Fix MCP JSON-RPC input handling

DEVELOPER_GUIDE.md

@@ -1436,6 +1436,13 @@ frames cannot switch encodings, output is BOM-less UTF-8, 64 KiB buffer,
 `AutoFlush = true`). Malformed UTF-8 bytes
 raise a transport decode failure that the MCP loop maps to JSON-RPC `-32700`
 with an invalid-UTF-8 hint instead of silently replacing bytes with U+FFFD.
+JSON-RPC frames are also bounded before dispatch: at most 1,000,000 UTF-16
+characters, at most 1,048,576 UTF-8 bytes, and JSON nesting depth 32. Oversized,
+malformed, or too-deep frames return `-32700` with `id: null`; MCP `status`
+surfaces the active limits under `mcp.limits`. JSON-RPC batch arrays are
+supported up to 100 items: each item is dispatched through the same single-request
+path, notification-only batches produce no response, and empty or nested batches
+return `-32600`.
 
 `HttpMcpTransport` (also #1558) wraps `System.Net.HttpListener`:
 

changelog.d/unreleased/1799-1996.fixed.md

@@ -0,0 +1,19 @@
+---
+category: fixed
+issues:
+  - 1799
+  - 1996
+affected:
+  - src/CodeIndex/Mcp/McpServer.cs
+  - src/CodeIndex/Mcp/McpToolHandlers.cs
+  - tests/CodeIndex.Tests/McpServerTests.cs
+  - DEVELOPER_GUIDE.md
+---
+
+## English
+
+- **MCP JSON-RPC input now enforces character, UTF-8 byte, and nesting-depth limits (#1799, #1996)** - request frames are capped before dispatch, multi-byte UTF-8 payloads can no longer bypass the byte ceiling, and MCP `status` exposes the configured limits under `mcp.limits`.
+
+## 日本語
+
+- **MCP JSON-RPC 入力で文字数・UTF-8 バイト数・ネスト深さの上限を適用するようになりました (#1799, #1996)** - request frame は dispatch 前に制限され、マルチバイト UTF-8 payload がバイト上限をすり抜けなくなり、MCP `status` は設定済み上限を `mcp.limits` に表示します。

changelog.d/unreleased/1816.fixed.md

@@ -0,0 +1,16 @@
+---
+category: fixed
+issues:
+  - 1816
+affected:
+  - src/CodeIndex/Mcp/McpServer.cs
+  - tests/CodeIndex.Tests/McpServerTests.cs
+---
+
+## English
+
+- **Malformed MCP JSON-RPC frames now return parse errors with `id: null` (#1816)** - invalid JSON now produces JSON-RPC `-32700` parse-error responses with a spec-compliant null id instead of omitting the id field.
+
+## 日本語
+
+- **不正な MCP JSON-RPC フレームが `id: null` 付きの parse error を返すようになりました (#1816)** - 不正な JSON は id フィールドを省略せず、JSON-RPC 仕様どおり null id を持つ `-32700` parse error として返ります。

changelog.d/unreleased/1892.added.md

@@ -0,0 +1,17 @@
+---
+category: added
+issues:
+  - 1892
+affected:
+  - src/CodeIndex/Mcp/McpServer.cs
+  - tests/CodeIndex.Tests/McpServerTests.cs
+  - DEVELOPER_GUIDE.md
+---
+
+## English
+
+- **MCP JSON-RPC batch arrays are now supported (#1892)** - batch frames dispatch each request through the normal MCP path, omit responses for notification-only batches, and reject empty or nested batches with clear `-32600` errors.
+
+## 日本語
+
+- **MCP JSON-RPC の batch array に対応しました (#1892)** - batch frame は各 request を通常の MCP 経路で dispatch し、notification のみの batch は応答なし、空またはネストした batch は明確な `-32600` エラーで拒否します。

src/CodeIndex/Mcp/McpServer.cs

@@ -135,9 +135,12 @@ public partial class McpServer : IDisposable
     // `startLine - before` が underflow、`endLine + after` が overflow し、`Math.Max/Min` で clamp
     // する前に slice 経路が破綻していたため、CLI の `--before` / `--after` 上限と揃える（#1528）。
     private const int MaxContextLines = 1000;
-    private const int MaxLineLength = 1_000_000; // 1 MB per JSON-RPC message / 1メッセージあたり最大1MB
+    internal const int MaxLineCharacterCount = 1_000_000;
+    internal const int MaxLineByteLength = 1_048_576;
+    internal const int MaxJsonDepth = 32;
+    internal const int MaxBatchRequestCount = 100;
     // 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 MaxLineLength.
+    // 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、
     // StreamReaderのデフォルト1KBから繰り返し拡張されるのを避けるサイズ。
     private const int StdioBufferSize = 64 * 1024;
@@ -664,12 +667,13 @@ internal static string BuildInvalidUtf8ErrorLog(string detail)
 
