diff --git a/changelog.d/unreleased/3113.security.md b/changelog.d/unreleased/3113.security.md
new file mode 100644
index 0000000000..1d5fa6772a
--- /dev/null
+++ b/changelog.d/unreleased/3113.security.md
@@ -0,0 +1,16 @@
+---
+category: security
+issues:
+  - 3113
+affected:
+  - src/CodeIndex/Lsp/LspServer.cs
+  - tests/CodeIndex.Tests/LspServerRequestIdTests.cs
+---
+
+## English
+
+- **LSP request ids now reject oversized raw JSON before echoing (#3113)** — JSON-RPC ids above the LSP raw byte limit are rejected without parsing them into retained response state or reflecting the oversized value.
+
+## 日本語
+
+- **LSP request id は oversized raw JSON を echo 前に拒否するようになりました (#3113)** — LSP の raw byte 上限を超える JSON-RPC id は、レスポンス保持用の状態へ parse したり oversized value を反映したりせず拒否されます。
diff --git a/src/CodeIndex/Lsp/LspServer.cs b/src/CodeIndex/Lsp/LspServer.cs
index d706360d08..5677275341 100644
--- a/src/CodeIndex/Lsp/LspServer.cs
+++ b/src/CodeIndex/Lsp/LspServer.cs
@@ -17,12 +17,17 @@ internal sealed class LspServer : IDisposable
     internal const int MaxLspHeaderLineBytes = 8 * 1024;
     internal const int MaxPositionDocumentBytes = 4 * 1024 * 1024;
     internal const int MaxTextDocumentUriChars = McpBoundedText.MaxResourceUriChars;
+    internal const int MaxLspRequestIdRawBytes = 4 * 1024;
     internal const int MaxJsonDepth = 32;
     internal const int MaxUnknownMethodDiagnosticChars = 240;
     private const int JsonRpcInvalidParamsCode = -32602;
     private const int JsonRpcInternalErrorCode = -32603;
     private const string JsonRpcInvalidParamsMessage = "Invalid params";
     private const string JsonRpcInternalErrorMessage = "Internal error";
+    private static readonly JsonReaderOptions LspJsonReaderOptions = new()
+    {
+        MaxDepth = MaxJsonDepth,
+    };
     private static readonly JsonDocumentOptions LspJsonDocumentOptions = new()
     {
         MaxDepth = MaxJsonDepth,
@@ -87,7 +92,8 @@ public int Run(Stream input, Stream output)
 
                 var method = root.TryGetProperty("method", out var methodElement) ? methodElement.GetString() : null;
                 hasId = root.TryGetProperty("id", out var idElement);
-                id = hasId ? JsonNode.Parse(idElement.GetRawText(), documentOptions: LspJsonDocumentOptions) : null;
+                if (hasId && !TryParseRequestId(payload, idElement, out id))
+                    return Error(null, -32600, $"Request id must be {MaxLspRequestIdRawBytes} raw JSON bytes or fewer.");
 
                 if (method == null)
                     return hasId ? Error(id, -32600, "Invalid Request") : null;
@@ -116,6 +122,66 @@ public int Run(Stream input, Stream output)
         }
     }
 
+    private static bool TryParseRequestId(string payload, JsonElement idElement, out JsonNode? id)
+    {
+        id = null;
+        if (!TryGetTopLevelRequestIdRawByteCount(payload, out var rawIdBytes) || rawIdBytes > MaxLspRequestIdRawBytes)
+            return false;
+
+        var rawId = idElement.GetRawText();
+        if (Encoding.UTF8.GetByteCount(rawId) > MaxLspRequestIdRawBytes)
+            return false;
+
+        id = JsonNode.Parse(rawId, documentOptions: LspJsonDocumentOptions);
+        return true;
+    }
+
+    private static bool TryGetTopLevelRequestIdRawByteCount(string payload, out int rawIdBytes)
+    {
+        rawIdBytes = 0;
+        var payloadByteCount = Encoding.UTF8.GetByteCount(payload);
+        var buffer = ArrayPool<byte>.Shared.Rent(payloadByteCount);
+        try
+        {
+            _ = Encoding.UTF8.GetBytes(payload.AsSpan(), buffer);
+            var reader = new Utf8JsonReader(buffer.AsSpan(0, payloadByteCount), LspJsonReaderOptions);
+            if (!reader.Read() || reader.TokenType != JsonTokenType.StartObject)
+                return true;
+
+            while (reader.Read())
+            {
+                if (reader.TokenType == JsonTokenType.EndObject && reader.CurrentDepth == 0)
+                    break;
+                if (reader.TokenType != JsonTokenType.PropertyName || reader.CurrentDepth != 1)
+                    continue;
+
+                var isId = reader.ValueTextEquals("id"u8);
+                if (!reader.Read())
+                    return false;
+
+                var valueStart = reader.TokenStartIndex;
+                reader.Skip();
+                if (isId)
+                {
+                    var rawLength = reader.BytesConsumed - valueStart;
+                    if (rawLength > int.MaxValue)
+                        return false;
+                    rawIdBytes = (int)rawLength;
+                }
+            }
+
+            return true;
+        }
+        catch (JsonException)
+        {
+            return false;
+        }
+        finally
+        {
+            ArrayPool<byte>.Shared.Return(buffer);
+        }
+    }
+
     private static string SanitizeUnknownMethod(string method)
     {
         var wasTruncated = method.Length > MaxUnknownMethodDiagnosticChars;
diff --git a/tests/CodeIndex.Tests/LspServerRequestIdTests.cs b/tests/CodeIndex.Tests/LspServerRequestIdTests.cs
new file mode 100644
index 0000000000..be72febd48
--- /dev/null
+++ b/tests/CodeIndex.Tests/LspServerRequestIdTests.cs
@@ -0,0 +1,42 @@
+using CodeIndex.Cli;
+using CodeIndex.Database;
+using CodeIndex.Lsp;
+
+namespace CodeIndex.Tests;
+
+[Collection("SQLite pool sensitive")]
+public class LspServerRequestIdTests
+{
+    [Fact]
+    public void HandleMessage_OversizedRequestId_ReturnsInvalidRequestWithoutEcho_Issue3113()
+    {
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_lsp_large_id");
+        try
+        {
+            var dbPath = TestProjectHelper.CreateProjectDb(projectRoot);
+            using var db = new DbContext(dbPath);
+            using var server = new LspServer(
+                new DbReader(db),
+                "1.2.3",
+                ProgramRunner.CreateDefaultJsonOptions(),
+                projectRoot);
+            var oversizedId = new string('\u00e9', LspServer.MaxLspRequestIdRawBytes / 2);
+            var request = $"{{\"jsonrpc\":\"2.0\",\"id\":\"{oversizedId}\",\"method\":\"initialize\",\"params\":{{}}}}";
+
+            var response = server.HandleMessage(request);
+
+            Assert.NotNull(response);
+            Assert.Equal(-32600, response!["error"]!["code"]!.GetValue<int>());
+            Assert.Contains(
+                "Request id must be",
+                response["error"]!["message"]!.GetValue<string>(),
+                StringComparison.Ordinal);
+            Assert.Null(response["id"]);
+            Assert.DoesNotContain(oversizedId, response.ToJsonString());
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+}