills its fields from that. The typed
 loaders fail on exactly the values the window is there to fix, such as the shipped empty password, so a
 typed read would open the window empty on the startup-failure path.
 
-The failure window offers Settings only when both sections read and carry every key the dialog edits:
-`StartupFailureViewModel.OffersSettings` is true when `SettingsSave.ReadOwned` succeeds for both and
+The failure window offers Settings only when all three sections read and carry every key the dialog edits:
+`StartupFailureViewModel.OffersSettings` is true when `SettingsSave.ReadOwned` succeeds for all three and
 `SettingsSave.FirstAbsentKey` finds nothing absent when the window opens. A missing configuration
 directory, a section folder with no `*.yaml` file, an unreadable file, and a required key that no file of
 its section carries leave the button hidden, and the
 failure text, which names the path and the keys to create, alone instructs the operator; the window
 creates no file, so a dialog over a section it cannot read, or over an absent key, has nothing to save into.
+An `opc/` fault behind a reported `connection/` failure also hides the button. `StartupProbe.Run` loads
+`connection/` before `opc/`, so the failure text names only `connection/`, and the `opc/` fault is reported at
+the next start.
 `Edit` -> `Settings` over such a folder opens with Save disabled and the reason on the message line. An empty password
-still offers Settings, and a save writes it into the existing `connection.yaml`. `App.RunStarted` and
+still offers Settings, and a save writes it into the existing `connection.yaml`. An `opc/` value the loader
+refuses, such as port 70000 or a host name, offers Settings too: the dialog opens it as an invalid field and
+saves the fix (`StartupFailureWindowTests.TheSettingsButton_IsShownOverAnOpcValueTheLoaderRefuses`).
+`App.RunStarted` and
 `App.RunFailed` hand the parsed launch options to the window they show. When the argument parse failed, the options
 are null and the failure window shows neither Settings nor Restart. A `LogFileTarget.Prepare` failure
 follows a successful parse, so `Program` passes the parsed options and the window offers both.
@@ -475,14 +483,15 @@ A save goes through `Settings/SettingsSave.Save`:
 1. The view model sends only the keys whose text differs from what it loaded. A key the file spells in
    another case, such as `Host:`, counts as changed: the window reads keys ignoring case and the loaders
    do not, so the save rewrites it under the loader's spelling.
-2. `ReadOwned` re-reads both section folders at save time.
+2. `ReadOwned` re-reads the three section folders at save time.
 3. `SettingsSave` creates `<config-dir>/.settings-staging-<random>/` with a folder per section; one
    that cannot be created fails the save with `Unwritable` naming `<config-dir>` under the `App`
    title, because that folder belongs to no section.
    `ConfigurationSectionWriter.Stage` copies every file of a section into it, and rewrites each file
    that owns an edited key. A key goes into the file that already carries it. A file with no edited
    key is copied byte for byte and never rewritten.
-4. `AppSettingsLoader.Load` and `PostgresConnectionLoader.Load` run over the staged folders. A refusal
+4. `AppSettingsLoader.Load`, `PostgresConnectionLoader.Load` and `OpcUaConnectionLoader.Load` run over
+   the staged folders. A refusal
    is returned with the real section directory in place of the staging one.
 5. Every rewritten target is opened for writing and closed, sharing as a section read does
    (`#the-live-theme`). One that refuses fails the save with `SectionProblem.Unwritable`, and nothing moves.
@@ -938,7 +947,8 @@ root registers `AddPostgresData`, and every member of `IDataProvider` is impleme
 the archive extent, the archive days read and the windowed history read. `AddOpcUaLiveSource` registers
 `OpcUaLiveSource`, which implements `ILiveSource` over the SCADA's OPC UA server. The chart draws history
 from the archive and the live edge from OPC UA; the archive replaces a live point only when a history
-read covers it (`[DEC:opc-live-edge]`). The settings window does not edit the `opc/` section yet. The pen editor writes the catalogue through `IPenCatalogueEditor`,
+read covers it (`[DEC:opc-live-edge]`). The settings window edits the `opc/` section's `host` and `port`
+(`#the-settings-window`). The pen editor writes the catalogue through `IPenCatalogueEditor`,
 and every running chart follows what it writes within 5 s (`#the-live-catalogue`), except the start
 values, which change nothing on screen (`#what-a-read-changes`). The chart places, drags, renames and
 deletes time markers stored in `semiplot_markers`, and every window follows them within 5 s
diff --git a/docs/architecture/ui-text.md b/docs/architecture/ui-text.md
index d0e99e1..792f55b 100644
--- a/docs/architecture/ui-text.md
+++ b/docs/architecture/ui-text.md
@@ -200,18 +200,23 @@ come from the mapper (`#what-the-mappers-two-consumers-read`):
 ## The settings window's text
 
 The `Edit` menu is `MenuEdit` with two items, `MenuEditSettings` and `MenuEditPensAndGroups`
-(`#the-pen-editors-text`). `Settings/SettingsDialog.axaml` reads `SettingsTitle`, the two section headers
-`SettingsInterfaceHeader` and `SettingsConnectionHeader`, one `Settings*Label` per field, `SettingsSave`
-and `SettingsClose`. The language and theme choices read `SettingsLanguageRussian`,
+(`#the-pen-editors-text`). `Settings/SettingsDialog.axaml` reads `SettingsTitle`, the three section headers
+`SettingsInterfaceHeader`, `SettingsConnectionHeader` and `SettingsOpcHeader`, one `Settings*Label` per
+field, `SettingsSave` and `SettingsClose`. The OPC UA block reuses `SettingsHostLabel` and
+`SettingsPortLabel` under its own header. The language and theme choices read `SettingsLanguageRussian`,
 `SettingsLanguageEnglish`, `SettingsThemeLight` and `SettingsThemeDark`: the combo box shows the label
 and the file keeps the token `SettingsVocabulary` pairs with it. The dialog's message line, left of the
 buttons, shows `SettingsSectionUnreadable` with the section header as `{0}` when a section fails to read,
-else `SettingsKeyAbsent` with the first edited key no file of its section carries as `{0}`, else the rule
-the first invalid field breaks, in form order: `SettingsFieldRequired` for an unchosen language or theme,
-`SettingsHostInvalid`, `SettingsPortInvalid`, and `SettingsFieldRequired` for a blank database, user or
-password. `SettingsFieldRequired` takes the field's label as `{0}`.
+else `SettingsKeyAbsent` with the section header as `{0}` and the first edited key no file of that section
+carries as `{1}`, else the rule the first invalid field breaks, in form order: `SettingsFieldRequired` for
+an unchosen language or theme, `SettingsHostInvalid`, `SettingsPortInvalid`, `SettingsFieldRequired` for a
+blank database, user or password, `SettingsOpcHostInvalid` and `SettingsOpcPortInvalid`.
+`SettingsFieldRequired` takes the field's label as `{0}`.
 `SettingsViewModel.ValidationMessage` picks it. The line is not under the field, so each message opens with the field it names, and each is one
-short line in both languages ("Host: expected an IP address such as 127.0.0.1");
+short line in both languages ("Host: expected an IP address such as 127.0.0.1"). The OPC UA messages
+open with "OPC UA host" and "OPC UA port", because "Host" names the archive host. `SettingsKeyAbsent`,
+which opens with a section header, can wrap into the second reserved line (with `SettingsOpcHeader` and
+`port` it does in both languages);
 `ui-theme.md#a-form-never-resizes-on-validation` holds the width it has to fit. `SettingsRestartNotice`
 is the dialog's own text after a save that changed any key but `theme`, on the same line while no field is invalid;
 `SettingsRestartNoticeThemeApplied` replaces it when that save also changed `theme`, which already applies,
diff --git a/docs/architecture/ui-theme.md b/docs/architecture/ui-theme.md
index 202eccc..90e1038 100644
--- a/docs/architecture/ui-theme.md
+++ b/docs/architecture/ui-theme.md
@@ -190,7 +190,8 @@ colour and text, never size:
   order, and is empty while every field is valid. A notice that is not an error, such as the settings
   dialog's restart notice with its Restart now button in the same cell, shares the same line and gives
   way to an error, the button with it.
-- Each message is one short line in both languages, `ui-text.md#the-settings-windows-text`.
+- Each field-rule message is one short line in both languages; `SettingsKeyAbsent`, which opens with a
+  section header, may use the second reserved line (`ui-text.md#the-settings-windows-text`).
 
 The invalid style targets the template part, not the control. Semi paints a `TextBox` border on
 `Border#PART_ContentPresenterBorder` from its own state styles: `Transparent` at rest and
@@ -236,6 +237,23 @@ line in English, in a 334 px cell, and 420 px in Russian, which wraps in its 241
 "Сохранено. Тема применена," at 208.6 px and "остальное после перезапуска." at 210.5 px, two lines with
 nothing trimmed.
 
+The OPC UA block and its messages were measured on 2026-10-21 in the realised 640 px dialog, at 14 px with
+20 px lines, on the headless platform with real drawing over Skia and HarfBuzz. That run reproduced the
+2026-09-30 message cells and Save and Close buttons above; the theme's font resolved to Microsoft YaHei, as
+Inter is not installed on the measuring machine. Each message is its single-line width against the message
+cell:
+
+| | English | Russian |
+| --- | --- | --- |
+| Dialog, every field valid and with each message below | 640 x 575 px | 640 x 575 px |
+| `SettingsOpcHostInvalid` | 365.2 px, one line | 354.3 px, one line |
+| `SettingsOpcPortInvalid` | 263.8 px, one line | 248.8 px, one line |
+| `SettingsKeyAbsent` with `SettingsOpcHeader` and `port` | 487.0 px, two lines, the first 445.6 px | 727.0 px, two lines, 344.5 and 370.1 px |
+
+Nothing is trimmed: the 457 px English cell holds 914 px in its two lines and the 388 px Russian cell
+776 px. `SettingsViewTests.TheDialog_KeepsItsSizeAndItsButtonsWhenAnOpcFieldTurnsInvalid` gates the block's
+effect on the dialog's and the Save button's `Bounds`.
+
 The export window (`Export/CsvExportWindow`) is 390 px wide with `SizeToContent="Height"` and
 `CanResize="False"`, a 20 px margin and 12 px between its three rows: the form grid, the message line and the
 buttons. The message line is its own full-width row above a right-aligned button row, as in the axis scale
diff --git a/docs/plans/backlog.md b/docs/plans/backlog.md
index b8b3d70..830708d 100644
--- a/docs/plans/backlog.md
+++ b/docs/plans/backlog.md
@@ -74,6 +74,15 @@ each is a scoped future task.
   nested interaction sub-VM (mirror the existing `Navigation` sub-VM pattern). Deferred during the fix pass
   as risky public-surface churn with no net saving; do it as a focused refactor.
 
+- **Splitting the settings view model.** `Settings/SettingsViewModel` is 512 lines, past the 300-line
+  preference. The seams that cut the size are the edit tracking (`_loaded*`, `Current`, `SpelledAsInTheFile`,
+  `EditsOf`) and the validation (`FirstBrokenRule`, 49 lines, at the method cap, with the `Is*Valid` rules).
+  A host-and-port child view model is a separate change for the duplicate, not for the size: the archive and
+  OPC UA blocks repeat one shape (`Host`/`OpcHost` and `Port`/`OpcPort` carry the same setter, each raising
+  its `Is*Valid` flag and `ValidationMessage`, over the same `NetworkAddress` rule). It removes about 25
+  lines, needs `ValidationMessage` to follow a child's change, and moves the dialog's bindings and about forty
+  test references. Both were deferred out of `20261021-opc-settings.md` as focused refactors.
+
 - **~~Latent: coordinator history sequence frozen at 1.~~ Resolved (20260618-post-review-fixes, Task 4).**
   The standalone coordinator history path (`RequestHistory`/`SetLayer`/`HistoryResults`) was removed.
   `RequestInitialHistory` now awaits `QueryHistoryAsync` directly and applies through the same
diff --git a/docs/plans/completed/20261021-opc-settings.md b/docs/plans/completed/20261021-opc-settings.md
new file mode 100644
index 0000000..8194b51
--- /dev/null
+++ b/docs/plans/completed/20261021-opc-settings.md
@@ -0,0 +1,490 @@
+# OPC UA block in the settings window
+
+## Overview
+
+- The settings window edits the `opc/` section: an OPC UA block after the database block, in the same
+  window, with the fields `host` and `port`. The operator no longer edits `opc/opc.yaml` by hand.
+- The block follows the rules the window already applies to `app/` and `connection/`: it edits only keys
+  that already exist, each in the file that owns it, and only the keys the operator changed. The
+  production loader `OpcUaConnectionLoader` validates a staged copy before any file is replaced, and a
+  saved change takes effect at the next start.
+- `host` follows `NetworkAddress.IsIPv4`, `port` the shared bounds `NetworkAddress.LowestPort` to
+  `NetworkAddress.HighestPort`, as the `connection/` fields do.
