[omartelo]

What

The New worktree dialog's name field only ever took a branch name: anything with a space was refused as Invalid branch name, so naming the work meant inventing the hyphens by hand — and the blank field, the path of least resistance, handed the branch a random calm-glacier.

That name is fine right up until it is the name on a pull request, and by then it cannot be fixed: renaming a branch through the GitHub API closes the PR that was open on it. So this settles the name at the only cheap moment there is — before the worktree exists.

Type fix the auth redirect and the branch is fix-the-auth-redirect, printed under the field before anything is created. A name git already accepts is still used exactly as typed; a blank field still auto-generates one.

How

toBranchName (frontend/src/lib/git/branch-name.ts) — pure, no model, no network:

• git already accepts it → returned untouched (feat/x stays feat/x).
• otherwise → lowercase, everything that is not a letter or a number in any script to separators (git takes a non-ASCII branch name, and stripping the accents off a phrase spells it wrong), grown one word at a time to at least 2 words / 10 chars (so a run of very short words still reads as a phrase) and stopped before 5 words / 40 chars (so a pasted sentence does not become the branch), joined with -.
• nothing usable → "", which is the same blank the dialog auto-generates a name for.

The Invalid branch name state is deleted: there is no longer such a thing to type. The helper line under the field already printed the branch that would be created — it now prints the slug, so nothing is rewritten out of sight.

Where it came from, and what was left out

The growth loop is borrowed from JetBrains/thinkrail's naiveWorkspaceName (packages/server/src/assist). Their pattern has a second half — a cheap-model pass that refines the name from the session's first turn, plus the host flow that renames the workspace once that turn settles. Both are deliberately out:

• No model pass. It costs a token round trip and a session per name, for a string the user is looking straight at and can edit.
• No rename after the first turn. lich's worktree dialog has no prompt field — the first turn happens after the session exists — so the derived name could only arrive as a rename, and renaming the branch is exactly the move that kills the PR. thinkrail can afford it because it keeps a display name (Title Case, spaces) apart from the branch slug; lich has one string that is both, and deriving it at typing time means it never has to move.
• No display/branch split. It only pays for itself if something renames, and nothing does.

The CLI and MCP paths (lich open --worktree <branch> --prompt <task>, open_session) are untouched: a blank worktree there means no worktree, and an agent opening one already names it knowing the task.

Test plan

• frontend/src/lib/git/branch-name.test.ts — passthrough of names git takes, the slug, both minimums, both maximums, the clamp on a single over-long word, the trailing separator the clamp can land on, a non-ASCII phrase, and the empty results that fall back to auto-generate.
• Gate green: gofmt -l ., go vet ./..., go test ./..., biome check ., vitest run (1224 tests), tsc -b, vite build.
• Not covered: the dialog's own render path (the frontend gate is node-only). The change there is one deleted conditional and one placeholder string.