What is SubagentStart?

A startup hook configuration that runs shell commands when an AI subagent begins work. A hook is an automatic action tied to a specific event.

Use it to run an advisor-context script for the mainframe-advisor subagent and a telemetry script for every subagent start.

It lets a project prepare context for certain subagents and record activity automatically, without relying on someone to run these commands by hand.

SubagentStart — Hook by CATWILLgh

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/catwillgh/mainframe/subagent-start

Clone the repo

git clone --depth 1 https://github.com/CATWILLgh/MAINFRAME

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 88% copy Near-identical to another mod in the catalogue.

Security

Grade A, and why

SubagentStart 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.

Origin

This is a copy

88% identical to UserPromptExpansion — 18 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

adapters/claude-code/plugin/hooks/hooks.json · 32 lines

What it actually says

{
  "SubagentStart": [
    {
      "matcher": "mainframe-advisor",
      "hooks": [
        {
          "type": "command",
          "command": "sh",
          "args": [
            "${CLAUDE_PLUGIN_ROOT}/hooks/scripts/run-hook.sh",
            "SubagentStart",
            "${CLAUDE_PLUGIN_ROOT}/hooks/scripts/advisor-context.py"
          ]
        }
      ]
    },
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "command",
          "command": "sh",
          "args": [
            "${CLAUDE_PLUGIN_ROOT}/hooks/scripts/run-telemetry-hook.sh",
            "SubagentStart",
            "${CLAUDE_PLUGIN_ROOT}/hooks/scripts/telemetry.py"
          ]
        }
      ]
    }
  ]
}

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 · 32 lines scan A 34afe916b2ae

Subscribe to this mod's changes

SubagentStart is a hook published in the GitHub repository CATWILLgh/MAINFRAME (2 stars, last pushed 11d 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. It is 88% identical to UserPromptExpansion, differing in 18 lines, and is treated as a copy.

Analytics and metrics Building agents ai-coding-agent claude-code developer-tools hooks skills subagents

SubagentStart

Getting it into your agent

Grade A, and why

Nothing flagged

This is a copy

What it actually says

What this file has done since we first saw it

Other hooks, from other repositories

SubagentStart

UserPromptSubmit

SubagentStart

SubagentStop

SubagentStop

SubagentStop