+- The startup-failure window offers Settings only when all three sections read and carry every key the
+  dialog edits. Today it reads two: an `opc/` folder that is missing, holds no `*.yaml`, holds an
+  unreadable file or lacks a key still offers a dialog that cannot fix it
+  (`SemiPlot.UI/Startup/StartupFailureViewModel.cs:36`, `:99-104`). This slice closes that gap.
+- Roadmap thesis, for orientation only: while the OPC UA session is up, every point past a pen's newest
+  archived row comes from OPC UA; the archive replaces a live point only when a history read covers it.
+  This slice touches no part of that path.
+
+## Context (from discovery)
+
+Roadmap: docs/plans/roadmaps/20261019-opc-live-edge-roadmap.md — slice opc-settings
+
+- Base: master `5f8ea2c` (#145, slice `opc-live-source`), whose tree is identical to `c9201ef`. Every
+  citation below is read there.
+- The `opc/` section as it stands:
+  - `ConfigFiles/opc/opc.yaml` carries `host: 127.0.0.1` and `port: 49875`.
+  - `SemiPlot.DataSource.OpcUa/Configuration/OpcUaConnectionLoader.cs:21-60`: `HostKey` and `PortKey` are
+    `ConnectionFile.HostKey` and `ConnectionFile.PortKey` (`:23`, `:25`); `Load` reads the folder with
+    `ConfigurationSection.Read` (`:33`) and checks it with `ConnectionFile.Validate` (`:48`). Every failure
+    is a `ConnectionFileError` or a `ConfigurationSectionError` of section `Opc` (`:16-20`).
+  - The certificate store root is `<opc dir>/pki` (`:27`, `:58`); the loader only computes the path.
+  - `SemiPlot.Core/Configuration/ConfigurationSectionError.cs:6-11`: `ConfigurationSectionName` has `App`,
+    `Connection`, `Opc`.
+  - `SemiPlot.UI/Startup/StartupProbe.cs:25` declares `OpcDirectoryName = "opc"`.
+  - `SemiPlot.Core/Configuration/ConnectionFile.cs:13`, `:15`: `HostKey = "host"`, `PortKey = "port"`, shared
+    by both loaders; `PostgresConnectionLoader.cs:33`, `:35` alias the same constants.
+  - `SemiPlot.Core/Configuration/NetworkAddress.cs:8`, `:10`, `:17`: `LowestPort`, `HighestPort`, `IsIPv4`.
+- The settings window:
+  - `SemiPlot.UI/Settings/SettingsSave.cs:28-38`: `_sections` holds two `SectionLoader`s, `App` and
+    `Connection`. `ReadOwned` returns a two-element tuple (`:41-49`), and its summary says "Both sections"
+    (`:40`). `FirstAbsentKey(app, connection)` returns a bare key (`:52-58`). `StageAndPromote` (`:103`),
+    `CreateStagingDirectory` (`:244`) and `SectionDirectory` (`:62`) iterate `_sections`. `InRealDirectory`
+    (`:155-169`) rebuilds `ConfigurationSectionError`, `AppSettingsError` and `ConnectionFileError`, the last
+    keeping `file.Section` (`:163-164`), so an `Opc` refusal already maps to the real `opc/` folder.
+  - `SemiPlot.Core/Configuration/ConfigurationSectionWriter.cs:91` lists only top-level `*.yaml` files of a
+    section, so staging never copies or touches `opc/pki`.
+  - `SemiPlot.UI/Settings/SettingsViewModel.cs` (470 lines): the constructor (`:38-92`, 55 lines) takes `app`
+    and `connection` results; `AppKeys` and `ConnectionKeys` (`:95-108`); `Host`, `Port` and their
+    `Is*Valid` (`:139-160`, `:195-197`); `canSave` over seven `WhenAnyValue` inputs (`:69-85`);
+    `FirstBrokenRule` in form order (`:305-345`); `SaveAsync` builds the per-section edit map and sets
+    `IsRestartPending` when a key other than `theme` changed (`:271-303`); `RefusalOf` (`:389-404`); `PortOf`
+    reads `PostgresConnectionLoader.PortKey` (`:418-427`).
+  - `SemiPlot.UI/Settings/SettingsDialog.axaml`: width 640, `SizeToContent="Height"` (`:12-13`); the form
+    grid has nine `Auto` rows (`:31`); the connection header row carries `Margin="0,10,0,0"` (`:66-70`); each
+    field binds `Classes.invalid` to its `Is*Valid` (`:80`, `:92`); the message line is
+    `SettingsValidationMessage` (`:143-146`).
+  - `SemiPlot.UI/Startup/StartupFailureViewModel.cs:36` sets `OffersSettings` from `EverySavedKeyPresent`,
+    which reads `app` and `connection` only (`:99-104`); its summary says "both configuration sections"
+    (`:61-64`).
+  - Callers of `SettingsSave.ReadOwned`: `SettingsViewModel.OpenAsync` (`SettingsViewModel.cs:236`),
+    `StartupFailureViewModel.EverySavedKeyPresent` (`:101`), `SettingsViewModelStand.Build`
+    (`Tests.Unit/UI/Settings/SettingsViewModelStand.cs:16`). `MainWindowViewModel.cs:218` reaches the dialog
+    through `OpenAsync` only.
+- Text: `Resources.resx:621-680` holds the `Settings*` keys. `SettingsHostInvalid` reads "Host: expected an
+  IP address such as 127.0.0.1" (`:666-668`), `SettingsPortInvalid` "Port: a whole number 1–65535"
+  (`:669-671`), `SettingsKeyAbsent` "Key '{0}': no file of its section carries it. Add it to a file by
+  hand." (`:678-680`). The OPC failure titles name the section "OPC UA connection settings" /
+  "подключения к серверу OPC UA" (`Resources.resx:279-280`, `Resources.ru.resx:279-280`).
+  `ResourcesTests` pins composite placeholders in `CompositeFormats_KeepTheirPlaceholder` (`:78-90`),
+  requires every neutral key in the Russian set (`:93`), a Russian value that differs (`:112`) and the
+  same placeholder indices (`:132`).
+- Layout rule: `docs/architecture/ui-theme.md:176-193` (a form never resizes on validation; one message
+  line, two reserved lines, wrapped, trimmed past the second), `:205-214` (the 640 px width and the bounds
+  gate `SettingsViewTests.TheDialog_KeepsItsSizeAndItsButtonsWhenAFieldTurnsInvalid`) and `:216-231` (the
+  dated Skia and HarfBuzz measurements; the message cell is 457 px in English and 388 px in Russian; the
+  headless test font is about twice as wide, so no test asserts trimming; a longer label or message is
+  measured in both languages before it ships).
