Widthdom · Widthdom · Jun 5, 2026 · Jun 5, 2026
diff --git a/DEVELOPER_GUIDE.md b/DEVELOPER_GUIDE.md
@@ -1907,7 +1907,7 @@ The following categories ride the standard JSON-RPC codes:
 | --- | --- | --- | --- |
 | `-32700` | `parse_error` | `false` | Frame was not valid JSON. |
 | `-32700` | `message_too_large` | `false` | Frame is above the per-frame byte cap and is rejected before parsing. The frame reader uses the parse-error code because the frame is unreadable in the same sense as malformed JSON. |
-| `-32600` | `invalid_request` | `false` | Frame parsed but is not a JSON object, is missing required JSON-RPC fields, or carries an invalid `id`. |
+| `-32600` | `invalid_request` | `false` | Frame parsed but is not a JSON object, is missing required JSON-RPC fields, or carries an invalid `id`. String and numeric request ids are capped before echo/audit serialization; oversized ids return `id:null` plus `data.max_request_id_chars` / `data.max_request_id_bytes`. |
 | `-32601` | `method_not_found` | `false` | Unknown JSON-RPC method (not one of `initialize` / `tools/list` / `tools/call` / `ping` / supported `notifications/*`). |
 | `-32601` | `tool_disabled` | `false` | Known MCP tool is disabled by `CDIDX_MCP_TOOLS_ALLOW` / `CDIDX_MCP_TOOLS_DENY` (#1561). The `-32601` wire code is preserved so the pre-#1581 client contract still holds; only the envelope is additive. `data.tool` carries the disabled tool name. |
 | `-32602` | `tool_unknown` | `false` | `tools/call` received an MCP tool name the server does not implement (typo or version mismatch). `data.tool` carries the unknown name. |
@@ -3524,7 +3524,7 @@ JSON-RPC 2.0 は `-32700` と `-32600..-32603` を仕様自身、`-32000..-32099
 | --- | --- | --- | --- |
 | `-32700` | `parse_error` | `false` | フレームが JSON として解析できなかった。 |
 | `-32700` | `message_too_large` | `false` | フレームがパース前にバイト上限を超えて拒否された。フレームリーダーは「読めない」という意味で parse-error と同じコードを使う。 |
-| `-32600` | `invalid_request` | `false` | パースはできたが JSON オブジェクトでない、JSON-RPC 必須フィールド欠落、または不正な `id` を含む。 |
+| `-32600` | `invalid_request` | `false` | パースはできたが JSON オブジェクトでない、JSON-RPC 必須フィールド欠落、または不正な `id` を含む。文字列と数値の request id は echo / audit serialization 前に上限を適用し、過大な id は `id:null` と `data.max_request_id_chars` / `data.max_request_id_bytes` 付きで返す。 |
 | `-32601` | `method_not_found` | `false` | 未知 JSON-RPC メソッド（`initialize` / `tools/list` / `tools/call` / `ping` / サポート対象 `notifications/*` 以外）。 |
 | `-32601` | `tool_disabled` | `false` | `CDIDX_MCP_TOOLS_ALLOW` / `CDIDX_MCP_TOOLS_DENY`（#1561）で無効化された既知ツール。ワイヤコードは #1581 以前のクライアント契約を保つため `-32601` のまま維持し、envelope のみ additive に追加する。`data.tool` に無効化されたツール名を含める。 |
 | `-32602` | `tool_unknown` | `false` | `tools/call` がサーバー未実装の MCP ツール名を指定した（typo またはバージョン不整合）。`data.tool` に未知の名前を含める。 |

