diff --git a/src/protocols/ioxide.http3/Http3Connection.cs b/src/protocols/ioxide.http3/Http3Connection.cs
index 0b7e3fc5..ef063d82 100644
--- a/src/protocols/ioxide.http3/Http3Connection.cs
+++ b/src/protocols/ioxide.http3/Http3Connection.cs
@@ -545,6 +545,12 @@ private void OnHeadersComplete(long sid, ReqStream rs)
             return;
         }
 
+        if (rs.Request.Malformed)
+        {
+            RefuseMalformed(sid, rs, "a pseudo-header in the request is repeated, unknown or after a regular field");
+            return;
+        }
+
         if (_streamBodies)
         {
             var sink = new Http3BodyReader(this, sid);
diff --git a/src/protocols/ioxide.http3/Http3Request.cs b/src/protocols/ioxide.http3/Http3Request.cs
index 11435dbc..2c297cf1 100644
--- a/src/protocols/ioxide.http3/Http3Request.cs
+++ b/src/protocols/ioxide.http3/Http3Request.cs
@@ -37,16 +37,19 @@ public sealed class Http3Request
     // The DATA payload content-length still owes: -1 without one, -2 for an invalid or repeated one.
     internal long ContentLengthLeft = -1;
 
+    // A pseudo-header out of place: the stream is refused once the section is decoded, never dispatched.
+    internal bool Malformed;
+
     // The QPACK decoder lands every decoded field here; pseudo-headers route by byte compare.
     internal void AddField(ReadOnlySpan<byte> name, ReadOnlySpan<byte> value)
     {
         if (name.Length > 0 && name[0] == (byte)':')
         {
-            (int Off, int Len) r = Append(value);
-            if      (name.SequenceEqual(":method"u8))    MethodR = r;
-            else if (name.SequenceEqual(":path"u8))      PathR = r;
-            else if (name.SequenceEqual(":scheme"u8))    SchemeR = r;
-            else if (name.SequenceEqual(":authority"u8)) AuthorityR = r;
+            if      (name.SequenceEqual(":method"u8))    Pseudo(ref MethodR, value);
+            else if (name.SequenceEqual(":path"u8))      Pseudo(ref PathR, value);
+            else if (name.SequenceEqual(":scheme"u8))    Pseudo(ref SchemeR, value);
+            else if (name.SequenceEqual(":authority"u8)) Pseudo(ref AuthorityR, value);
+            else Malformed = true;
             return;
         }
 
@@ -64,6 +67,13 @@ internal void AddField(ReadOnlySpan<byte> name, ReadOnlySpan<byte> value)
         }
     }
 
+    // RFC 9114 4.3: each at most once, and ahead of every regular field.
+    private void Pseudo(ref (int Off, int Len) range, ReadOnlySpan<byte> value)
+    {
+        Malformed |= range.Len >= 0 || HeaderRanges.Count > 0;
+        range = Append(value);
+    }
+
     private (int Off, int Len) Append(ReadOnlySpan<byte> data)
     {
         if (Arena.Length - ArenaUsed < data.Length)
diff --git a/tests/Ioxide.Tests.E2E/Protocols/H3ErrorCodeTests.cs b/tests/Ioxide.Tests.E2E/Protocols/H3ErrorCodeTests.cs
index 70d0da08..3586d7e0 100644
--- a/tests/Ioxide.Tests.E2E/Protocols/H3ErrorCodeTests.cs
+++ b/tests/Ioxide.Tests.E2E/Protocols/H3ErrorCodeTests.cs
@@ -39,6 +39,7 @@ internal static class H3ErrorCodeTests
     private const ulong H3FrameError             = 0x0106;
     private const ulong H3ExcessiveLoad          = 0x0107;
     private const ulong H3RequestIncomplete      = 0x010d;
+    private const ulong H3MessageError           = 0x010e;
     private const ulong QpackDecompressionFailed = 0x0200;
 
     // A GET framed by hand: HEADERS(len 6), field-section prefix 00 00 (Required Insert Count 0,
@@ -62,6 +63,52 @@ public static void Register(Runner runner)
 
     private static void RegisterCodeSeam(Runner runner)
     {
+        // RFC 9114 4.3. Hand-framed like WellFormedGet: 0xD1 :method GET, 0xD7 :scheme https, 0xC1 :path /, 0xC0 :authority "".
+        byte[] field = [0x23, (byte)'x', (byte)'-', (byte)'a', 0x01, (byte)'v'];
+        byte[] admin = [0x51, 6, .. "/admin"u8.ToArray()];   // :path, as a literal on its static name
+        foreach ((string name, byte[] fields) in new[]
+        {
+            ("a repeated :method", (byte[])[0xD1, 0xD7, 0xC1, 0xC0, 0xD1]),
+            ("a repeated :scheme", (byte[])[0xD1, 0xD7, 0xC1, 0xC0, 0xD7]),
+            ("a repeated :path", (byte[])[0xD1, 0xD7, 0xC1, 0xC0, .. admin]),
+            ("a repeated :authority", (byte[])[0xD1, 0xD7, 0xC1, 0xC0, 0xC0]),
+            ("a pseudo-header after a regular field", (byte[])[0xD1, 0xD7, 0xC1, .. field, 0xC0]),
+            ("an unknown pseudo-header", (byte[])[0xD1, 0xD7, 0xC1, 0xC0, 0x22, (byte)':', (byte)'x', 0x01, (byte)'v']),
+        })
+        {
+            string shape = name;
+            byte[] headers = [0x01, (byte)(fields.Length + 2), 0x00, 0x00, .. fields];
+
+            runner.Test($"http3/codes: {shape} is H3_MESSAGE_ERROR on its stream, which no handler sees", () =>
+            {
+                var quic = new RecordingQuic();
+                Task run = StartPure(quic, out Served served, closeTransport: true, (0, headers, true), (4, WellFormedGet, true));
+
+                Assert.True(run.IsCompleted, "the run loop should complete inline once the transport closes");
+                Assert.True(quic.Resets.SequenceEqual([(0L, H3MessageError)]),
+                    $"expected stream 0 reset with H3_MESSAGE_ERROR and nothing else, got [{string.Join(", ", quic.Resets)}]");
+                Assert.Equal(1, served.Count);           // stream 4 alone
+                Assert.True(quic.ClosedWith is null, "one malformed request closed the connection");
+            });
+
+            runner.Test($"http3/codes: {shape} never reaches a handler taking a body reader", () =>
+            {
+                var quic = new RecordingQuic();
+                Assert.True(quic.EnqueueStreamData(0, headers, true), "the recv ring rejected the item");
+                quic.MarkClosed();
+
+                int served = 0;
+                Task run = new Http3Connection(quic).RunAsync((_, _) =>
+                {
+                    served++;
+                    return new ValueTask<Http3Response>(Http3Response.Text("ok"));
+                });
+
+                Assert.True(run.IsCompleted, "the run loop should complete inline once the transport closes");
+                Assert.Equal(0, served);
+            });
+        }
+
         runner.Test("http3/codes: control - a hand-framed GET is served through the recording transport", () =>
         {
             var quic = new RecordingQuic();