fern wires directly into your language server, your debugger, and your git history — so it plans changes instead of guessing at them.
Every feature exists because a large, messy, real repository demanded it.
fern drafts a change plan before touching a file, so you approve the approach — not just the diff.
Spin off focused subagents for research, testing, or refactors that run in parallel without polluting context.
fern remembers what it tried last time in this repo, and won't repeat a fix that already failed.
Edits are addressed by content hash, not line number — so they still apply cleanly after upstream changes.
Go-to-definition, diagnostics, and live debugging are first-class tools, not text scraped from your editor.
Project rules can apply retroactively — flag old code against a rule you only just wrote.
Before fern proposes a change, it walks the call graph, checks type signatures through your LSP, and reviews recent commits touching that file.
Works with any git repo. No config file required to start.