         // Reject oversized messages to prevent memory exhaustion
         // メモリ枯渇を防ぐため巨大メッセージを拒否
-        if (line.Length > MaxLineLength)
+        var byteLength = Encoding.UTF8.GetByteCount(line);
+        if (line.Length > MaxLineCharacterCount || byteLength > MaxLineByteLength)
         {
-            DeferFrameLog(BuildOversizedMessageLog(line.Length));
-            var errorResponse = CreateErrorResponse(null, -32700, "Message too large",
+            DeferFrameLog(BuildOversizedMessageLog(line.Length, byteLength));
+            var errorResponse = CreateErrorResponse(hasId: true, id: null, code: -32700, message: "Message too large",
                 category: McpErrorEnvelope.CategoryMessageTooLarge,
-                suggestion: $"JSON-RPC frame exceeds the {MaxLineLength} byte cap. Split the request into smaller calls or use `batch_query` with smaller slots.",
+                suggestion: $"JSON-RPC frame exceeds the {MaxLineCharacterCount} character or {MaxLineByteLength} byte cap. Split the request into smaller calls or use `batch_query` with smaller slots.",
                 retrySafe: false);
             return errorResponse.ToJsonString(_jsonOptions);
         }
@@ -679,7 +683,7 @@ internal static string BuildInvalidUtf8ErrorLog(string detail)
         JsonNode? responseId = null;
         try
         {
-            request = JsonNode.Parse(line);
+            request = JsonNode.Parse(line, documentOptions: new JsonDocumentOptions { MaxDepth = MaxJsonDepth });
             if (request == null)
                 return null;
 
@@ -691,9 +695,9 @@ internal static string BuildInvalidUtf8ErrorLog(string detail)
         {
             // Parse error / パースエラー
             DeferFrameLog(BuildJsonParseErrorLog(ex.Message));
-            var errorResponse = CreateErrorResponse(null, -32700, "Parse error",
+            var errorResponse = CreateErrorResponse(hasId: true, id: null, code: -32700, message: "Parse error",
                 category: McpErrorEnvelope.CategoryParseError,
-                suggestion: "Send valid JSON-RPC 2.0 framed as a single line of UTF-8 JSON.",
+                suggestion: $"Send valid JSON-RPC 2.0 framed as a single line of UTF-8 JSON with nesting depth <= {MaxJsonDepth}.",
                 retrySafe: false);
             return errorResponse.ToJsonString(_jsonOptions);
         }
@@ -797,6 +801,9 @@ private static string BuildMinimalInternalErrorResponse(bool hasId, JsonNode? id
     /// </summary>
     internal JsonNode? HandleMessage(JsonNode request)
     {
+        if (request is JsonArray batch)
+            return HandleBatchMessage(batch);
+
         if (request is not JsonObject obj)
             return CreateErrorResponse(hasId: false, id: null, code: -32600, message: "Invalid request: expected JSON object",
                 category: McpErrorEnvelope.CategoryInvalidRequest,
@@ -908,6 +915,57 @@ private static string BuildMinimalInternalErrorResponse(bool hasId, JsonNode? id
         });
     }
 
