diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index 1c77125..e7d72af 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -166,3 +166,5 @@ Full doctrine: `../docs/ARCHITECTURE.md#framework-vs-consumer-boundary`
 19. **Documentation layering** — write `.md` documentation at the **defining layer** (where the code lives). Higher-layer `.md` files reference the base docs (e.g. `see AyCode.Services/docs/SIGNALR/README.md`) and document only project-specific overrides or extensions. Never duplicate base-layer descriptions in consumer-level docs.
 20. **Do not re-read .md files** already in your context window. They only change if you modify them yourself (new content is already in context) or if the developer tells you they changed — in that case re-read them once.
 21. **Folder navigation** — start from the root `README.md` for solution-level navigation. When you need to understand a folder's contents or find a type/class, read the `README.md` in that folder first — it indexes the local files and sub-folders. Follow this before grepping or reading source files.
+
+22. **Language Preference**: Communicate in Hungarian as requested by the user.
diff --git a/AyCode.Core.Tests/Serialization/Utf8TranscoderTests.cs b/AyCode.Core.Tests/Serialization/Utf8TranscoderTests.cs
new file mode 100644
index 0000000..0318c29
--- /dev/null
+++ b/AyCode.Core.Tests/Serialization/Utf8TranscoderTests.cs
@@ -0,0 +1,310 @@
+using System.Text;
+using AyCode.Core.Serializers.Binaries;
+
+namespace AyCode.Core.Tests.Serialization;
+
+/// <summary>
+/// Round-trip and correctness tests for <see cref="Utf8Transcoder"/>'s SIMD path tiers.
+///
+/// <para><b>Critical coverage</b>: each path tier (Vector512 / Vector256 / Vector128 / scalar) has
+/// minimum-size and boundary-crossing inputs to ensure the path is actually exercised. The
+/// Hungarian benchmark in <c>BenchmarkTestDataProvider</c> bails out of the AVX2 ASCII-prefix
+/// path early (first non-ASCII byte at position 4-5), so it cannot validate the long-ASCII path
+/// on its own. These tests fill that gap.</para>
+/// </summary>
+[TestClass]
+public class Utf8TranscoderTests
+{
+    private static readonly Encoding Utf8 = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);
+
+    // ──────────────────────────────────────────────────────────────────────
+    // CountUtf8Chars — content classes
+    // ──────────────────────────────────────────────────────────────────────
+
+    [TestMethod]
+    public void CountUtf8Chars_AsciiOnly_MatchesStringLength()
+    {
+        var s = "Hello, World! This is plain ASCII.";
+        var bytes = Utf8.GetBytes(s);
+        Assert.AreEqual(s.Length, Utf8Transcoder.CountUtf8Chars(bytes));
+    }
+
+    [TestMethod]
+    public void CountUtf8Chars_HungarianMixed_MatchesStringLength()
+    {
+        var s = "árvíztűrő tükörfúrógép";
+        var bytes = Utf8.GetBytes(s);
+        Assert.AreEqual(s.Length, Utf8Transcoder.CountUtf8Chars(bytes));
+    }
+
+    [TestMethod]
+    public void CountUtf8Chars_CjkBmp_MatchesStringLength()
+    {
+        var s = "你好世界 こんにちは 안녕하세요";
+        var bytes = Utf8.GetBytes(s);
+        Assert.AreEqual(s.Length, Utf8Transcoder.CountUtf8Chars(bytes));
+    }
+
+    [TestMethod]
+    public void CountUtf8Chars_SupplementaryPlane_CountsSurrogatePairs()
+    {
+        // Each emoji is U+1F600-range (4-byte UTF-8 → 2-char surrogate pair in UTF-16)
