fix: reject initialize protocol version conflicts

src/mcp/server/streamable_http.py

@@ -476,23 +476,11 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
                 await response(scope, receive, send)
                 return
 
-            # Check if this is an initialization request
-            is_initialization_request = isinstance(message, JSONRPCRequest) and message.method == "initialize"
-
-            if is_initialization_request:
-                # Check if the server already has an established session
-                if self.mcp_session_id:
-                    # Check if request has a session ID
-                    request_session_id = self._get_session_id(request)
-
-                    # If request has a session ID but doesn't match, return 404
-                    if request_session_id and request_session_id != self.mcp_session_id:  # pragma: no cover
-                        response = self._create_error_response(
-                            "Not Found: Invalid or expired session ID",
-                            HTTPStatus.NOT_FOUND,
-                        )
-                        await response(scope, receive, send)
-                        return
+            is_initialization_request = False
+            if isinstance(message, JSONRPCRequest) and message.method == "initialize":
+                is_initialization_request = True
+                if not await self._validate_initialization_request(message, request, send):
+                    return
             elif not await self._validate_request_headers(request, send):
                 return
 
@@ -865,6 +853,44 @@ async def _validate_protocol_version(self, request: Request, send: Send) -> bool
 
         return True
 
+    async def _validate_initialization_request(self, message: JSONRPCRequest, request: Request, send: Send) -> bool:
+        if not await self._validate_initialization_protocol_version(message, request, send):
+            return False
+
+        if not self.mcp_session_id:
+            return True
+
+        request_session_id = self._get_session_id(request)
+        if request_session_id and request_session_id != self.mcp_session_id:  # pragma: no cover
+            response = self._create_error_response(
+                "Not Found: Invalid or expired session ID",
+                HTTPStatus.NOT_FOUND,
+            )
+            await response(request.scope, request.receive, send)
+            return False
+
+        return True
+
+    async def _validate_initialization_protocol_version(
+        self, message: JSONRPCRequest, request: Request, send: Send
+    ) -> bool:
+        header_protocol_version = request.headers.get(MCP_PROTOCOL_VERSION_HEADER)
+        body_protocol_version = str(message.params.get("protocolVersion")) if message.params else None
+        if (
+            header_protocol_version is not None
+            and body_protocol_version is not None
+            and header_protocol_version != body_protocol_version
+        ):
+            response = self._create_error_response(
+                f"Bad Request: {MCP_PROTOCOL_VERSION_HEADER} header does not match initialize.params.protocolVersion",
+                HTTPStatus.BAD_REQUEST,
+                INVALID_REQUEST,
+            )
+            await response(request.scope, request.receive, send)
+            return False
+
+        return True
+
     async def _replay_events(self, last_event_id: str, request: Request, send: Send) -> None:
         """Replays events that would have been sent after the specified event ID.
 

tests/shared/test_streamable_http.py

@@ -1489,6 +1489,45 @@ async def test_server_validates_protocol_version_header(basic_app: Starlette) ->
         assert response.status_code == 200
 
 
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    ("header_version", "body_version"),
+    [
+        ("2025-03-26", "2025-06-18"),
+        ("2025-06-18", "2025-03-26"),
+    ],
+)
+async def test_server_rejects_initialize_protocol_version_mismatch(
+    basic_app: Starlette, header_version: str, body_version: str
+) -> None:
+    """Initialize rejects conflicting protocol versions in header and body."""
+    init_request: dict[str, Any] = {
+        "jsonrpc": "2.0",
+        "method": "initialize",
+        "params": {
+            "clientInfo": {"name": "test-client", "version": "1.0"},
+            "protocolVersion": body_version,
+            "capabilities": {},
+        },
+        "id": "init-1",
+    }
+
+    async with make_client(basic_app) as client:
+        response = await client.post(
+            "/mcp",
+            headers={
+                "Accept": "application/json, text/event-stream",
+                "Content-Type": "application/json",
+                MCP_PROTOCOL_VERSION_HEADER: header_version,
+            },
+            json=init_request,
+        )
+
+    assert response.status_code == 400
+    assert MCP_PROTOCOL_VERSION_HEADER in response.text
+    assert "protocolVersion" in response.text
+
+
 @pytest.mark.anyio
 async def test_server_backwards_compatibility_no_protocol_version(basic_app: Starlette) -> None:
     """A request without a protocol version header is accepted for backwards compatibility."""