diff --git a/docs/fetch-cmd.md b/docs/fetch-cmd.md
index 32fa8d1..b125ed3 100644
--- a/docs/fetch-cmd.md
+++ b/docs/fetch-cmd.md
@@ -6,6 +6,9 @@
 filesystems or lazy checkouts where fetching metadata and downloading file
 contents should be decoupled.
 
+The checkout half of a sync has a counterpart, `repo.reprojectcmd`; see
+`docs/reproject-cmd.md`.
+
 ## Configuration
 
 To use this feature, set the following in `.repo/manifests.git/config`:
diff --git a/docs/reproject-cmd.md b/docs/reproject-cmd.md
new file mode 100644
index 0000000..76e1de8
--- /dev/null
+++ b/docs/reproject-cmd.md
@@ -0,0 +1,126 @@
+# Reproject Command Contract
+
+The `repo.reprojectcmd` configuration names a command that `repo sync` runs
+instead of Git to move a project's index and worktree to the tree of the
+target commit. It is the checkout-side counterpart of `repo.fetchcmd` (see
+`docs/fetch-cmd.md`): together they let an external tool take over both the
+network fetch and the materialization of a project. This is useful on
+virtualized filesystems that address content by hash, where a tree can be
+materialized far faster than `git checkout` can write every file.
+
+The command only materializes the tree. `repo` then makes the ref write that
+Git would have made, using `git update-ref`.
+
+## Configuration
+
+To use this feature, set the following in `.repo/manifests.git/config`:
+```ini
+[repo]
+	reprojectcmd = "your custom command here"
+	uselocalgitdirs = true
+```
+Setting `repo.reprojectcmd` **requires** `repo.uselocalgitdirs` to be set to
+`true`.
+
+For reference, this command does with Git what `repo` would otherwise do
+itself:
+```ini
+[repo]
+	reprojectcmd = "git -C $REPO_PATH read-tree -m -u $REPO_TREV"
+	uselocalgitdirs = true
+```
+The one-tree merge applies the change to the target, keeps local changes to
+every other path, and refuses to overwrite a modified or untracked file, so it
+enforces the preconditions below by itself. It also works for a project that
+has nothing checked out yet.
+
+## Environment Variables
+
+The command is executed in a subshell, from the root of the client, populated
+with standard project-context environment variables. For details on standard
+variables (such as `REPO_PROJECT`, `REPO_PATH`, `REPO_REMOTE`, etc.), see the
+Environment section in `repo help forall` or `subcmds/forall.py`.
+
+The variables the command typically needs are:
+
+*   `REPO_PATH`: The project path relative to the root of the client.
+*   `REPO_TREV`: The target revision resolved to a full commit hash. Match this
+    commit's tree.
+
+There is no force mode: a project that would need one never reaches the
+command (see the preconditions below).
+
+## When the command runs
+
+`repo sync` already classifies each project and picks a Git operation. The
+command replaces the three that are a materialization of a target tree:
+
+1.  The checkout that detaches HEAD at the target. This is the common case: a
+    project on a detached HEAD, a project on a branch that does not track
+    upstream, and `repo sync -d`.
+2.  The fast-forward of the checked out branch to the target.
+3.  The hard reset of the checked out branch to the target, when the commits
+    it carried were dropped upstream.
+
+After the command exits 0, `repo` writes the ref itself: it detaches `HEAD` at
+`REPO_TREV`, or moves the checked out branch to `REPO_TREV`.
+
+The command is **not** run:
+
+*   When `HEAD` already names `REPO_TREV`.
+*   At the fast-forward step when `HEAD` is ahead of `REPO_TREV`, where Git's
+    merge would be a no-op.
+*   For a rebase. A branch carrying local commits has them replayed onto the
+    target by `git rebase`, which is not a materialization of a target tree.
+*   For `MetaProject`s (i.e. the internal `repo` repository itself at
+    `.repo/repo` and the `manifests` repository at `.repo/manifests`).
+
+## Contract
+
+### Preconditions
+
+Before invoking the command, `repo` ensures that:
+
+*   The index has no staged changes (the index matches `HEAD`, or is empty on an unborn `HEAD`).
+*   No rebase, cherry-pick, merge, or revert is in progress.
+
+Detecting collisions with untracked files or unstaged working-tree modifications is the responsibility of the reproject command itself (e.g. via `git read-tree -m -u $REPO_TREV` or a custom virtual filesystem checkout tool). If local changes collide with the target tree, the command must abort with a non-zero exit code. Local modifications and untracked files outside the diff between `HEAD` and `REPO_TREV` must be preserved.
+
+### Postconditions on exit 0
+
+After the command exits with status 0, `repo` expects the following
+postconditions to be met:
+
+1.  `git diff-index --quiet --cached REPO_TREV^{tree}` exits 0 (the index
+    matches the target tree).
+2.  `HEAD` still names what it did before the command, and its resolved commit
+    object ID has not changed.
+
+### Invariants
+
+The command may modify the worktree and the index, and may write project-local
+Git config. The command must:
+
+*   Apply the change from `HEAD`'s tree to `REPO_TREV`'s tree and leave every
+    other path alone. Local modifications and untracked files outside that
+    change must survive: the command applies a diff, it does not reset the
+    tree.
+*   Not write any ref, including `HEAD` and `ORIG_HEAD`. `repo` owns every ref
+    write.
+*   Not create or replace `.git/`, and not touch anything under `.repo/`.
+*   Not require the Git remote, to preserve `repo sync --local-only`.
+*   Be idempotent. Running it twice on the same target is a no-op.
+
+### Failure
+
+*   A non-zero exit status, a failed precondition or a failed postcondition
+    fails that project's sync, and the command's or Git's output is surfaced
+    to the user.
+*   Other projects continue, and `repo sync` exits non-zero.
+
+## Limitations
+
+Nested projects are out of scope: a project whose path lies inside another
+project's path, a `<project>` nested in another `<project>` in the manifest,
+and a submodule discovered with `sync-s` or `--recurse-submodules`. `repo sync`
+fails if the manifest has one while `repo.reprojectcmd` is set.
diff --git a/project.py b/project.py
index a084d8d..783aead 100644
--- a/project.py
+++ b/project.py
@@ -110,6 +110,14 @@
 _ALTERNATES = os.environ.get("REPO_USE_ALTERNATES") == "1"
 
 
+def _FirstLines(lines: List[str], limit: int = 10) -> str:
+    """Join |lines|, eliding all but the first |limit| of them."""
+    shown = list(lines[:limit])
+    if len(lines) > limit:
+        shown.append(f"... and {len(lines) - limit} more")
+    return "\n".join(shown)
+
+
 def _lwrite(path, content):
     lock = "%s.lock" % path
 
@@ -811,6 +819,17 @@
         """Returns True if a cherry-pick is in progress."""
         return os.path.exists(self.work_git.GetDotgitPath("CHERRY_PICK_HEAD"))
 
+    def _OperationInProgress(self) -> Optional[str]:
+        """Return the name of the Git operation in progress, if any."""
+        if self.IsRebaseInProgress():
+            return "rebase"
+        if self.IsCherryPickInProgress():
+            return "cherry-pick"
+        for state, name in (("MERGE_HEAD", "merge"), ("REVERT_HEAD", "revert")):
+            if os.path.exists(self.work_git.GetDotgitPath(state)):
+                return name
+        return None
+
     def _AbortRebase(self):
         """Abort ongoing rebase, cherry-pick or patch apply (am).
 
