fix(client): catch httpx transport errors in streamable HTTP post_writer#3025
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When an unreachable streamable HTTP server causes httpx.HTTPError inside a request task, complete the pending JSON-RPC waiter with an error response instead of letting the exception crash the outer transport task group. Fixes modelcontextprotocol#915
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Summary
httpx.HTTPErrorinside streamable HTTPpost_writerrequest tasks and reply with a JSON-RPC error on the read stream instead of letting the exception escape into the outer transport task group.RuntimeError: Attempted to exit cancel scope in a different task than it was entered inwhenClientSessionGroup.connect_to_server()targets an unreachable streamable HTTP server (Exception inClientSessionGroupifstreamable_httpMCP server is not available #915).tests/issues/test_915_streamable_http_unreachable.py.Test plan
uv run pytest tests/issues/test_915_streamable_http_unreachable.py -vuv run ruff check src/mcp/client/streamable_http.py tests/issues/test_915_streamable_http_unreachable.pyFixes #915