faultlines 1.0.0

Map features in any codebase from git history alone. No Jira required.

🗺️ Faultlines

Your codebase has a map. You just can't see it yet.

Faultlines turns raw git history into a living map of every feature and user flow in your repo — with bug hotspots, test coverage, and the precise context your AI coding agent actually needs.

No Jira. No annotations. No manual tagging. Just git log and your code.

Website · Quick start · For AI agents · How it works

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The problem

Every engineer rediscovers the same thing on a new codebase:

"Which files actually implement checkout?" "What breaks if I change this?" "Who owns the billing flow?" "Where are the bugs hiding?"

Your issue tracker doesn't know — it's full of aspirational tickets. Static analysis sees imports, not intent. And your AI agent just greps 15 files and burns your context window guessing.

The answers were in your git history the whole time. Faultlines reads them.

What Faultlines does

$ faultlines scan-v2 ./my-app --llm --flows --symbols

  ✓ 472 commits analysed · 1,284 files mapped

  FEATURE                  HEALTH  COVERAGE  HOTSPOTS  FLOWS
  ─────────────────────────────────────────────────────────
  Billing & Subscriptions    62      48%        3        7
  Authentication             88      91%        0        4
  Document E-Sign            41 ⚠     22%        5        9
  Team & Permissions         79      66%        1        5
  …

  → Highest blast radius:  src/payments/charge.ts (touches 4 features)
  → Riskiest flow:         e-sign/finalize  (low coverage · 5 recent bug-fixes)

A two-layer feature map:

• Developer features — code-grounded, the units your engineers actually work in.
• Product features — the customer-facing capabilities those roll up into.

…each broken into flows (real user journeys), scored, attributed down to the function and line range, and served to humans and AI agents.

✨ Features

• 🧭 Feature & flow detection — from git history + code structure, on any stack (Next.js, Rails, Django, FastAPI, Express, Spring, Laravel, Phoenix, and more).
• 🔥 Bug hotspots & health scores — find what's rotting before it pages you.
• 🎯 Behavioral test coverage — coverage per user flow, inferred from history even when there's no lcov.
• 💥 Change-impact / blast radius — "if I touch these files, here's what breaks and who to add as reviewer."
• 🔬 Symbol-level attribution — functions, classes and line ranges per flow, not just file lists.
• 👥 Ownership & bus-factor — who maintains each feature, and where the knowledge is dangerously concentrated.
• 🤖 MCP server for AI agents — 13 typed tools your coding agent calls instead of grepping.
• 📡 Runtime overlays — map Sentry errors and PostHog usage onto features (which features actually fail and get used).
• 🔒 Local-first & private — runs on your machine; your source code never has to leave it.

🚀 Quick start

pip install faultlines

# Scan a repo — deterministic by default, add --llm for richer naming
faultlines scan-v2 /path/to/your/repo --llm --flows --symbols

That writes a versioned feature-map JSON to ~/.faultline/. Explore it, diff it across runs, ship it to CI, or hand it to your AI agent (below).

🤖 Built for AI coding agents

This is the wedge. Install the companion MCP server and your agent stops guessing:

pip install faultlines-mcp

// ~/.cursor/mcp.json  (or: claude mcp add faultlines -- faultlines-mcp)
{
  "mcpServers": {
    "faultlines": { "command": "faultlines-mcp" }
  }
}

Now Cursor / Claude Code / Cline / Windsurf can call 13 tools:

	Tools
🔎 Discover	list_features · find_feature · get_repo_summary
📁 Files & symbols	get_feature_files · get_flow_files · find_symbols_in_flow · find_symbols_for_feature
⚠️ Risk & impact	get_hotspots · get_feature_owners · analyze_change_impact · get_regression_risk
📡 Runtime	get_feature_errors (Sentry) · get_feature_pageviews (PostHog)

Typical result: ~90% fewer tokens per query than a naive grep-and-read loop — your agent reads the right functions, with line ranges, on the first try.

📊 The metrics — and why they matter

Metric	What it tells you	Why you care
Health score	Composite of churn, bug-fixes, coverage & ownership	One number to triage what to refactor next
Bug-fix ratio	Share of commits that fix bugs	High = fragile, defect-prone code
Churn	How often a feature changes	Hotspot detection; instability signal
Impact score	Structural blast radius − coverage	What a change here actually endangers
Coverage	Behavioral test coverage per flow	Find untested user journeys, not just untested lines
Ownership / bus factor	Who holds the knowledge	Spot single-points-of-failure before they leave

🧠 How it works

 git history ─┐
              ├─▶  deterministic extractors  ─┐
 code/config ─┘    (routes · MVC · schema ·   │
                    package · stack patterns)  ├─▶  feature & flow map
                                               │     + metrics + symbols
        optional LLM pass (naming · flows) ────┘            │
                                                            ▼
                              feature-map JSON  ──▶  CLI · CI · dashboard · MCP

Deterministic-first. The structure comes from your routing conventions, configs, schemas and git co-change patterns. An optional LLM pass adds human-readable names and flow detection. The output is a single versioned JSON — the stable contract every consumer reads.

🔌 Integrations

• GitHub — PR comments with risk, coverage gaps and runtime signal on the exact features a diff touches.
• Sentry — production errors mapped to features.
• PostHog — real usage & traffic per feature.
• Slack — weekly digest of top risks, coverage gaps and hotspots.

🆚 Why not just…

• …grep / read the files? Burns context and misses cross-boundary, runtime and historical coupling that static analysis can't see.
• …SonarQube / linters? Great for line-level issues; blind to features, flows and blast radius.
• …your issue tracker? Describes intent, not reality. Faultlines is grounded in what the code and history actually say.

Faultlines is the only layer that joins structure + git history + runtime into one map — and serves it to your AI agent.

🗺️ Roadmap

• Two-layer feature/flow map on any stack
• Behavioral test coverage & health scoring
• Symbol-level attribution
• MCP server (13 tools) — Local · Hosted · VPC
• Sentry + PostHog runtime overlays
• Incremental, sub-second re-scans on every commit
• Native plugins for more agents & IDEs

🤝 Contributing

Issues, ideas and PRs are welcome. Faultlines is built to map any codebase — if it mis-reads your stack, that's a bug we want to hear about.

📄 License

MIT — see LICENSE.

⭐ Star this repo if Faultlines helps you (or your agent) understand a codebase faster.

Made for engineers and the AI agents that work alongside them · faultlines.dev