@@ -1829,6 +1848,18 @@
 
         self.revisionId = revisionId
 
+    @property
+    def UseReprojectCmd(self) -> bool:
+        """Whether repo.reprojectcmd materializes this project's tree.
+
+        MetaProjects (repo itself and the manifests) always use Git. See
+        docs/reproject-cmd.md.
+        """
+        if isinstance(self, MetaProject):
+            return False
+        mp = self.manifest.manifestProject
+        return bool(mp.use_local_gitdirs and mp.reproject_cmd)
+
     def Sync_LocalHalf(
         self,
         syncbuf,
@@ -1856,6 +1887,29 @@
             )
             return
 
+        if not isinstance(self, MetaProject):
+            mp = self.manifest.manifestProject
+            if mp.reproject_cmd and not mp.use_local_gitdirs:
+                fail(
+                    LocalSyncFail(
+                        "repo.reprojectcmd requires repo.uselocalgitdirs to be "
+                        "enabled",
+                        project=self.name,
+                    )
+                )
+                return
+
+        reproject = self.UseReprojectCmd
+        if reproject and self.parent:
+            fail(
+                LocalSyncFail(
+                    "repo.reprojectcmd does not support nested projects or "
+                    "submodules",
+                    project=self.name,
+                )
+            )
+            return
+
         self._InitWorkTree(force_sync=force_sync, submodules=submodules)
         # TODO(https://git-scm.com/docs/git-worktree#_bugs): Re-evaluate if
         # submodules can be init when using worktrees once its support is
@@ -1886,8 +1940,30 @@
                 )
                 return
 
+        head = self._GetHead()
+        if head and head.startswith(R_HEADS):
+            branch = head[len(R_HEADS) :]
+            try:
+                head = all_refs[head]
+            except KeyError:
+                head = None
+        else:
+            branch = None
+
+        def _checkout() -> None:
+            """Detach HEAD at revid, like `git checkout <revid>`."""
+            if reproject:
+                self._ReprojectCheckout(revid, head, verbose=verbose)
+            else:
+                self._Checkout(revid, force_checkout=force_checkout, quiet=True)
+
         def _doff():
-            self._FastForward(revid)
+            if reproject:
+                self._ReprojectBranch(
+                    revid, head, f"merge {revid}: Fast-forward", verbose=verbose
+                )
+            else:
+                self._FastForward(revid)
             self._CopyAndLinkFiles()
 
         def _dorebase():
@@ -1913,16 +1989,6 @@
             if p.Wait() != 0:
                 logger.warning("warn: %s: stateless gc failed", self.name)
 
-        head = self._GetHead()
-        if head and head.startswith(R_HEADS):
-            branch = head[len(R_HEADS) :]
-            try:
-                head = all_refs[head]
-            except KeyError:
-                head = None
-        else:
-            branch = None
-
         if branch is None or syncbuf.detach_head:
             # Currently on a detached HEAD.  The user is assumed to
             # not have any local modifications worth worrying about.
@@ -1951,10 +2017,10 @@
                     syncbuf.info(self, "discarding %d commits", len(lost))
 
             try:
-                self._Checkout(revid, force_checkout=force_checkout, quiet=True)
+                _checkout()
                 if submodules:
                     self._SyncSubmodules(quiet=True)
-            except GitError as e:
+            except (GitError, LocalSyncFail) as e:
                 fail(e)
                 return
             self._CopyAndLinkFiles()
@@ -1978,10 +2044,10 @@
                 self, "leaving %s; does not track upstream", branch.name
             )
             try:
-                self._Checkout(revid, force_checkout=force_checkout, quiet=True)
+                _checkout()
                 if submodules:
                     self._SyncSubmodules(quiet=True)
-            except GitError as e:
+            except (GitError, LocalSyncFail) as e:
                 fail(e)
                 return
             self._CopyAndLinkFiles()
@@ -2022,7 +2088,12 @@
                             )
                         )
                     return
-                syncbuf.later1(self, _doff, not verbose)
+                if reproject:
+                    # HEAD is ahead of revid, so there is no tree to
+                    # materialize: Git's fast-forward would be a no-op.
+                    self._CopyAndLinkFiles()
+                else:
+                    syncbuf.later1(self, _doff, not verbose)
                 return
             elif pub == head:
                 # All published commits are merged, and thus we are a
@@ -2048,7 +2119,9 @@
             self._CopyAndLinkFiles()
             return
 
-        if self.IsDirty(consider_untracked=False):
+        if (not reproject or (cnt_mine > 0 and self.rebase)) and self.IsDirty(
+            consider_untracked=False
+        ):
             fail(_DirtyError(project=self.name))
             return
 
@@ -2094,11 +2167,19 @@
             syncbuf.later2(self, _docopyandlink, not verbose)
         elif local_changes:
             try:
-                self._ResetHard(revid)
+                if reproject:
+                    self._ReprojectBranch(
+                        revid,
+                        head,
+                        f"reset: moving to {revid}",
+                        verbose=verbose,
+                    )
+                else:
+                    self._ResetHard(revid)
                 if submodules:
                     self._SyncSubmodules(quiet=True)
                 self._CopyAndLinkFiles()
-            except GitError as e:
+            except (GitError, LocalSyncFail) as e:
                 fail(e)
                 return
         else:
@@ -3696,6 +3777,184 @@
         if GitCommand(self, cmd).Wait() != 0:
             raise GitError(f"{self.name} merge {head} ", project=self.name)
 
