feat(maestro): worktree on by default with new-or-existing branch picker (#61)

- toggle defaults on for any target the project scan reports (repo or
  worktree); only the free-text escape hatch pays for /api/gitroot
- branch mode control switches between creating a new branch (named
  after the session name, as before) and picking an existing one from
  GET /api/branches; branches held by another worktree stay visible but
  disabled, labelled with that worktree's path
- emits create_branch: false on the wire for existing-mode picks
- branch-list load follows the form's ready/destroyed discipline so a
  late response cannot corrupt a rebuilt draft

crates/maestro-daemon/src/branches.rs

@@ -0,0 +1,167 @@
+//! `GET /api/branches` — local branches of one repository (#55).
+//!
+//! Spawning into a worktree on an *existing* branch needs to know which
+//! branches the repository has and which are taken: git refuses to check the
+//! same branch out in two worktrees at once. Each [`BranchInfo`] answers for
+//! one local branch — its name, whether it is the main checkout's HEAD, and
+//! which linked worktree (if any) holds it.
+//!
+//! Read-only, like [`crate::gitroot`]: two git probes (`for-each-ref`,
+//! `worktree list --porcelain`) with no write path anywhere. A path outside
+//! any repository yields an empty list rather than an error, matching how
+//! `/api/gitroot` reports "not a repository".
+
+use std::collections::HashMap;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+use axum::extract::{Query, State};
+use axum::response::{IntoResponse, Response};
+use axum::Json;
+use maestro_protocol::BranchInfo;
+use serde::Deserialize;
+
+use crate::local_api::LocalApiState;
+
+/// Query string for `GET /api/branches`. Same `path` convention as
+/// `/api/dirs` and `/api/gitroot`: resolved against the daemon's home dir.
+#[derive(Debug, Deserialize)]
+pub struct BranchesQuery {
+    #[serde(default)]
+    pub path: String,
+}
+
+pub async fn handler(State(s): State<LocalApiState>, Query(q): Query<BranchesQuery>) -> Response {
+    let dir = crate::dirlist::resolve(&s.home_dir, &q.path);
+    Json(list(&dir)).into_response()
+}
+
+/// List the enclosing repository's local branches. Empty when `dir` is not
+/// inside a repository.
+pub fn list(dir: &Path) -> Vec<BranchInfo> {
+    let Some(root) = crate::gitroot::find_root(dir) else {
+        return Vec::new();
+    };
+    let worktrees = worktree_branches(&root);
+    // The first porcelain record is always the main checkout; its branch is
+    // the HEAD answer. Linked worktrees are the occupancy map.
+    let head_branch = worktrees.first().and_then(|(branch, _)| branch.clone());
+    let occupied: HashMap<&str, &Path> = worktrees
+        .iter()
+        .skip(1)
+        .filter_map(|(branch, path)| branch.as_deref().map(|b| (b, path.as_path())))
+        .collect();
+
+    branch_names(&root)
+        .into_iter()
+        .map(|name| BranchInfo {
+            is_head: head_branch.as_deref() == Some(name.as_str()),
+            worktree: occupied.get(name.as_str()).copied().map(Path::to_path_buf),
+            name,
+        })
+        .collect()
+}
+
+fn run_git(root: &Path, args: &[&str]) -> Option<String> {
+    let out = Command::new("git")
+        .arg("-C")
+        .arg(root)
+        .args(args)
+        .output()
+        .ok()?;
+    if !out.status.success() {
+        return None;
+    }
+    Some(String::from_utf8_lossy(&out.stdout).into_owned())
+}
+
+fn branch_names(root: &Path) -> Vec<String> {
+    run_git(
+        root,
+        &["for-each-ref", "refs/heads", "--format=%(refname:short)"],
+    )
+    .map(|out| {
+        out.lines()
+            .filter(|l| !l.is_empty())
+            .map(str::to_owned)
+            .collect()
+    })
+    .unwrap_or_default()
+}
+
+fn worktree_branches(root: &Path) -> Vec<(Option<String>, PathBuf)> {
+    let out = match run_git(root, &["worktree", "list", "--porcelain"]) {
+        Some(out) => out,
+        None => return Vec::new(),
+    };
+    parse_worktrees(&out)
+}
+
+/// Parse `git worktree list --porcelain` into `(branch, toplevel)` records,
+/// branch-less for detached/bare checkouts. Records are separated by blank
+/// lines; each `worktree` line starts a new one.
+fn parse_worktrees(out: &str) -> Vec<(Option<String>, PathBuf)> {
+    let mut result: Vec<(Option<String>, PathBuf)> = Vec::new();
+    let mut pending: Option<PathBuf> = None;
+    let mut branch: Option<String> = None;
+
+    for line in out.lines() {
+        if let Some(path) = line.strip_prefix("worktree ") {
+            if let Some(prev) = pending.take() {
+                result.push((branch.take(), prev));
+            }
+            pending = Some(PathBuf::from(path));
+        } else if let Some(b) = line.strip_prefix("branch ") {
+            branch = Some(b.trim_start_matches("refs/heads/").to_owned());
+        }
+    }
+    if let Some(path) = pending {
+        result.push((branch.take(), path));
+    }
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parses_main_and_linked_worktrees() {
+        let out = "\
+worktree /repos/maestro
+HEAD 0123abcd
+branch refs/heads/main
+
+worktree /repos/wt-feature
+HEAD 4567abcd
+branch refs/heads/spawn-v2
+
+";
+        assert_eq!(
+            parse_worktrees(out),
+            vec![
+                (Some("main".into()), PathBuf::from("/repos/maestro")),
+                (Some("spawn-v2".into()), PathBuf::from("/repos/wt-feature")),
+            ]
+        );
+    }
+
+    #[test]
+    fn detached_head_yields_no_branch() {
+        let out = "\
+worktree /repos/detached
+HEAD 0123abcd
+detached
+
+";
+        assert_eq!(
+            parse_worktrees(out),
+            vec![(None, PathBuf::from("/repos/detached"))]
+        );
+    }
+
+    #[test]
+    fn empty_output_yields_no_records() {
+        assert!(parse_worktrees("").is_empty());
+    }
+}

