@fern-api/replay

Automatically preserves SDK customizations across Fern regenerations.

What it does

Fern Replay detects your edits to generated SDK code, stores them as patches in .fern/replay.lock, and re-applies them after each regeneration using 3-way merge. Your customizations survive fern generate without manual intervention.

The replay loop

Three phases. The generator runs between detect and apply, overwriting everything in the SDK directory.

1. Detect — find new customizations in the customer's git history since the last generation
2. Apply — put them back onto the freshly generated code via 3-way merge
3. Commit — record the result as a [fern-replay] commit on top of the [fern-generated] one

Vocabulary

The team's canonical vocabulary — customization, patch, generation, the three merge sides, the patch lifecycle verbs — lives in CONTEXT.md.

Documentation

• CONTEXT.md — domain language and team voice
• docs/architecture.md — module structure, design decisions, integration with fern/fern
• docs/reference.md — types, customer flows, CLI commands, key algorithms
• docs/known-divergences.md — places where multiple implementations exist intentionally
• docs/migration-guide.md — upgrade and migration notes

Install

npm install @fern-api/replay

Development

npm install
npm run build
npm test

Local Development with Fern CLI

To use a local version of @fern-api/replay with the Fern CLI monorepo:

# From the fern-replay directory
cd /path/to/fern-replay
pnpm link --global

# From the fern monorepo root
cd /path/to/fern
pnpm link --global @fern-api/replay

Verify the link is working:

ls -la node_modules/@fern-api/replay
# Should show a symlink pointing to your local fern-replay directory

Any changes you make in fern-replay will be picked up immediately without reinstalling.

To unlink and restore the published npm version:

pnpm unlink @fern-api/replay
pnpm install

License

MIT