1.x: streamable_http client leaves a request unresolved when the SSE response stream ends without a response (fixed on 2.x by #3047 — backport request for v1.x)

Summary

On the 1.x line (v1.29.0, v1.29.1, and the v1.x branch as of 92120b4b), the Streamable HTTP client leaves a pending request unresolved forever when the text/event-stream response for that request ends without a response event. The caller learns of the disconnect only when its own deadline fires, so a dead server is indistinguishable from a slow one — and it is reported as a timeout, not a disconnect.

This is already fixed on 2.x: #3047 (merged 2026-07-07, shipped in v2.0.0) added StreamableHTTPTransport._resolve_abandoned_request(...) and calls it at exactly the two sites below with "SSE stream ended without a response" and "SSE stream ended and reconnection attempts were exhausted". The v1.x branch does not carry it (grep -c _resolve_abandoned_request src/mcp/client/streamable_http.py → 0 at 92120b4b), and neither did the v1.29.1 release (2026-08-24).

Ask: backport the _resolve_abandoned_request resolution from #3047 onto v1.x and cut a 1.29.x/1.30 release. Consumers pinned to mcp<2 (e.g. IBM/mcp-context-forge, mcp>=1.28.1,<2 at v1.0.8/v1.0.9/main) cannot pick the fix up from 2.x.

Mechanism (src/mcp/client/streamable_http.py @ v1.29.0 == v1.29.1, sha256 8eb01c3d…)

1. _handle_sse_response (:397-435): a completed response returns inside the try. When the server dies mid-stream, EventSource.aiter_sse() raises httpx.RemoteProtocolError("peer closed connection without sending complete message body (incomplete chunked read)"), which except Exception as e: # pragma: no cover (:429-430) logs at DEBUG (SSE stream ended: …). The tail (:432-435) reconnects only if last_event_id is not None; a server that sends no event ids (FastMCP's default, no event store) makes the coroutine return without writing anything to ctx.read_stream_writer. ClientSession.send_request therefore waits forever on its response stream.
2. _handle_reconnection (:446-448) gives up after MAX_RECONNECTION_ATTEMPTS with a bare return — the same unresolved-request hang for servers that do send ids.

The SDK already has the channel to resolve this: _send_session_terminated_error (:510-522) writes a JSONRPCError for a request id to read_stream_writer, and _receive_loop routes it to the pending request, where send_request raises McpError.

Reproduction (measured 2026-09-04, mcp 1.29.0, httpx 0.28.1, anyio 4.14.2, Python 3.12.13)

Server: a FastMCP 3.x Streamable HTTP server with a tool slow(seconds) that sleeps. Client: streamablehttp_client + ClientSession, initialize(), then

with anyio.fail_after(5):
    await session.call_tool("slow", {"seconds": 4})

and SIGKILL the server ≈1.0 s after the call starts (response headers arrive at ~0.002 s; no event before the kill).

The SDK's own debug log confirms the swallow: SSE stream ended: peer closed connection without sending complete message body (incomplete chunked read) at +1.11 s, then silence until the caller's deadline.

Related

• #3047 — the 2.x fix (_resolve_abandoned_request), part of the subscriptions/listen driver PR.
• #1401 — ClientSession error handling (broader; this is one concrete instance on 1.x).
• Downstream consumer report: IBM/mcp-context-forge#6592 (a gateway on mcp<2 that renders this hang as Tool invocation timed out after {tool_timeout}s).

We are carrying a build-time backport of the two-site resolution in our ContextForge image until a 1.x release ships it; happy to open the backport PR against v1.x if maintainers want it.

Start in src/mcp/client/streamable_http.py, reading _handle_sse_response, _handle_reconnection, and _send_session_terminated_error; compare the corresponding resolution in 2.x from #3047. Reproduce the no-response SSE disconnect and verify that pending requests receive the expected McpError promptly, then run the relevant client tests and confirm the v1.x backport is ready for release.