crates/maestro-daemon/src/lib.rs

@@ -9,6 +9,7 @@
 
 pub mod activity;
 pub mod attach;
+pub mod branches;
 pub mod cli;
 pub mod config;
 pub mod dirlist;

crates/maestro-daemon/src/local_api.rs

@@ -48,6 +48,7 @@ pub fn router(state: LocalApiState) -> Router {
         .route(routes::RECENT_DIRS, get(recent_dirs))
         .route(routes::PROJECTS, get(projects))
         .route(routes::GITROOT, get(git_root))
+        .route(routes::BRANCHES, get(crate::branches::handler))
         .route(routes::NAMES, get(names))
         .route(routes::INFO, get(daemon_info))
         .route(routes::EVENTS, any(events_ws))

crates/maestro-daemon/tests/branches.rs

@@ -0,0 +1,141 @@
+//! Integration tests for `GET /api/branches` (#55).
+
+mod common;
+
+use std::net::{Ipv4Addr, SocketAddr};
+use std::path::Path;
+use std::process::Command;
+use std::sync::Arc;
+
+use common::DaemonFixture;
+use maestro_daemon::local_api;
+use tokio::net::TcpListener;
+use tokio::sync::oneshot;
+
+struct Harness {
+    addr: SocketAddr,
+    _shutdown: oneshot::Sender<()>,
+    _fx: DaemonFixture,
+}
+
+async fn spawn() -> Harness {
+    let fx = DaemonFixture::new();
+    let presets = maestro_config::SpawnSection::default().resolve().unwrap();
+    let router = local_api::router(local_api::state_for(
+        fx.manager.clone(),
+        "naps-laptop",
+        "/home/test",
+        presets,
+        Arc::new(maestro_daemon::projects::ProjectScanner::disabled()),
+    ));
+    let listener = TcpListener::bind(SocketAddr::from((Ipv4Addr::LOCALHOST, 0)))
+        .await
+        .unwrap();
+    let addr = listener.local_addr().unwrap();
+    let (tx, rx) = oneshot::channel::<()>();
+    tokio::spawn(async move {
+        let _ = axum::serve(listener, router)
+            .with_graceful_shutdown(async move {
+                let _ = rx.await;
+            })
+            .await;
+    });
+    Harness {
+        addr,
+        _shutdown: tx,
+        _fx: fx,
+    }
+}
+
+fn git(repo: &Path, args: &[&str]) {
+    let out = Command::new("git")
+        .arg("-C")
+        .arg(repo)
+        .args(args)
+        .output()
+        .unwrap();
+    assert!(
+        out.status.success(),
+        "git {args:?} failed: {}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+}
+
+/// A real repo on `main` with a second branch `feature`, and a linked
+/// worktree at `<tmp>/wt-feature` holding `feature`.
+fn repo_with_occupied_branch() -> tempfile::TempDir {
+    let tmp = tempfile::tempdir().unwrap();
+    let repo = tmp.path();
+    git(repo, &["init", "-q", "-b", "main"]);
+    git(
+        repo,
+        &[
+            "-c",
+            "user.email=t@t",
+            "-c",
+            "user.name=t",
+            "commit",
+            "-qm",
+            "init",
+            "--allow-empty",
+        ],
+    );
+    git(repo, &["branch", "feature"]);
+    let wt = tmp.path().join("wt-feature");
+    git(repo, &["worktree", "add", &wt.to_string_lossy(), "feature"]);
+    tmp
+}
+
+async fn get_branches(addr: SocketAddr, path: &Path) -> Vec<maestro_protocol::BranchInfo> {
+    reqwest::get(format!(
+        "http://{addr}/api/branches?path={}",
+        urlencoding::encode(&path.to_string_lossy())
+    ))
+    .await
+    .unwrap()
+    .json()
+    .await
+    .unwrap()
+}
+
+/// #55: every local branch is listed; the main checkout's HEAD reports
+/// `is_head`, and a branch held by a linked worktree reports that worktree's
+/// path so the client can explain why the entry is disabled.
+#[tokio::test]
+async fn branches_lists_local_branches_and_worktree_occupancy() {
+    let tmp = repo_with_occupied_branch();
+    let h = spawn().await;
+
+    let mut branches = get_branches(h.addr, tmp.path()).await;
+    branches.sort_by(|a, b| a.name.cmp(&b.name));
+
+    assert_eq!(
+        branches.iter().map(|b| b.name.as_str()).collect::<Vec<_>>(),
+        ["feature", "main"]
+    );
+
+    let main = branches.iter().find(|b| b.name == "main").unwrap();
+    assert!(main.is_head, "main is the main checkout's HEAD");
+    assert_eq!(main.worktree, None, "HEAD occupancy rides `is_head`");
+
+    let feature = branches.iter().find(|b| b.name == "feature").unwrap();
+    assert!(!feature.is_head);
+    assert_eq!(
+        feature.worktree.as_deref(),
+        Some(tmp.path().join("wt-feature").as_path()),
+        "the linked worktree holding the branch must be reported"
+    );
+}
+
+/// #55: a path outside any repository is not an error — an empty list,
+/// matching how `/api/gitroot` reports "not a repository".
+#[tokio::test]
+async fn branches_returns_empty_for_a_path_outside_a_repository() {
+    let h = spawn().await;
+    let tmp = tempfile::tempdir().unwrap();
+    let plain = tmp.path().join("plain");
+    std::fs::create_dir_all(&plain).unwrap();
+
+    let branches = get_branches(h.addr, &plain).await;
+    assert!(branches.is_empty());
+}

crates/maestro-protocol/src/lib.rs

@@ -212,6 +212,21 @@ pub struct Project {
     pub last_command: Option<String>,
 }
 
+/// One local branch of a repository for `GET /api/branches` (#55). A client
+/// disables the entry when `is_head` is set or `worktree` is present — git
+/// refuses to check the same branch out in two worktrees at once.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct BranchInfo {
+    /// Short branch name (`main`, `feature/foo`).
+    pub name: String,
+    /// Whether this branch is the main checkout's current HEAD. The main
+    /// checkout's occupancy reports here rather than through `worktree`.
+    pub is_head: bool,
+    /// Toplevel path of the linked worktree holding this branch, if any.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub worktree: Option<PathBuf>,
+}
+
 /// Structured failure used everywhere we don't want to leak a string-only error.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct ErrorInfo {

crates/maestro-protocol/src/routes.rs

@@ -21,13 +21,15 @@ pub const RECENT_DIRS: &str = "/api/recent-dirs";
 pub const PROJECTS: &str = "/api/projects";
 /// `GET` the enclosing repository of a path. Takes a `path` query param.
 pub const GITROOT: &str = "/api/gitroot";
+/// `GET` one repository's local branches. Takes a `path` query param.
+pub const BRANCHES: &str = "/api/branches";
 /// `GET` a batch of generated session handles (#53). Takes a `count` query
 /// param (default 1, capped at [`maestro_names`]' batch cap).
 pub const NAMES: &str = "/api/names";
 /// `WS` catalog snapshot + session deltas.
 pub const EVENTS: &str = "/api/events";
 
-/// Name of the query parameter [`DIRS`] and [`GITROOT`] both take.
+/// Name of the query parameter [`DIRS`], [`GITROOT`], and [`BRANCHES`] take.
 pub const PATH_QUERY: &str = "path";
 
 /// Name of the query parameter [`NAMES`] takes.
@@ -187,6 +189,7 @@ mod tests {
             RECENT_DIRS,
             PROJECTS,
             GITROOT,
+            BRANCHES,
             NAMES,
             EVENTS,
         ] {

web/src/api.ts

@@ -1,6 +1,7 @@
 import * as routes from "./routes.js";
 import type {
   ApiError,
+  Branches,
   CreateSessionRequest,
   DaemonInfo,
   DirListing,
@@ -89,4 +90,11 @@ export async function fetchNames(count: number): Promise<NameBatch> {
   return jsonOrError(await fetch(url, { credentials: "same-origin" }));
 }
 
+/** One repository's local branches, with worktree occupancy (#55). Empty for
+ * a path outside any repo. Drives the spawn form's existing-branch picker. */
+export async function fetchBranches(path: string): Promise<Branches> {
+  const url = routes.withPathQuery(routes.BRANCHES, path);
+  return jsonOrError(await fetch(url, { credentials: "same-origin" }));
+}
+
 export { eventsWsUrl } from "./routes.js";

web/src/routes.ts

@@ -12,6 +12,7 @@ export const RECENT_DIRS = "/api/recent-dirs";
 export const PROJECTS = "/api/projects";
 export const GITROOT = "/api/gitroot";
 export const NAMES = "/api/names";
+export const BRANCHES = "/api/branches";
 export const EVENTS = "/api/events";
 
 /** Name of the query parameter `DIRS` and `GITROOT` both take. */

web/src/spawnForm.test.ts

@@ -8,6 +8,7 @@ const base: SpawnFormValues = {
   name: "amber-fox",
   worktreeEnabled: false,
   worktreeBranch: "",
+  worktreeBranchMode: "new",
 };
 
 describe("buildSpawnRequest", () => {
@@ -33,11 +34,50 @@ describe("buildSpawnRequest", () => {
         cwd: base.cwd,
         command: base.command,
         metadata: { name: "amber-fox" },
-        worktree: { base_repo: "/home/user/project", branch: "feat/foo", cleanup_on_kill: true },
+        worktree: {
+          base_repo: "/home/user/project",
+          branch: "feat/foo",
+          cleanup_on_kill: true,
+          create_branch: true,
+        },
       },
     });
   });
 
+  it("checks out an existing branch when the mode is existing (#61)", () => {
+    const res = buildSpawnRequest(
+      {
+        ...base,
+        worktreeEnabled: true,
+        worktreeBranch: "spawn-v2",
+        worktreeBranchMode: "existing",
+      },
+      "/home/user/project",
+    );
+    expect(res).toEqual({
+      req: {
+        cwd: base.cwd,
+        command: base.command,
+        metadata: { name: "amber-fox" },
+        worktree: {
+          base_repo: "/home/user/project",
+          branch: "spawn-v2",
+          cleanup_on_kill: true,
+          create_branch: false,
+        },
+      },
+    });
+  });
+
+  it("rejects an existing-mode worktree with no branch picked", () => {
+    expect(
+      buildSpawnRequest(
+        { ...base, worktreeEnabled: true, worktreeBranch: "", worktreeBranchMode: "existing" },
+        "/repo",
+      ),
+    ).toHaveProperty("error");
+  });
+
   it("rejects a missing working directory", () => {
     expect(buildSpawnRequest({ ...base, cwd: "" }, null)).toHaveProperty("error");
   });

web/src/spawnForm.ts

@@ -1,10 +1,21 @@
 import { escapeHtml } from "./html.js";
-import { createSession, fetchNames, getDaemonInfo, projects as fetchProjects, gitRoot } from "./api.js";
+import {
+  createSession,
+  fetchBranches,
+  fetchNames,
+  getDaemonInfo,
+  projects as fetchProjects,
+  gitRoot,
+} from "./api.js";
 import { attachDirAutocomplete } from "./dirAutocomplete.js";
 import { filterProjects, groupProjects, isStale, sortProjects } from "./projectList.js";
-import type { CreateSessionRequest, Project, Session, SpawnPreset } from "./types.js";
+import type { BranchInfo, CreateSessionRequest, Project, Session, SpawnPreset } from "./types.js";
 import { abbreviateCwd, abbreviateFolderName } from "./types.js";
 
+/** Where a worktree's branch comes from: freshly created (default) or picked
+ * from the repository's existing branches (#61). */
+export type WorktreeBranchMode = "new" | "existing";
+
 /** Seed values for the spawn form — used by "clone" to reopen with the same
  * cwd, command, and name as an existing session, and by draft panes to restore
  * their own half-filled state after a re-render. */
@@ -14,6 +25,7 @@ export interface SpawnFormSeed {
   name?: string;
   worktreeEnabled?: boolean;
   worktreeBranch?: string;
+  worktreeBranchMode?: WorktreeBranchMode;
   cwdRawVisible?: boolean;
 }
 
@@ -25,6 +37,8 @@ export interface SpawnFormValues {
   name: string;
   worktreeEnabled: boolean;
   worktreeBranch: string;
+  /** "new" creates the branch; "existing" checks out a picked one (#61). */
+  worktreeBranchMode: WorktreeBranchMode;
   /** Whether the cwd lives in the raw text input (vs a picked recent row). */
   cwdRawVisible?: boolean;
 }
@@ -47,15 +61,21 @@ export function buildSpawnRequest(
   };
 
   if (values.worktreeEnabled) {
+    const mode = values.worktreeBranchMode ?? "new";
     const branch = values.worktreeBranch.trim();
     if (!baseRepo) return { error: "Worktree needs a directory inside a git repository." };
-    if (!branch) return { error: "Worktree requires a branch name." };
+    if (!branch) {
+      return { error: mode === "existing" ? "Pick an existing branch." : "Worktree requires a branch name." };
+    }
     req.worktree = {
       base_repo: baseRepo,
       branch,
       // The daemon always cleans up, guarded against unsaved work (#56);
       // the wire field stays for older daemons.
       cleanup_on_kill: true,
+      // Existing-mode picks must check out as-is; the daemon creates
+      // anything else with `worktree add -b` (#56/#61).
+      create_branch: mode !== "existing",
     };
   }
 
