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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
75 lines
3.8 KiB
Bash
75 lines
3.8 KiB
Bash
#!/usr/bin/env bash
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# Scribe plugin — PreCompact: tell the summarizer what must survive (#3680).
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#
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# WHAT THIS HOOK ACTUALLY DOES, and why it is not what the design first assumed.
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#
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# Spike #3680 asked whether a PreCompact hook can reach the model, and note
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# #3679 assumed the channel would be `hookSpecificOutput.additionalContext`
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# plus a block. Both halves of that are wrong, read out of the installed build
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# (Claude Code 2.1.273):
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#
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# * `additionalContext` is NEVER read on PreCompact. The hook-output schema
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# has no PreCompact variant at all; the field is honoured for SessionStart,
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# SubagentStart, Stop and friends, and silently dropped here.
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#
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# * A PreCompact hook's STDOUT becomes the compaction's custom instructions.
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# The handler collects every hook that exited 0 with non-empty stdout and
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# returns it as `newCustomInstructions`, which is merged with whatever the
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# operator typed after `/compact` and passed into the prompt that writes
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# the summary. Every path does this — manual, auto and partial compaction.
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#
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# So there IS a model-reaching channel, and it is better than the one designed:
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# we do not interrupt the compaction, we steer the summary it produces. The
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# summary is what the next turn reads, so text that survives into it survives
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# the compaction.
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#
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# WHAT NOT TO DO HERE: never block. A PreCompact block (exit 2, or
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# `{"decision":"block"}`) does not pause for the model and cannot tell it
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# anything — the compaction is SKIPPED, a warning goes to the operator's
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# screen, and the session continues uncompacted toward its context limit with
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# no summary at all. That failure is silent from the model's side, which is
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# exactly the outcome #3680 existed to avoid shipping. This hook exits 0 on
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# every path.
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#
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# SCOPE: in a subagent the handler discards hook stdout and keeps only a block,
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# so this steers the main session's compaction and nothing else. That is the
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# one we care about — a subagent's summary does not outlive it.
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#
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# NO NETWORK, NO CONFIG. What must be preserved is already in the conversation
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# being summarized; this hook's job is to say which parts of it are load-bearing
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# so the summarizer keeps them literally instead of compressing them away.
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# Naming the in-flight task ids from the instance would need a server round-trip
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# inside the compaction path, which is a separate, measurable question.
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#
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# PROOF THAT IT FIRED: on a manual `/compact` the handler shows the operator
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# `PreCompact [<command>] completed successfully: <stdout>`, so the hook's own
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# text is the receipt. (Auto-compaction suppresses that notification.)
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set -uo pipefail
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# Drain the event so the caller never sees a broken pipe. Nothing in it changes
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# what we emit: the instruction is the same whether the operator typed
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# `/compact` or the session hit its limit, and it composes with any custom
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# instructions they gave, which are merged ahead of ours.
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cat >/dev/null 2>&1 || true
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cat <<'EOF'
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Preserve the following literally in the summary — copied through, not
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paraphrased or counted:
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- Every Scribe record the conversation refers to, by id AND title: tasks,
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issues, milestones, notes, rules, systems. A bare "#4061" is not enough; a
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record whose name is lost has to be looked up again before it can be used.
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- Which task is in progress and what its status was last set to, plus the
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milestone it sits under.
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- Work that was done but NOT yet recorded in Scribe — an edit with no work-log,
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a fix not filed as an issue, a decision not written down. Carry these over as
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outstanding; they exist nowhere else once this conversation is summarized.
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- Any rule or preference that was retrieved and still governs the work, by id
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and title.
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- Anything the operator asked for that has not been done yet, in their words.
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Everything else here can be recovered from the repository or from Scribe. These
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cannot: they are this session's only copy.
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EOF
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exit 0
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