A pre-compaction hook that runs a Python script before an agent's conversation context is shortened. Context compaction is the process of reducing earlier messages so work can continue within the available space.

Use it to trigger `scripts/yarbrain.py` before context compaction in a Claude plugin.

It provides a defined action point before earlier context is compacted, so the configured script can run first. The input does not explain what the script itself changes.

PreCompact — Hook by yarlson

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add hooks/yarlson/yarstack/pre-compact

Clone the repo

git clone --depth 1 https://github.com/yarlson/yarstack

Per session not measured What this adds to a session before it is invoked.

When invoked not measured Not applicable: nothing here is loaded into a session.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Security

Grade A, and why

PreCompact scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the commands this hook runs, not an audit. A hook is shell that executes on your machine at the event it names, which is why every command in it is printed with what was found.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

plugins/yarbrain/hooks/hooks.json · 13 lines

What it actually says

{
  "PreCompact": [
    {
      "hooks": [
        {
          "type": "command",
          "command": "python3 \"${CLAUDE_PLUGIN_ROOT}/scripts/yarbrain.py\" hook",
          "timeout": 3
        }
      ]
    }
  ]
}

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

yesterday First seen · 13 lines scan A 56fb100272cf

Subscribe to this mod's changes

PreCompact is a hook published in the GitHub repository yarlson/yarstack (3 stars, last pushed 4d ago), licensed MIT. Its token cost is not measured: a hook is shell that never enters the context. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

Code review Languages agent-skills agent-workflows ai-coding coding-agents developer-tools

PreCompact

Getting it into your agent

Grade A, and why

Nothing flagged

What it actually says

What this file has done since we first saw it

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