fix: honor q=0 in streamable HTTP Accept negotiation

Author: raashish1601

src/mcp/server/streamable_http.py

@@ -397,9 +397,27 @@ def _check_accept_headers(self, request: Request) -> tuple[bool, bool]:
         - */* matches any media type
         - application/* matches any application/ subtype
         - text/* matches any text/ subtype
+        - media types with q=0 are treated as unacceptable
         """
         accept_header = request.headers.get("accept", "")
-        accept_types = [media_type.strip().split(";")[0].strip().lower() for media_type in accept_header.split(",")]
+        accept_types: list[str] = []
+        for media_range in accept_header.split(","):
+            parts = [part.strip().lower() for part in media_range.split(";")]
+            media_type = parts[0]
+            if not media_type:
+                continue
+
+            quality = 1.0
+            for param in parts[1:]:
+                if param.startswith("q="):
+                    try:
+                        quality = float(param[2:])
+                    except ValueError:
+                        pass
+                    break
+
+            if quality > 0:
+                accept_types.append(media_type)
 
         has_wildcard = "*/*" in accept_types
         has_json = has_wildcard or any(t in (CONTENT_TYPE_JSON, "application/*") for t in accept_types)

tests/shared/test_streamable_http.py

@@ -613,6 +613,8 @@ def test_accept_header_wildcard(basic_server: None, basic_server_url: str, accep
     [
         ("application/json", "application/json"),
         ("text/event-stream", "text/event-stream"),
+        ("application/json;q=0.9, text/event-stream;q=0", "application/json"),
+        ("text/event-stream;q=0.9, application/json;q=0", "text/event-stream"),
     ],
 )
 def test_accept_header_single_media_type_negotiates_response(
@@ -659,6 +661,7 @@ def test_accept_header_single_media_type_negotiates_response(
         "text/html",
         "text/plain",
         "application/xml",
+        "application/json;q=0, text/event-stream;q=0",
     ],
 )
 def test_accept_header_incompatible(basic_server: None, basic_server_url: str, accept_header: str):
@@ -933,14 +936,22 @@ def test_json_response_missing_accept_header(json_response_server: None, json_se
     assert "Not Acceptable" in response.text
 
 
-def test_json_response_incorrect_accept_header(json_response_server: None, json_server_url: str):
+@pytest.mark.parametrize(
+    "accept_header",
+    [
+        "text/event-stream",
+        "application/json;q=0, text/event-stream;q=1",
+    ],
+)
+def test_json_response_incorrect_accept_header(
+    json_response_server: None, json_server_url: str, accept_header: str
+):
     """Test that json_response servers reject requests with incorrect Accept header."""
     mcp_url = f"{json_server_url}/mcp"
-    # Test with only text/event-stream (wrong for JSON server)
     response = requests.post(
         mcp_url,
         headers={
-            "Accept": "text/event-stream",
+            "Accept": accept_header,
             "Content-Type": "application/json",
         },
         json=INIT_REQUEST,