diff --git a/changelog.d/unreleased/3104.security.md b/changelog.d/unreleased/3104.security.md
@@ -0,0 +1,18 @@
+---
+category: security
+issues:
+  - 3104
+affected:
+  - DEVELOPER_GUIDE.md
+  - src/CodeIndex/Mcp/McpServer.cs
+  - tests/CodeIndex.Tests/McpServerTests.cs
+  - tests/CodeIndex.Tests/McpAuditLogTests.cs
+---
+
+## English
+
+- **MCP JSON-RPC request ids are now length-limited before echoing (#3104)** — oversized string or numeric `id` values are rejected with a bounded `id:null` invalid-request response before response metadata, telemetry, or audit records can retain them.
+
+## 日本語
+
+- **MCP JSON-RPC request id を echo 前に長さ制限するようになりました (#3104)** — 過大な文字列または数値の `id` は、レスポンスメタデータ、telemetry、audit record に保持される前に、境界付きの `id:null` invalid-request response として拒否されます。
diff --git a/src/CodeIndex/Mcp/McpServer.cs b/src/CodeIndex/Mcp/McpServer.cs
@@ -174,6 +174,8 @@ public partial class McpServer : IDisposable
     private const string SamplingEnabledEnvironmentVariable = "CDIDX_MCP_SAMPLING";
     internal const int MaxJsonDepth = 32;
     internal const int MaxBatchRequestCount = 100;
+    internal const int MaxRequestIdCharacterCount = 128;
+    internal const int MaxRequestIdByteLength = 256;
     // Stdio buffer for the JSON-RPC loop. Sized to fit typical large MCP payloads (e.g. batch_query)
     // in a single read so the StreamReader does not grow from its 1 KB default toward MaxLineCharacterCount.
     // JSON-RPCループのstdioバッファ。大きめのMCPペイロードを1回の読み取りで吸収し、
@@ -968,7 +970,10 @@ private bool TryCompletePendingClientRequest(JsonNode request)
             || obj["method"] is not null)
             return false;
 
-        var key = id?.ToJsonString(_jsonOptions) ?? "null";
+        if (!TrySerializeRequestId(id, out var serializedId, out _))
+            return false;
+
+        var key = serializedId ?? "null";
         if (!_pendingClientRequests.TryRemove(key, out var pending))
             return false;
 
@@ -1223,11 +1228,12 @@ private static void AppendMinimalCorrelationData(StringBuilder builder)
         // で例外を投げると、認証ゲート前に -32603 が返ってしまい、未認証呼び出し元に dispatch
         // 内部まで届いた事実が漏れる (#1559)。
         var method = TryGetStringMember(obj, "method");
-        if (!TryGetRequestId(obj, out var hasId, out var id))
-            return CreateErrorResponse(hasId: true, id: null, code: -32600, message: "Invalid request: id must be string, number, or null",
+        if (!TryGetRequestId(obj, out var hasId, out var id, out var idError))
+            return CreateErrorResponse(hasId: true, id: null, code: -32600, message: BuildInvalidRequestIdMessage(idError),
                 category: McpErrorEnvelope.CategoryInvalidRequest,
-                suggestion: "JSON-RPC 2.0 `id` must be a string, integer, or null. Booleans/objects/arrays are not allowed.",
-                retrySafe: false);
+                suggestion: BuildInvalidRequestIdSuggestion(idError),
+                retrySafe: false,
+                extraData: BuildInvalidRequestIdData(idError));
 
         using var correlationScope = hasId && CurrentCorrelationContext.Value is null ? BeginRequestCorrelation(id) : null;
 
@@ -2841,16 +2847,7 @@ private static void AddArgKeyMetadata(JsonObject target, IReadOnlyList<KeyValueP
 
     private static string? SerializeRequestId(JsonNode? id)
     {
-        if (id is null)
-            return null;
-        try
-        {
-            return id.ToJsonString();
-        }
-        catch
-        {
-            return null;
-        }
+        return TrySerializeRequestId(id, out var serialized, out _) ? serialized : null;
     }
 
     private static string? TryReadStringArg(JsonNode? args, string key)
@@ -3092,28 +3089,142 @@ public void Dispose()
 
     // --- JSON-RPC helpers / JSON-RPCヘルパー ---
 
+    private enum RequestIdValidationError
+    {
+        None,
+        InvalidType,
+        TooLong,
+    }
+
     private static bool TryGetRequestId(JsonObject request, out bool hasId, out JsonNode? id)
+        => TryGetRequestId(request, out hasId, out id, out _);
+
+    private static bool TryGetRequestId(JsonObject request, out bool hasId, out JsonNode? id, out RequestIdValidationError error)
     {
+        error = RequestIdValidationError.None;
         hasId = request.TryGetPropertyValue("id", out id);
         if (!hasId)
             return true;
 
         if (id is null)
             return true;
 
-        if (id is JsonValue)
+        return TrySerializeRequestId(id, out _, out error);
+    }
+
+    private static bool TrySerializeRequestId(JsonNode? id, out string? serialized, out RequestIdValidationError error)
+    {
+        serialized = null;
+        error = RequestIdValidationError.None;
+        if (id is null)
+            return true;
+
+        if (id is not JsonValue value)
         {
-            var serialized = id.ToJsonString();
-            if (serialized.Length == 0)
-                return false;
+            error = RequestIdValidationError.InvalidType;
+            return false;
+        }
 