+- Tests that pin the surface: `SettingsSaveTests`, `SettingsViewModelTests` (fields `_appDirectory`,
+  `_connectionDirectory`, `:27-37`; `AMissingSectionFolderIsReported...(bool appAbsent, bool
+  connectionAbsent)`, `:145-167`; `ChangingOnlyTheTheme_LeavesTheConnectionFileByteIdentical...`, `:213`),
+  `SettingsViewTests`, `StartupFailureWindowTests` (`TheSettingsButton_IsHiddenWhenASectionCannotSave`,
+  `:92-123`), and the helpers `SettingsSaveSandbox` (`DirectoryOf` is a two-way ternary, `:62-65`),
+  `SettingsViewModelStand`, `ShippedConfiguration` (`CopyTo` copies every tracked file, `opc/` included,
+  `:21-34`).
+- Docs that state the window does not edit `opc/`: `AGENTS.md:33-39`, `docs/architecture/overview.md:430-432`
+  and `:941`, `docs/architecture/data-integration.md:1774-1775`, `readme.md:74-76`. Docs that count two
+  sections: `overview.md:206`, `:454-458`, `:460-462`, `:478`, `:485`; `data-integration.md:1821`;
+  `charting.md:648-649`; `ui-text.md:203-212`.
+
+## Development Approach
+
+- **testing approach**: Regular (code first, then tests), as the settings window's existing suites are
+  written.
+- complete each task fully before moving to the next
+- make small, focused changes
+- **CRITICAL: every task MUST include new/updated tests** for code changes in that task
+  - tests are not optional - they are a required part of the checklist
+  - write unit tests for new functions/methods
+  - write unit tests for modified functions/methods
+  - add new test cases for new code paths
+  - update existing test cases if behavior changes
+  - tests cover both success and error scenarios
+- **CRITICAL: all tests must pass before starting next task** - no exceptions
+- **CRITICAL: update this plan file when scope changes during implementation**
+- run tests after each change
+- maintain backward compatibility: an installed `opc/opc.yaml` loads unchanged, and a save rewrites only
+  the file that owns an edited key
+
+## Testing Strategy
+
+- **unit tests**: `SemiPlot.Tests.Unit` only. Every new test opens no socket and starts no container.
+  View-model and view tests are `[AvaloniaFact]`/`[AvaloniaTheory]`; `SettingsSaveTests` stays plain
+  `[Fact]`/`[Theory]`. Every new test is a member of an existing class, which already carries the three
+  traits.
+- **headless UI tests** stand in for e2e: every click, clear and typed text goes through
+  `HeadlessInput` (`Click`, `Clear`, `Type`, `Press`), per `AGENTS.md`.
+- Gate: `dotnet build SemiPlot.slnx` (warnings are errors) and
+  `dotnet test SemiPlot/SemiPlot.Tests.Unit/SemiPlot.Tests.Unit.csproj`. The integration project is not
+  touched and runs only in Task 3.
+
+## Acceptance Evidence
+
+The defect is reproducible from code, so the evidence is headless tests.
+
+1. The failure-window gap: `EverySavedKeyPresent` never reads `opc/` (`StartupFailureViewModel.cs:101-103`),
+   so with the copied `opc/` folder deleted `OffersSettings` is true. The new `opc-missing-directory` case of
+   `StartupFailureWindowTests.TheSettingsButton_IsHiddenWhenASectionCannotSave` asserts it is false. Task 1
+   runs that case red before the `StartupFailureViewModel` edit and green after it.
+2. The fix end to end: `StartupFailureWindowTests.TheSettingsDialog_OverARefusedOpcPort_WritesTheFixedPortIntoTheOpcFile`
+   (Task 2) writes `port: 70000` into `opc/opc.yaml` of a shipped copy with the password filled, builds the
+   failure view model, clicks `StartupFailureSettings`, finds `SettingsOpcPort` empty with the `invalid`
+   class and Save disabled, types `49875`, clicks Save, and asserts
+   `OpcUaConnectionLoader.Load(<copy>/opc)` succeeds with port 49875, `connection/connection.yaml` and
+   `app/app.yaml` byte-identical to before the save, and the window's panel empty.
+3. Command, run from the repository root; it must report zero failures:
+
+   ```powershell
+   dotnet test SemiPlot/SemiPlot.Tests.Unit/SemiPlot.Tests.Unit.csproj --filter "FullyQualifiedName~SemiPlot.Tests.Unit.UI.Settings|FullyQualifiedName~StartupFailureWindowTests|FullyQualifiedName~ResourcesTests"
+   ```
+
+4. What no headless test measures, because the test font is about twice as wide (`ui-theme.md:228-229`):
+   the dialog's height and whether each new message fits its two reserved lines. Task 3 measures both in
+   both languages; the smoke checklist in Post-Completion repeats them on the operator's screen.
+
+## Progress Tracking
+
+- mark completed items with `[x]` immediately when done
+- add newly discovered tasks with ➕ prefix
+- document issues/blockers with ⚠️ prefix
+- update plan if implementation deviates from original scope
+- keep plan in sync with actual work done
+
+## Solution Overview
+
+- `SettingsSave` gains a third `SectionLoader`: `ConfigurationSectionName.Opc`, `StartupProbe.OpcDirectoryName`,
+  `OpcUaConnectionLoader.Load(directory).ToResult()`. Staging, the write probe, promotion and the error
+  rebuild already loop `_sections` or handle the OPC loader's error types, so the save path needs no other
+  change.
+- `ReadOwned` returns a three-element tuple `(App, Connection, Opc)`. A tuple matches today's shape and its
+  three callers deconstruct it; a record buys nothing for three fields. (Fixer phase 2: replaced by
+  `IReadOnlyList<SectionRead>` in `_sections` order, `SectionRead(Section, Owned)`, because three same-typed
+  positional results let a swapped pair compile and widened every signature they passed through.)
+- `FirstAbsentKey(app, connection, opc)` returns the section with the key,
+  `(ConfigurationSectionName Section, string Key)?`. With `host` and `port` now in two sections, a message
+  that names only the key does not tell the operator which file to fix. (Fixer phase 2: it takes the
+  `SectionRead` list and returns `AbsentKey(Section, Key)?`, reading each section's keys through
+  `SettingsViewModel.KeysOf`.)
+- `SettingsKeyAbsent` names the section header first and the key second: "{0}: no file carries the key
+  '{1}'. Add it to a file by hand." The `ui-text.md` rule that each message opens with what it names holds,
+  and the section header is the name.
