modelcontextprotocol · GauthamPrabhuM · Jun 13, 2026
diff --git a/docs/migration.md b/docs/migration.md
@@ -531,6 +531,29 @@ await ctx.log(level="info", data="hello")
 
 Positional calls (`await ctx.info("hello")`) are unaffected.
 
+### Invalid JSON-RPC envelopes return `-32600` with a correlated request id
+
+**What changed:** A message that is valid JSON but not a valid JSON-RPC
+request object (wrong `jsonrpc` version, missing `jsonrpc`, non-string
+`method`, etc.) is now answered with an **Invalid Request (`-32600`)** error
+that echoes the original request `id` when it can be detected.
+
+In v1 this case was handled inconsistently: streamable HTTP replied with
+`-32602` (Invalid params) and `id: null`, while stdio sent no response at
+all (the validation error was dropped). Neither path let a client correlate
+the failure back to the request that caused it.
+
+**Why it changed:** Per JSON-RPC 2.0, an unparseable-as-a-request envelope is
+an Invalid Request, and the error response should carry the original `id` so
+clients can match it to the offending request.
+
+**How to migrate:** If you string-matched on the `-32602` code (or relied on
+`id: null`) to detect malformed requests over streamable HTTP, switch to
+`-32600` and read the echoed `id`. Lines over stdio that previously produced
+no response now produce a `-32600` error when an `id` is present; a line with
+no detectable `id` (parse error, malformed notification, or an `id` of an
+invalid type) still produces no response.
+
 ### Replace `RootModel` by union types with `TypeAdapter` validation
 
 The following union types are no longer `RootModel` subclasses:

diff --git a/src/mcp/server/stdio.py b/src/mcp/server/stdio.py
@@ -23,10 +23,35 @@ async def run_server():
 
 import anyio
 import anyio.lowlevel
+import pydantic_core
 
 from mcp import types
 from mcp.shared._context_streams import create_context_streams
 from mcp.shared.message import SessionMessage
+from mcp.types.jsonrpc import extract_request_id
+
+
+def _invalid_request_error(line: str) -> types.JSONRPCError | None:
+    """Build an Invalid Request error for an id-bearing line that failed envelope validation.
+
+    Per JSON-RPC 2.0, a request that is valid JSON but not a valid request
+    object gets a -32600 error response echoing the original request id, so
+    the client can correlate the failure. Returns None when the line is not
+    valid JSON (parse error, no response expected by existing consumers) or
+    when no id can be detected (a malformed notification gets no response).
+    """
+    try:
+        raw = pydantic_core.from_json(line)
+    except ValueError:
+        return None
+    request_id = extract_request_id(raw)
+    if request_id is None:
+        return None
+    return types.JSONRPCError(
+        jsonrpc="2.0",
+        id=request_id,
+        error=types.ErrorData(code=types.INVALID_REQUEST, message="Invalid Request"),
+    )
 
 
 @asynccontextmanager
@@ -53,6 +78,8 @@ async def stdin_reader():
                     try:
                         message = types.jsonrpc_message_adapter.validate_json(line, by_name=False)
                     except Exception as exc:
+                        if (error := _invalid_request_error(line)) is not None:
+                            await write_stream.send(SessionMessage(error))
                         await read_stream_writer.send(exc)
                         continue
 

diff --git a/src/mcp/server/streamable_http.py b/src/mcp/server/streamable_http.py
@@ -32,7 +32,6 @@
 from mcp.types import (
     DEFAULT_NEGOTIATED_VERSION,
     INTERNAL_ERROR,
-    INVALID_PARAMS,
     INVALID_REQUEST,
     PARSE_ERROR,
     ErrorData,
@@ -43,6 +42,7 @@
     RequestId,
     jsonrpc_message_adapter,
 )
+from mcp.types.jsonrpc import extract_request_id
 
 logger = logging.getLogger(__name__)
 
@@ -288,6 +288,7 @@ def _create_error_response(
         status_code: HTTPStatus,
         error_code: int = INVALID_REQUEST,
         headers: dict[str, str] | None = None,
+        request_id: RequestId | None = None,
     ) -> Response:
         """Create an error response with a simple string message."""
         response_headers = {"Content-Type": CONTENT_TYPE_JSON}
@@ -300,7 +301,7 @@ def _create_error_response(
         # Return a properly formatted JSON error response
         error_response = JSONRPCError(
             jsonrpc="2.0",
-            id=None,
+            id=request_id,
             error=ErrorData(code=error_code, message=error_message),
         )
 
