FabledScribe/plugin/hooks/scribe_precompact_preserve.sh

#!/usr/bin/env bash
# Scribe plugin — PreCompact: tell the summarizer what must survive (#3680).
#
# WHAT THIS HOOK ACTUALLY DOES, and why it is not what the design first assumed.
#
# Spike #3680 asked whether a PreCompact hook can reach the model, and note
# #3679 assumed the channel would be `hookSpecificOutput.additionalContext`
# plus a block. Both halves of that are wrong, read out of the installed build
# (Claude Code 2.1.273):
#
#   * `additionalContext` is NEVER read on PreCompact. The hook-output schema
#     has no PreCompact variant at all; the field is honoured for SessionStart,
#     SubagentStart, Stop and friends, and silently dropped here.
#
#   * A PreCompact hook's STDOUT becomes the compaction's custom instructions.
#     The handler collects every hook that exited 0 with non-empty stdout and
#     returns it as `newCustomInstructions`, which is merged with whatever the
#     operator typed after `/compact` and passed into the prompt that writes
#     the summary. Every path does this — manual, auto and partial compaction.
#
# So there IS a model-reaching channel, and it is better than the one designed:
# we do not interrupt the compaction, we steer the summary it produces. The
# summary is what the next turn reads, so text that survives into it survives
# the compaction.
#
# WHAT NOT TO DO HERE: never block. A PreCompact block (exit 2, or
# `{"decision":"block"}`) does not pause for the model and cannot tell it
# anything — the compaction is SKIPPED, a warning goes to the operator's
# screen, and the session continues uncompacted toward its context limit with
# no summary at all. That failure is silent from the model's side, which is
# exactly the outcome #3680 existed to avoid shipping. This hook exits 0 on
# every path.
#
# SCOPE: in a subagent the handler discards hook stdout and keeps only a block,
# so this steers the main session's compaction and nothing else. That is the
# one we care about — a subagent's summary does not outlive it.
#
# NO NETWORK, NO CONFIG. What must be preserved is already in the conversation
# being summarized; this hook's job is to say which parts of it are load-bearing
# so the summarizer keeps them literally instead of compressing them away.
# Naming the in-flight task ids from the instance would need a server round-trip
# inside the compaction path, which is a separate, measurable question.
#
# PROOF THAT IT FIRED: on a manual `/compact` the handler shows the operator
# `PreCompact [<command>] completed successfully: <stdout>`, so the hook's own
# text is the receipt. (Auto-compaction suppresses that notification.)
set -uo pipefail

# shellcheck source=plugin/hooks/scribe_defs.sh
. "$(dirname "${BASH_SOURCE[0]}")/scribe_defs.sh"

# THE EVENT IS NOW READ, where it used to be drained. The old comment here said
# nothing in it changes what we emit, and that was true while every line was
# static. It stopped being true at #4216: the slippage readout below is
# specific to THIS session, and `session_id` is the key to the ledgers that
# hold it. The instruction is still the same whether the operator typed
# `/compact` or the session hit its limit — what varies is the measurement
# appended to it.
event=$(cat 2>/dev/null || true)
session_id=""
if [ -n "$event" ]; then
  event_flat=$(printf '%s' "$event" | scribe_json_flat 2>/dev/null || true)
  session_id=$(scribe_json_pick "$event_flat" '.session_id' 2>/dev/null || true)
fi

cat <<'EOF'
Preserve the following literally in the summary — copied through, not
paraphrased or counted:

- Every Scribe record the conversation refers to, by id AND title: tasks,
  issues, milestones, notes, rules, systems. A bare "#4061" is not enough; a
  record whose name is lost has to be looked up again before it can be used.
- Which task is in progress and what its status was last set to, plus the
  milestone it sits under.
- Work that was done but NOT yet recorded in Scribe — an edit with no work-log,
  a fix not filed as an issue, a decision not written down. Carry these over as
  outstanding; they exist nowhere else once this conversation is summarized.
- Any rule or preference that was retrieved and still governs the work, by id
  and title.
- Anything the operator asked for that has not been done yet, in their words.

Everything else here can be recovered from the repository or from Scribe. These
cannot: they are this session's only copy.
EOF

# ── The slippage readout (#4216, milestone 419) ───────────────────────────
#
# WHY IT BELONGS HERE RATHER THAN IN A REPLY. A rule read and left unresolved
# is invisible by construction: it looks exactly like a rule that worked. The
# compaction is where that invisibility becomes permanent — the turns holding
# the evidence are summarised away, and an unjudged thing that survives as
# nothing is how a decision quietly becomes nobody's.
#
# STDOUT HERE IS THE SUMMARISER'S INSTRUCTIONS, not a message to the model
# (the header records how that was established). So this does not say "you
# slipped"; it says WHICH ids must be carried through, which is the one thing
# the summary can do about it. The next turn then reads them in the summary
# with the work still attached.
#
# SILENT WHEN THERE IS NOTHING TO SAY. A session that opened everything it was
# shown gets no extra lines. "0 rules unresolved" on every compaction is how a
# readout teaches its reader to skip it.
if [ -n "$session_id" ]; then
  safe_sid=$(printf '%s' "$session_id" | tr -c 'A-Za-z0-9._-' '_')
  slippage=$(scribe_slippage_lines "${TMPDIR:-/tmp}/scribe-priorart" "$safe_sid" 2>/dev/null || true)
  if [ -n "$slippage" ]; then
    printf '\n%s\n' "$slippage"
  fi
fi
exit 0