215 lines
5.5 KiB
Markdown
215 lines
5.5 KiB
Markdown
# 𝄐 dirigent_fermata
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**A fast, harness-agnostic policy gate for AI coding agents.**
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Drop a `.botignore` file in your project root. Fermata reads it and blocks your agent from reading, writing, or running things it shouldn't — before the tool call happens.
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```
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.env
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.env.*
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secrets/**
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conf/settings.local.yaml
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```
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That's all it takes.
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---
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## Why Fermata
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AI coding agents are powerful, but they don't have an innate sense of "don't touch `.env`." Native hook systems in tools like Claude Code let you intercept every file operation — but wiring up your own secure, fast hook for each project is friction. Fermata is that hook, ready to drop in.
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- **Fast** — written in Rust; ~1–5ms per call. Hooks fire on every read, write, and bash operation. Python cold-start (~50–150ms) compounds fast. Fermata doesn't.
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- **Familiar syntax** — `.botignore` uses gitignore rules via the `ignore` crate (the same engine powering ripgrep).
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- **Per-operation control** — `botignore.toml` lets you block writes to `vendor/**` while still allowing reads, or deny specific bash patterns without touching path rules.
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- **Harness-agnostic** — plain CLI exit codes work from any shell wrapper; the hook adapter speaks Claude Code's JSON natively.
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---
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## Status: v0.1
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| Component | Status |
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|-----------|--------|
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| Library (`Op`, `Decision`, `Policy::check`, `Policy::check_command`) | Done |
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| `.botignore` walker (project-root walk-up, gitignore semantics) | Done |
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| `botignore.toml` parser (read / write / bash namespaces) | Done |
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| Path identification heuristics | Done |
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| CLI: `fermata check <path>...` | Done |
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| CLI: `fermata hook --harness claude` | Done |
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| Claude Code PreToolUse adapter | Done |
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Out of scope for v0.1: Codex / Gemini hook adapters, MCP server mode, audit log, filesystem watcher.
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---
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## Install
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From source (this monorepo):
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```bash
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cargo install --path crates/dirigent_fermata --features cli
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```
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This installs the `fermata` binary into `~/.cargo/bin/`.
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---
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## Usage
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### Checking a path
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```bash
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fermata check --op read /path/to/.env
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# exit 1 — blocked
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# stderr: blocked by rule ".env" in /your/project/.botignore
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fermata check --op write /path/to/src/main.rs
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# exit 0 — allowed
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```
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### Claude Code hook adapter
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```bash
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fermata hook --harness claude < hook_payload.json
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```
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Reads the PreToolUse JSON from stdin, extracts the tool name and path or command, applies policy, and emits the Claude-shaped JSON response. The hook's exit code is always `0`; the verdict is in the JSON body.
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---
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## Configuration
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### `.botignore` — the 80% case
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Create a `.botignore` at your project root. Gitignore syntax. Blocks both reads and writes.
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```gitignore
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# Secrets
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.env
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.env.*
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secrets/**
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# Local config overrides
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conf/settings.local.yaml
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conf/settings.test.yaml
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# Generated files — let the tools rebuild them, not patch them
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dist/**
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*.lock
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```
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Fermata walks up from the target file to find the nearest `.botignore`, so it works correctly even when an agent changes directory.
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### `botignore.toml` — per-operation rules
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For cases where `.botignore`'s uniform read+write block isn't granular enough:
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```toml
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[read]
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# Block reading secrets outright
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patterns = [".env*", "secrets/**", "conf/settings.local.yaml"]
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[write]
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# Allow reading vendor code but block patching it
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patterns = ["vendor/**", "*.lock"]
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[bash]
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# Hard-block destructive or exfiltrating commands
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deny = [
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"rm -rf /",
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"curl * | sh",
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"git push --force*",
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]
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# Ask before any removal or move
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ask = ["rm:*", "mv:*"]
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# Narrow allowlist for automated commands
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allow_prefixes = ["make test", "git checkout:*"]
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```
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---
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## How it fits into Claude Code
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Add fermata as a `PreToolUse` hook in `.claude/settings.json`:
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```json
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{
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"hooks": {
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"PreToolUse": [
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{
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"matcher": "Bash|Read|Edit|Write",
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"hooks": [
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{
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"type": "command",
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"command": "fermata hook --harness claude"
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}
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]
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}
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]
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}
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}
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```
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When Claude attempts a `Read(.env)`, `Write(vendor/foo.js)`, or `Bash(rm ./secrets/key.pem)`, fermata intercepts the call, checks policy, and returns a deny with a human-readable reason — before any damage is done.
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---
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## Real-world scenario
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A project has `.env`, `conf/settings.local.yaml`, and a `vendor/` tree it doesn't want patched. With `.botignore`:
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```gitignore
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.env
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.env.*
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conf/settings.local.yaml
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vendor/**
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```
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Claude attempts to read credentials:
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```
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Tool: Read
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Path: ./conf/settings.local.yaml
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Decision: BLOCK — matched rule "conf/settings.local.yaml" (.botignore)
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```
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Claude attempts to read application code:
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```
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Tool: Read
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Path: ./src/app/main.rs
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Decision: ALLOW
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```
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Claude attempts to run `cat .env` via bash — which would bypass a path-only check:
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```toml
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# botignore.toml
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[bash]
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deny = ["cat .env*", "cat conf/settings.local*"]
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```
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```
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Tool: Bash
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Command: cat .env
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Decision: BLOCK — matched bash deny rule "cat .env*"
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```
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---
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## Architecture
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Three concentric layers; nothing inner imports from anything outer:
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- **`core/`** — harness-unaware, sync. Types, `.botignore` walker, `botignore.toml` parser, `Policy::check` / `check_command`, path extraction.
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- **`harness/`** — `HarnessAdapter` trait over a normalized `ToolCall`. Each adapter lives in its own submodule, feature-gated.
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- **`bin/fermata.rs`** — the only place `clap`, stdio, and exit codes appear.
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---
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## See also
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- `docs/tools/fermata.md` — Dirigent integration plan
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- `docs/workpad/brainstorm/fermata.md` — full product spec and field notes
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- `docs/architecture/crates.md` — crate dependency map
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