precompact-transcript-backup.sh: mapfile fails silently on stock macOS bash 3.2

[fuuuuuuma]

Summary

hooks/precompact-transcript-backup.sh uses mapfile, which is a bash 4+ builtin. macOS ships bash 3.2 by default (/bin/bash and /usr/bin/env bash both resolve to 3.2.57 unless the user has installed a newer bash via Homebrew). On a stock Mac, the rotation step fails every time:

precompact-transcript-backup.sh: line 23: mapfile: command not found

Why this is easy to miss

The script wraps everything in trap 'exit 0' ERR (correctly, for fail-open behavior), so the failure is completely silent — no error surfaces anywhere in Claude Code. The visible symptom is that ~/.claude/backups/transcripts/ grows without bound, one file per compaction, forever, for any user who hasn't separately installed a newer bash.

line 23: mapfile -t OLD_BACKUPS < <(find "$BACKUP_DIR" -maxdepth 1 -type f -name "*-${SESSION_ID}.jsonl" -print 2>/dev/null | sort -r | tail -n +21)
line 24: if [[ ${#OLD_BACKUPS[@]} -gt 0 ]]; then
line 25:   rm -f "${OLD_BACKUPS[@]}" 2>/dev/null || true
line 26: fi

Suggested fix

Replace the mapfile + array with a portable while read loop (no bash4+ dependency):

find "$BACKUP_DIR" -maxdepth 1 -type f -name "*-${SESSION_ID}.jsonl" -print 2>/dev/null | sort -r | tail -n +21 | while IFS= read -r old; do
  rm -f "$old" 2>/dev/null || true
done

I verified this fix locally: seeded 25 fake backup files for a session, ran the patched script, and confirmed it correctly pruned down to 20 (the intended retention count) with bash --version = 3.2.57(1)-release.

Environment

• macOS (Darwin 25.5.0, arm64), no Homebrew bash installed
• bash --version → GNU bash, version 3.2.57(1)-release (arm64-apple-darwin25)
• compact-plus commit: 098e8f7 (main, at time of testing)

Happy to open a PR with the fix above if useful.

[u-ichi]

Thanks for the detailed report and the working local fix, @fuuuuuuma — the root cause (bash 3.2 has no mapfile, and trap 'exit 0' ERR swallowed the failure silently) matched exactly.

Opened #3 with the while IFS= read -r rewrite. Verified on both /bin/bash 3.2.57 (stock macOS) and Homebrew bash 5.3.12 that a seeded 25-file directory is correctly pruned to 20 and other-session files are untouched. Also grepped the rest of the repo for other bash 4+ features (mapfile, readarray, declare -A, ${var^^}, ${var,,}) — no other occurrences.

Will close this once #3 lands.

[u-ichi]

Follow-up on #3:

• Rolled the plugin version 1.0.0 → 1.0.1 (patch, since this is a bug fix).
• Added a Version bump policy section to AGENTS.md, adapted from the reviewable-html-workbench convention, so future commits keep .claude-plugin/plugin.json version and .claude-plugin/marketplace.json plugins[0].version in sync under semver.

Both changes are included in the same PR #3 as the mapfile fix.