+        var s = "😀😁😂🎉";  // 4 codepoints, but 8 chars in UTF-16
+        var bytes = Utf8.GetBytes(s);
+        Assert.AreEqual(s.Length, Utf8Transcoder.CountUtf8Chars(bytes));
+        Assert.AreEqual(8, s.Length, "Sanity check: each emoji is a surrogate pair");
+    }
+
+    [TestMethod]
+    public void CountUtf8Chars_MixedAllClasses_MatchesStringLength()
+    {
+        var s = "ASCII Magyar:árvíz CJK:你好 Emoji:😀";
+        var bytes = Utf8.GetBytes(s);
+        Assert.AreEqual(s.Length, Utf8Transcoder.CountUtf8Chars(bytes));
+    }
+
+    [TestMethod]
+    public void CountUtf8Chars_Empty_ReturnsZero()
+    {
+        Assert.AreEqual(0, Utf8Transcoder.CountUtf8Chars(ReadOnlySpan<byte>.Empty));
+    }
+
+    // ──────────────────────────────────────────────────────────────────────
+    // EncodeUtf8SinglePass + DecodeUtf8SinglePass — round-trip per content class
+    // ──────────────────────────────────────────────────────────────────────
+
+    [TestMethod]
+    public void EncodeDecode_AsciiShort_RoundTrip()
+    {
+        AssertRoundTrip("Hello");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_AsciiExactly31Bytes_RoundTrip()
+    {
+        // Boundary: just below FixStr 31-byte limit, just below Vector256 threshold (32)
+        AssertRoundTrip(new string('a', 31));
+    }
+
+    [TestMethod]
+    public void EncodeDecode_AsciiExactly32Bytes_RoundTrip()
+    {
+        // Boundary: exactly Vector256<byte>.Count — Phase 1 AVX2 widen path triggers
+        // CRITICAL: this validates the Vector256.Widen upper-half store offset bug-fix.
+        AssertRoundTrip(new string('a', 32));
+    }
+
+    [TestMethod]
+    public void EncodeDecode_AsciiLong_64Bytes_RoundTrip()
+    {
+        // Boundary: Vector512 threshold for the encoder; 2× Vector256 iter for the decoder
+        AssertRoundTrip(new string('x', 64));
+    }
+
+    [TestMethod]
+    public void EncodeDecode_AsciiVeryLong_500Bytes_RoundTrip()
+    {
+        // Multi-iter SIMD widen on the decoder; AVX-512 path on capable hosts
+        AssertRoundTrip(new string('z', 500));
+    }
+
+    [TestMethod]
+    public void EncodeDecode_HungarianShort_RoundTrip()
+    {
+        AssertRoundTrip("Termék");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_HungarianMedium_RoundTrip()
+    {
+        AssertRoundTrip("árvíztűrő tükörfúrógép");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_HungarianLong_RoundTrip()
+    {
+        // Long enough to span multiple Vector128/256 iterations
+        AssertRoundTrip(string.Concat(Enumerable.Repeat("árvíztűrő tükörfúrógép ", 20)));
+    }
+
+    [TestMethod]
+    public void EncodeDecode_CjkBmp_RoundTrip()
+    {
+        AssertRoundTrip("你好世界 こんにちは 안녕하세요");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_CjkBmpLong_RoundTrip()
+    {
+        AssertRoundTrip(string.Concat(Enumerable.Repeat("你好世界 ", 30)));
+    }
+
+    [TestMethod]
+    public void EncodeDecode_SupplementaryPlane_RoundTrip()
+    {
+        AssertRoundTrip("😀😁😂🎉🌟");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_MixedAllClasses_RoundTrip()
+    {
+        AssertRoundTrip("Plain ASCII + Magyar (árvíztűrő) + CJK (你好世界) + Emoji (😀🎉)");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_LongMixed_RoundTrip()
+    {
+        // Long mixed content forcing all SIMD tiers + scalar tail to engage
+        var sb = new StringBuilder();
+        for (var i = 0; i < 50; i++)
+        {