+    def _ReprojectCheckout(
+        self, revid: str, head: Optional[str], verbose: bool = False
+    ) -> None:
+        """Detach HEAD at |revid| with repo.reprojectcmd.
+
+        This stands in for `git checkout <revid>`. |head| is the commit HEAD
+        names now, or None. The command is not run when that is already
+        |revid|, since there is then nothing to materialize.
+        """
+        old = self._GetHead()
+        if old and old.startswith(R_HEADS):
+            old = old[len(R_HEADS) :]
+        if head != revid:
+            self._Reproject(revid, verbose=verbose)
+        self.work_git.DetachHead(
+            revid, message=f"checkout: moving from {old or revid} to {revid}"
+        )
+
+    def _ReprojectBranch(
+        self,
+        revid: str,
+        head: Optional[str],
+        message: str,
+        verbose: bool = False,
+    ) -> None:
+        """Move the checked out branch to |revid| with repo.reprojectcmd.
+
+        This stands in for a fast-forward merge or a hard reset. |head| is the
+        commit the branch is at; the ref write fails if it moved meanwhile.
+        """
+        self._Reproject(revid, verbose=verbose)
+        self.work_git.UpdateRef(HEAD, revid, old=head, message=message)
+
+    def _Reproject(self, revid: str, verbose: bool = False) -> None:
+        """Make the index and worktree match |revid| with repo.reprojectcmd.
+
+        The command stands in for the tree materialization of a checkout, a
+        fast-forward or a hard reset. It must leave every ref alone; the
+        caller writes the ref Git would have written.
+
+        For the contract the command has to honor, see docs/reproject-cmd.md.
+
+        Raises:
+            LocalSyncFail: An operation is in progress, the index has staged
+                changes, the command failed, or it left the project in a state
+                that breaks the contract.
+        """
+        in_progress = self._OperationInProgress()
+        if in_progress:
+            raise LocalSyncFail(
+                f"{in_progress} in progress; reprojectcmd cannot run",
+                project=self.name,
+            )
+
+        try:
+            head_tree = self.work_git.rev_parse(
+                "-q", "--verify", "HEAD^{tree}", log_as_error=False
+            )
+        except GitError:
+            head_tree = None
+
+        if head_tree:
+            p = GitCommand(
+                self,
+                ["diff-index", "-z", "--cached", "--name-only", head_tree],
+                capture_stdout=True,
+                capture_stderr=True,
+            )
+            if p.Wait() != 0:
+                raise LocalSyncFail(
+                    f"cannot check the index for staged changes: "
+                    f"{p.stderr.strip()}",
+                    project=self.name,
+                )
+            staged = p.stdout.split("\0")[:-1]
+        else:
+            p = GitCommand(
+                self,
+                ["ls-files", "-z", "--cached"],
+                capture_stdout=True,
+                capture_stderr=True,
+            )
+            if p.Wait() != 0:
+                raise LocalSyncFail(
+                    f"cannot check the index for staged changes: "
+                    f"{p.stderr.strip()}",
+                    project=self.name,
+                )
+            staged = p.stdout.split("\0")[:-1]
+
+        if staged:
+            raise LocalSyncFail(
+                "reprojectcmd cannot run with staged changes:\n"
+                + _FirstLines(staged),
+                project=self.name,
+            )
+
+        head = self.work_git.GetHead()
+        try:
+            head_oid = self.work_git.rev_parse(
+                "-q", "--verify", "HEAD", log_as_error=False
+            )
+        except GitError:
+            head_oid = None
+
+        env = os.environ.copy()
+        env.update(self.GetEnvVars())
+        env["REPO_TREV"] = revid
+        cmd_str = self.manifest.manifestProject.reproject_cmd
+        if verbose:
+            print(f"Running reprojectcmd: {cmd_str} for {self.name}")
+
+        output = None if verbose else subprocess.PIPE
+        try:
+            p = subprocess.run(
+                cmd_str,
+                shell=True,
+                cwd=self.manifest.topdir,
+                env=env,
+                stdout=output,
+                stderr=None if verbose else subprocess.STDOUT,
+                text=True,
+            )
+        except OSError as e:
+            raise LocalSyncFail(
+                f"failed to run reprojectcmd: {e}", project=self.name
+            )
+        if p.returncode != 0:
+            msg = f"reprojectcmd exited with {p.returncode}"
+            if p.stdout:
+                msg += ":\n" + p.stdout.rstrip()
+            raise LocalSyncFail(msg, project=self.name)
+
+        new_head = self.work_git.GetHead()
+        try:
+            new_head_oid = self.work_git.rev_parse(
+                "-q", "--verify", "HEAD", log_as_error=False
+            )
+        except GitError:
+            new_head_oid = None
+
+        if new_head != head or new_head_oid != head_oid:
+            from_desc = (
+                f"{head} ({head_oid})"
+                if head_oid and head != head_oid
+                else f"{head}"
+            )
+            to_desc = (
+                f"{new_head} ({new_head_oid})"
+                if new_head_oid and new_head != new_head_oid
+                else f"{new_head}"
+            )
+            raise LocalSyncFail(
+                f"reprojectcmd moved HEAD from {from_desc} to {to_desc}; repo "
+                "owns every ref write",
+                project=self.name,
+            )
+
+        p = GitCommand(
+            self,
+            ["diff-index", "--cached", "--name-only", f"{revid}^{{tree}}"],
+            capture_stdout=True,
+            capture_stderr=True,
+        )
+        if p.Wait() != 0:
+            raise LocalSyncFail(
+                f"cannot compare the index against {revid}: "
+                f"{p.stderr.strip()}",
+                project=self.name,
+            )
+        mismatched = p.stdout.splitlines()
+        if mismatched:
+            raise LocalSyncFail(
+                f"reprojectcmd left the index different from {revid}:\n"
+                + _FirstLines(mismatched),
+                project=self.name,
+            )
+
     def _InitGitDir(self, mirror_git=None, force_sync=False, quiet=False):
         # Prefix for temporary directories created during gitdir initialization.
         TMP_GITDIR_PREFIX = ".tmp-project-initgitdir-"
@@ -4806,7 +5065,7 @@
             if not self.quiet:
                 out.nl()
             return True
-        except GitError as e:
+        except (GitError, LocalSyncFail) as e:
             syncbuf.fail(self.project, e)
             out.nl()
             return False
@@ -5086,6 +5345,11 @@
         return self.config.GetString("repo.fetchcmd")
 
     @property
+    def reproject_cmd(self) -> Optional[str]:
+        """The command that materializes a project's tree instead of Git."""
+        return self.config.GetString("repo.reprojectcmd")
+
+    @property
     def clone_bundle(self):
         """Whether we use clone_bundle."""
         return self.config.GetBoolean("repo.clonebundle")
@@ -5505,6 +5769,15 @@
             )
             return False
 
+        if self.reproject_cmd and not (
+            use_local_gitdirs or self.use_local_gitdirs
+        ):
+            logger.error(
+                "fatal: repo.reprojectcmd is set but repo.uselocalgitdirs is "
+                "not enabled"
+            )
+            return False
+
         if archive:
             if is_new:
                 self.config.SetBoolean("repo.archive", archive)
diff --git a/subcmds/sync.py b/subcmds/sync.py
index d88ec88..c51ecf3 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -72,6 +72,7 @@
 from git_refs import R_HEADS
 import git_superproject
 from hooks import RepoHook
+from manifest_xml import XmlManifest
 import platform_utils
 from progress import elapsed_str
 from progress import jobs_str
@@ -156,6 +157,11 @@
     return res
 
 
+def _NestedProjects(projects: List[Project]) -> List[Project]:
+    """Return the projects in |projects| living inside another one's path."""
+    return [p for level in _SafeCheckoutOrder(projects)[1:] for p in level]
+
+
 def _ParentFirstBatches(projects: List[Project]) -> List[List[Project]]:
     """Group |projects| so that a parent is fetched before its submodules.
 
@@ -2478,6 +2484,7 @@
             manifest=manifest,
             all_manifests=not opt.this_manifest_only,
         )
+        self._CheckReprojectCmdNesting(opt, args, manifest, all_projects)
 
         # Log the repo projects by existing and new.
         existing = [x for x in all_projects if x.Exists]
@@ -2541,6 +2548,46 @@
         if not opt.quiet:
             print("repo sync has finished successfully.")
 
+    def _CheckReprojectCmdNesting(
+        self,
+        opt: optparse.Values,
+        args: List[str],
+        manifest: XmlManifest,
+        all_projects: List[Project],
+    ) -> None:
+        """Fail when repo.reprojectcmd is used on a manifest nesting projects.
+
+        The command materializes a project's tree without Git, so nothing
+        keeps it from clobbering a project or submodule checked out inside
+        that tree. See docs/reproject-cmd.md.
+        """
+        if not any(p.UseReprojectCmd for p in all_projects):
+            return
+        projects = all_projects
+        if args:
+            # Nesting is a property of the manifest, not of the projects
+            # picked on the command line.
+            projects = self.GetProjects(
+                [],
+                groups=opt.groups,
+                missing_ok=True,
+                submodules_ok=opt.recurse_submodules,
+                manifest=manifest,
+                all_manifests=not opt.this_manifest_only,
+            )
+        nested = _NestedProjects(projects)
+        if not nested:
+            return
+        e = SyncError(
+            "error: repo.reprojectcmd does not support nested projects or "
+            "submodules; found:\n"
+            + "\n".join(
+                f" - {p.RelPath(local=opt.this_manifest_only)}" for p in nested
+            )
+        )
+        logger.error(e)
+        raise e
+
     def _CreateSyncProgressThread(
         self, pm: Progress, stop_event: _threading.Event
     ) -> _threading.Thread:
diff --git a/tests/test_project.py b/tests/test_project.py
index 911bf72..764fbc4 100644
--- a/tests/test_project.py
+++ b/tests/test_project.py
@@ -20,7 +20,7 @@
 import shutil
 import subprocess
 import tempfile
-from typing import Dict, List, Optional, Tuple
+from typing import Any, Callable, Dict, List, Optional, Sequence, Tuple
 import unittest
 from unittest import mock
 
