diff --git a/SECURITY.md b/SECURITY.md
index 0e49834..05c9354 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -104,9 +104,9 @@ Transfer-Encoding: chunked\r\n
 
 **Attack:** Invalid version strings enable protocol downgrade attacks and undefined behavior.
 
-**RFC:** 9112 Section 2.6 — `HTTP-version = "HTTP/" DIGIT "." DIGIT`
+**RFC:** 9112 Section 2.3 — `HTTP-version = "HTTP/" DIGIT "." DIGIT`
 
-**Protection:** The version string must be exactly 8 bytes matching `HTTP/X.Y` where X and Y are ASCII digits.
+**Protection:** Only `HTTP/1.0` and `HTTP/1.1` are accepted, as exactly 8 case-sensitive bytes. A well-formed version the parser does not implement (`HTTP/2.0`, `HTTP/1.2`, `HTTP/0.9`, …) is rejected with 505 (RFC 9110 Section 15.6.6); anything that does not match the grammar at all (`http/1.1`, `HTTP/1.x`, `HTTP/11`, …) is a malformed message and rejected with 400.
 
 ### Resource Limits (DoS Prevention)
 
diff --git a/src/Glyph11/HttpParseException.cs b/src/Glyph11/HttpParseException.cs
index d327f36..b84e1ba 100644
--- a/src/Glyph11/HttpParseException.cs
+++ b/src/Glyph11/HttpParseException.cs
@@ -12,9 +12,12 @@ public HttpParseException(string message, int statusCode = 400) : base(message)
         StatusCode = statusCode;
     }
 
-    /// <summary>The HTTP status code to return (400 for structural errors, 431 for limit breaches).</summary>
+    /// <summary>
+    /// The HTTP status code to return: 400 for structural or semantic errors, 431 for limit
+    /// breaches, 505 for a well-formed HTTP version the parser does not implement.
+    /// </summary>
     public int StatusCode { get; }
 
-    /// <summary>True when the error is a size/count limit breach (→ 431), false for structural errors (→ 400).</summary>
-    public bool IsLimitViolation => StatusCode != 400;
+    /// <summary>True when the error is a size/count limit breach (→ 431); false otherwise (400, 505).</summary>
+    public bool IsLimitViolation => StatusCode == 431;
 }
diff --git a/src/Glyph11/Parser/ParserConstants.cs b/src/Glyph11/Parser/ParserConstants.cs
index 0a26492..9767479 100644
--- a/src/Glyph11/Parser/ParserConstants.cs
+++ b/src/Glyph11/Parser/ParserConstants.cs
@@ -63,8 +63,9 @@ public static bool IsValidFieldValue(ReadOnlySpan<byte> span)
         => span.IndexOfAnyExcept(FieldValueSearchValues) < 0;
 
     /// <summary>
-    /// HTTP-version = "HTTP/" DIGIT "." DIGIT  (RFC 9112 §2.6)
-    /// Exactly 8 bytes, case-sensitive.
+    /// HTTP-version = "HTTP/" DIGIT "." DIGIT  (RFC 9112 §2.3)
+    /// Exactly 8 bytes, case-sensitive — and one of the two versions this parser implements,
+    /// <c>HTTP/1.0</c> or <c>HTTP/1.1</c>.
     /// </summary>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static bool IsValidHttpVersion(ReadOnlySpan<byte> span)
@@ -80,6 +81,25 @@ public static bool IsValidHttpVersion(ReadOnlySpan<byte> span)
             && (span[7] == (byte)'0' || span[7] == (byte)'1');
     }
 
+    /// <summary>
+    /// True if <paramref name="span"/> matches the HTTP-version grammar
+    /// (<c>"HTTP/" DIGIT "." DIGIT</c>, exactly 8 bytes, case-sensitive — RFC 9112 §2.3), whatever
+    /// the digits. Separates a version the parser does not implement (505) from bytes that are not
+    /// an HTTP-version at all (400).
+    /// </summary>
+    internal static bool IsWellFormedHttpVersion(ReadOnlySpan<byte> span)
+    {
+        return span.Length == 8
+            && span[0] == (byte)'H'
+            && span[1] == (byte)'T'
+            && span[2] == (byte)'T'
+            && span[3] == (byte)'P'
+            && span[4] == (byte)'/'
+            && IsDigit(span[5])
+            && span[6] == (byte)'.'
+            && IsDigit(span[7]);
+    }
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static bool IsDigit(byte b) => (uint)(b - '0') <= 9;
 
diff --git a/src/Glyph11/Parser/UltraHardened/UltraHardenedParser.FullHeader.ROM.cs b/src/Glyph11/Parser/UltraHardened/UltraHardenedParser.FullHeader.ROM.cs
index 228bc69..0ad5271 100644
--- a/src/Glyph11/Parser/UltraHardened/UltraHardenedParser.FullHeader.ROM.cs
+++ b/src/Glyph11/Parser/UltraHardened/UltraHardenedParser.FullHeader.ROM.cs
@@ -94,9 +94,13 @@ public static bool TryExtractFullHeaderROM(
             throw new HttpParseException("Request-target contains invalid characters.");
 
         // --- Version ---
+        // A well-formed HTTP-version this parser does not implement is 505 (RFC 9110 §15.6.6);
+        // bytes that are not an HTTP-version at all make a malformed request line, 400.
         var versionSpan = requestLine[(secondSpace + 1)..];
         if (!ParserConstants.IsValidHttpVersion(versionSpan))
-            throw new HttpParseException("Invalid HTTP version.", 505);
+            throw ParserConstants.IsWellFormedHttpVersion(versionSpan)
+                ? new HttpParseException("Unsupported HTTP version.", 505)
+                : new HttpParseException("Invalid HTTP version.");
 
         request.Version = input.Slice(secondSpace + 1, versionSpan.Length);
 
diff --git a/src/Glyph11/Parser/UltraHardened/UltraHardenedParser.FullResponse.ROM.cs b/src/Glyph11/Parser/UltraHardened/UltraHardenedParser.FullResponse.ROM.cs
index 078e681..97ed7ee 100644
--- a/src/Glyph11/Parser/UltraHardened/UltraHardenedParser.FullResponse.ROM.cs
+++ b/src/Glyph11/Parser/UltraHardened/UltraHardenedParser.FullResponse.ROM.cs
@@ -59,9 +59,13 @@ public static bool TryExtractFullResponseHeaderROM(
             throw new HttpParseException("Invalid status line: missing status code.");
 
         // --- Version ---
+        // Same rule as the request line: a well-formed version this parser does not implement is
+        // 505, anything else in this position is a malformed status line, 400.
         var versionSpan = statusLine[..firstSpace];
         if (!ParserConstants.IsValidHttpVersion(versionSpan))
-            throw new HttpParseException("Invalid HTTP version.", 505);
+            throw ParserConstants.IsWellFormedHttpVersion(versionSpan)
+                ? new HttpParseException("Unsupported HTTP version.", 505)
+                : new HttpParseException("Invalid HTTP version.");
 
         response.Version = input[..firstSpace];
 
