diff --git a/crates/maestro-daemon/src/gitroot.rs b/crates/maestro-daemon/src/gitroot.rs
new file mode 100644
index 0000000..07dfca1
--- /dev/null
+++ b/crates/maestro-daemon/src/gitroot.rs
@@ -0,0 +1,118 @@
+//! Git-toplevel probe for the spawn-form worktree section (#25).
+//!
+//! When the user picks a working directory, the spawn form wants to offer a
+//! "git worktree" toggle that needs only a branch name — the `base_repo` is
+//! *inferred* from the chosen dir rather than typed. This module answers that
+//! one question: "what git repository, if any, encloses this path?"
+//!
+//! [`find_root`] walks up from a path to the nearest ancestor containing a
+//! `.git` entry and returns that ancestor (the repo toplevel). A regular `.git`
+//! directory and a `.git` *file* (the linked-worktree / submodule form) both
+//! count, so a dir that is itself inside a checked-out worktree still resolves
+//! to its worktree toplevel.
+//!
+//! It's a pure filesystem walk with no `git` subprocess — same shape as
+//! [`crate::dirlist`]: unit-tested in isolation, and the registry-client task /
+//! HTTP layer (#26) just wraps its `Option` for the wire.
+
+use std::path::{Path, PathBuf};
+
+/// Walk up from `start` to the nearest ancestor that holds a `.git` entry and
+/// return that ancestor — the enclosing repository's toplevel. Returns `None`
+/// when no ancestor is a git repo.
+///
+/// Both a `.git` directory (ordinary checkout) and a `.git` file (linked
+/// worktree or submodule, which stores a `gitdir:` pointer) count as a repo
+/// marker, matching what `git rev-parse --show-toplevel` would resolve. `start`
+/// itself is checked first, so passing a repo root returns that root.
+///
+/// The input is used as-is: callers that accept abbreviated client paths
+/// should resolve them (e.g. via [`crate::dirlist`]'s home-dir expansion)
+/// before probing, so an empty or `~` path doesn't walk up from the process
+/// working directory.
+pub fn find_root(start: &Path) -> Option<PathBuf> {
+    // `ancestors()` yields `start`, its parent, grandparent, … up to the root,
+    // so a repo toplevel is matched at the first hit (closest enclosing repo).
+    start
+        .ancestors()
+        .find(|dir| is_git_root(dir))
+        .map(Path::to_path_buf)
+}
+
+/// Whether `dir` holds a `.git` entry — directory or file. A bare `exists()`
+/// check is deliberate: we don't open or parse it, only detect the marker.
+fn is_git_root(dir: &Path) -> bool {
+    dir.join(".git").exists()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+
+    /// A temp repo: `<root>/.git/` plus a nested `a/b/c` subtree. Returns the
+    /// temp dir (its `Drop` cleans up) so callers keep it alive for the test.
+    fn repo_fixture() -> tempfile::TempDir {
+        let tmp = tempfile::tempdir().unwrap();
+        let root = tmp.path();
+        fs::create_dir(root.join(".git")).unwrap();
+        fs::create_dir_all(root.join("a/b/c")).unwrap();
+        tmp
+    }
+
+    #[test]
+    fn dir_inside_repo_resolves_to_toplevel() {
+        let tmp = repo_fixture();
+        // The repo root itself resolves to the repo root.
+        assert_eq!(find_root(tmp.path()).as_deref(), Some(tmp.path()));
+        // An immediate subdir resolves to the toplevel, not itself.
+        assert_eq!(
+            find_root(&tmp.path().join("a")).as_deref(),
+            Some(tmp.path())
+        );
+    }
+
+    #[test]
+    fn nested_subdir_resolves_to_toplevel() {
+        let tmp = repo_fixture();
+        let deep = tmp.path().join("a/b/c");
+        assert_eq!(find_root(&deep).as_deref(), Some(tmp.path()));
+    }
+
+    #[test]
+    fn non_repo_dir_is_none() {
+        // A temp dir with no `.git` anywhere up the chain. Use the temp root so
+        // the walk can't accidentally hit a real repo above the test's cwd.
+        let tmp = tempfile::tempdir().unwrap();
+        let sub = tmp.path().join("plain/dir");
+        fs::create_dir_all(&sub).unwrap();
+        assert_eq!(find_root(&sub), None);
+    }
+
+    #[test]
+    fn git_file_worktree_marker_counts_as_root() {
+        // Linked worktrees and submodules store `.git` as a *file* holding a
+        // `gitdir:` pointer rather than a directory. It must still resolve.
+        let tmp = tempfile::tempdir().unwrap();
+        let root = tmp.path();
+        fs::write(root.join(".git"), b"gitdir: /somewhere/.git/worktrees/wt\n").unwrap();
+        let sub = root.join("src");
+        fs::create_dir(&sub).unwrap();
+        assert_eq!(find_root(&sub).as_deref(), Some(root));
+    }
+
+    #[test]
+    fn nearest_enclosing_repo_wins() {
+        // An inner repo nested inside an outer repo resolves to the *inner*
+        // toplevel — the walk stops at the first `.git` it meets going up.
+        let tmp = tempfile::tempdir().unwrap();
+        let outer = tmp.path();
+        fs::create_dir(outer.join(".git")).unwrap();
+        let inner = outer.join("vendored");
+        fs::create_dir(&inner).unwrap();
+        fs::create_dir(inner.join(".git")).unwrap();
+        let inner_sub = inner.join("src");
+        fs::create_dir(&inner_sub).unwrap();
+        assert_eq!(find_root(&inner_sub).as_deref(), Some(inner.as_path()));
+    }
+}
diff --git a/crates/maestro-daemon/src/lib.rs b/crates/maestro-daemon/src/lib.rs
index 6a08763..a910bb3 100644
--- a/crates/maestro-daemon/src/lib.rs
+++ b/crates/maestro-daemon/src/lib.rs
@@ -13,6 +13,7 @@ pub mod cli;
 pub mod config;
 pub mod configure;
 pub mod dirlist;
+pub mod gitroot;
 pub mod jwt;
 pub mod local_api;
 pub mod local_events;