@@ -1866,6 +1866,7 @@
     tempdir,
     use_local_gitdirs=False,
     fetch_cmd=None,
+    reproject_cmd: Optional[str] = None,
     depth=None,
     gitdir=None,
     objdir=None,
@@ -1875,6 +1876,7 @@
     manifest = mock.MagicMock()
     manifest.manifestProject.use_local_gitdirs = use_local_gitdirs
     manifest.manifestProject.fetch_cmd = fetch_cmd
+    manifest.manifestProject.reproject_cmd = reproject_cmd
     manifest.manifestProject.depth = depth
     manifest.manifestProject.dissociate = False
     manifest.manifestProject.clone_filter = None
@@ -2839,20 +2841,25 @@
             self.assertEqual(env["REPO_LREV"], "")
 
 
+def _create_manifest_project(tempdir: str) -> project.ManifestProject:
+    """Return a ManifestProject for a new .repo/ under |tempdir|."""
+    repodir = os.path.join(tempdir, ".repo")
+    manifest_dir = os.path.join(repodir, "manifests")
+    manifest_file = os.path.join(repodir, manifest_xml.MANIFEST_FILE_NAME)
+    os.mkdir(repodir)
+    os.mkdir(manifest_dir)
+    manifest = manifest_xml.XmlManifest(repodir, manifest_file)
+
+    return project.ManifestProject(
+        manifest, "test/manifest", os.path.join(tempdir, ".git"), tempdir
+    )
+
+
 class FetchCmdTests(unittest.TestCase):
     """Tests for fetch_cmd feature."""
 
     def setUpManifest(self, tempdir):
-        repodir = os.path.join(tempdir, ".repo")
-        manifest_dir = os.path.join(repodir, "manifests")
-        manifest_file = os.path.join(repodir, manifest_xml.MANIFEST_FILE_NAME)
-        os.mkdir(repodir)
-        os.mkdir(manifest_dir)
-        manifest = manifest_xml.XmlManifest(repodir, manifest_file)
-
-        return project.ManifestProject(
-            manifest, "test/manifest", os.path.join(tempdir, ".git"), tempdir
-        )
+        return _create_manifest_project(tempdir)
 
     def _get_project(self, tempdir):
         proj = _create_mock_project(
@@ -2905,3 +2912,752 @@
 