+- `SettingsViewModel` gains the block as flat members beside the connection fields: `OpcHost`, `OpcPort`,
+  `IsOpcHostValid`, `IsOpcPortValid`, `OpcKeys`, and a third loaded snapshot. The names are prefixed because
+  `Host` and `Port` already belong to the archive connection.
+- The block's rows reuse the labels `SettingsHostLabel` and `SettingsPortLabel` under a new header
+  `SettingsOpcHeader`. Its two rules get their own messages, `SettingsOpcHostInvalid` and
+  `SettingsOpcPortInvalid`, because the message line sits left of the buttons, not under the field, and
+  "Host: ..." would name the archive host.
+- The startup-failure gate reads all three sections. An `opc/` value the loader refuses (port 70000, a host
+  name) still offers Settings: the dialog opens that value as an invalid field and saves the fix, which is
+  the case the gate exists for.
+- Size: the constructor is already 55 lines, past the 50-line method cap, and the slice adds to it, so the
+  slice extracts two named private methods from it: `Populate(app, connection, opc)` seeds every field from
+  the section values, and `CanSave(bool canWriteSections)` returns the save's `IObservable<bool>`. Neither
+  changes a binding. (Fixer phase 2: with `CanSave` gone the constructor is about 35 lines, so `Populate` and
+  its `[MemberNotNull]` were inlined back into it.) The class grew from 470 to 562 lines, past the 300-line preference; the review fixes
+  then dropped `CanSave` for `ValidationMessage.Length == 0`, inlined the `Current*` wrappers and
+  `Populate`, passed the sections as one list and dropped restating summaries, leaving 512. It is not split here: a host-and-port child view model, which would also remove the duplicated
+  `Host`/`OpcHost` setter shape, moves the existing bindings, the `ValidationMessage` notifications and
+  about forty test references with it, and cuts little of the size. `docs/plans/backlog.md` records the
+  split and the seams that do cut it.
+
+## Technical Details
+
+### `SettingsSave` (Task 1)
+
+```csharp
+new(
+	ConfigurationSectionName.Opc,
+	StartupProbe.OpcDirectoryName,
+	directory => OpcUaConnectionLoader.Load(directory).ToResult())
+
+internal static (Result<OwnedSection> App, Result<OwnedSection> Connection, Result<OwnedSection> Opc)
+	ReadOwned(string configDirectory)
+
+internal static (ConfigurationSectionName Section, string Key)? FirstAbsentKey(
+	OwnedSection app, OwnedSection connection, OwnedSection opc)
+```
+
+`FirstAbsentKey` checks `AppKeys`, then `ConnectionKeys`, then `OpcKeys`, the dialog's order.
+
+Fixer phase 2 changed both signatures; the order holds:
+
+```csharp
+internal sealed record SectionRead(ConfigurationSectionName Section, Result<OwnedSection> Owned);
+internal sealed record AbsentKey(ConfigurationSectionName Section, string Key);
+
+internal static IReadOnlyList<SectionRead> ReadOwned(string configDirectory)
+internal static AbsentKey? FirstAbsentKey(IReadOnlyList<SectionRead> sections)
+```
+
+### `SettingsViewModel`
+
+| Member | Rule | Task |
+| --- | --- | --- |
+| constructor | takes `Result<OwnedSection> opc` after `connection`; `OpenAsync` and `SettingsViewModelStand` pass it (fixer phase 2: takes the one `IReadOnlyList<SectionRead>`, and is internal because `SectionRead` is) | 1 |
+| `Populate(app, connection, opc)` | private; seeds every field, the OPC pair included, from the section values (fixer phase 2: inlined back into the constructor) | 1 |
+| `RefusalOf(app, connection, opc)` | unreadable `app`, then `connection`, then `opc`, then the first absent key with its section header (fixer phase 2: `RefusalOf(sections)`, the first failed `SectionRead` in the list's order) | 1 |
+| `OpcKeys` | `host` -> `vm.OpcHost`, `port` -> `TextOf(vm.OpcPort)`; keys `OpcUaConnectionLoader.HostKey`, `PortKey` | 1 |
+| `OpcHost` | setter raises `IsOpcHostValid` and `ValidationMessage`, the shape of `Host` (`:139-148`) | 1 |
+| `OpcPort` | `decimal?`, null while not a whole number in range; setter raises `IsOpcPortValid` and `ValidationMessage` | 1 |
+| `IsOpcHostValid` | `NetworkAddress.IsIPv4(OpcHost)` | 1 |
+| `IsOpcPortValid` | `IsPort(OpcPort)` | 1 |
+| `PortOf(values)` | reads `ConnectionFile.PortKey`, the constant both loaders alias; called for both sections (review: takes the text, read through each loader's `PortKey`) | 1 |
+| `CanSave(bool canWriteSections)` | private; nine `WhenAnyValue` inputs, the seven today plus `IsOpcHostValid`, `IsOpcPortValid`. ReactiveUI 23.2.28 ships overloads up to 12 selectors (the package's `ReactiveUI.xml` lists `WhenAnyValue``14`) (review: replaced by `ValidationMessage.Length == 0`, the same conjunction in one place) | 2 |
+| `FirstBrokenRule` | language, theme, host, port, database, user, password, OPC host, OPC port | 2 |
+| `SaveAsync` | adds `[ConfigurationSectionName.Opc] = opcEdits`; `IsRestartPending` also when `opcEdits.Count > 0` | 2 |
+
+In Task 1 the OPC members exist, fill from the file and validate, but nothing binds them, `canSave` does not
+read them and the save does not send them.
+
+### `SettingsDialog.axaml` (Task 2)
+
+- `RowDefinitions` at `:31` gains three `Auto` rows (twelve in all).
+- Row 9: `TextBlock` with `SettingsOpcHeader`, `Grid.ColumnSpan="2"`, `Margin="0,10,0,0"`, bold, as `:66-70`.
+- Row 10: `SettingsHostLabel`; `TextBox Name="SettingsOpcHost"`, `Classes.invalid="{Binding !IsOpcHostValid}"`,
+  `Text="{Binding OpcHost}"`.
+- Row 11: `SettingsPortLabel`; `NumericUpDown Name="SettingsOpcPort"` with the attributes of `:88-98`,
+  bound to `OpcPort` and `!IsOpcPortValid`.