+    private JsonNode? HandleBatchMessage(JsonArray batch)
+    {
+        if (batch.Count == 0)
+            return CreateErrorResponse(hasId: true, id: null, code: -32600, message: "Invalid request: empty batch",
+                category: McpErrorEnvelope.CategoryInvalidRequest,
+                suggestion: "JSON-RPC 2.0 batch requests must contain at least one request object.",
+                retrySafe: false);
+
+        if (batch.Count > MaxBatchRequestCount)
+            return CreateErrorResponse(hasId: true, id: null, code: -32600, message: "Invalid request: batch too large",
+                category: McpErrorEnvelope.CategoryInvalidRequest,
+                suggestion: $"JSON-RPC batch requests are limited to {MaxBatchRequestCount} items.",
+                retrySafe: false);
+
+        var responses = new JsonArray();
+        foreach (var item in batch)
+        {
+            JsonNode? response;
+            if (item is null)
+            {
+                response = CreateErrorResponse(hasId: true, id: null, code: -32600, message: "Invalid request: expected JSON object",
+                    category: McpErrorEnvelope.CategoryInvalidRequest,
+                    suggestion: "Each JSON-RPC batch item must be a request object.",
+                    retrySafe: false);
+            }
+            else if (item is JsonArray)
+            {
+                response = CreateErrorResponse(hasId: true, id: null, code: -32600, message: "Invalid request: nested batches are not supported",
+                    category: McpErrorEnvelope.CategoryInvalidRequest,
+                    suggestion: "JSON-RPC batch items must be request objects, not nested arrays.",
+                    retrySafe: false);
+            }
+            else if (item is not JsonObject)
+            {
+                response = CreateErrorResponse(hasId: true, id: null, code: -32600, message: "Invalid request: expected JSON object",
+                    category: McpErrorEnvelope.CategoryInvalidRequest,
+                    suggestion: "Each JSON-RPC batch item must be a request object.",
+                    retrySafe: false);
+            }
+            else
+            {
+                response = HandleMessage(item);
+            }
+
+            if (response != null)
+                responses.Add(response);
+        }
+
+        return responses.Count == 0 ? null : responses;
+    }
+
     private JsonNode DispatchWithRequestCancellation(JsonNode? id, Func<JsonNode> action)
     {
         var requestKey = SerializeRequestId(id);
@@ -959,7 +1017,7 @@ private static bool IsCancellationFrame(string frame)
     {
         try
         {
-            var node = JsonNode.Parse(frame);
+            var node = JsonNode.Parse(frame, documentOptions: new JsonDocumentOptions { MaxDepth = MaxJsonDepth });
             if (node is not JsonObject obj)
                 return false;
             var method = obj["method"]?.GetValue<string>();
@@ -1897,8 +1955,8 @@ internal static (IReadOnlyList<string> Keys, IReadOnlyList<KeyValuePair<string,
         return null;
     }
 
-    internal static string BuildOversizedMessageLog(int lineLength) =>
-        $"[cdidx-mcp] Message too large ({lineLength} bytes), rejecting. Split the request into smaller JSON-RPC messages or shorter arguments, then retry.";
+    internal static string BuildOversizedMessageLog(int characterCount, int byteCount) =>
+        $"[cdidx-mcp] Message too large ({characterCount} chars / {byteCount} bytes), rejecting. Split the request into smaller JSON-RPC messages or shorter arguments, then retry.";
 
     internal static string BuildJsonParseErrorLog(string detail) =>
         $"[cdidx-mcp] JSON parse error: {detail}. Send one UTF-8 JSON-RPC object per line and retry.";

src/CodeIndex/Mcp/McpToolHandlers.cs

@@ -16,7 +16,7 @@ namespace CodeIndex.Mcp;
 /// </summary>
 public partial class McpServer
 {
-    private const int DefaultBatchQueryResponseByteLimit = MaxLineLength;
+    private const int DefaultBatchQueryResponseByteLimit = MaxLineByteLength;
     private const string BatchQueryResponseByteLimitEnvVar = "CDIDX_MCP_BATCH_RESPONSE_MAX_BYTES";
     internal const int MaxMcpArrayFilterCount = 100;
     internal const int MaxMcpArrayFilterStringLength = 4096;
@@ -1344,6 +1344,16 @@ private JsonNode ExecuteStatus(JsonNode? id)
             structured["sqlGraphContractReady"] = status.SqlGraphContractReady;
             if (status.SqlGraphContractDegradedReason != null)
                 structured["sqlGraphContractDegradedReason"] = status.SqlGraphContractDegradedReason;
+            structured["mcp"] = new JsonObject
+            {
+                ["limits"] = new JsonObject
+                {
+                    ["max_request_characters"] = MaxLineCharacterCount,
+                    ["max_request_bytes"] = MaxLineByteLength,
+                    ["max_json_depth"] = MaxJsonDepth,
+                    ["max_batch_requests"] = MaxBatchRequestCount,
+                }
+            };
             return CreateToolResult(id, "Database stats returned.", structured);
         });
     }

tests/CodeIndex.Tests/McpServerTests.cs

@@ -858,9 +858,11 @@ public void McpAuthenticatorFactory_TokenSet_ReturnsTokenAuthenticator()
     [Fact]
     public void BuildOversizedMessageLog_IsActionable()
     {
-        var message = McpServer.BuildOversizedMessageLog(1_234_567);
+        var message = McpServer.BuildOversizedMessageLog(1_234_567, 1_500_000);
 
         Assert.Contains("Message too large", message);
+        Assert.Contains("chars", message);
+        Assert.Contains("bytes", message);
         Assert.Contains("Split the request into smaller JSON-RPC messages", message);
         Assert.Contains("retry", message);
     }
@@ -9078,9 +9080,84 @@ public void ErrorResponse_ProcessFrame_ParseError_CarriesEnvelope()
         var response = JsonNode.Parse(raw!)!;
         var error = response["error"]!;
         Assert.Equal(-32700, error["code"]!.GetValue<int>());
+        AssertJsonNullId(response);
         AssertEnvelope(error["data"], "parse_error", expectedRetrySafe: false);
     }
 