             result = fakeproj.Sync(use_local_gitdirs=False)
             self.assertFalse(result)
+
+
+class ReprojectCmdTests(unittest.TestCase):
+    """Tests for the repo.reprojectcmd feature."""
+
+    REVID = "1234abcd" * 5
+    HEAD_ID = "5678abcd" * 5
+    HEAD_TREE = "cafe0000" * 5
+    OTHER_ID = "9abcdef0" * 5
+    PUB_ID = "0fedcba9" * 5
+
+    def _get_project(self, tempdir: str) -> project.Project:
+        proj = _create_mock_project(
+            tempdir, use_local_gitdirs=True, reproject_cmd="echo reproject"
+        )
+        proj.manifest.path_prefix = ""
+        proj.GetRevisionId = mock.MagicMock(return_value=self.REVID)
+        proj.IsRebaseInProgress = mock.MagicMock(return_value=False)
+        proj.IsCherryPickInProgress = mock.MagicMock(return_value=False)
+        proj.work_git = mock.MagicMock()
+        proj.work_git.GetHead.return_value = self.HEAD_ID
+        proj.work_git.rev_parse.return_value = self.HEAD_TREE
+        proj.work_git.GetDotgitPath.side_effect = lambda subpath: os.path.join(
+            tempdir, ".git", subpath
+        )
+        return proj
+
+    @staticmethod
+    def _z(*items: str) -> str:
+        """Return |items| as NUL-delimited Git output."""
+        return "".join(item + "\0" for item in items)
+
+    @staticmethod
+    def _git_command(
+        staged: str = "", diff: str = "", returncode: int = 0
+    ) -> Callable[..., mock.MagicMock]:
+        """Return a GitCommand stand-in answering the reproject queries.
+
+        Args:
+            staged: `git diff-index -z --cached` or `git ls-files -z` output.
+            diff: `git diff-index --cached` postcondition output.
+            returncode: exit code of GitCommand.
+        """
+
+        def make(
+            project: project.Project, cmdv: List[str], **kwargs: Any
+        ) -> mock.MagicMock:
+            cmd = mock.MagicMock()
+            cmd.stderr = ""
+            if cmdv[0] == "diff-index":
+                if any("^{tree}" in arg for arg in cmdv):
+                    cmd.stdout = diff
+                else:
+                    cmd.stdout = staged
+            elif cmdv[0] == "ls-files":
+                cmd.stdout = staged
+            else:
+                cmd.stdout = ""
+            cmd.Wait.return_value = returncode
+            return cmd
+
+        return make
+
+    def _reproject(
+        self, proj: project.Project, **outputs: Any
+    ) -> mock.MagicMock:
+        """Run _Reproject against mocked Git; return the subprocess mock."""
+        with mock.patch(
+            "project.GitCommand", side_effect=self._git_command(**outputs)
+        ), mock.patch("subprocess.run") as mock_run:
+            mock_run.return_value = mock.MagicMock(returncode=0, stdout="")
+            proj._Reproject(self.REVID)
+        return mock_run
+
+    def test_reproject_runs_the_command_with_project_env(self) -> None:
+        """Test the command runs from the client root with REPO_TREV set."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            with mock.patch(
+                "project.GitCommand", side_effect=self._git_command()
+            ), mock.patch("subprocess.run") as mock_run:
+                mock_run.return_value = mock.MagicMock(returncode=0, stdout="")
+                proj._Reproject(self.REVID)
+
+            mock_run.assert_called_once()
+            args, kwargs = mock_run.call_args
+            self.assertEqual(args[0], "echo reproject")
+            self.assertTrue(kwargs["shell"])
+            self.assertEqual(kwargs["cwd"], tempdir)
+            self.assertEqual(kwargs["env"]["REPO_TREV"], self.REVID)
+            self.assertEqual(kwargs["env"]["REPO_PATH"], "test-project")
+
+    def test_reproject_rejects_a_staged_change(self) -> None:
+        """Test a staged change fails before the command runs."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            with self.assertRaises(project.LocalSyncFail) as e:
+                self._reproject(proj, staged=self._z("lib/a.c"))
+            self.assertIn(
+                "reprojectcmd cannot run with staged changes", str(e.exception)
+            )
+            self.assertIn("lib/a.c", str(e.exception))
+
+    def test_reproject_unborn_head_rejects_a_staged_change(self) -> None:
+        """Test a staged change on unborn HEAD fails before command runs."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            proj.work_git.rev_parse.side_effect = error.GitError("unborn")
+            with self.assertRaises(project.LocalSyncFail) as e:
+                self._reproject(proj, staged=self._z("lib/a.c"))
+            self.assertIn(
+                "reprojectcmd cannot run with staged changes", str(e.exception)
+            )
+            self.assertIn("lib/a.c", str(e.exception))
+
+    def test_reproject_unborn_head_runs_clean(self) -> None:
+        """Test a clean unborn HEAD allows the command to run."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            proj.work_git.rev_parse.side_effect = error.GitError("unborn")
+            mock_run = self._reproject(proj)
+            mock_run.assert_called_once()
+
+    def test_reproject_rejects_an_operation_in_progress(self) -> None:
+        """Test an unfinished rebase fails the sync before the command runs."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            proj.IsRebaseInProgress.return_value = True
+            with mock.patch("subprocess.run") as mock_run:
+                with self.assertRaises(project.LocalSyncFail) as e:
+                    proj._Reproject(self.REVID)
+            self.assertIn("rebase in progress", str(e.exception))
+            mock_run.assert_not_called()
+
+    def test_reproject_surfaces_a_failed_command(self) -> None:
+        """Test a non-zero exit fails the sync with the command's output."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            with mock.patch(
+                "project.GitCommand", side_effect=self._git_command()
+            ), mock.patch("subprocess.run") as mock_run:
+                mock_run.return_value = mock.MagicMock(
+                    returncode=3, stdout="disk on fire\n"
+                )
+                with self.assertRaises(project.LocalSyncFail) as e:
+                    proj._Reproject(self.REVID)
+            self.assertIn("exited with 3", str(e.exception))
+            self.assertIn("disk on fire", str(e.exception))
+
+    def test_reproject_rejects_a_moved_head_ref(self) -> None:
+        """Test a command that wrote HEAD fails the sync."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            proj.work_git.GetHead.side_effect = [self.HEAD_ID, self.REVID]
+            with mock.patch(
+                "project.GitCommand", side_effect=self._git_command()
+            ), mock.patch("subprocess.run") as mock_run:
+                mock_run.return_value = mock.MagicMock(returncode=0, stdout="")
+                with self.assertRaises(project.LocalSyncFail) as e:
+                    proj._Reproject(self.REVID)
+            self.assertIn("moved HEAD", str(e.exception))
+
+    def test_reproject_rejects_a_moved_head_oid_on_branch(self) -> None:
+        """Test a command that moved a branch pointer fails the sync."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            proj.work_git.GetHead.return_value = "refs/heads/main"
+            proj.work_git.rev_parse.side_effect = [
+                "tree123",  # HEAD^{tree}
+                self.HEAD_ID,  # HEAD before command
+                self.REVID,  # HEAD after command
+            ]
+            with mock.patch(
+                "project.GitCommand", side_effect=self._git_command()
+            ), mock.patch("subprocess.run") as mock_run:
+                mock_run.return_value = mock.MagicMock(returncode=0, stdout="")
+                with self.assertRaises(project.LocalSyncFail) as e:
+                    proj._Reproject(self.REVID)
+            self.assertIn("moved HEAD", str(e.exception))
+
+    def test_reproject_rejects_an_index_mismatch(self) -> None:
+        """Test a command that left the index off the target fails the sync."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+            with mock.patch(
+                "project.GitCommand",
+                side_effect=self._git_command(diff="lib/a.c\n"),
+            ), mock.patch("subprocess.run") as mock_run:
+                mock_run.return_value = mock.MagicMock(returncode=0, stdout="")
+                with self.assertRaises(project.LocalSyncFail) as e:
+                    proj._Reproject(self.REVID)
+            self.assertIn(self.REVID, str(e.exception))
+            self.assertIn("lib/a.c", str(e.exception))
+
+    def _get_synced_project(
+        self,
+        tempdir: str,
+        head: Optional[str],
+        branch: Optional[str] = None,
+        upstream_gain: Sequence[str] = (),
+        local_changes: Sequence[str] = (),
+    ) -> project.Project:
+        """Return a project ready for Sync_LocalHalf with Git mocked out.
+
+        Args:
+            head: The commit HEAD is at, or None for an unborn branch.
+            branch: The name of the checked out branch, or None if detached.
+            upstream_gain: The commits the target has that HEAD lacks.
+            local_changes: The "<sha> <email>" lines HEAD has that the target
+                lacks.
+        """
+        proj = self._get_project(tempdir)
+        for name in (
+            "_InitWorkTree",
+            "CleanPublishedCache",
+            "_CopyAndLinkFiles",
+            "_Reproject",
+            "_Checkout",
+            "_FastForward",
+            "_ResetHard",
+            "_Rebase",
+        ):
+            setattr(proj, name, mock.MagicMock())
+        proj.IsDirty = mock.MagicMock(return_value=False)
+        proj._userident_name = "Me"
+        proj._userident_email = "me@example.com"
+
+        proj.bare_ref = mock.MagicMock()
+        if branch:
+            proj.bare_ref.head = project.R_HEADS + branch
+            proj.bare_ref.all = {project.R_HEADS + branch: head} if head else {}
+            # A branch that does not track upstream; tests that need one
+            # tracking upstream replace this with _tracking_branch().
+            proj.GetBranch = mock.MagicMock(
+                return_value=self._tracking_branch(branch, merge=None)
+            )
+        else:
+            proj.bare_ref.head = head
+            proj.bare_ref.all = {}
+
+        def _revlist(*args: Any, **kwargs: Any) -> List[str]:
+            if kwargs.get("format"):
+                return list(local_changes)
+            if args[0] == project.not_rev(project.HEAD):
+                return list(upstream_gain)
+            if args[1] == self.PUB_ID:
+                return [self.PUB_ID]
+            return []
+
+        proj._revlist = mock.MagicMock(side_effect=_revlist)
+        return proj
+
+    @staticmethod
+    def _tracking_branch(
+        name: str = "topic", merge: Optional[str] = "main"
+    ) -> mock.MagicMock:
+        """Return a branch tracking |merge| upstream, or nothing if None."""
+        branch = mock.MagicMock()
+        branch.name = name
+        branch.merge = merge
+        branch.LocalMerge = f"refs/remotes/origin/{merge}" if merge else None
+        return branch
+
+    def test_sync_local_half_materializes_a_fresh_project(self) -> None:
+        """Test a project with an unborn HEAD is checked out by the command."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(tempdir, head=None, branch="main")
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertTrue(syncbuf.Finish())
+            proj._Reproject.assert_called_once_with(self.REVID, verbose=False)
+            proj.work_git.DetachHead.assert_called_once_with(
+                self.REVID,
+                message=f"checkout: moving from main to {self.REVID}",
+            )
+            proj._Checkout.assert_not_called()
+            proj._CopyAndLinkFiles.assert_called_once_with()
+
+    def test_sync_local_half_detached_head_uses_the_command(self) -> None:
+        """Test a detached HEAD is moved to the target by the command."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(tempdir, head=self.HEAD_ID)
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertTrue(syncbuf.Finish())
+            proj._Reproject.assert_called_once_with(self.REVID, verbose=False)
+            proj.work_git.DetachHead.assert_called_once_with(
+                self.REVID,
+                message=f"checkout: moving from {self.HEAD_ID} to {self.REVID}",
+            )
+            proj._Checkout.assert_not_called()
+
+    def test_sync_local_half_head_at_target_skips_the_command(self) -> None:
+        """Test `repo sync -d` at the target only detaches HEAD."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(
+                tempdir, head=self.REVID, branch="topic"
+            )
+            syncbuf = project.SyncBuffer(proj.config, detach_head=True)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertTrue(syncbuf.Finish())
+            proj._Reproject.assert_not_called()
+            proj.work_git.DetachHead.assert_called_once()
+            self.assertEqual(
+                proj.work_git.DetachHead.call_args[0][0], self.REVID
+            )
+
+    def test_sync_local_half_command_failure_fails_the_project(self) -> None:
+        """Test a failed command is reported and leaves the ref alone."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(tempdir, head=self.HEAD_ID)
+            proj._Reproject.side_effect = project.LocalSyncFail(
+                "reprojectcmd exited with 1", project=proj.name
+            )
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertFalse(syncbuf.Finish())
+            self.assertEqual(len(syncbuf.errors), 1)
+            self.assertIn("exited with 1", str(syncbuf.errors[0]))
+            proj.work_git.DetachHead.assert_not_called()
+            proj._CopyAndLinkFiles.assert_not_called()
+
+    def test_sync_local_half_fast_forward_uses_the_command(self) -> None:
+        """Test a branch behind the target is fast-forwarded by the command."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(
+                tempdir,
+                head=self.HEAD_ID,
+                branch="topic",
+                upstream_gain=[self.REVID],
+            )
+            proj.GetBranch = mock.MagicMock(
+                return_value=self._tracking_branch()
+            )
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertTrue(syncbuf.Finish())
+            proj._Reproject.assert_called_once_with(self.REVID, verbose=False)
+            proj.work_git.UpdateRef.assert_called_once_with(
+                project.HEAD,
+                self.REVID,
+                old=self.HEAD_ID,
+                message=f"merge {self.REVID}: Fast-forward",
+            )
+            proj._FastForward.assert_not_called()
+            proj._CopyAndLinkFiles.assert_called_once_with()
+
+    def test_sync_local_half_head_ahead_skips_the_command(self) -> None:
+        """Test a published branch ahead of the target is left alone."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(
+                tempdir, head=self.HEAD_ID, branch="topic"
+            )
+            proj.GetBranch = mock.MagicMock(
+                return_value=self._tracking_branch()
+            )
+            proj.work_git.merge_base.side_effect = error.GitError("no")
+            proj.WasPublished = mock.MagicMock(return_value=self.PUB_ID)
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertTrue(syncbuf.Finish())
+            proj._Reproject.assert_not_called()
+            proj.work_git.UpdateRef.assert_not_called()
+            proj._FastForward.assert_not_called()
+            proj._CopyAndLinkFiles.assert_called_once_with()
+
+    def test_sync_local_half_hard_reset_uses_the_command(self) -> None:
+        """Test a branch whose commits upstream dropped is reset by it."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(
+                tempdir,
+                head=self.HEAD_ID,
+                branch="topic",
+                upstream_gain=[self.REVID],
+                local_changes=[f"{self.OTHER_ID} other@example.com"],
+            )
+            proj.GetBranch = mock.MagicMock(
+                return_value=self._tracking_branch()
+            )
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertTrue(syncbuf.Finish())
+            proj._Reproject.assert_called_once_with(self.REVID, verbose=False)
+            proj.work_git.UpdateRef.assert_called_once_with(
+                project.HEAD,
+                self.REVID,
+                old=self.HEAD_ID,
+                message=f"reset: moving to {self.REVID}",
+            )
+            proj._ResetHard.assert_not_called()
+
+    def test_sync_local_half_rebase_is_not_delegated(self) -> None:
+        """Test a branch carrying the user's commits is rebased by Git."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(
+                tempdir,
+                head=self.HEAD_ID,
+                branch="topic",
+                upstream_gain=[self.REVID],
+                local_changes=[f"{self.OTHER_ID} me@example.com"],
+            )
+            proj.GetBranch = mock.MagicMock(
+                return_value=self._tracking_branch()
+            )
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertTrue(syncbuf.Finish())
+            proj._Rebase.assert_called_once_with(
+                upstream=f"{self.OTHER_ID}^1", onto=self.REVID
+            )
+            proj._Reproject.assert_not_called()
+            proj.work_git.UpdateRef.assert_not_called()
+
+    def test_sync_local_half_rejects_a_nested_project(self) -> None:
+        """Test a submodule or nested project fails before any checkout."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(tempdir, head=self.HEAD_ID)
+            proj.parent = mock.MagicMock()
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertFalse(syncbuf.Finish())
+            self.assertIn("nested", str(syncbuf.errors[0]))
+            proj._InitWorkTree.assert_not_called()
+            proj._Reproject.assert_not_called()
+
+    def test_sync_local_half_without_the_command_uses_git(self) -> None:
+        """Test Git keeps doing the checkout when the command is not set."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_synced_project(tempdir, head=self.HEAD_ID)
+            proj.manifest.manifestProject.reproject_cmd = None
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+
+            self.assertTrue(syncbuf.Finish())
+            proj._Checkout.assert_called_once_with(
+                self.REVID, force_checkout=False, quiet=True
+            )
+            proj._Reproject.assert_not_called()
+            proj.work_git.DetachHead.assert_not_called()
+
+    def test_metaproject_never_uses_the_command(self) -> None:
+        """Test .repo/manifests and .repo/repo are checked out by Git."""
+        with utils_for_test.TempGitTree() as tempdir:
+            fakeproj = _create_manifest_project(tempdir)
+            fakeproj.config.SetString("repo.reprojectcmd", "echo hi")
+            fakeproj.config.SetBoolean("repo.uselocalgitdirs", True)
+            self.assertFalse(fakeproj.UseReprojectCmd)
+
+    def test_sync_reproject_cmd_requires_use_local_gitdirs(self) -> None:
+        """Test that repo.reprojectcmd requires repo.uselocalgitdirs."""
+        with utils_for_test.TempGitTree() as tempdir:
+            fakeproj = _create_manifest_project(tempdir)
+
+            class DummyManifest:
+                is_submanifest = False
+
+                def GetDefaultGroupsStr(
+                    self, with_platform: bool = False
+                ) -> str:
+                    return ""
+
+            fakeproj.manifest = DummyManifest()
+
+            fakeproj.config.SetString("repo.reprojectcmd", "echo hi")
+            fakeproj.config.SetBoolean("repo.uselocalgitdirs", False)
+
+            result = fakeproj.Sync(use_local_gitdirs=False)
+            self.assertFalse(result)
+
+
+class ReprojectCmdGitTests(unittest.TestCase):
+    """Tests running the reprojectcmd contract against a real Git checkout."""
+
+    READ_TREE = "git -C $REPO_PATH read-tree -m -u $REPO_TREV"
+
+    @staticmethod
+    def _git(cwd: str, *args: str) -> str:
+        return subprocess.run(
+            ["git", "-C", cwd] + list(args),
+            check=True,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            universal_newlines=True,
+        ).stdout.rstrip("\n")
+
+    def _make_client(
+        self, topdir: str, reproject_cmd: Optional[str]
+    ) -> Tuple[project.Project, str]:
+        """Set up a fetched, never checked out project under |topdir|.
+
+        Returns:
+            The project and the commit its manifest revision names.
+        """
+        # A remote holding the history the project fetches.
+        remote = os.path.join(topdir, "remote")
+        os.mkdir(remote)
+        self._git(remote, "init", "-q")
+        self._git(remote, "symbolic-ref", "HEAD", "refs/heads/main")
+        for msg, files in (
+            ("one", {"one.txt": "one\n", "keep.txt": "keep\n"}),
+            ("two", {"one.txt": "one, revised\n", "two.txt": "two\n"}),
+        ):
+            for name, content in files.items():
+                with open(os.path.join(remote, name), "w") as fp:
+                    fp.write(content)
+                self._git(remote, "add", name)
+            self._git(
+                remote,
+                "-c",
+                "user.name=Test",
+                "-c",
+                "user.email=test@example.com",
+                "commit",
+                "-q",
+                "-m",
+                msg,
+            )
+        revid = self._git(remote, "rev-parse", "HEAD")
+
+        # The project as repo.fetchcmd leaves it: objects fetched, HEAD on an
+        # unborn branch, nothing in the index or the worktree.
+        worktree = os.path.join(topdir, "proj")
+        os.mkdir(worktree)
+        self._git(worktree, "init", "-q")
+        self._git(worktree, "symbolic-ref", "HEAD", "refs/heads/main")
+        self._git(worktree, "fetch", "-q", remote, "refs/heads/main")
+
+        manifest = mock.MagicMock()
+        manifest.manifestProject.use_local_gitdirs = True
+        manifest.manifestProject.reproject_cmd = reproject_cmd
+        manifest.UseLocalGitDirs = True
+        manifest.IsMirror = False
+        manifest.is_multimanifest = False
+        manifest.topdir = topdir
+        manifest.path_prefix = ""
+        manifest.globalConfig = None
+
+        remote_spec = mock.MagicMock()
+        remote_spec.name = "origin"
+        remote_spec.url = remote
+
+        proj = project.Project(
+            manifest=manifest,
+            name="proj",
+            remote=remote_spec,
+            gitdir=os.path.join(worktree, ".git"),
+            objdir=os.path.join(worktree, ".git"),
+            worktree=worktree,
+            relpath="proj",
+            revisionExpr="main",
+            revisionId=revid,
+        )
+        proj._Checkout = mock.MagicMock(
+            side_effect=AssertionError("Git must not do the checkout")
+        )
+        return proj, revid
+
+    def _sync(self, proj: project.Project) -> Tuple[bool, List[Any]]:
+        """Run Sync_LocalHalf; return whether it succeeded and its errors."""
+        syncbuf = project.SyncBuffer(proj.config)
+        proj.Sync_LocalHalf(syncbuf)
+        return syncbuf.Finish(), syncbuf.errors
+
+    def _step_back(self, proj: project.Project, revid: str) -> str:
+        """Put the project cleanly at the commit before |revid|, with Git."""
+        parent = self._git(proj.worktree, "rev-parse", revid + "~1")
+        self._git(proj.worktree, "update-ref", "--no-deref", "HEAD", parent)
+        self._git(proj.worktree, "read-tree", "-u", "--reset", "HEAD")
+        return parent
+
+    def test_sync_local_half_checks_out_a_fresh_project(self) -> None:
+        """Test the read-tree command materializes a project like Git would."""
+        with tempfile.TemporaryDirectory(prefix="repo-tests") as topdir:
+            marker = os.path.join(topdir, "ran")
+            proj, revid = self._make_client(
+                topdir, f"{self.READ_TREE} && touch {marker}"
+            )
+            worktree = proj.worktree
+
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+            self.assertTrue(syncbuf.Finish(), syncbuf.errors)
+
+            self.assertTrue(os.path.exists(marker))
+            self.assertEqual(self._git(worktree, "rev-parse", "HEAD"), revid)
+            with self.assertRaises(subprocess.CalledProcessError):
+                self._git(worktree, "symbolic-ref", "-q", "HEAD")
+            self.assertEqual(self._git(worktree, "status", "--porcelain"), "")
+            with open(os.path.join(worktree, "one.txt")) as fp:
+                self.assertEqual(fp.read(), "one, revised\n")
+            with open(os.path.join(worktree, "two.txt")) as fp:
+                self.assertEqual(fp.read(), "two\n")
+
+            # Syncing again finds HEAD at the target and leaves it alone.
+            os.remove(marker)
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+            self.assertTrue(syncbuf.Finish(), syncbuf.errors)
+            self.assertFalse(os.path.exists(marker))
+            self.assertEqual(self._git(worktree, "rev-parse", "HEAD"), revid)
+
+    def test_sync_local_half_leaves_an_untracked_file_in_the_way_alone(
+        self,
+    ) -> None:
+        """Test an untracked file the target adds fails without any change."""
+        with tempfile.TemporaryDirectory(prefix="repo-tests") as topdir:
+            proj, revid = self._make_client(topdir, self.READ_TREE)
+            worktree = proj.worktree
+            with open(os.path.join(worktree, "two.txt"), "w") as fp:
+                fp.write("mine\n")
+
+            clean, errors = self._sync(proj)
+            self.assertFalse(clean)
+            self.assertIn("two.txt", str(errors[0]))
+
+            self.assertEqual(
+                self._git(worktree, "symbolic-ref", "HEAD"), "refs/heads/main"
+            )
+            self.assertEqual(sorted(os.listdir(worktree)), [".git", "two.txt"])
+            with open(os.path.join(worktree, "two.txt")) as fp:
+                self.assertEqual(fp.read(), "mine\n")
+
+    def test_sync_local_half_keeps_local_changes_out_of_the_way(self) -> None:
+        """Test edits and untracked files the target leaves alone survive."""
+        with tempfile.TemporaryDirectory(prefix="repo-tests") as topdir:
+            proj, revid = self._make_client(topdir, self.READ_TREE)
+            worktree = proj.worktree
+            self.assertTrue(self._sync(proj)[0])
+            self._step_back(proj, revid)
+            with open(os.path.join(worktree, "keep.txt"), "w") as fp:
+                fp.write("edited\n")
+            with open(os.path.join(worktree, "junk"), "w") as fp:
+                fp.write("junk\n")
+
+            clean, errors = self._sync(proj)
+            self.assertTrue(clean, errors)
+
+            self.assertEqual(self._git(worktree, "rev-parse", "HEAD"), revid)
+            with open(os.path.join(worktree, "one.txt")) as fp:
+                self.assertEqual(fp.read(), "one, revised\n")
+            with open(os.path.join(worktree, "keep.txt")) as fp:
+                self.assertEqual(fp.read(), "edited\n")
+            self.assertEqual(
+                self._git(worktree, "status", "--porcelain").splitlines(),
+                [" M keep.txt", "?? junk"],
+            )
+
+    def test_sync_local_half_rejects_a_local_change_in_the_way(self) -> None:
+        """Test an edit to a file the target changes fails without a change."""
+        with tempfile.TemporaryDirectory(prefix="repo-tests") as topdir:
+            proj, revid = self._make_client(topdir, self.READ_TREE)
+            worktree = proj.worktree
+            self.assertTrue(self._sync(proj)[0])
+            parent = self._step_back(proj, revid)
+            with open(os.path.join(worktree, "one.txt"), "w") as fp:
+                fp.write("edited\n")
+
+            clean, errors = self._sync(proj)
+            self.assertFalse(clean)
+            self.assertIn("one.txt", str(errors[0]))
+
+            self.assertEqual(self._git(worktree, "rev-parse", "HEAD"), parent)
+            with open(os.path.join(worktree, "one.txt")) as fp:
+                self.assertEqual(fp.read(), "edited\n")
+            self.assertNotIn("two.txt", os.listdir(worktree))
+
+    def test_sync_local_half_rejects_a_command_that_moves_head(self) -> None:
+        """Test a command that writes HEAD fails the postcondition."""
+        with tempfile.TemporaryDirectory(prefix="repo-tests") as topdir:
+            proj, revid = self._make_client(
+                topdir,
+                f"{self.READ_TREE} && git -C $REPO_PATH update-ref "
+                "--no-deref HEAD $REPO_TREV",
+            )
+            syncbuf = project.SyncBuffer(proj.config)
+            proj.Sync_LocalHalf(syncbuf)
+            self.assertFalse(syncbuf.Finish())
+            self.assertIn("moved HEAD", str(syncbuf.errors[0]))
+
+    def test_sync_local_half_rejects_staged_changes(self) -> None:
+        """Test a staged change fails before the command runs."""
+        with tempfile.TemporaryDirectory(prefix="repo-tests") as topdir:
+            proj, revid = self._make_client(topdir, self.READ_TREE)
+            worktree = proj.worktree
+            self.assertTrue(self._sync(proj)[0])
+            self._step_back(proj, revid)
+            with open(os.path.join(worktree, "keep.txt"), "w") as fp:
+                fp.write("staged\n")
+            self._git(worktree, "add", "keep.txt")
+
+            clean, errors = self._sync(proj)
+            self.assertFalse(clean)
+            self.assertIn("staged changes", str(errors[0]))
+            self.assertIn("keep.txt", str(errors[0]))
+
+    def test_sync_local_half_rejects_a_command_that_moves_branch_tip(
+        self,
+    ) -> None:
+        """Test a command that moves the branch commit fails postcondition."""
+        with tempfile.TemporaryDirectory(prefix="repo-tests") as topdir:
+            proj, revid = self._make_client(
+                topdir,
+                f"{self.READ_TREE} && git -C $REPO_PATH update-ref "
+                "refs/heads/main $REPO_TREV",
+            )
+            clean, errors = self._sync(proj)
+            self.assertFalse(clean)
+            self.assertIn("moved HEAD", str(errors[0]))
+
+    def test_sync_local_half_keeps_local_changes_on_tracking_branch(
+        self,
+    ) -> None:
+        """Test benign edits survive fast-forward on a tracking branch."""
+        with tempfile.TemporaryDirectory(prefix="repo-tests") as topdir:
+            proj, revid = self._make_client(topdir, self.READ_TREE)
+            worktree = proj.worktree
+            self.assertTrue(self._sync(proj)[0])
+            parent = self._step_back(proj, revid)
+            self._git(worktree, "checkout", "-q", "-b", "main", parent)
+            self._git(worktree, "config", "branch.main.remote", "origin")
+            self._git(
+                worktree, "config", "branch.main.merge", "refs/heads/main"
+            )
+            with open(os.path.join(worktree, "keep.txt"), "w") as fp:
+                fp.write("edited\n")
+            with open(os.path.join(worktree, "junk"), "w") as fp:
+                fp.write("junk\n")
+
+            clean, errors = self._sync(proj)
+            self.assertTrue(clean, errors)
+
+            self.assertEqual(self._git(worktree, "rev-parse", "HEAD"), revid)
+            with open(os.path.join(worktree, "one.txt")) as fp:
+                self.assertEqual(fp.read(), "one, revised\n")
+            with open(os.path.join(worktree, "keep.txt")) as fp:
+                self.assertEqual(fp.read(), "edited\n")
+            self.assertEqual(
+                self._git(worktree, "status", "--porcelain").splitlines(),
+                [" M keep.txt", "?? junk"],
+            )
diff --git a/tests/test_subcmds_sync.py b/tests/test_subcmds_sync.py
index e929f22..39d9f03 100644
--- a/tests/test_subcmds_sync.py
+++ b/tests/test_subcmds_sync.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 """Unittests for the subcmds/sync.py module."""
 