+
+### Text (resx, both files)
+
+| Key | English | Russian | Task |
+| --- | --- | --- | --- |
+| `SettingsOpcHeader` (new) | OPC UA connection | Подключение к серверу OPC UA | 1 |
+| `SettingsKeyAbsent` (changed, two placeholders) | {0}: no file carries the key '{1}'. Add it to a file by hand. | {0}: ни один файл не содержит ключ '{1}'. Добавьте его в файл вручную. | 1 |
+| `SettingsOpcHostInvalid` (new) | OPC UA host: expected an IP address such as 127.0.0.1 | Сервер OPC UA: ожидается IP-адрес вида 127.0.0.1 | 2 |
+| `SettingsOpcPortInvalid` (new) | OPC UA port: a whole number 1–65535 | Порт OPC UA: целое число 1–65535 | 2 |
+
+The range dash in `SettingsOpcPortInvalid` is the one `SettingsPortInvalid` carries, so the two port
+messages read alike.
+
+### Save flow
+
+Unchanged steps of `overview.md#the-settings-window` 1-7, now over three folders: the view model sends
+the changed keys of each section; `ReadOwned` re-reads three folders; the staging folder gets
+`app/`, `connection/` and `opc/`; `AppSettingsLoader.Load`, `PostgresConnectionLoader.Load` and
+`OpcUaConnectionLoader.Load` run over the staged folders; every rewritten target is probed, then replaced.
+An `opc/` file with no edited key is copied byte for byte and never rewritten, so an archive-only save
+leaves `opc/opc.yaml` untouched.
+
+## What Goes Where
+
+- **Implementation Steps** (`[ ]` checkboxes): code, tests, the measurement and documentation in this
+  repository.
+- **Post-Completion** (no checkboxes): the smoke on the operator's screen, the screenshot recapture, and the
+  remaining roadmap slice.
+
+## Implementation Steps
+
+### Task 1: Read, gate and save the `opc/` section
+
+**Files:**
+- Modify: `SemiPlot/SemiPlot.UI/Settings/SettingsSave.cs`
+- Modify: `SemiPlot/SemiPlot.UI/Settings/SettingsViewModel.cs`
+- Modify: `SemiPlot/SemiPlot.UI/Startup/StartupFailureViewModel.cs`
+- Modify: `SemiPlot/SemiPlot.UI/Localization/Resources.resx`, `Resources.ru.resx`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveSandbox.cs`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelStand.cs`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsSaveTests.cs`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelTests.cs`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Startup/StartupFailureWindowTests.cs`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Localization/ResourcesTests.cs`
+
+- [x] `SettingsSave`: add the `Opc` `SectionLoader` to `_sections`; `ReadOwned` returns `(App, Connection,
+      Opc)` and its summary (`:40`) says "Every section the dialog edits"; `FirstAbsentKey(app, connection,
+      opc)` returns `(Section, Key)?` over `AppKeys`, `ConnectionKeys`, `OpcKeys`
+- [x] `SettingsViewModel` state: the constructor takes `Result<OwnedSection> opc`; `OpenAsync` passes it;
+      `ValuesOf(opc)` reports an unreadable `opc/` to the panel as the other two sections do; extract
+      `Populate(app, connection, opc)`; add `OpcHost`, `OpcPort`, `IsOpcHostValid`, `IsOpcPortValid` and
+      `OpcKeys`; `PortOf` reads `ConnectionFile.PortKey`
+- [x] `SettingsViewModel.RefusalOf` gains the `opc` arm and names the section header in the absent-key
+      message (`App` -> `SettingsInterfaceHeader`, `Connection` -> `SettingsConnectionHeader`, `Opc` ->
+      `SettingsOpcHeader`)
+- [x] resx: add `SettingsOpcHeader`; change `SettingsKeyAbsent` to the two-placeholder text in both files
+- [x] write the `opc-missing-directory` case of `StartupFailureWindowTests.TheSettingsButton_IsHiddenWhenASectionCannotSave`
+      and run it before the next step: it fails
+- [x] `StartupFailureViewModel.EverySavedKeyPresent` reads all three sections and requires all three to
+      succeed and `FirstAbsentKey` to be null; the `OffersSettings` summary says "every configuration
+      section the dialog edits" instead of "both"; the `opc-missing-directory` case now passes
+- [x] test helpers: `SettingsSaveSandbox` gains `OpcDirectory` and `LoadOpc()`, and `DirectoryOf` becomes a
+      switch over the three sections; `SettingsViewModelStand.Build` passes the third result;
+      `SettingsViewModelTests` gains an `_opcDirectory` field beside `_appDirectory` and
+      `_connectionDirectory`
+- [x] `SettingsSaveTests`: `AnInvalidValueIsRefusedAndNoFileChanges` gains `(Opc, "port", "70000",
+      OutOfRange)` and `(Opc, "host", "scada-01", HostNotIPv4)`;
+      `ARefusedSaveNamesTheOperatorsFolderAndNeverTheStagingOne` gains `(Opc, "port", "70000")`; both theories
+      also assert `(error as ConnectionFileError)?.Section.Should().Be(section)`; new
+      `AnOpcEditReachesTheOpcFileAndLeavesTheOtherSectionsByteIdentical` (edit `port` to `55875`, load it
+      through `OpcUaConnectionLoader`, compare `app/` and `connection/` bytes with a snapshot); new
+      `TheCertificateStoreUnderTheOpcSectionSurvivesASaveUntouched` (a file under `opc/pki/own/certs/` is
+      byte-identical after an `opc` save and no staging folder remains)
+- [x] `SettingsViewModelTests`, refusal: `AnAppFileWithoutALocale...` and `AConnectionFileWithoutAPort...`
+      assert the two-argument `FormatSettingsKeyAbsent(header, key)`; new
+      `AnOpcFileWithoutAHostRefusesTheSaveAndNamesTheSectionAndTheKey` (message is
+      `FormatSettingsKeyAbsent(SettingsOpcHeader, "host")`); `AMissingSectionFolderIsReported...` takes a
+      `ConfigurationSectionName` parameter in place of its two bools, with one row per section; new
+      `AnUnreadableOpcSectionIsReportedOnceAndBlocksTheSave` (`host: [unclosed`, one panel entry, message
+      `FormatSettingsSectionUnreadable(SettingsOpcHeader)`)
+- [x] `SettingsViewModelTests`, the OPC fields: `ADirectoryPopulatesEveryField` writes `opc/opc.yaml` and
+      asserts `OpcHost`, `OpcPort`; `TheShippedSetPopulatesEveryField...` asserts `127.0.0.1` and `49875`; new
+      `AnOpcPortTheRuleRejectsLoadsAsAnEmptyField` (`port: not-a-number`, `port: 70000`); new
+      `AHostNameInTheOpcFileLoadsIntoAnInvalidOpcHostField`
+- [x] `StartupFailureWindowTests.TheSettingsButton_IsHiddenWhenASectionCannotSave` gains `opc-no-files`,
+      `opc-unreadable-file` and `opc-key-absent`; new `TheSettingsButton_IsShownOverAnOpcValueTheLoaderRefuses`
+      (`port: 70000` in `opc/opc.yaml`, `OffersSettings` true)
+- [x] `ResourcesTests.CompositeFormats_KeepTheirPlaceholder` adds
+      `Resources.SettingsKeyAbsent.Should().Contain("{0}").And.Contain("{1}")`
+- [x] run `dotnet build SemiPlot.slnx` and `dotnet test SemiPlot/SemiPlot.Tests.Unit/SemiPlot.Tests.Unit.csproj`
+      - must pass before Task 2
+
+### Task 2: The OPC UA block in the dialog
+
+**Files:**
+- Modify: `SemiPlot/SemiPlot.UI/Settings/SettingsViewModel.cs`
+- Modify: `SemiPlot/SemiPlot.UI/Settings/SettingsDialog.axaml`
+- Modify: `SemiPlot/SemiPlot.UI/Localization/Resources.resx`, `Resources.ru.resx`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewModelTests.cs`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Settings/SettingsViewTests.cs`
+- Modify: `SemiPlot/SemiPlot.Tests.Unit/UI/Startup/StartupFailureWindowTests.cs`
+
+- [x] `SettingsViewModel`: the third loaded snapshot through `SpelledAsInTheFile`; extract
+      `CanSave(bool canWriteSections)` over nine inputs; `FirstBrokenRule` arms for the OPC host and port
+      after the password; `SaveAsync` sends `opcEdits` and sets `IsRestartPending` when any `opc` key changed
+- [x] `SettingsDialog.axaml`: three rows after the password row as in Technical Details
+- [x] resx: add `SettingsOpcHostInvalid` and `SettingsOpcPortInvalid` in both files
+- [x] `SettingsViewModelTests`: `EveryFieldTheWindowEditsReachesTheFileTheLoadersRead` sets and loads the OPC
+      pair; `TheSaveCannotExecuteWhileATextFieldIsBlank` gains `OpcHost`; new
+      `TheSaveCannotExecuteWhileTheOpcPortIsNotAWholeNumberInRange` (null, 0, 65536, 49875.5); new
+      `AnOpcSaveSetsTheRestartNotice`; `ChangingOnlyTheTheme_LeavesTheConnectionFileByteIdentical...` also
+      compares `opc/opc.yaml`; `TheValidationMessageNamesTheFirstInvalidFieldInFormOrder` ends with an invalid
+      OPC host, then an empty OPC port, then empty
+- [x] `SettingsViewTests`: `TheRealisedDialog_ShowsTheLoadedValues` asserts `SettingsOpcHost` `127.0.0.1` and
+      `SettingsOpcPort` `49875`; new `TheDialog_KeepsItsSizeAndItsButtonsWhenAnOpcFieldTurnsInvalid` (types
+      `scada-01` into `SettingsOpcHost`, compares dialog and Save `Bounds`); new
+      `AnInvalidOpcHost_NamesTheOpcFieldOnTheMessageLine` (`SettingsValidationMessage` reads
+      `SettingsOpcHostInvalid`, `SettingsOpcHost` carries `invalid`, `SettingsHost` does not)
+- [x] `StartupFailureWindowTests`: new
+      `TheSettingsDialog_OverARefusedOpcPort_WritesTheFixedPortIntoTheOpcFile`, the journey of Acceptance
+      Evidence item 2
+- [x] run `dotnet build SemiPlot.slnx` and `dotnet test SemiPlot/SemiPlot.Tests.Unit/SemiPlot.Tests.Unit.csproj`
+      - must pass before Task 3
+
+### Task 3: Verify acceptance criteria
+
+- [x] verify every Overview point: the block edits `host` and `port` of `opc/`, existing keys only, only
+      changed keys, validated on a staged copy, the restart notice after an OPC save, the three-section gate
+      (each point read in the code and backed by a named test of Tasks 1-2)
+- [x] verify the scope guard: `git diff --stat origin/master...HEAD` lists no file under
+      `SemiPlot.DataSource.OpcUa`, `Chart/`, `Bridge/` or `ConfigFiles/`, and no change to
+      `OpcUaConnectionLoader`, `ConnectionFile` or `NetworkAddress` (14 files, none matches)
+- [x] measure the realised dialog with Skia and HarfBuzz, the real theme, at 14 px with 20 px lines, the
+      method of `ui-theme.md:216-231`, in English and Russian: the dialog's height with every field valid; the
+      single-line width of `SettingsOpcHostInvalid`, `SettingsOpcPortInvalid` and `SettingsKeyAbsent` with
+      `SettingsOpcHeader` and `port`, each against the message cell (457 px English, 388 px Russian). Each
+      fits its two reserved lines untrimmed; a message that does not is shortened in both files and measured
+      again (2026-10-21: dialog 640 x 575 px in both languages, every state; English 365.2, 263.8 and
+      487.0 px, Russian 354.3, 248.8 and 727.0 px; all untrimmed, the key-absent message in two lines in both;
+      nothing shortened)
+- [x] run the Acceptance Evidence command; zero failures (132/132)
+- [x] run the full suite: `dotnet test SemiPlot.slnx` (the integration project needs a container runtime)
+      (unit 2523/2523, integration 205/205)
+- [x] run `dotnet format SemiPlot.slnx --verify-no-changes` and `dotnet terse` over the touched `.cs` files;
+      both exit 0 (terse over 10 files)
+
+### Task 4: [Final] Update documentation
+
+**Files:**
+- Modify: `AGENTS.md`
+- Modify: `docs/architecture/overview.md`
+- Modify: `docs/architecture/data-integration.md`
+- Modify: `docs/architecture/ui-text.md`
+- Modify: `docs/architecture/ui-theme.md`
+- Modify: `docs/architecture/charting.md`
+- Modify: `docs/plans/backlog.md`
+- Modify: `readme.md`
+
+- [x] `AGENTS.md:33-39`: drop "and it edits no key of `opc/` yet"; the failure window offers it "only when
+      all three sections it edits, `app/`, `connection/` and `opc/`, read and carry every key it edits"
+- [x] `overview.md#the-settings-window`: `:430-432` lists `host` and `port` in `opc/` and drops the by-hand
+      sentence; `:445-452` says the OPC host and port follow the same rules; `:454-458`, `:460-462` and `:478`
+      count three sections; `:485` adds `OpcUaConnectionLoader.Load`; `:206` and `:941` lose "both"/"yet"
+- [x] `data-integration.md:1774-1775`: the settings window edits `opc/` under its usual rules, effective at
+      the next start; `:1821` counts three sections
+- [x] `ui-text.md:203-212`: three section headers including `SettingsOpcHeader`; `SettingsKeyAbsent` takes
+      the section header as `{0}` and the key as `{1}`; the form-order list ends with `SettingsOpcHostInvalid`,
+      `SettingsOpcPortInvalid`
+- [x] `ui-theme.md:216-231`: the dated rows of the Task 3 measurement (dialog height, the three messages,
+      both languages)
+- [x] `charting.md:648-649`: the modal edits the `app/`, `connection/` and `opc/` section folders
+- [x] `backlog.md`: one entry for splitting `SettingsViewModel` into a host-and-port child view model shared
+      by the archive and OPC UA blocks, with the class size and the duplicated setter shape as the reason
+- [x] `readme.md:74-76`: the settings window sets the OPC UA server address; drop the sentence that sends the
+      operator to `opc/opc.yaml`
+- [x] `git grep -n -i "opc.*by hand\|edits no key of\|does not edit .*opc\|both sections\|both section folders\|two section headers" -- AGENTS.md readme.md docs/architecture`
+      prints only `data-integration.md:1493` and `ui-text.md:239`, which name the port bounds of
+      `connection/` and `opc/` and stay (2026-10-21: those two lines only, `ui-text.md:243` after the edits
+      above it)
+- [x] move this plan to `docs/plans/completed/` (left in place: archiving belongs to the delivery step)
+
+## Post-Completion
+
+*Items requiring manual intervention or external systems - no checkboxes, informational only*
+
+**Manual verification:**
+
+1. Start the viewer against `SemiPlot/Artifacts/dev-config` in English: `Edit` -> `Settings` shows the
+   "OPC UA connection" block after the archive block with `127.0.0.1` and the copy's port.
