Spike #3680 asked whether a PreCompact hook can reach the model. Read out of
the installed Claude Code build (2.1.273), the answer is yes — through a
different channel than note #3679 assumed:
* `hookSpecificOutput.additionalContext` is NEVER read on PreCompact. The
hook-output schema has no PreCompact variant; the field is honoured for
SessionStart, SubagentStart and Stop, and silently dropped here.
* A PreCompact hook's STDOUT becomes `newCustomInstructions`, merged with
the operator's own `/compact` instructions and passed into the prompt that
writes the summary. Manual, auto and partial compaction all do this.
So the hook does not interrupt the compaction — it steers the summary, which
is what the next turn reads. Blocking is the thing not to do: a PreCompact
block SKIPS compaction, tells the model nothing, and leaves the session
running on uncompacted with no summary at all.
scribe_precompact_preserve.sh names what the summary is the only copy of:
Scribe record ids WITH titles, the in-progress task and its milestone, work
done but not yet recorded, governing rules, and unfinished operator asks.
No network and no config — what must survive is already in the conversation
being summarized; the hook only says which parts are load-bearing.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy
