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.
npx agentmods add instructions/m4ttstack/fast-browser/claude-mdgit clone --depth 1 https://github.com/m4ttstack/fast-browserWhat it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.01446 | $0.01446 |
| Opus 5 | $0.00723 | $0.00723 |
| Sonnet 5 | $0.00289 | $0.00289 |
| Haiku 4.5 | $0.00145 | $0.00145 |
Grade A, and why
fast-browser CLAUDE.md 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 body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
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.
How it starts
The opening of the file, as written. The whole thing — 131 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Fast Browser plugin
This repo is the plugin: skills, agents, macros, the installer, and the
runtime-lock.json that pins which runtime and Chrome extension get installed.
It does not contain the code that drives the browser. That lives in a fork of Playwright and ships as prebuilt artifacts. Most "Fast Browser is behaving wrong" bugs cannot be fixed here.
Where a change belongs
| Symptom | Repo |
|---|---|
| relay drops, CDP, tab groups, extension behavior, MCP tools | the fork (see below) |
| setup, doctor, uninstall, migrate, config, annotate, gif | here, lib/ |
| skills, agents, macros, routing rules | here |
| which runtime version is installed | here, runtime-lock.json |
The fork is m4ttheweric/playwright, normally cloned as a sibling of this repo.
Browser-driving code is mostly packages/extension/src/ and
packages/playwright/src/mcp/.
Fork branch: use fast-browser-runtime
All Fast Browser work, including the packaging and release tooling, is on the
fast-browser-runtime branch. The fork usually keeps a worktree for it under
.worktrees/fast-browser-runtime, parked on the commit the current lock names.
main and multi-connection-extension are behind that branch and have none
of the release tooling. Landing a fix there produces a runtime that cannot be
built or released. Some files are byte-identical across the branches, so a clean
git diff proves nothing. Check git log HEAD..fast-browser-runtime before
picking a branch.
Verify with runtime-lock.json's sourceCommit, which is the exact commit the
installed artifacts were built from.
Releasing a new runtime
GitHub Actions is disabled on the fork on purpose: it is a Playwright fork,
so enabling it runs the entire upstream Playwright CI suite. Do not turn it on,
and do not "fix" publish_fast_browser.yml expecting it to run. Artifacts are
built locally.
From the fork worktree, with the fix committed:
node utils/build/build.js
npm run test-extension # real Chrome, ~40s
node utils/fast_browser/build_artifacts.mjs --version <v> --out-dir fast-browser-dist
gh release create fast-browser-v<v> --repo m4ttheweric/playwright \
--target fast-browser-runtime --title "Fast Browser <v>" --notes-file <notes>
gh release upload fast-browser-v<v> fast-browser-dist/* --repo m4ttheweric/playwright
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 · 131 lines · 1,446 tokens per session scan A ef16b7a5838c
fast-browser CLAUDE.md is an instructions file published in the GitHub repository m4ttstack/fast-browser (0 stars, last pushed 7d ago), licensed MIT. It adds 1,446 tokens to every session, about $0.0072 per session on Opus 5. 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.
Other instructions, from other repositories
neobrowser AGENTS.md
Instructions for pitiflautico/neobrowser, covering agents.md — orientation for ai agents & contributors, build, run, test (rust — primary), how it works (rust module map), conventions and ci and release gates.
apitap CLAUDE.md
Instructions for n1byn1kt/apitap, covering claude.md, using apitap as an mcp tool, commands, architecture and module map.
scout CLAUDE.md
Claude Code instructions for klarlabs-studio/scout, covering claude.md, repository, what this is, commands and build & verify.
opentabs CLAUDE.md
Instructions for opentabs-dev/opentabs, covering project instructions for claude, project overview, architecture, you are connected to this project's mcp server and workflow skills.
fanqie-publisher-skill AGENTS.md
Instructions for amm10090/fanqie-publisher-skill, covering fanqie-publisher-skill — codex cli agent adapter, read first, prerequisites, entry points and utf-8 / unicode safeguards.
worbrow AGENTS.md
AGENTS.md instructions for noisystreet/worbrow, covering agents.md, project identity, hard constraints (revert on violation), modifying architecture docs and commit conventions.