lane — Copy-on-write Git worktrees with memory that survives them.

A second worktree that costs no disk and no cold build.

Lane asks git for every ignored path — nested ones like packages/a/node_modules, single files like .env — and clones each by reference. On APFS, btrfs, and XFS with reflink=1, the two copies share the same extents. The second tree takes no new space, and the build cache is already warm.

Edit a file and only that file's blocks un-share. One source edit plus its incremental rebuild rewrote 550 files — 106.5 MiB. The other 1251.6 MiB stayed shared. The 106.5 MiB goes away when the lane does.

Reflink works on APFS, btrfs, XFS with reflink=1, bcachefs, and recent ZFS. Everywhere else, lane init says so up front and lane new leaves a plain worktree — byte-copying a build cache is the expensive thing lane exists to avoid. Opt a path out with git config --add lane.exclude target.

The reason you did it that way, kept next to the code it constrains.

A note records what must stay true, not what you changed.

$ lane note add src/auth.rs -a "fn verify" \
    "must stay constant-time; early return leaks token length"
noted -> src/auth.rs#fn verify

$ lane why src/auth.rs

[fn verify]
  - 01M0B9MBYB · 2026-08-14
    must stay constant-time; early return leaks token length

Or leave it in the commit you were already writing. lane install hooks reads the Why: trailer and nothing else — no diff, no commit body, no history import:

$ git commit -am "make verify constant-time"
  ...
  Why: src/auth.rs#fn verify | early return leaks token length

The subject says what you did. The trailer says what must stay true.

Anchors — what the note is about

Tree-sitter resolves these, not a regular expression. A span ends where the declaration ends. A # inside a code fence is not a heading.

Freshness — what happens as the code moves

Each anchor is hashed on its own span, normalized, so a formatter stales nothing. Editing #script leaves a note on #style alone. A renamed file is followed — lane audit reads git's own rename detection. A drifted note stays flagged until you confirm, replace, or retire it.

The next agent reads the constraints before it edits.

Two agents can note the same file, and the same anchor, at the same time. Each note is a new file, so nothing locks. Land them in any order; both notes are there afterwards.

$ lane why src/auth.rs

[fn verify]
  - 01M0B9MBYB · 2026-08-14
    must stay constant-time; early return leaks token length
  - 01M0B9MZQ4 · 2026-08-14
    the token bytes must never reach a log, even at debug

lane init writes a four-line protocol into AGENTS.md. Notes are plain markdown at predictable paths, so an agent finds them with no plugin.

Get the binary

Once per machine

Once per repository

$ cargo run -p lane-tour -- start
builds a throwaway repo next to this checkout and prints its path

The tour runs the real workflows — a note drifting, three lanes landing out of order, two landings colliding — and prints every command before it runs.