+    [Fact]
+    public void ProcessFrame_OversizedUtf8Bytes_ReturnsParseErrorWithNullId()
+    {
+        var multibyte = new string('\u3042', (McpServer.MaxLineByteLength / 3) + 1);
+        var raw = _server.ProcessFrame("{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"ping\",\"params\":{\"q\":\"" + multibyte + "\"}}");
+
+        var response = JsonNode.Parse(raw!)!;
+        Assert.Equal(-32700, response["error"]!["code"]!.GetValue<int>());
+        Assert.Equal("message_too_large", response["error"]!["data"]!["category"]!.GetValue<string>());
+        AssertJsonNullId(response);
+    }
+
+    [Fact]
+    public void ProcessFrame_TooDeepJson_ReturnsParseErrorWithNullId()
+    {
+        var raw = _server.ProcessFrame(new string('[', McpServer.MaxJsonDepth + 2) + "0" + new string(']', McpServer.MaxJsonDepth + 2));
+
+        var response = JsonNode.Parse(raw!)!;
+        Assert.Equal(-32700, response["error"]!["code"]!.GetValue<int>());
+        Assert.Equal("parse_error", response["error"]!["data"]!["category"]!.GetValue<string>());
+        AssertJsonNullId(response);
+    }
+
+    [Fact]
+    public void HandleMessage_BatchMixedRequests_ReturnsResponseArray()
+    {
+        var batch = JsonNode.Parse("""[{"jsonrpc":"2.0","id":1,"method":"ping"},{"jsonrpc":"2.0","method":"notifications/initialized"},{"jsonrpc":"2.0","id":2,"method":"nope"}]""")!;
+
+        var response = _server.HandleMessage(batch)!.AsArray();
+
+        Assert.Equal(2, response.Count);
+        Assert.Equal(1, response[0]!["id"]!.GetValue<int>());
+        Assert.NotNull(response[0]!["result"]);
+        Assert.Equal(2, response[1]!["id"]!.GetValue<int>());
+        Assert.Equal(-32601, response[1]!["error"]!["code"]!.GetValue<int>());
+    }
+
+    [Fact]
+    public void HandleMessage_AllNotificationBatch_ReturnsNull()
+    {
+        var batch = JsonNode.Parse("""[{"jsonrpc":"2.0","method":"notifications/initialized"}]""")!;
+
+        Assert.Null(_server.HandleMessage(batch));
+    }
+
+    [Fact]
+    public void HandleMessage_EmptyBatch_ReturnsInvalidRequest()
+    {
+        var response = _server.HandleMessage(JsonNode.Parse("[]")!)!;
+
+        Assert.Equal(-32600, response["error"]!["code"]!.GetValue<int>());
+        AssertJsonNullId(response);
+    }
+
+    [Fact]
+    public void HandleMessage_NestedBatchItem_ReturnsInvalidRequestInBatchResponse()
+    {
+        var response = _server.HandleMessage(JsonNode.Parse("""[[{"jsonrpc":"2.0","id":1,"method":"ping"}]]""")!)!.AsArray();
+
+        Assert.Single(response);
+        Assert.Equal(-32600, response[0]!["error"]!["code"]!.GetValue<int>());
+        AssertJsonNullId(response[0]!);
+    }
+
+    [Fact]
+    public void HandleMessage_ScalarBatchItem_ReturnsInvalidRequestWithNullId()
+    {
+        var response = _server.HandleMessage(JsonNode.Parse("""[1]""")!)!.AsArray();
+
+        Assert.Single(response);
+        Assert.Equal(-32600, response[0]!["error"]!["code"]!.GetValue<int>());
+        AssertJsonNullId(response[0]!);
+    }
+
     [Fact]
     public void BuildData_ExtraDataCannotShadowCanonicalKeys()
     {
@@ -9101,6 +9178,13 @@ public void BuildData_ExtraDataCannotShadowCanonicalKeys()
         Assert.Equal("search", data["tool"]!.GetValue<string>());
     }
 
+    private static void AssertJsonNullId(JsonNode node)
+    {
+        var obj = Assert.IsType<JsonObject>(node);
+        Assert.True(obj.ContainsKey("id"));
+        Assert.Null(obj["id"]);
+    }
+
     private static void WriteOversizedAsciiFile(string path)
     {
         const int targetBytes = 10 * 1024 * 1024 + 1;