@@ -128,10 +148,24 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
       <p class="wt-hint" hidden></p>
       <div class="wt-body" hidden>
         <p class="wt-repo"></p>
-        <label>
+        <div class="wt-mode" role="radiogroup" aria-label="Branch source">
+          <label class="row-inline">
+            <input type="radio" name="worktree_branch_mode" value="new" />
+            New branch
+          </label>
+          <label class="row-inline">
+            <input type="radio" name="worktree_branch_mode" value="existing" />
+            Existing branch
+          </label>
+        </div>
+        <label class="wt-new-wrap">
           Branch
           <input name="worktree_branch" type="text" placeholder="feat/foo" autocomplete="off" />
         </label>
+        <label class="wt-exist-wrap" hidden>
+          Branch
+          <select class="branch-select"></select>
+        </label>
       </div>
     </fieldset>
     <p class="form-error" role="alert" aria-live="polite"></p>
@@ -153,7 +187,13 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
   const wtHint = form.querySelector(".wt-hint") as HTMLElement;
   const wtBody = form.querySelector(".wt-body") as HTMLElement;
   const wtRepo = form.querySelector(".wt-repo") as HTMLElement;
+  const modeRadios = [
+    ...form.querySelectorAll<HTMLInputElement>("input[name=worktree_branch_mode]"),
+  ];
+  const newWrap = form.querySelector(".wt-new-wrap") as HTMLElement;
+  const existWrap = form.querySelector(".wt-exist-wrap") as HTMLElement;
   const branchInput = form.querySelector("input[name=worktree_branch]") as HTMLInputElement;
+  const branchSelect = form.querySelector(".branch-select") as HTMLSelectElement;
   const nameInput = form.querySelector("input[name=name]") as HTMLInputElement;
 
   // ---- form state ------------------------------------------------------
@@ -162,11 +202,15 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
   let selectedPresetId = "";
   let command = ""; // resolved command (preset's, or the raw custom input)
   let cwd = ""; // chosen working directory
-  let inferredBaseRepo: string | null = null; // from the gitroot probe
+  let inferredBaseRepo: string | null = null; // from the gitroot probe or the project scan
   let probeToken = 0; // guards async gitroot probes against races
   // A seed's cwd, applied once the (async) projects load for the initial host
   // settles — so the fetch can't race past and clobber the seeded directory.
   let seedCwd: string | null = prefill.cwd ?? null;
+  let branchMode: WorktreeBranchMode = prefill.worktreeBranchMode ?? "new";
+  let branchToken = 0; // guards async branch-list loads against races
+  // Branch list for the current cwd — refetched whenever the target changes.
+  let branchCache: { cwd: string; branches: BranchInfo[] } | null = null;
 
   // `emit` stays silent until the initial load settles and goes silent again
   // once the form is destroyed. Construction runs with `cwd === ""` and the
@@ -176,6 +220,7 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
   // clobber the live one's leaf (#57 review).
   let ready = false;
   let destroyed = false;
+  const branchValue = () => (branchMode === "existing" ? branchSelect.value : branchInput.value);
   const emit = () => {
     if (!ready || destroyed) return;
     opts.onChange?.({
@@ -183,7 +228,8 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
       cwd,
       name: nameInput.value,
       worktreeEnabled: wtEnabled.checked,
-      worktreeBranch: branchInput.value,
+      worktreeBranch: branchValue(),
+      worktreeBranchMode: branchMode,
       cwdRawVisible: !cwdRawWrap.hidden,
     });
   };
@@ -191,10 +237,10 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
   // ---- name (also seeds the worktree branch) ---------------------------
   // The worktree branch defaults to the session name — including a clone's
   // prefilled name — and tracks it on reroll/edit until the user types their
-  // own branch.
+  // own branch. Only in "new" mode: an existing-branch pick is not a name.
   let branchEdited = Boolean(prefill.worktreeBranch);
   const syncBranchToName = () => {
-    if (!branchEdited) branchInput.value = nameInput.value;
+    if (!branchEdited && branchMode === "new") branchInput.value = nameInput.value;
   };
   // Handles come from `GET /api/names` in batches (#53): the first one seeds
   // the field, rerolls consume the rest locally so they stay instant, and a
@@ -357,6 +403,7 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
 
   cwdRaw.addEventListener("input", () => {
     cwd = cwdRaw.value.trim();
+    branchCache = null;
     emit();
     if (wtEnabled.checked) probeGitRoot();
   });
@@ -374,11 +421,14 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
     cwd = project.path;
     cwdRawWrap.hidden = true;
     markActiveProject(project.path);
+    // The scan already knows this directory is inside a repo (#61): the
+    // toggle defaults back on and no /api/gitroot round trip is needed.
+    wtEnabled.checked = true;
+    applyProjectRepo(project.base_repo ?? project.path);
     emit();
     // Picking a project pre-selects the command it was last used with.
     const match = presets.find((p) => p.command === project.last_command && p.command !== "");
     if (match) selectPreset(match.id);
-    if (wtEnabled.checked) probeGitRoot();
   }
 
   let discovered: Project[] = []; // daemon snapshot, daemon-sorted
@@ -432,7 +482,7 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
     }
     // Everything seeded: the form may publish from here on.
     ready = true;
-    if (wtEnabled.checked) void probeGitRoot();
+    resolveBaseRepo();
     emit();
   }
 
@@ -473,7 +523,37 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
     return p;
   }
 
-  // ---- worktree (branch-only; base_repo inferred via /gitroot) ----------
+  // ---- worktree (branch-only; base_repo inferred via scan or /gitroot) --
+  // The project scan already reports repo membership, so a picked project
+  // resolves its base repo locally (#61); only the free-text escape hatch
+  // pays for a /api/gitroot probe.
+  function applyProjectRepo(repo: string): void {
+    inferredBaseRepo = repo;
+    branchCache = null;
+    wtHint.hidden = true;
+    wtBody.hidden = false;
+    wtRepo.textContent = `repo: ${repo}`;
+    if (branchMode === "existing") void loadBranches();
+  }
+
+  /** Decide how the worktree section describes the current target. Called
+   * once the initial load has settled `cwd`. */
+  function resolveBaseRepo(): void {
+    if (!cwd) {
+      inferredBaseRepo = null;
+      wtBody.hidden = true;
+      wtHint.hidden = false;
+      wtHint.textContent = "Choose a working directory first.";
+      return;
+    }
+    const project = discovered.find((p) => p.path === cwd);
+    if (project) {
+      applyProjectRepo(project.base_repo ?? project.path);
+      return;
+    }
+    void probeGitRoot();
+  }
+
   async function probeGitRoot(): Promise<void> {
     const token = ++probeToken;
     if (!cwd) {
@@ -494,10 +574,7 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
     }
     if (token !== probeToken) return; // a newer probe superseded us
     if (root) {
-      inferredBaseRepo = root;
-      wtHint.hidden = true;
-      wtBody.hidden = false;
-      wtRepo.textContent = `repo: ${root} (inferred)`;
+      applyProjectRepo(root);
     } else {
       inferredBaseRepo = null;
       // Publish the untick too, or the next render seeds the box checked with
@@ -510,10 +587,72 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
     }
   }
 
+  // ---- existing-branch picker (#61) -------------------------------------
+  function setBranchMode(mode: WorktreeBranchMode): void {
+    branchMode = mode;
+    for (const radio of modeRadios) radio.checked = radio.value === mode;
+    newWrap.hidden = mode !== "new";
+    existWrap.hidden = mode !== "existing";
+    if (mode === "existing") void loadBranches();
+    emit();
+  }
+
+  /** Fetch the target repo's branches once per cwd. Like every async load in
+   * this form: token-guarded, and nothing touches the DOM (or emits) after
+   * destroy() — a late response into a rebuilt form would corrupt that
+   * form's draft (#57 review). */
+  async function loadBranches(): Promise<void> {
+    if (!cwd || !inferredBaseRepo) return;
+    const token = ++branchToken;
+    if (branchCache?.cwd !== cwd) {
+      const target = cwd;
+      let branches: BranchInfo[];
+      try {
+        branches = await fetchBranches(target);
+      } catch {
+        return; // picker just stays empty; "new" mode still works
+      }
+      if (destroyed || token !== branchToken || cwd !== target) return;
+      branchCache = { cwd: target, branches };
+    }
+    renderBranchOptions();
+  }
+
+  function renderBranchOptions(): void {
+    if (destroyed) return;
+    const branches = branchCache?.cwd === cwd ? branchCache.branches : [];
+    branchSelect.replaceChildren();
+    const opt = (label: string, value: string) => {
+      const o = document.createElement("option");
+      o.textContent = label;
+      o.value = value;
+      return o;
+    };
+    if (branches.length === 0) {
+      branchSelect.appendChild(opt("No local branches found.", ""));
+      return;
+    }
+    for (const b of branches) {
+      // A branch another worktree holds stays visible but unpickable — git
+      // would refuse to check it out twice; the path says who holds it.
+      const o = opt(b.worktree ? `${b.name} — held by ${b.worktree}` : b.name, b.name);
+      o.disabled = Boolean(b.worktree);
+      o.title = b.worktree ?? "";
+      branchSelect.appendChild(o);
+    }
+  }
+
+  for (const radio of modeRadios) {
+    radio.addEventListener("change", () => {
+      if (radio.checked) setBranchMode(radio.value as WorktreeBranchMode);
+    });
+  }
+  branchSelect.addEventListener("change", () => emit());
+
   wtEnabled.addEventListener("change", () => {
     emit();
     if (wtEnabled.checked) {
-      probeGitRoot();
+      resolveBaseRepo();
     } else {
       inferredBaseRepo = null;
       wtBody.hidden = true;
@@ -521,15 +660,11 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
     }
   });
 
