A long-running gateway session could permanently lose an MCP server: once a
stdio subprocess died (or transient drops accumulated over the session), the
run loop exhausted its reconnect budget and returned, orphaning the task. With
no listener for _reconnect_event, the circuit breaker's half-open probe could
never revive the server — every probe hit a dead/absent session, re-armed the
60s cooldown, and looped forever until a full gateway restart (#16788).
Root cause was split ownership of transport liveness between the run loop and
the tool handler, plus a permanent give-up path. Fixed by one invariant: a
non-shutdown server task is always reconnectable.
- run loop parks (deregisters phantom tools, then awaits _reconnect_event)
instead of returning when the reconnect budget is exhausted, so the task
stays alive as a dormant listener
- retry budget resets on every successful (re)connect, so a healthy
long-lived server can't accumulate lifetime drops into a death sentence
- half-open probe with no live session signals a reconnect (reviving a
parked/dead task and respawning a dead stdio subprocess) and returns a
clean 'reconnecting' error instead of writing into a dead pipe
- breaker resets on successful session init across all transports
(stdio/HTTP/SSE) — fully transport-agnostic, no PID/pipe polling
Builds on the closed-PR cluster for this issue: keeps #49255's deregister-on-
exhaustion insight and #21006's signal-don't-probe insight, discards the racy
os.kill PID machinery.
Co-authored-by: LeonSGP43 <LeonSGP43@users.noreply.github.com>
Co-authored-by: srojk34 <srojk34@users.noreply.github.com>
The MCP circuit breaker in tools/mcp_tool.py has no half-open state and
no reset-on-reconnect behavior, so once it trips after 3 consecutive
failures it stays tripped for the process lifetime. These tests lock
in the intended recovery behavior:
1. test_circuit_breaker_half_opens_after_cooldown — after the cooldown
elapses, the next call must actually probe the session; success
closes the breaker.
2. test_circuit_breaker_reopens_on_probe_failure — a failed probe
re-arms the cooldown instead of letting every subsequent call
through.
3. test_circuit_breaker_cleared_on_reconnect — a successful OAuth
recovery resets the breaker even if the post-reconnect retry
fails (a successful reconnect is sufficient evidence the server
is viable again).
All three currently fail, as expected.
