Headroom is a library, proxy, and MCP server that compresses tool outputs, logs, files, retrieval chunks, and conversation history before they reach a language model. It helps coding agents use fewer context tokens while keeping the original content available locally when needed. Its catalogue add-ons support using Headroom with agent workflows.
Getting it into your agent
This one installs as part of its plugin. Adding the marketplace and installing the plugin brings it with everything else the plugin ships.
/plugin marketplace add headroomlabs-ai/headroom/plugin install headroomWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/hooks/headroomlabs-ai/headroom/session-start)<a href="https://agentmods.dev/hooks/headroomlabs-ai/headroom/session-start"><img src="https://agentmods.dev/badge/hooks/headroomlabs-ai/headroom/session-start/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/hooks/headroomlabs-ai/headroom/session-start"><img src="https://agentmods.dev/badge/hooks/headroomlabs-ai/headroom/session-start.svg" alt="Reviewed on agentmods" width="80" height="20"></a>Grade A, and why
SessionStart 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 10d ago.
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.
What it actually says
{
"SessionStart": [
{
"matcher": "startup|resume",
"hooks": [
{
"type": "command",
"command": "sh -c '[ -n \"${CLAUDE_PLUGIN_ROOT:-}\" ] && launcher=\"${CLAUDE_PLUGIN_ROOT}/bin/headroom-hook.sh\" && [ -r \"$launcher\" ] && exec sh \"$launcher\"; exec headroom init hook ensure'",
"powershell": "headroom init hook ensure",
"timeout": 60
}
]
}
]
}What else hooks.json configures
This page is one entry in a file that holds 2. Installing the file brings all of them; each is measured and scanned on its own page.
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.
- 10d ago Changed · +1 lines c0d6badd3042
- 21d ago Changed 33f6c20ce123
- 1mo ago First seen · 14 lines scan A 8a935bc9c8fd
SessionStart is a hook published in the GitHub repository headroomlabs-ai/headroom (75,453 stars, last pushed today), licensed Apache-2.0. 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 2026-08-30.
Other hooks, from other repositories
SessionStart
Runs when a session starts on startup, executing install_pkgs.sh. From supabase/supabase.
PostToolUse
Runs after a tool call finishes for Write and Edit tool calls, executing format_and_lint.sh. From supabase/supabase.
SessionStart
Runs when a session starts, executing on-start.js via node. From google-gemini/gemini-cli.
PreToolUse
Runs before the agent uses a tool for Bash tool calls, running an inline shell check. From apache/superset.
PreToolUse
Runs before the agent uses a tool, executing pre-write.sh and pre-bash.sh (2 commands). From anthropics/claude-cookbooks.
SessionStart
Runs when a session starts, executing session-start.sh. From anthropics/claude-cookbooks.