Both input paths recorded the (clientId, seq) pair as applied and acknowledged the
frame BEFORE knowing whether the write had landed: the POST route because its mux
write is fire-and-forget so the response never waits on a tmux child, the
WebSocket handler because it ACKed unconditionally.
When the write then failed, the client dropped the frame from its durable queue
and the server rejected the retry as a duplicate. The reliable-delivery layer was
guaranteeing exactly-once delivery of something that had never been delivered —
and `Session.write()` returned void, so a session whose PTY was gone swallowed the
data with no signal at all.
- `forgetInputSeq()` rolls the bookkeeping back on failure, but only when that seq
is still the newest one; a later input has superseded it and must not re-open.
- The WebSocket handler withholds its ACK when the write did not land, so the
client redelivers.
- `Session.write()` reports whether it reached a PTY.
Response codes are unchanged, deliberately: a session can legitimately have no PTY
yet, and turning that into a failure status would be a contract change of its own.
What this does NOT do: remove the root cause. The POST still answers 200 before
the mux write is attempted, so a client that treats any 2xx as final cannot learn
about that failure. What closes is the narrower window — the write failed AND the
ACK never reached the client — plus the whole WebSocket path. Closing the rest
would mean awaiting the tmux child inside the request.
9 tests. They drive the HTTP route, not only the Session primitives: with the
rollback removed from the route, 2 of them fail.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claudia
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