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Ignore pre-2026 protocol_version pins at the StreamableHTTP transport (#2923)

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Max 2026-06-20 16:29:57 +01:00 • committed by GitHub
commit 5a3412ddc1
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1 changed files with 37 additions and 9 deletions
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12
src/mcp/client/streamable_http.py
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@ -93,13 +93,14 @@ class StreamableHTTPTransport:
Args:
url: The endpoint URL.
protocol_version: Pin the MCP-Protocol-Version header from the first request
instead of waiting to snoop it from an InitializeResult. Required for
stateless 2026-07-28 sessions that never send initialize.
protocol_version: Pin the MCP-Protocol-Version header from the first request.
Only honoured for stateless 2026-07-28+ sessions that never send
initialize; for earlier (stateful) versions the header is populated
from the negotiated InitializeResult, so a pre-2026 value is ignored.
"""
self.url = url
self.session_id: str | None = None
self.protocol_version: str | None = protocol_version
self.protocol_version: str | None = protocol_version if protocol_version in MODERN_PROTOCOL_VERSIONS else None
def _per_message_headers(self, message: JSONRPCMessage) -> dict[str, str]:
"""Per-POST routing headers (Mcp-Method, Mcp-Name) for 2026-07-28+ pinned transports.
@ -158,7 +159,8 @@ class StreamableHTTPTransport:
def _maybe_extract_protocol_version_from_message(self, message: JSONRPCMessage) -> None:
"""Extract protocol version from initialization response message."""
if self.protocol_version is not None:
# Constructor pin wins over snooping the InitializeResult.
# Only a modern constructor pin reaches here (pre-2026 values are dropped
# in __init__), and a modern pin never sends initialize.
return
if isinstance(message, JSONRPCResponse) and message.result: # pragma: no branch
try: