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fix(client): catch httpx transport errors in streamable HTTP post_writer by devansh-dek · Pull Request #3025 · modelcontextprotocol/python-sdk · GitHub

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fix(client): catch httpx transport errors in streamable HTTP post_writer - #3025

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modelcontextprotocol:mainfrom
devansh-dek:fix/915-streamable-http-unreachable-server
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fix(client): catch httpx transport errors in streamable HTTP post_writer#3025
devansh-dek wants to merge 1 commit into
modelcontextprotocol:mainfrom
devansh-dek:fix/915-streamable-http-unreachable-server

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devansh-dek commented Jun 30, 2026 •
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Summary

  • Catch httpx.HTTPError inside streamable HTTP post_writer request tasks and reply with a JSON-RPC error on the read stream instead of letting the exception escape into the outer transport task group.
  • Fixes the uncatchable RuntimeError: Attempted to exit cancel scope in a different task than it was entered in when ClientSessionGroup.connect_to_server() targets an unreachable streamable HTTP server (Exception in ClientSessionGroup if streamable_http MCP server is not available #915).
  • Adds a regression test in tests/issues/test_915_streamable_http_unreachable.py.

Test plan

  • uv run pytest tests/issues/test_915_streamable_http_unreachable.py -v
  • uv run ruff check src/mcp/client/streamable_http.py tests/issues/test_915_streamable_http_unreachable.py

Fixes #915

fix(client): catch httpx transport errors in streamable HTTP post_writer
9e3d580 
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
cubic-dev-ai Bot reviewed Jun 30, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

No issues found across 2 files

Tip: cubic could auto-approve low-risk PRs like this, if it thinks it's safe to merge. Learn more

Re-trigger cubic

jlacour-git mentioned this pull request Jul 15, 2026
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Exception in ClientSessionGroup if streamable_http MCP server is not available

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