@@ -468,10 +469,13 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
             try:
                 message = jsonrpc_message_adapter.validate_python(raw_message, by_name=False)
             except ValidationError as e:
+                # Per JSON-RPC 2.0, an invalid envelope is an Invalid Request
+                # error, echoing the original request id when it is detectable.
                 response = self._create_error_response(
                     f"Validation error: {str(e)}",
                     HTTPStatus.BAD_REQUEST,
-                    INVALID_PARAMS,
+                    INVALID_REQUEST,
+                    request_id=extract_request_id(raw_message),
                 )
                 await response(scope, receive, send)
                 return

diff --git a/src/mcp/types/jsonrpc.py b/src/mcp/types/jsonrpc.py
@@ -82,3 +82,17 @@ class JSONRPCError(BaseModel):
 
 JSONRPCMessage = JSONRPCRequest | JSONRPCNotification | JSONRPCResponse | JSONRPCError
 jsonrpc_message_adapter: TypeAdapter[JSONRPCMessage] = TypeAdapter(JSONRPCMessage)
+
+
+def extract_request_id(raw: Any) -> RequestId | None:
+    """Best-effort extraction of the request id from an invalid JSON-RPC envelope.
+
+    Per JSON-RPC 2.0, an Invalid Request error response must echo the original
+    request id when it can be detected, and null otherwise. The bool guard
+    matters: `bool` is an `int` subclass but not a valid id type.
+    """
+    match raw:
+        case {"id": str() | int() as request_id} if not isinstance(request_id, bool):
+            return request_id
+        case _:
+            return None
diff --git a/tests/interaction/transports/test_hosting_http.py b/tests/interaction/transports/test_hosting_http.py
@@ -15,7 +15,7 @@
 from mcp.server import Server, ServerRequestContext
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.types import (
-    INVALID_PARAMS,
+    INVALID_REQUEST,
     PARSE_ERROR,
     CallToolRequestParams,
     CallToolResult,
@@ -129,7 +129,7 @@ async def test_non_json_content_type_is_rejected() -> None:
 @requirement("hosting:http:parse-error-400")
 @requirement("hosting:http:batch")
 async def test_malformed_and_batched_bodies_return_400() -> None:
-    """A non-JSON body returns 400 Parse error; a JSON array of requests returns 400 Invalid params."""
+    """A non-JSON body returns 400 Parse error; a JSON array of requests returns 400 Invalid Request."""
     async with mounted_app(_server()) as (http, _):
         session_id = await initialize_via_http(http)
         not_json = await http.post(
@@ -149,7 +149,7 @@ async def test_malformed_and_batched_bodies_return_400() -> None:
     assert not_json.status_code == 400
     assert JSONRPCError.model_validate_json(not_json.text).error.code == PARSE_ERROR
     assert batched.status_code == 400
-    assert JSONRPCError.model_validate_json(batched.text).error.code == INVALID_PARAMS
+    assert JSONRPCError.model_validate_json(batched.text).error.code == INVALID_REQUEST
 
 
 @requirement("hosting:http:protocol-version-400")

diff --git a/tests/server/test_stdio.py b/tests/server/test_stdio.py
@@ -11,7 +11,15 @@
 from mcp.server.mcpserver import MCPServer
 from mcp.server.stdio import stdio_server
 from mcp.shared.message import SessionMessage
-from mcp.types import JSONRPCMessage, JSONRPCRequest, JSONRPCResponse, jsonrpc_message_adapter
+from mcp.types import (
+    INVALID_REQUEST,
+    ErrorData,
+    JSONRPCError,
+    JSONRPCMessage,
+    JSONRPCRequest,
+    JSONRPCResponse,
+    jsonrpc_message_adapter,
+)
 
 
 @pytest.mark.anyio
@@ -66,6 +74,60 @@ async def test_stdio_server_round_trips_messages_over_injected_streams() -> None
     assert received_responses[1] == JSONRPCResponse(jsonrpc="2.0", id=4, result={})
 
