Declare your systems and the rules between them. faultline redraws the map on every edit and stops coding agents at the boundary.
The Astro monorepo, 22 systems, with one simulated edit that crosses a rule. Click the red edge for the import behind it, or double-click a system to open it.
Coding agents did two real tasks in the Astro repo, three times each, where the obvious import breaks a declared rule. The same rules were given to them as written docs, or through faultline.
| Task | Rules in docs | faultline |
|---|---|---|
| Warn when a page takes over 500 ms 1import { getTimeStat } from '../../../core/build/util.js' | crossed | held |
| Warn when a page takes over 500 ms 2 | crossed | held |
| Warn when a page takes over 500 ms 3 | crossed | held |
| Add an error header for coding agents 1import { isRunByAgent } from '../../cli/agent.js' | held | held |
| Add an error header for coding agents 2 | crossed | held |
| Add an error header for coding agents 3 | crossed | held |
| Crossed | 5 of 6 | 0 of 6 |
Every run finished its task. Runs through faultline used more tokens. It is a small pilot; the prompts, the rules and every diff are in bench/agent-ab.
fault init drafts faultline.yml from your folders and imports. You name the systems and write the rules that matter.
fault map redraws on every edit. A new dependency appears in green, a crossed rule in red, each with the import behind it.
Agents read the map before they write and hear about a crossed rule the moment it lands. fault check holds it in CI.
fault setup connects the MCP server, hooks, AGENTS.md and a pre-commit check for the agents your repo uses.
Claude Code, Codex, Cursor, Copilot, Gemini CLI, Kiro, Zed and opencode, and anything with a shell.
The same faultline.yml gates every pull request, with one comment that says what changed in the architecture and why.
# .github/workflows/faultline.yml
on: pull_request
jobs:
faultline:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
with: { fetch-depth: 0 }
- uses: anzal1/faultline@v0
Where code lives is a fact, so faultline keeps it up to date. What may depend on what is a decision, so it stays with people.
fault sync --apply writes it, and agents may run it.fault check fails until someone approves it.Pick an entry point and the map shows everything it loads at startup: how much of each system, which packages, the shortest import chain behind any file, and the one import that would drop the most.
On Astro's production entry it found zod imported at startup to read a couple of constants.
Node 20 or newer. It runs on your machine, and nothing leaves it.
$ npm install -g @anzalabidi/faultline $ fault init # draft faultline.yml $ fault map # open the live map $ fault setup # connect your agents $ fault check # the CI gate
Reads TypeScript, JavaScript, Astro, Vue, Svelte, Python, Go, Rust, Java, Kotlin, Scala, C#, C, C++, Ruby, PHP, Swift, Dart, Elixir, Lua, Haskell and Zig.