+            sb.Append("ASCII run-").Append(i).Append(" Magyar:árvíz CJK:你好 ");
+        }
+        AssertRoundTrip(sb.ToString());
+    }
+
+    [TestMethod]
+    public void EncodeDecode_BoundaryAsciiToHungarian_RoundTrip()
+    {
+        // ASCII prefix exactly at common boundaries, then non-ASCII switch
+        for (var asciiLen = 0; asciiLen <= 64; asciiLen++)
+        {
+            var s = new string('a', asciiLen) + "árvíz";
+            AssertRoundTrip(s, $"asciiLen={asciiLen}");
+        }
+    }
+
+    [TestMethod]
+    public void EncodeDecode_BoundaryAsciiToCjk_RoundTrip()
+    {
+        // 3-byte sequence boundary stress
+        for (var asciiLen = 0; asciiLen <= 64; asciiLen++)
+        {
+            var s = new string('a', asciiLen) + "你好世界";
+            AssertRoundTrip(s, $"asciiLen={asciiLen}");
+        }
+    }
+
+    [TestMethod]
+    public void EncodeDecode_BoundaryAsciiToEmoji_RoundTrip()
+    {
+        // 4-byte sequence boundary (surrogate pair in UTF-16)
+        for (var asciiLen = 0; asciiLen <= 64; asciiLen++)
+        {
+            var s = new string('a', asciiLen) + "😀";
+            AssertRoundTrip(s, $"asciiLen={asciiLen}");
+        }
+    }
+
+    [TestMethod]
+    public void EncodeDecode_Empty_RoundTrip()
+    {
+        AssertRoundTrip(string.Empty);
+    }
+
+    [TestMethod]
+    public void EncodeDecode_SingleAsciiChar_RoundTrip()
+    {
+        AssertRoundTrip("X");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_SingleHungarianChar_RoundTrip()
+    {
+        AssertRoundTrip("é");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_SingleCjkChar_RoundTrip()
+    {
+        AssertRoundTrip("好");
+    }
+
+    [TestMethod]
+    public void EncodeDecode_SingleEmoji_RoundTrip()
+    {
+        AssertRoundTrip("😀");
+    }
+
+    // ──────────────────────────────────────────────────────────────────────
+    // Decoder-side cross-check: BCL Encoding.UTF8.GetString reference
+    // ──────────────────────────────────────────────────────────────────────
+
+    [TestMethod]
+    public void DecodeUtf8SinglePass_MatchesBclGetString_Ascii()
+    {
+        AssertDecodeMatchesBcl("ASCII test string with spaces and digits 0123456789.");
+    }
+
+    [TestMethod]
+    public void DecodeUtf8SinglePass_MatchesBclGetString_LongAscii32Plus()
+    {
+        // CRITICAL — exercises the Vector256 ASCII prefix widen path that had the offset bug
+        AssertDecodeMatchesBcl(new string('A', 32));
+        AssertDecodeMatchesBcl(new string('A', 33));
+        AssertDecodeMatchesBcl(new string('A', 64));
+        AssertDecodeMatchesBcl(new string('A', 65));
+        AssertDecodeMatchesBcl(new string('B', 100));
+        AssertDecodeMatchesBcl(new string('C', 256));
+    }
+
+    [TestMethod]
+    public void DecodeUtf8SinglePass_MatchesBclGetString_Hungarian()
+    {
+        AssertDecodeMatchesBcl("árvíztűrő tükörfúrógép");
+    }
+
+    [TestMethod]
+    public void DecodeUtf8SinglePass_MatchesBclGetString_Mixed()
+    {
+        AssertDecodeMatchesBcl("Plain ASCII + Magyar (árvíz) + CJK (你好) + Emoji (😀)");
+    }
+
+    // ──────────────────────────────────────────────────────────────────────
+    // Helpers
+    // ──────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Verifies that EncodeUtf8SinglePass produces bytes identical to <see cref="Encoding.UTF8.GetBytes"/>,
+    /// and that DecodeUtf8SinglePass on those bytes reconstructs the original string exactly.
