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Save compaction summary to log

Keep a readable trail of every compaction

Appends the compaction summary generated by Claude Code to a markdown log file inside the project. Provides a persistent record of what was summarized across context window resets, useful for long-running sessions and post-session review.

What does the Save compaction summary to log hook do?

Save compaction summary to log is a Claude Code PostCompact hook. It fires automatically at that lifecycle event — outside the model, so it can't be skipped or forgotten. Keep a readable trail of every compaction.

Because it is a deterministic Node.js script, it executes on every matching event without relying on the model to remember — the guarantee that makes agentic workflows safe to automate.

Use cases

  • Keep a human-readable history of every compaction event in the project
  • Review what Claude summarized at context boundaries for debugging or audit
  • Reconstruct the timeline of a long session from compaction summaries

Tags

#compaction#logging#documentation#context

settings.json fragment

{
  "hooks": {
    "PostCompact": [
      {
        "hooks": [
          {
            "command": "node $CLAUDE_PROJECT_DIR/.claude/hooks/post-compact-save-summary.mjs",
            "type": "command"
          }
        ]
      }
    ]
  }
}

Script · .claude/hooks/post-compact-save-summary.mjs

#!/usr/bin/env node
// @hookstack post-compact-save-summary
// Journalise le résumé de compaction dans .claude/compaction-log.md (PostCompact)
import { appendFileSync, mkdirSync, readFileSync } from "node:fs";
import { dirname, join } from "node:path";
import { fileURLToPath } from "node:url";

export function run(
	input,
	{
		append = appendFileSync,
		mkdir = mkdirSync,
		projectDir = process.env.CLAUDE_PROJECT_DIR ?? ".",
		now = () => new Date().toISOString(),
	} = {},
) {
	const summary = input.compact_summary ?? "";
	if (!summary.trim()) return null;

	const logPath = join(projectDir, ".claude", "compaction-log.md");
	try {
		mkdir(dirname(logPath), { recursive: true });
	} catch {
		/* exists */
	}

	const entry = `\n## ${now()} (${input.trigger ?? "auto"})\n${summary}\n`;
	append(logPath, entry);
	return entry;
}

/* v8 ignore next 4 */
if (process.argv[1] === fileURLToPath(import.meta.url)) {
	const input = JSON.parse(readFileSync(0, "utf8"));
	run(input);
}

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