+import contextlib
 import json
 import optparse
 import os
@@ -20,7 +21,7 @@
 import shutil
 import tempfile
 import time
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, Tuple
 import unittest
 from unittest import mock
 
@@ -516,6 +517,7 @@
 
         self.use_git_worktrees = False
         self.UseAlternates = False
+        self.UseReprojectCmd = False
         self.manifest = mock.MagicMock()
         self.manifest.GetProjectsWithName.return_value = [self]
         self.config = mock.MagicMock()
@@ -642,6 +644,27 @@
         )
 
 
+class NestedProjects(unittest.TestCase):
+    def test_flat_manifest(self) -> None:
+        p_foo = FakeProject("foo")
+        p_foo_bar = FakeProject("foo-bar")
+        self.assertEqual(sync._NestedProjects([p_foo, p_foo_bar]), [])
+
+    def test_nested_paths(self) -> None:
+        p_foo = FakeProject("foo")
+        p_foo_bar = FakeProject("foo/bar")
+        p_foo_bar_baz = FakeProject("foo/bar/baz")
+        self.assertEqual(
+            sync._NestedProjects([p_foo_bar_baz, p_foo, p_foo_bar]),
+            [p_foo_bar, p_foo_bar_baz],
+        )
+
+    def test_submodule_of_a_parent(self) -> None:
+        parent = FakeProject("foo")
+        sub = FakeProject("foo/sub", parent=parent, is_derived=True)
+        self.assertEqual(sync._NestedProjects([parent, sub]), [sub])
+
+
 class ParentFirstBatches(unittest.TestCase):
     def test_no_submodules(self) -> None:
         p_a = FakeProject("a")
@@ -1083,7 +1106,10 @@
         self.project = p = mock.MagicMock(
             use_git_worktrees=False,
             UseAlternates=False,
+            UseReprojectCmd=False,
             name="project",
+            relpath="rel_path",
+            parent=None,
             Sync_NetworkHalf=Sync_NetworkHalf,
             Sync_LocalHalf=Sync_LocalHalf,
             RelPath=mock.Mock(return_value="rel_path"),
@@ -1142,6 +1168,78 @@
         _, kwargs = self.cmd.GetProjects.call_args
         self.assertEqual(kwargs.get("groups"), "my_group")
 
+    def _ExecuteUntilSync(
+        self, args: List[str]
+    ) -> Tuple[mock.MagicMock, mock.MagicMock]:
+        """Run Execute up to the sync itself, returning the sync mocks."""
+        self.opt.mp_update = False
+        with contextlib.ExitStack() as stack:
+            for name in (
+                "_UpdateRepoProject",
+                "_UpdateProjectsRevisionId",
+                "_ValidateOptionsWithManifest",
+                "_RunPostSyncHook",
+            ):
+                stack.enter_context(mock.patch.object(self.cmd, name))
+            phased = stack.enter_context(
+                mock.patch.object(self.cmd, "_SyncPhased")
+            )
+            interleaved = stack.enter_context(
+                mock.patch.object(self.cmd, "_SyncInterleaved")
+            )
+            self.cmd.Execute(self.opt, args)
+        return phased, interleaved
+
+    def test_reproject_cmd_allows_a_flat_manifest(self) -> None:
+        """Ensure repo.reprojectcmd syncs a manifest without nesting."""
+        self.project.UseReprojectCmd = True
+        phased, interleaved = self._ExecuteUntilSync([])
+        self.assertTrue(phased.called or interleaved.called)
+
+    def test_reproject_cmd_rejects_nested_projects(self) -> None:
+        """Ensure repo.reprojectcmd fails a manifest with nested projects."""
+        p_foo = FakeProject("foo")
+        p_foo_bar = FakeProject("foo/bar")
+        p_foo.UseReprojectCmd = p_foo_bar.UseReprojectCmd = True
+        self.cmd.GetProjects.return_value = [p_foo, p_foo_bar]
+        with self.assertRaises(sync.SyncError) as e:
+            self._ExecuteUntilSync([])
+        self.assertIn("foo/bar", str(e.exception))
+        self.assertNotIn(" - foo\n", str(e.exception))
+
+    def test_reproject_cmd_rejects_a_submodule(self) -> None:
+        """Ensure repo.reprojectcmd fails a manifest with a submodule."""
+        p_foo = FakeProject("foo")
+        p_sub = FakeProject("foo/sub", parent=p_foo, is_derived=True)
+        p_foo.UseReprojectCmd = p_sub.UseReprojectCmd = True
+        self.cmd.GetProjects.return_value = [p_foo, p_sub]
+        with self.assertRaises(sync.SyncError) as e:
+            self._ExecuteUntilSync([])
+        self.assertIn("foo/sub", str(e.exception))
+
+    def test_reproject_cmd_checks_the_whole_manifest(self) -> None:
+        """Ensure nesting is checked beyond the projects given as args."""
+        p_foo = FakeProject("foo")
+        p_foo_bar = FakeProject("foo/bar")
+        p_foo.UseReprojectCmd = p_foo_bar.UseReprojectCmd = True
+        self.cmd.GetProjects.side_effect = lambda args, **kwargs: (
+            [p_foo_bar] if args else [p_foo, p_foo_bar]
+        )
+        with self.assertRaises(sync.SyncError):
+            self._ExecuteUntilSync(["foo/bar"])
+        self.assertEqual(self.cmd.GetProjects.call_count, 2)
+        _, kwargs = self.cmd.GetProjects.call_args
+        self.assertEqual(kwargs.get("missing_ok"), True)
+
+    def test_reproject_cmd_off_ignores_nested_projects(self) -> None:
+        """Ensure nesting is only checked with repo.reprojectcmd in use."""
+        projects = [FakeProject("foo"), FakeProject("foo/bar")]
+        for p in projects:
+            p.Exists = False
+        self.cmd.GetProjects.return_value = projects
+        phased, interleaved = self._ExecuteUntilSync([])
+        self.assertTrue(phased.called or interleaved.called)
+
 
 class SyncUpdateRepoProject(unittest.TestCase):
     """Tests for Sync._UpdateRepoProject."""