 
+@pytest.mark.anyio
+async def test_stdio_server_replies_invalid_request_for_invalid_envelope() -> None:
+    """An id-bearing line that fails envelope validation gets a correlated -32600 response.
+
+    Lines that are valid JSON but invalid JSON-RPC envelopes get an Invalid Request
+    error response echoing the original request id. Lines without a detectable id
+    (parse errors, malformed notifications, ids of an invalid type) get no response;
+    every invalid line still surfaces as an in-stream exception and later valid
+    messages keep flowing. Regression test for issue #2848.
+    """
+    stdin = io.StringIO()
+    stdout = io.StringIO()
+
+    invalid_lines = [
+        '{"jsonrpc": "1.0", "id": 3, "method": "ping", "params": {}}',  # wrong jsonrpc version
+        '{"id": 4, "method": "ping", "params": {}}',  # missing jsonrpc field
+        '{"jsonrpc": "2.0", "id": 8, "method": 12345, "params": {}}',  # method is not a string
+        '{"jsonrpc": "2.0", "id": "abc", "method": 12345}',  # string id is echoed as-is
+        "this is not json",  # parse error: no response
+        '{"jsonrpc": "1.0", "method": "ping"}',  # no id (malformed notification): no response
+        '{"jsonrpc": "2.0", "id": true, "method": 12345}',  # bool is not a valid id type: no response
+        '{"jsonrpc": "2.0", "id": 1.5, "method": 12345}',  # fractional id is not a valid id type: no response
+    ]
+    valid = JSONRPCRequest(jsonrpc="2.0", id=99, method="ping")
+    for line in invalid_lines:
+        stdin.write(line + "\n")
+    stdin.write(valid.model_dump_json(by_alias=True, exclude_none=True) + "\n")
+    stdin.seek(0)
+
+    with anyio.fail_after(5):
+        async with stdio_server(stdin=anyio.AsyncFile(stdin), stdout=anyio.AsyncFile(stdout)) as (
+            read_stream,
+            write_stream,
+        ):
+            async with read_stream:
+                for _ in invalid_lines:
+                    received = await read_stream.receive()
+                    assert isinstance(received, Exception)
+                final = await read_stream.receive()
+                assert isinstance(final, SessionMessage)
+                assert final.message == valid
+            await write_stream.aclose()
+
+    stdout.seek(0)
+    responses = [jsonrpc_message_adapter.validate_json(line.strip()) for line in stdout.readlines()]
+    invalid_request = ErrorData(code=INVALID_REQUEST, message="Invalid Request")
+    assert responses == [
+        JSONRPCError(jsonrpc="2.0", id=3, error=invalid_request),
+        JSONRPCError(jsonrpc="2.0", id=4, error=invalid_request),
+        JSONRPCError(jsonrpc="2.0", id=8, error=invalid_request),
+        JSONRPCError(jsonrpc="2.0", id="abc", error=invalid_request),
+    ]
+
+
 @pytest.mark.anyio
 async def test_stdio_server_invalid_utf8(monkeypatch: pytest.MonkeyPatch) -> None:
     """Non-UTF-8 stdin bytes surface as an in-stream exception without killing the stream.

diff --git a/tests/shared/test_streamable_http.py b/tests/shared/test_streamable_http.py
@@ -46,6 +46,7 @@
 from mcp.shared.message import ClientMessageMetadata, ServerMessageMetadata, SessionMessage
 from mcp.shared.session import RequestResponder
 from mcp.types import (
+    INVALID_REQUEST,
     CallToolRequestParams,
     CallToolResult,
     InitializeResult,
@@ -499,6 +500,44 @@ async def test_json_parsing(basic_app: Starlette) -> None:
         assert "Validation error" in response.text
 
 
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    ("body", "expected_id"),
+    [
+        pytest.param({"jsonrpc": "1.0", "id": 3, "method": "ping", "params": {}}, 3, id="wrong-jsonrpc-version"),
+        pytest.param({"id": 4, "method": "ping", "params": {}}, 4, id="missing-jsonrpc-field"),
+        pytest.param({"jsonrpc": "2.0", "id": 8, "method": 12345, "params": {}}, 8, id="method-not-a-string"),
+        pytest.param({"jsonrpc": "2.0", "id": "abc", "method": 12345}, "abc", id="string-id"),
+        pytest.param({"foo": "bar"}, None, id="no-id-detectable"),
+        pytest.param({"jsonrpc": "2.0", "id": True, "method": 12345}, None, id="bool-is-not-a-valid-id"),
+        pytest.param({"jsonrpc": "2.0", "id": 1.5, "method": 12345}, None, id="fractional-id-is-not-valid"),
+    ],
+)
+async def test_invalid_envelope_error_echoes_request_id(
+    basic_app: Starlette, body: dict[str, Any], expected_id: int | str | None
+) -> None:
+    """An invalid JSON-RPC envelope gets a -32600 error echoing the original request id.
+
+    Valid JSON that fails envelope validation is an Invalid Request per JSON-RPC 2.0;
+    the error response must carry the original request id when it is detectable (and
+    null otherwise) so the client can correlate the failure. Regression test for
+    issue #2848.
+    """
+    async with make_client(basic_app) as client:
+        response = await client.post(
+            "/mcp",
+            headers={
+                "Accept": "application/json, text/event-stream",
+                "Content-Type": "application/json",
+            },
+            json=body,
+        )
+        assert response.status_code == 400
+        error_response = response.json()
+        assert error_response["id"] == expected_id
+        assert error_response["error"]["code"] == INVALID_REQUEST
+
+
 @pytest.mark.anyio
 async def test_method_not_allowed(basic_app: Starlette) -> None:
     """Unsupported HTTP methods are rejected with 405."""