FitFix

FIT activity toolkit, in your browser. Fix, merge and convert .fit files from Garmin, Wahoo, Coros, Polar, Bryton, Lezyne, Hammerhead, Stages, Zwift, Suunto and any other device that writes the FIT format. 100 % local — your files never touch a server.

🔗 fitfix.vercel.app · 📦 PWA (works offline)

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Why

Watches and bike computers split long activities into two .fit files (battery save, firmware quirks, mid-activity reboot). When you stop for a break, GPS keeps recording and your position drifts in spaghetti loops — or it bounces between canyon walls and draws phantom kilometres while you're moving. Barometric elevation drifts indoors. Activities flagged "in a vehicle" by Strava when they were just indoor on Zwift. Forgot to start. Forgot to stop and drove home with the watch on.

FitFix is one unified editor for all of that. Drop a file, get suggested fixes, apply the ones you want, export.

How it works

• Pure browser app. Vite + React + TypeScript, deployed as a static PWA on Vercel. No backend, no API, no upload — your .fit bytes stay in your tab.
• Byte-level FIT manipulation. Custom walker reads the FIT binary directly so we can preserve every vendor-proprietary undocumented message during round-trip. The output file matches the input bit-for-bit except for the fields we explicitly modify. Vendor-specific developer fields from Polar / Coros / Wahoo are passed through opaquely.
• Endianness aware. Watches write little-endian; Garmin Connect re-encodes uploads as big-endian. The walker reads the per-definition arch flag.
• CRC recomputation. Header and file CRCs are recalculated using the standard FIT polynomial after edits.
• Re-upload friendly. file_id.time_created and serial_number are bumped on every output so Garmin Connect, Strava, etc. treat the result as a new activity instead of refusing it as a duplicate.
• No FIT SDK dependency. We do not bundle the proprietary Garmin FIT SDK, which forbids OSS redistribution. The parser is hand-written from the public protocol documentation, the same approach GoldenCheetah takes.

Privacy

• 🔒 Files are processed entirely client-side. There is no upload endpoint.
• 🌐 Map tiles come from CartoDB / Esri / OpenTopoMap. Those servers see the tile coordinates you're viewing while the map is on screen — same as any web map. They do not see your activity data.
• 🚫 No analytics, no tracking pixels, no third-party scripts beyond what you can see in the bundle.
• ✅ Open source under MIT — audit anything.

Run locally

git clone https://github.com/otaprochazka/fitfix.git
cd fitfix
npm install
npm run dev

Production build:

npm run build
npm run preview

Tests (Vitest):

npm test               # one-shot
npm run test:watch     # watch mode
npm run test:coverage  # with coverage

Deploy

Anywhere static. The dist/ directory is a self-contained PWA. Vercel autodetects Vite — just import the repo.

Acknowledgments

• Garmin FIT SDK for the open file format.
• Leaflet for the map.
• CartoDB, OpenStreetMap, OpenTopoMap, Esri for tiles.

Roadmap

• MCP server for Claude Desktop / Claude Code — drive FitFix's detectors and edits by prompt, files stay on your disk (stdio transport, distributed via npx). Tracked in docs/roadmap/mcp-server.md.
• Garmin Connect integration — link a Garmin account, pull activities into the editor, push fixes back. Tracked in docs/roadmap/garmin-connect.md.

For background research and engineering lessons, see docs/.

Contributing / agents

Working on the codebase (human or AI)? Read AGENTS.md first — it covers the repo layout, the FIT format gotchas, the encoder LRU bug that keeps trying to come back, the dev / build / deploy commands, and the PR workflow. Saves a few hours of re-deriving them.

Claude Code reads AGENTS.md directly when no CLAUDE.md is present, so the same file orients both human contributors and AI agents — no need for a parallel CLAUDE.md.

License

MIT — see LICENSE.