-  // A seeded worktree restores its checkbox/body up front; its gitroot probe
-  // waits for loadRecents to apply the seeded cwd.
-  if (prefill.worktreeEnabled || prefill.worktreeBranch) {
-    wtEnabled.checked = Boolean(prefill.worktreeEnabled);
-    if (prefill.worktreeBranch) branchInput.value = prefill.worktreeBranch;
-    wtBody.hidden = !wtEnabled.checked;
-    wtHint.hidden = true;
-    emit();
-  }
+  // The toggle defaults ON (#61): the operator almost always wants a
+  // worktree, and every non-repo target unticks it via the gitroot probe.
+  wtEnabled.checked = prefill.worktreeEnabled ?? true;
+  if (prefill.worktreeBranch) branchInput.value = prefill.worktreeBranch;
+  setBranchMode(branchMode);
 
   // Seed ordering: the raw-cwd application races the async info load (which
   // provides homeDir for the projects' abbreviations), so projects wait for it.
@@ -548,8 +683,9 @@ export function createSpawnForm(opts: SpawnFormOptions): SpawnFormHandle {
         cwd,
         name: nameInput.value,
         worktreeEnabled: wtEnabled.checked,
-        worktreeBranch: branchInput.value,
-        },
+        worktreeBranch: branchValue(),
+        worktreeBranchMode: branchMode,
+      },
       inferredBaseRepo,
     );
     if ("error" in built) {

web/src/spawnFormWorktree.test.ts

@@ -0,0 +1,172 @@
+// @vitest-environment happy-dom
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import { createSpawnForm, type SpawnFormHandle, type SpawnFormValues } from "./spawnForm.js";
+
+const PROJECTS = [
+  { path: "/home/user/maestro", name: "maestro", last_activity: "2026-08-20T10:00:00Z" },
+];
+
+const BRANCHES = [
+  { name: "main", is_head: true },
+  { name: "spawn-v2", is_head: false, worktree: "/home/user/wt-spawn" },
+  { name: "free-branch", is_head: false },
+];
+
+/** Stub the fetches the form makes. `gateBranches` (when given) holds the
+ * /api/branches response until released, so tests can destroy the form while
+ * its branch list is still in flight. */
+function stubFetch(opts?: {
+  gateBranches?: (release: () => void) => void;
+}): string[] {
+  const urls: string[] = [];
+  const gated = new Promise<void>((resolve) => {
+    if (opts?.gateBranches) opts.gateBranches(resolve);
+    else resolve();
+  });
+  vi.stubGlobal(
+    "fetch",
+    vi.fn(async (input: RequestInfo | URL) => {
+      const url = String(input);
+      urls.push(url);
+      if (url.includes("/api/names")) {
+        return new Response(JSON.stringify({ names: ["amber-fox"] }), { status: 200 });
+      }
+      if (url.includes("/api/branches")) {
+        await gated;
+        return new Response(JSON.stringify(BRANCHES), { status: 200 });
+      }
+      if (url.includes("/api/projects")) {
+        return new Response(JSON.stringify(PROJECTS), { status: 200 });
+      }
+      return new Response(
+        JSON.stringify({ hostname: "h", version: "0", home_dir: "/home/user", presets: [] }),
+        { status: 200 },
+      );
+    }),
+  );
+  return urls;
+}
+
+describe("spawnForm worktree defaults and branch picker (#61)", () => {
+  let handle: SpawnFormHandle | null = null;
+  let urls: string[];
+
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    handle?.destroy();
+    handle = null;
+    vi.unstubAllGlobals();
+    vi.useRealTimers();
+  });
+
+  async function mount(
+    seed?: Parameters<typeof createSpawnForm>[0]["seed"],
+    onChange?: (values: SpawnFormValues) => void,
+  ) {
+    urls = stubFetch();
+    handle = createSpawnForm({ seed, onSuccess() {}, onChange });
+    document.body.appendChild(handle.el);
+    for (let i = 0; i < 40; i++) await Promise.resolve();
+    return handle.el;
+  }
+
+  const drain = async () => {
+    for (let i = 0; i < 40; i++) await Promise.resolve();
+  };
+
+  function pickProject(form: HTMLFormElement): void {
+    form.querySelector<HTMLButtonElement>(".project-row.project-other")!.click();
+    form
+      .querySelectorAll<HTMLButtonElement>(".project-row")
+      .forEach((row) => row.dataset.cwd === "/home/user/maestro" && row.click());
+  }
+
+  it("defaults the worktree toggle on for a picked project with no gitroot probe", async () => {
+    const form = await mount();
+    expect(form.querySelector<HTMLInputElement>("input[name=worktree_enabled]")!.checked).toBe(
+      true,
+    );
+
+    pickProject(form);
+    await drain();
+
+    expect(urls.some((u) => u.includes("/api/gitroot"))).toBe(false);
+    expect(form.querySelector<HTMLElement>(".wt-repo")!.textContent).toContain(
+      "/home/user/maestro",
+    );
+  });
+
+  it("still probes the gitroot for a free-text directory", async () => {
+    const form = await mount();
+
+    // Escape hatch: switch to a free-text dir; only now does /api/gitroot fire.
+    form.querySelector<HTMLElement>(".project-row.project-other")!.click();
+    const raw = form.querySelector<HTMLInputElement>("input[name=cwd_raw]")!;
+    raw.value = "/opt/somewhere";
+    raw.dispatchEvent(new Event("input"));
+    await drain();
+
+    expect(urls.some((u) => u.includes("/api/gitroot"))).toBe(true);
+  });
+
+  it("populates existing branches, disabling ones held by another worktree", async () => {
+    const form = await mount();
+    pickProject(form);
+
+    const radio = form.querySelector<HTMLInputElement>(
+      'input[name=worktree_branch_mode][value="existing"]',
+    )!;
+    radio.click();
+    await drain();
+
+    const select = form.querySelector<HTMLSelectElement>(".branch-select")!;
+    expect(urls.some((u) => u.includes("/api/branches?path="))).toBe(true);
+    const options = [...select.options];
+    expect(options.map((o) => o.value)).toEqual(["main", "spawn-v2", "free-branch"]);
+    const held = options.find((o) => o.value === "spawn-v2")!;
+    expect(held.disabled).toBe(true);
+    expect(held.textContent).toContain("/home/user/wt-spawn");
+    expect(options.find((o) => o.value === "free-branch")!.disabled).toBe(false);
+  });
+
+  it("emits the branch mode as draft state when it changes", async () => {
+    const changes: SpawnFormValues[] = [];
+    const form = await mount(undefined, (v) => changes.push(v));
+    await drain();
+
+    const radio = form.querySelector<HTMLInputElement>(
+      'input[name=worktree_branch_mode][value="existing"]',
+    )!;
+    radio.click();
+    await drain();
+
+    const modes = changes.map((c) => (c as { worktreeBranchMode?: string }).worktreeBranchMode);
+    expect(modes).toContain("new");
+    expect(modes.at(-1)).toBe("existing");
+  });
+
+  it("does not repopulate a destroyed form when the branch list lands late", async () => {
+    const gate: { release?: () => void } = {};
+    urls = stubFetch({ gateBranches: (r) => (gate.release = r) });
+    handle = createSpawnForm({ onSuccess() {} });
+    document.body.appendChild(handle.el);
+    for (let i = 0; i < 40; i++) await Promise.resolve();
+
+    pickProject(handle.el);
+    handle.el
+      .querySelector<HTMLInputElement>('input[name=worktree_branch_mode][value="existing"]')!
+      .click();
+
+    handle.destroy();
+    gate.release?.();
+    await drain();
+
+    expect(
+      handle.el.querySelector<HTMLSelectElement>(".branch-select")!.options.length,
+    ).toBe(0);
+  });
+});

web/src/styles.css

@@ -1575,6 +1575,16 @@ textarea {
   color: var(--muted);
 }
 
+/* New-vs-existing branch source (#61). */
+.wt-mode {
+  display: flex;
+  gap: var(--sp-4);
+}
+
+.branch-select {
+  width: 100%;
+}
+
 /* Generated-name field with a reroll button. */
 .name-row {
   display: flex;

web/src/types.ts

@@ -114,8 +114,25 @@ export interface WorktreeSpec {
   base_repo: string;
   branch: string;
   cleanup_on_kill?: boolean;
+  /** When false, `branch` already exists and is checked out as-is (#61).
+   * Defaults to true (create a new branch) on older daemons. */
+  create_branch?: boolean;
 }
 
+/** One local branch of the enclosing repository, from `GET /api/branches`
+ * (#55). Mirrors `maestro_protocol::BranchInfo`. */
+export interface BranchInfo {
+  /** Short branch name (`main`, `feature/foo`). */
+  name: string;
+  /** Whether this branch is the main checkout's current HEAD. */
+  is_head: boolean;
+  /** Toplevel path of the linked worktree holding this branch, if any. */
+  worktree?: string | null;
+}
+
+/** `GET /api/branches?path=` — the local branches of one repository (#55). */
+export type Branches = BranchInfo[];
+
 export interface CreateSessionRequest {
   cwd: string;
   command: string;