Fix Streamable HTTP Accept negotiation

Author: raashish1601

src/mcp/server/streamable_http.py

@@ -426,16 +426,24 @@ async def _validate_accept_header(self, request: Request, scope: Scope, send: Se
                 )
                 await response(scope, request.receive, send)
                 return False
-        # For SSE responses, require both content types
-        elif not (has_json and has_sse):
+        # For SSE-capable responses, accept either JSON or SSE and negotiate later.
+        elif not (has_json or has_sse):
             response = self._create_error_response(
-                "Not Acceptable: Client must accept both application/json and text/event-stream",
+                "Not Acceptable: Client must accept application/json or text/event-stream",
                 HTTPStatus.NOT_ACCEPTABLE,
             )
             await response(scope, request.receive, send)
             return False
         return True
 
+    def _should_use_json_response(self, request: Request) -> bool:
+        """Choose JSON when required or when the client does not accept SSE."""
+        if self.is_json_response_enabled:
+            return True
+
+        has_json, has_sse = self._check_accept_headers(request)
+        return has_json and not has_sse
+
     async def _handle_post_request(self, scope: Scope, request: Request, receive: Receive, send: Send) -> None:
         """Handle POST requests containing JSON-RPC messages."""
         writer = self._read_stream_writer
@@ -476,6 +484,8 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
                 await response(scope, receive, send)
                 return
 
+            use_json_response = self._should_use_json_response(request)
+
             # Check if this is an initialization request
             is_initialization_request = isinstance(message, JSONRPCRequest) and message.method == "initialize"
 
@@ -527,7 +537,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
             self._request_streams[request_id] = anyio.create_memory_object_stream[EventMessage](0)
             request_stream_reader = self._request_streams[request_id][1]
 
-            if self.is_json_response_enabled:
+            if use_json_response:
                 # Process the message
                 metadata = ServerMessageMetadata(request_context=request)
                 session_message = SessionMessage(message, metadata=metadata)

tests/issues/test_1363_race_condition_streamable_http.py

@@ -123,7 +123,7 @@ async def test_race_condition_invalid_accept_headers(caplog: pytest.LogCaptureFi
     """Test the race condition with invalid Accept headers.
 
     This test reproduces the exact scenario described in issue #1363:
-    - Send POST request with incorrect Accept headers (missing either application/json or text/event-stream)
+    - Send POST request with incorrect Accept headers that match neither JSON nor SSE
     - Request fails validation early and returns quickly
     - This should trigger the race condition where message_router encounters ClosedResourceError
     """
@@ -137,34 +137,34 @@ async def test_race_condition_invalid_accept_headers(caplog: pytest.LogCaptureFi
 
         # Suppress WARNING logs (expected validation errors) and capture ERROR logs
         with caplog.at_level(logging.ERROR):
-            # Test with missing text/event-stream in Accept header
+            # Test with an incompatible text media type
             async with httpx.AsyncClient(
                 transport=httpx.ASGITransport(app=app), base_url="http://testserver", timeout=5.0
             ) as client:
                 response = await client.post(
                     "/",
                     json={"jsonrpc": "2.0", "method": "initialize", "id": 1, "params": {}},
                     headers={
-                        "Accept": "application/json",  # Missing text/event-stream
+                        "Accept": "text/plain",
                         "Content-Type": "application/json",
                     },
                 )
-                # Should get 406 Not Acceptable due to missing text/event-stream
+                # Should get 406 Not Acceptable for an unsupported media type
                 assert response.status_code == 406
 
-            # Test with missing application/json in Accept header
+            # Test with an incompatible application media type
             async with httpx.AsyncClient(
                 transport=httpx.ASGITransport(app=app), base_url="http://testserver", timeout=5.0
             ) as client:
                 response = await client.post(
                     "/",
                     json={"jsonrpc": "2.0", "method": "initialize", "id": 1, "params": {}},
                     headers={
-                        "Accept": "text/event-stream",  # Missing application/json
+                        "Accept": "application/xml",
                         "Content-Type": "application/json",
                     },
                 )
-                # Should get 406 Not Acceptable due to missing application/json
+                # Should get 406 Not Acceptable for an unsupported media type
                 assert response.status_code == 406
 
             # Test with completely invalid Accept header

tests/shared/test_streamable_http.py

@@ -608,12 +608,57 @@ def test_accept_header_wildcard(basic_server: None, basic_server_url: str, accep
     assert response.status_code == 200
 
 
+@pytest.mark.parametrize(
+    ("accept_header", "expected_content_type"),
+    [
+        ("application/json", "application/json"),
+        ("text/event-stream", "text/event-stream"),
+    ],
+)
+def test_accept_header_single_media_type_negotiates_response(
+    basic_server: None, basic_server_url: str, accept_header: str, expected_content_type: str
+):
+    """Test that SSE-capable servers negotiate JSON or SSE from a single accepted media type."""
+    mcp_url = f"{basic_server_url}/mcp"
+    init_response = requests.post(
+        mcp_url,
+        headers={
+            "Accept": accept_header,
+            "Content-Type": "application/json",
+        },
+        json=INIT_REQUEST,
+    )
+    assert init_response.status_code == 200
+    assert init_response.headers.get("Content-Type") == expected_content_type
+
+    session_id = init_response.headers.get(MCP_SESSION_ID_HEADER)
+    assert session_id is not None
+
+    if expected_content_type == "application/json":
+        negotiated_version = init_response.json()["result"]["protocolVersion"]
+    else:
+        negotiated_version = extract_protocol_version_from_sse(init_response)
+
+    tools_response = requests.post(
+        mcp_url,
+        headers={
+            "Accept": accept_header,
+            "Content-Type": "application/json",
+            MCP_SESSION_ID_HEADER: session_id,
+            MCP_PROTOCOL_VERSION_HEADER: negotiated_version,
+        },
+        json={"jsonrpc": "2.0", "method": "tools/list", "id": "tools-accept-single-media"},
+    )
+    assert tools_response.status_code == 200
+    assert tools_response.headers.get("Content-Type") == expected_content_type
+
+
 @pytest.mark.parametrize(
     "accept_header",
     [
         "text/html",
-        "application/*",
-        "text/*",
+        "text/plain",
+        "application/xml",
     ],
 )
 def test_accept_header_incompatible(basic_server: None, basic_server_url: str, accept_header: str):