+2. Type `scada-01` into the OPC host: the field takes the error border, the message line reads the OPC
+   host rule untrimmed, and the dialog does not change size.
+3. Switch the language to Russian, restart, repeat step 2: the Russian message fits the two reserved lines
+   untrimmed.
+4. Remove `port` from `opc/opc.yaml`, reopen the dialog: the message line names "Подключение к серверу OPC
+   UA" and `port`, untrimmed, and Save is disabled.
+5. Restore the file, set the OPC port to the demo stand's 55875, save, press Restart now: the status bar's
+   OPC indicator reaches "OPC UA connected".
+6. Write `port: 70000` into the copy's `opc/opc.yaml` and start: the failure window offers Settings, the
+   dialog fixes the port, Restart now starts a copy that connects.
+
+**Screenshot:** `readme.md:87` embeds `docs/img/settings.png`, which still shows the removed poll field and
+no OPC block. It needs a recapture by hand after this slice merges.
+
+**Remaining slices:**
+
+- live-hold-to-edge: the grey ahead-break span and the held line to the live edge, render only.
+
+**Executed by exec:**
+
+- branch: opc-settings
+
+## Verify it yourself
+
+1. The failure-window gap. On master `5f8ea2c` `StartupFailureViewModel.EverySavedKeyPresent` reads
+   `app/` and `connection/` only, so a copy whose `opc/` folder is deleted still offers Settings. Task 1
+   ran the new `opc-missing-directory` row red before the gate edit ("Expected viewModel.OffersSettings
+   to be False, but found True") and green after it; the row ships in `67d7979`. Run:
+
+   ```powershell
+   dotnet test SemiPlot/SemiPlot.Tests.Unit/SemiPlot.Tests.Unit.csproj --filter "FullyQualifiedName~TheSettingsButton_IsHiddenWhenASectionCannotSave|FullyQualifiedName~TheSettingsButton_IsShownOverAnOpcValueTheLoaderRefuses"
+   ```
+
+   By hand: delete `opc/` from a copy of `ConfigFiles` with the password filled and start the viewer on
+   it. The failure window names the `opc/` failure and shows no Settings button. Before this branch it
+   showed the button over a dialog that could not fix the fault.
+2. The fix end to end. `TheSettingsDialog_OverARefusedOpcPort_WritesTheFixedPortIntoTheOpcFile` writes
+   `port: 70000` into the copy's `opc/opc.yaml`, opens the dialog from the failure window, types 49875 and
+   saves; `OpcUaConnectionLoader.Load` then reads 49875, and `app.yaml` and `connection.yaml` stay
+   byte-identical. By hand this is Post-Completion step 6. Before this branch the dialog had no OPC UA
+   block, so the failure window offered no way to fix the port.
+3. The whole acceptance set, from the repository root; it reports zero failures (138 on `16a57f5`):
+
+   ```powershell
+   dotnet test SemiPlot/SemiPlot.Tests.Unit/SemiPlot.Tests.Unit.csproj --filter "FullyQualifiedName~SemiPlot.Tests.Unit.UI.Settings|FullyQualifiedName~StartupFailureWindowTests|FullyQualifiedName~ResourcesTests"
+   ```
+
+4. What no headless test shows: the dialog's height and whether the new messages fit. Task 3 measured
+   640 x 575 px in both languages and every message untrimmed; the tightest is the English
+   `SettingsKeyAbsent` with the OPC UA header, whose first line is 445.6 of 457 px. Post-Completion steps
+   1 to 4 check this on the operator's screen.
diff --git a/readme.md b/readme.md
index ddb8935..f775b7f 100644
--- a/readme.md
+++ b/readme.md
@@ -71,9 +71,8 @@ SemiPlot — программа просмотра трендов техноло
 - Команда «Файл → Экспорт в CSV...» записывает в файл CSV исходные строки архива выбранных перьев за
   интервал до 24 часов одной таблицей по времени: перо, время, значение. Заголовок следует языку интерфейса, а
   разделитель и десятичный знак берутся из региональных настроек Windows.
-- Окно «Правка → Настройки» задаёт язык интерфейса, тему оформления и параметры подключения к архиву.
-  Смена темы применяется во всех открытых окнах сразу. Адрес сервера OPC UA задаётся в файле
-  `opc/opc.yaml` каталога конфигурации.
+- Окно «Правка → Настройки» задаёт язык интерфейса, тему оформления, параметры подключения к архиву и
+  адрес сервера OPC UA. Смена темы применяется во всех открытых окнах сразу.
 - В строке состояния два индикатора: связь с базой данных и связь с сервером OPC UA.
 - Панель сообщений собирает все ошибки чтения и записи.
 - При ошибке запуска программа открывает окно ошибки, из которого доступны настройки и перезапуск.