-            var first = serialized[0];
-            return first == '"' || first == '-' || char.IsDigit(first) || first == 'n';
+        return TrySerializeRequestIdValue(value, out serialized, out error);
+    }
+
+    private static bool TrySerializeRequestIdValue(JsonValue value, out string? serialized, out RequestIdValidationError error)
+    {
+        serialized = null;
+        error = RequestIdValidationError.None;
+        JsonValueKind kind;
+        try
+        {
+            kind = value.GetValueKind();
+        }
+        catch
+        {
+            error = RequestIdValidationError.InvalidType;
+            return false;
         }
 
-        return false;
+        switch (kind)
+        {
+            case JsonValueKind.String:
+                try
+                {
+                    var requestId = value.GetValue<string>();
+                    if (!IsRequestIdWithinBounds(requestId))
+                    {
+                        error = RequestIdValidationError.TooLong;
+                        return false;
+                    }
+
+                    serialized = JsonSerializer.Serialize(requestId);
+                    return true;
+                }
+                catch
+                {
+                    error = RequestIdValidationError.InvalidType;
+                    return false;
+                }
+
+            case JsonValueKind.Number:
+                try
+                {
+                    serialized = value.TryGetValue<JsonElement>(out var element) && element.ValueKind == JsonValueKind.Number
+                        ? element.GetRawText()
+                        : value.ToJsonString();
+                }
+                catch
+                {
+                    error = RequestIdValidationError.InvalidType;
+                    return false;
+                }
+
+                if (serialized.Length == 0 || !(serialized[0] == '-' || char.IsDigit(serialized[0])))
+                {
+                    error = RequestIdValidationError.InvalidType;
+                    serialized = null;
+                    return false;
+                }
+
+                if (!IsRequestIdWithinBounds(serialized))
+                {
+                    error = RequestIdValidationError.TooLong;
+                    serialized = null;
+                    return false;
+                }
+
+                return true;
+
+            case JsonValueKind.Null:
+                return true;
+
+            default:
+                error = RequestIdValidationError.InvalidType;
+                return false;
+        }
     }
 
+    private static bool IsRequestIdWithinBounds(string value)
+        => value.Length <= MaxRequestIdCharacterCount
+            && Encoding.UTF8.GetByteCount(value) <= MaxRequestIdByteLength;
+
+    private static string BuildInvalidRequestIdMessage(RequestIdValidationError error)
+        => error == RequestIdValidationError.TooLong
+            ? "Invalid request: id exceeds the request-id length limit"
+            : "Invalid request: id must be string, number, or null";
+
+    private static string BuildInvalidRequestIdSuggestion(RequestIdValidationError error)
+        => error == RequestIdValidationError.TooLong
+            ? $"JSON-RPC 2.0 `id` must be no more than {MaxRequestIdCharacterCount} characters and {MaxRequestIdByteLength} UTF-8 bytes. Use a compact string or number id."
+            : "JSON-RPC 2.0 `id` must be a string, integer, or null. Booleans/objects/arrays are not allowed.";
+
+    private static JsonObject? BuildInvalidRequestIdData(RequestIdValidationError error)
+        => error == RequestIdValidationError.TooLong
+            ? new JsonObject
+            {
+                ["max_request_id_chars"] = MaxRequestIdCharacterCount,
+                ["max_request_id_bytes"] = MaxRequestIdByteLength,
+            }
+            : null;
+
     private static JsonObject CreateSuccessResponse(JsonNode? id, JsonNode result)
         => CreateSuccessResponse(id is not null, id, result);
 

diff --git a/tests/CodeIndex.Tests/McpAuditLogTests.cs b/tests/CodeIndex.Tests/McpAuditLogTests.cs
@@ -124,6 +124,37 @@ public void ToolsCall_MissingToolName_StillEmitsAuditRecord()
         Assert.Equal("missing_tool_name", record.GetProperty("error").GetString());
     }
 
+    [Fact]
+    public void ToolsCall_OversizedRequestId_IsRejectedBeforeAuditRecord_Issue3104()
+    {
+        using var sink = new AuditLogSink(_auditPath, AuditLogSink.DefaultMaxBytes, includeValues: false);
+        using var server = CreateServer(sink);
+        var oversizedId = new string('r', McpServer.MaxRequestIdCharacterCount + 1);
+        var request = new JsonObject
+        {
+            ["jsonrpc"] = "2.0",
+            ["id"] = oversizedId,
+            ["method"] = "tools/call",
+            ["params"] = new JsonObject
+            {
+                ["name"] = "ping",
+                ["arguments"] = new JsonObject(),
+            },
+        };
+
+        var response = server.HandleMessage(request)!;
+
+        Assert.Equal(-32600, response["error"]!["code"]!.GetValue<int>());
+        Assert.Equal("invalid_request", response["error"]!["data"]!["category"]!.GetValue<string>());
+        AssertJsonNullId(response);
+        if (File.Exists(_auditPath))
+        {
+            var rawLog = File.ReadAllText(_auditPath);
+            Assert.DoesNotContain(oversizedId, rawLog, StringComparison.Ordinal);
+            Assert.True(string.IsNullOrWhiteSpace(rawLog), "oversized request ids must be rejected before tool audit emission");
+        }
+    }
+
     [Fact]
     public void ToolsCall_DisabledTool_EmitsAuditRecordWithToolDisabledError()
     {
@@ -384,4 +415,10 @@ private JsonElement ReadOnlyRecord()
         Assert.Single(lines);
         return JsonDocument.Parse(lines[0]).RootElement.Clone();
     }
+
+    private static void AssertJsonNullId(JsonNode response)
+    {
+        Assert.True(response.AsObject().ContainsKey("id"), "JSON-RPC error responses must include id:null");
+        Assert.Null(response["id"]);
+    }
 }
diff --git a/tests/CodeIndex.Tests/McpServerTests.cs b/tests/CodeIndex.Tests/McpServerTests.cs
@@ -12252,6 +12252,40 @@ public void ProcessFrame_OversizedUtf8Bytes_ReturnsParseErrorWithNullId()
         AssertJsonNullId(response);
     }
 
+    [Fact]
+    public void ProcessFrame_OversizedStringRequestId_ReturnsInvalidRequestWithNullId_Issue3104()
+    {
+        var oversizedId = new string('x', McpServer.MaxRequestIdCharacterCount + 1);
+        var raw = _server.ProcessFrame("{\"jsonrpc\":\"2.0\",\"id\":\"" + oversizedId + "\",\"method\":\"tools/list\"}");
+
+        Assert.NotNull(raw);
+        Assert.DoesNotContain(oversizedId, raw, StringComparison.Ordinal);
+        var response = JsonNode.Parse(raw!)!;
+        var error = response["error"]!;
+        Assert.Equal(-32600, error["code"]!.GetValue<int>());
+        Assert.Equal("invalid_request", error["data"]!["category"]!.GetValue<string>());
+        Assert.Equal(McpServer.MaxRequestIdCharacterCount, error["data"]!["max_request_id_chars"]!.GetValue<int>());
+        Assert.Equal(McpServer.MaxRequestIdByteLength, error["data"]!["max_request_id_bytes"]!.GetValue<int>());
+        AssertJsonNullId(response);
+    }
+
+    [Fact]
+    public void ProcessFrame_OversizedNumericRequestId_ReturnsInvalidRequestWithNullId_Issue3104()
+    {
+        var oversizedId = new string('9', McpServer.MaxRequestIdCharacterCount + 1);
+        var raw = _server.ProcessFrame("{\"jsonrpc\":\"2.0\",\"id\":" + oversizedId + ",\"method\":\"tools/list\"}");
+
+        Assert.NotNull(raw);
+        Assert.DoesNotContain(oversizedId, raw, StringComparison.Ordinal);
+        var response = JsonNode.Parse(raw!)!;
+        var error = response["error"]!;
+        Assert.Equal(-32600, error["code"]!.GetValue<int>());
+        Assert.Equal("invalid_request", error["data"]!["category"]!.GetValue<string>());
+        Assert.Equal(McpServer.MaxRequestIdCharacterCount, error["data"]!["max_request_id_chars"]!.GetValue<int>());
+        Assert.Equal(McpServer.MaxRequestIdByteLength, error["data"]!["max_request_id_bytes"]!.GetValue<int>());
+        AssertJsonNullId(response);
+    }
+
     [Fact]
     public void ProcessFrame_TooDeepJson_ReturnsParseErrorWithNullId()
     {