+    /// </summary>
+    private static void AssertRoundTrip(string original, string? context = null)
+    {
+        var ctx = context is null ? string.Empty : $" [{context}]";
+
+        // 1. Encoder produces bytes identical to BCL Encoding.UTF8
+        var dst = new byte[original.Length * 4];  // worst-case UTF-8
+        var bytesWritten = Utf8Transcoder.EncodeUtf8SinglePass(original.AsSpan(), dst.AsSpan());
+        var encoded = dst.AsSpan(0, bytesWritten).ToArray();
+        var bclEncoded = Utf8.GetBytes(original);
+        CollectionAssert.AreEqual(bclEncoded, encoded, $"Encoder output mismatch{ctx}");
+
+        // 2. CountUtf8Chars matches the original char count
+        var charCount = Utf8Transcoder.CountUtf8Chars(encoded);
+        Assert.AreEqual(original.Length, charCount, $"Char count mismatch{ctx}");
+
+        // 3. DecodeUtf8SinglePass reconstructs the original string exactly
+        var decoded = string.Create(charCount, encoded, static (chars, bytes) =>
+        {
+            Utf8Transcoder.DecodeUtf8SinglePass(bytes, chars);
+        });
+        Assert.AreEqual(original, decoded, $"Decoder output mismatch{ctx}");
+    }
+
+    /// <summary>
+    /// Verifies that DecodeUtf8SinglePass produces output identical to <see cref="Encoding.UTF8.GetString"/>
+    /// for the same byte input. Catches silent decoder bugs that pass the round-trip test
+    /// (e.g. write-overlap that happens to land back on the right value by accident).
+    /// </summary>
+    private static void AssertDecodeMatchesBcl(string original)
+    {
+        var bytes = Utf8.GetBytes(original);
+        var bclDecoded = Utf8.GetString(bytes);
+        var charCount = Utf8Transcoder.CountUtf8Chars(bytes);
+        var ourDecoded = string.Create(charCount, bytes, static (chars, b) =>
+        {
+            Utf8Transcoder.DecodeUtf8SinglePass(b, chars);
+        });
+        Assert.AreEqual(bclDecoded, ourDecoded, $"Decoder mismatch for input length {bytes.Length}");
+    }
+}
diff --git a/AyCode.Core/Serializers/Binaries/Utf8Transcoder.cs b/AyCode.Core/Serializers/Binaries/Utf8Transcoder.cs
index a69adab..da833e8 100644
--- a/AyCode.Core/Serializers/Binaries/Utf8Transcoder.cs
+++ b/AyCode.Core/Serializers/Binaries/Utf8Transcoder.cs
@@ -337,10 +337,15 @@ internal static class Utf8Transcoder
                 // ASCII detect: any high bit set among the 32 bytes?
                 if (v.ExtractMostSignificantBits() != 0) break;
 
-                // Widen 32 bytes → 2 × Vector256<ushort> (32 chars total)
+                // Widen 32 bytes → 2 × Vector256<ushort> (32 chars total). Each Vector256<ushort>
+                // holds 16 ushort, so the upper half stores at dstIdx + 16 (= Vector256<ushort>.Count).
+                // Earlier latent bug used Vector128<ushort>.Count (= 8) here, causing overlap on
+                // indices 8-15 and uninitialized 24-31 — hidden by the Hungarian benchmark's early
+                // ASCII bail-out (no 32+ byte ASCII run). Validated by Utf8TranscoderTests
+                // LongAscii32Plus + AsciiExactly32Bytes round-trips.
                 var (lower, upper) = Vector256.Widen(v);
                 lower.StoreUnsafe(ref dstRef, (uint)dstIdx);
-                upper.StoreUnsafe(ref dstRef, (uint)(dstIdx + Vector128<ushort>.Count));
+                upper.StoreUnsafe(ref dstRef, (uint)(dstIdx + Vector256<ushort>.Count));
                 srcIdx += Vector256<byte>.Count;
                 dstIdx += Vector256<byte>.Count;  // 32 bytes → 32 chars
             }