fix: accept wildcard media types in Accept header per RFC 7231 (#2152)

Co-authored-by: Shivam <shivam@Shivams-MacBook-Air-2.local>

src/mcp/server/streamable_http.py

@@ -391,12 +391,19 @@ class StreamableHTTPServerTransport:
             await self._handle_unsupported_request(request, send)
 
     def _check_accept_headers(self, request: Request) -> tuple[bool, bool]:
-        """Check if the request accepts the required media types."""
+        """Check if the request accepts the required media types.
-        accept_header = request.headers.get("accept", "")
-        accept_types = [media_type.strip() for media_type in accept_header.split(",")]
 
-        has_json = any(media_type.startswith(CONTENT_TYPE_JSON) for media_type in accept_types)
+        Supports wildcard media types per RFC 7231, section 5.3.2:
-        has_sse = any(media_type.startswith(CONTENT_TYPE_SSE) for media_type in accept_types)
+        - */* matches any media type
+        - application/* matches any application/ subtype
+        - text/* matches any text/ subtype
+        """
+        accept_header = request.headers.get("accept", "")
+        accept_types = [media_type.strip().split(";")[0].strip().lower() for media_type in accept_header.split(",")]
+
+        has_wildcard = "*/*" in accept_types
+        has_json = has_wildcard or any(t in (CONTENT_TYPE_JSON, "application/*") for t in accept_types)
+        has_sse = has_wildcard or any(t in (CONTENT_TYPE_SSE, "text/*") for t in accept_types)
 
         return has_json, has_sse
 

tests/shared/test_streamable_http.py

@@ -572,8 +572,10 @@ def json_server_url(json_server_port: int) -> str:
 # Basic request validation tests
 def test_accept_header_validation(basic_server: None, basic_server_url: str):
     """Test that Accept header is properly validated."""
-    # Test without Accept header
+    # Test without Accept header (suppress requests library default Accept: */*)
-    response = requests.post(
+    session = requests.Session()
+    session.headers.pop("Accept")
+    response = session.post(
         f"{basic_server_url}/mcp",
         headers={"Content-Type": "application/json"},
         json={"jsonrpc": "2.0", "method": "initialize", "id": 1},
@@ -582,6 +584,52 @@ def test_accept_header_validation(basic_server: None, basic_server_url: str):
     assert "Not Acceptable" in response.text
 
 
+@pytest.mark.parametrize(
+    "accept_header",
+    [
+        "*/*",
+        "application/*, text/*",
+        "text/*, application/json",
+        "application/json, text/*",
+        "*/*;q=0.8",
+        "application/*;q=0.9, text/*;q=0.8",
+    ],
+)
+def test_accept_header_wildcard(basic_server: None, basic_server_url: str, accept_header: str):
+    """Test that wildcard Accept headers are accepted per RFC 7231."""
+    response = requests.post(
+        f"{basic_server_url}/mcp",
+        headers={
+            "Accept": accept_header,
+            "Content-Type": "application/json",
+        },
+        json=INIT_REQUEST,
+    )
+    assert response.status_code == 200
+
+
+@pytest.mark.parametrize(
+    "accept_header",
+    [
+        "text/html",
+        "application/*",
+        "text/*",
+    ],
+)
+def test_accept_header_incompatible(basic_server: None, basic_server_url: str, accept_header: str):
+    """Test that incompatible Accept headers are rejected for SSE mode."""
+    response = requests.post(
+        f"{basic_server_url}/mcp",
+        headers={
+            "Accept": accept_header,
+            "Content-Type": "application/json",
+        },
+        json=INIT_REQUEST,
+    )
+    assert response.status_code == 406
+    assert "Not Acceptable" in response.text
+
+
 def test_content_type_validation(basic_server: None, basic_server_url: str):
     """Test that Content-Type header is properly validated."""
     # Test with incorrect Content-Type
@@ -826,7 +874,10 @@ def test_json_response_accept_json_only(json_response_server: None, json_server_
 def test_json_response_missing_accept_header(json_response_server: None, json_server_url: str):
     """Test that json_response servers reject requests without Accept header."""
     mcp_url = f"{json_server_url}/mcp"
-    response = requests.post(
+    # Suppress requests library default Accept: */* header
+    session = requests.Session()
+    session.headers.pop("Accept")
+    response = session.post(
         mcp_url,
         headers={
             "Content-Type": "application/json",
@@ -853,6 +904,29 @@ def test_json_response_incorrect_accept_header(json_response_server: None, json_
     assert "Not Acceptable" in response.text
 
 
+@pytest.mark.parametrize(
+    "accept_header",
+    [
+        "*/*",
+        "application/*",
+        "application/*;q=0.9",
+    ],
+)
+def test_json_response_wildcard_accept_header(json_response_server: None, json_server_url: str, accept_header: str):
+    """Test that json_response servers accept wildcard Accept headers per RFC 7231."""
+    mcp_url = f"{json_server_url}/mcp"
+    response = requests.post(
+        mcp_url,
+        headers={
+            "Accept": accept_header,
+            "Content-Type": "application/json",
+        },
+        json=INIT_REQUEST,
+    )
+    assert response.status_code == 200
+    assert response.headers.get("Content-Type") == "application/json"
+
+
 def test_get_sse_stream(basic_server: None, basic_server_url: str):
     """Test establishing an SSE stream via GET request."""
     # First, we need to initialize a session
@@ -941,8 +1015,10 @@ def test_get_validation(basic_server: None, basic_server_url: str):
     assert init_data is not None
     negotiated_version = init_data["result"]["protocolVersion"]
 
-    # Test without Accept header
+    # Test without Accept header (suppress requests library default Accept: */*)
-    response = requests.get(
+    session = requests.Session()
+    session.headers.pop("Accept")
+    response = session.get(
         mcp_url,
         headers={
             MCP_SESSION_ID_HEADER: session_id,