diff --git a/tests/Tests/UltraHardenedParser.VersionStatus.cs b/tests/Tests/UltraHardenedParser.VersionStatus.cs
new file mode 100644
index 0000000..3f8dcf7
--- /dev/null
+++ b/tests/Tests/UltraHardenedParser.VersionStatus.cs
@@ -0,0 +1,190 @@
+using System.Buffers;
+using System.Text;
+using Glyph11;
+using Glyph11.Parser;
+using Glyph11.Parser.UltraHardened;
+using Glyph11.Protocol;
+using Glyph11.Utils;
+
+namespace Tests;
+
+/// <summary>
+/// The status code carried by a bad HTTP-version: a version that matches the grammar
+/// (<c>"HTTP/" DIGIT "." DIGIT</c>) but is not one the parser implements is 505; bytes that are not
+/// an HTTP-version at all are a malformed message, 400. The rule is the same on the request line
+/// and the status line.
+/// </summary>
+public class HttpVersionStatusTests : IDisposable
+{
+    private readonly BinaryRequest _request = new();
+    private readonly BinaryResponse _response = new();
+    private static readonly ParserLimits Defaults = ParserLimits.Default;
+
+    public void Dispose()
+    {
+        _request.Dispose();
+        _response.Dispose();
+        GC.SuppressFinalize(this);
+    }
+
+    public static TheoryData<string, bool> Unsupported =>
+        BothSegmentations("HTTP/2.0", "HTTP/1.2", "HTTP/0.9", "HTTP/3.0");
+
+    public static TheoryData<string, bool> Malformed =>
+        BothSegmentations("http/1.1", "HTTP/1.x", "HTTP1.1", "HTTP/11", "HTTP/1.1x", "HTTP/1.", "HTTP/");
+
+    public static TheoryData<string, bool> Implemented =>
+        BothSegmentations("HTTP/1.0", "HTTP/1.1");
+
+    // ================================================================
+    // Request line
+    // ================================================================
+
+    [Theory]
+    [MemberData(nameof(Unsupported))]
+    public void Request_WellFormedUnsupportedVersion_Throws505(string version, bool multi)
+    {
+        var ex = Assert.Throws<HttpParseException>(
+            () => ParseRequest($"GET / {version}\r\nHost: x\r\n\r\n", multi));
+
+        Assert.Equal(505, ex.StatusCode);
+        Assert.False(ex.IsLimitViolation);
+    }
+
+    [Theory]
+    [MemberData(nameof(Malformed))]
+    public void Request_MalformedVersion_Throws400(string version, bool multi)
+    {
+        var ex = Assert.Throws<HttpParseException>(
+            () => ParseRequest($"GET / {version}\r\nHost: x\r\n\r\n", multi));
+
+        Assert.Equal(400, ex.StatusCode);
+        Assert.False(ex.IsLimitViolation);
+    }
+
+    [Theory]
+    [MemberData(nameof(Implemented))]
+    public void Request_ImplementedVersion_Parses(string version, bool multi)
+    {
+        Assert.True(ParseRequest($"GET / {version}\r\nHost: x\r\n\r\n", multi));
+        AssertAscii.Equal(version, _request.Version);
+    }
+
+    // ================================================================
+    // Status line
+    // ================================================================
+
+    [Theory]
+    [MemberData(nameof(Unsupported))]
+    public void Response_WellFormedUnsupportedVersion_Throws505(string version, bool multi)
+    {
+        var ex = Assert.Throws<HttpParseException>(
+            () => ParseResponse($"{version} 200 OK\r\nContent-Length: 0\r\n\r\n", multi));
+
+        Assert.Equal(505, ex.StatusCode);
+        Assert.False(ex.IsLimitViolation);
+    }
+
+    [Theory]
+    [MemberData(nameof(Malformed))]
+    public void Response_MalformedVersion_Throws400(string version, bool multi)
+    {
+        var ex = Assert.Throws<HttpParseException>(
+            () => ParseResponse($"{version} 200 OK\r\nContent-Length: 0\r\n\r\n", multi));
+
+        Assert.Equal(400, ex.StatusCode);
+        Assert.False(ex.IsLimitViolation);
+    }
+
+    [Theory]
+    [MemberData(nameof(Implemented))]
+    public void Response_ImplementedVersion_Parses(string version, bool multi)
+    {
+        Assert.True(ParseResponse($"{version} 200 OK\r\nContent-Length: 0\r\n\r\n", multi));
+        AssertAscii.Equal(version, _response.Version);
+    }
+
+    // ================================================================
+    // IsLimitViolation means 431 and nothing else
+    // ================================================================
+
+    [Fact]
+    public void IsLimitViolation_OnlyFor431()
+    {
+        Assert.False(new HttpParseException("structural").IsLimitViolation);
+        Assert.False(new HttpParseException("unsupported version", 505).IsLimitViolation);
+        Assert.True(new HttpParseException("too large", 431).IsLimitViolation);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void Request_LimitBreach_IsLimitViolation(bool multi)
+    {
+        var limits = Defaults with { MaxTotalHeaderBytes = 20 };
+
+        var ex = Assert.Throws<HttpParseException>(
+            () => ParseRequest("GET / HTTP/1.1\r\nHost: localhost\r\n\r\n", multi, limits));
+
+        Assert.Equal(431, ex.StatusCode);
+        Assert.True(ex.IsLimitViolation);
+    }
+
+    // ================================================================
+    // Helpers
+    // ================================================================
+
+    private static TheoryData<string, bool> BothSegmentations(params string[] versions)
+    {
+        var data = new TheoryData<string, bool>();
+        foreach (var v in versions)
+        {
+            data.Add(v, false);
+            data.Add(v, true);
+        }
+        return data;
+    }
+
+    private bool ParseRequest(string raw, bool multiSegment)
+        => ParseRequest(raw, multiSegment, Defaults);
+
+    private bool ParseRequest(string raw, bool multiSegment, ParserLimits limits)
+    {
+        var bytes = Encoding.ASCII.GetBytes(raw);
+
+        if (multiSegment)
+        {
+            var seq = SplitIntoSegments(bytes);
+            return UltraHardenedParser.TryExtractFullHeaderValidated(ref seq, _request, in limits, out _);
+        }
+
+        ReadOnlyMemory<byte> rom = bytes;
+        return UltraHardenedParser.TryExtractFullHeaderROM(ref rom, _request, in limits, out _);
+    }
+
+    private bool ParseResponse(string raw, bool multiSegment)
+    {
+        var bytes = Encoding.ASCII.GetBytes(raw);
+        var limits = Defaults;
+
+        if (multiSegment)
+        {
+            var seq = SplitIntoSegments(bytes);
+            return UltraHardenedParser.TryExtractFullResponseHeaderValidated(ref seq, _response, in limits, out _);
+        }
+
+        ReadOnlyMemory<byte> rom = bytes;
+        return UltraHardenedParser.TryExtractFullResponseHeaderROM(ref rom, _response, in limits, out _);
+    }
+
+    private static ReadOnlySequence<byte> SplitIntoSegments(byte[] data)
+    {
+        int split1 = data.Length / 3;
+        int split2 = 2 * data.Length / 3;
+
+        var first = new BufferSegment(data.AsMemory(0, split1));
+        var last = first.Append(data.AsMemory(split1, split2 - split1)).Append(data.AsMemory(split2));
+
+        return new ReadOnlySequence<byte>(first, 0, last, last.Memory.Length);
+    }
+}