fix(server): match Content-Type case-insensitively in StreamableHTTP

Media types are case-insensitive (RFC 9110, section 8.3.1), and
StreamableHTTPServerTransport._check_accept_headers already lowercases the
Accept media types before comparing. _check_content_type did not, so a
spec-valid request with a mixed/upper-case Content-Type (e.g.
"Application/JSON") was rejected with 415 Unsupported Media Type.

Lowercase the parsed Content-Type media type before comparing to
CONTENT_TYPE_JSON, consistent with _check_accept_headers. Adds a unit test for
case-insensitive matching (the _check_content_type path was previously no-cover).

Author: ly-wang19

src/mcp/server/streamable_http.py

@@ -411,7 +411,9 @@ def _check_accept_headers(self, request: Request) -> tuple[bool, bool]:
     def _check_content_type(self, request: Request) -> bool:
         """Check if the request has the correct Content-Type."""
         content_type = request.headers.get("content-type", "")
-        content_type_parts = [part.strip() for part in content_type.split(";")[0].split(",")]
+        # Media types are case-insensitive (RFC 9110, section 8.3.1), so normalize
+        # to lower case before comparing — consistent with _check_accept_headers.
+        content_type_parts = [part.strip().lower() for part in content_type.split(";")[0].split(",")]
 
         return any(part == CONTENT_TYPE_JSON for part in content_type_parts)
 

tests/shared/test_streamable_http.py

@@ -597,6 +597,30 @@ def test_streamable_http_transport_init_validation() -> None:
         StreamableHTTPServerTransport(mcp_session_id="test\n")
 
 
+def test_check_content_type_is_case_insensitive() -> None:
+    """Content-Type media types are case-insensitive (RFC 9110, section 8.3.1).
+
+    A spec-valid request such as ``Content-Type: Application/JSON`` must be
+    accepted, consistent with ``_check_accept_headers`` (which already lowercases).
+    """
+    transport = StreamableHTTPServerTransport(mcp_session_id=None)
+
+    def request_with(content_type: str) -> Request:
+        return Request({"type": "http", "headers": [(b"content-type", content_type.encode())]})
+
+    for value in (
+        "application/json",
+        "Application/JSON",
+        "APPLICATION/JSON",
+        "application/json; charset=utf-8",
+        "Application/Json; charset=utf-8",
+    ):
+        assert transport._check_content_type(request_with(value)) is True, value
+
+    # A genuinely different media type is still rejected.
+    assert transport._check_content_type(request_with("text/plain")) is False
+
+
 @pytest.mark.anyio
 async def test_session_termination(basic_app: Starlette) -> None:
     """DELETE terminates the session, after which requests for it return 404."""