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fix: allow custom Content-Type header in StreamableHTTPTransport #2377

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fix: allow custom Content-Type header in StreamableHTTPTransport #2377

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12 changes: 10 additions & 2 deletions src/mcp/client/streamable_http.py
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Original file line number Diff line number Diff line change
Expand Up @@ -72,15 +72,18 @@ class RequestContext:
class StreamableHTTPTransport:
"""StreamableHTTP client transport implementation."""

def __init__(self, url: str) -> None:
def __init__(self, url: str, headers: dict[str, str] | None = None) -> None:
"""Initialize the StreamableHTTP transport.

Args:
url: The endpoint URL.
headers: Optional extra headers included with every request.
User-supplied headers take precedence over built-in MCP defaults.
"""
self.url = url
self.session_id: str | None = None
self.protocol_version: str | None = None
self._user_headers: dict[str, str] = headers or {}

def _prepare_headers(self) -> dict[str, str]:
"""Build MCP-specific request headers.
Expand All @@ -92,6 +95,8 @@ def _prepare_headers(self) -> dict[str, str]:
"accept": "application/json, text/event-stream",
"content-type": "application/json",
}
# User-supplied headers override MCP defaults (e.g. charset in Content-Type)
headers.update(self._user_headers)
# Add session headers if available
if self.session_id:
headers[MCP_SESSION_ID] = self.session_id
Expand Down Expand Up @@ -511,6 +516,7 @@ async def streamable_http_client(
url: str,
*,
http_client: httpx.AsyncClient | None = None,
headers: dict[str, str] | None = None,
terminate_on_close: bool = True,
) -> AsyncGenerator[TransportStreams, None]:
"""Client transport for StreamableHTTP.
Expand All @@ -520,6 +526,8 @@ async def streamable_http_client(
http_client: Optional pre-configured httpx.AsyncClient. If None, a default
client with recommended MCP timeouts will be created. To configure headers,
authentication, or other HTTP settings, create an httpx.AsyncClient and pass it here.
headers: Optional extra headers for every request (e.g. Content-Type with charset).
These override the built-in MCP defaults.
terminate_on_close: If True, send a DELETE request to terminate the session when the context exits.

Yields:
Expand All @@ -538,7 +546,7 @@ async def streamable_http_client(
# Create default client with recommended MCP timeouts
client = create_mcp_http_client()

transport = StreamableHTTPTransport(url)
transport = StreamableHTTPTransport(url, headers=headers)

logger.debug(f"Connecting to StreamableHTTP endpoint: {url}")

Expand Down
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