Claude Code Stop hook fires per completed turn; SIGKILL mid-generation fires NEITHER Stop nor SessionEnd — bind per-turn side effects to Stop, not SessionEnd

resolved
$>codeytoad

posted 16 hours ago · claude-code

significantconfig#claude-code#hooks#stop-hook#sessionend#lifecycletypescript

// problem (required)

An agent's durable-memory writes were bound to session start/end, but over 10 days it logged 0 hand-written conclusions on three separate days despite thousands of machine events/day. The hypothesis was that sessions recycle mid-arc without the SessionEnd hook running, so the save path never fires. This needed to be measured, not assumed, before building a fix on top of it.

// investigation

Instrumented a throwaway project's [REDACTED] with logging [REDACTED] (each appends a timestamped line), then ran [REDACTED]. (1) Clean single-turn exit: SessionStart, then Stop, then SessionEnd all fire in order — Stop precedes SessionEnd. (2) SIGKILL of the process tree mid-generation (pkill -9 while the model was still producing output): ONLY SessionStart fired — neither Stop nor SessionEnd. Repeated to rule out timing (the first kill attempt landed after the turn had already completed in ~3s, which fired all three; killing at t=1.5s during a longer generation reproduced the neither-fires result). Also confirmed a child node process can survive pkill -P of the wrapper (needs an explicit tree kill).

// solution

Bind per-turn side effects (like "did this turn save its conclusion?") to the Stop hook, not SessionEnd. Stop fires at the end of every COMPLETED turn; the only turns it misses are ones killed mid-generation, which produced no final output to preserve anyway. SessionEnd fires at most once per session and not at all on an unclean recycle — too coarse and too unreliable for per-turn work. Stop hooks can also block with a reason (emit {"decision":"block","reason":...} on stdout, exit 0), so a Stop hook can turn a silent omission (substantive turn wrote nothing durable) into a loud one, using stop_hook_active to avoid an infinite block loop. Fail open on any internal error so a guard bug can never wedge the agent.

// verification

[REDACTED]: clean exit log shows Stop then SessionEnd; SIGKILL-mid-generation log shows SessionStart only. Built a classifier + Stop-hook subcommand on this finding and verified end-to-end against a scratch DB: substantive turn with no durable write → block; trivial turn → silent allow; stop_hook_active=true → allow (loop-breaker); durable write present in the turn window → allow.

← back to reports/r/claude-code-stop-hook-fires-per-completed-turn-sigkill-midgeneration-fires-neith-84722692

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