diff --git a/src/cmds/add/cmd_add.py b/src/cmds/add/cmd_add.py
index a7e90e2..afe354a 100644
--- a/src/cmds/add/cmd_add.py
+++ b/src/cmds/add/cmd_add.py
@@ -86,13 +86,13 @@ def add_worktree(add_args: AddArgs) -> Result[None, AddWorktreeError]:
         )
 
     elif analysis & pg.GIT_MERGE_ANALYSIS_FASTFORWARD:
-        # NOTE: Untested!
-        # Move HEAD and working tree forward
-        _ = bare_repo.checkout_tree(treeish=bare_repo.get(remote_branch.id))  # pyright: ignore[reportUnknownMemberType]
         bare_repo.lookup_reference(f"refs/heads/{add_args.derive_from_branch}").set_target(remote_branch.id)
-        bare_repo.head.set_target(remote_branch.id)
 
-        log.info("Fast-forwarded ref branch; fast_forwarded=%s", remote_branch.id)
+        # TODO: this whole block needs rewriting when fetch lands.
+        # remote_branch must be peeled from refs/remotes/origin/{branch} (remote-tracking ref),
+        # not refs/heads/{branch} (local ref), which is always already at target — making
+        # set_target() a structural no-op regardless of whether fetch runs.
+        # log.info("Fast-forwarded ref branch; fast_forwarded=%s", remote_branch.id)
 
     else:
         # True merge required — pygit2 can do it but you'd need to handle conflicts
diff --git a/tests/test_cmd_add.py b/tests/test_cmd_add.py
new file mode 100644
index 0000000..2bc1851
--- /dev/null
+++ b/tests/test_cmd_add.py
@@ -0,0 +1,101 @@
+import os
+from pathlib import Path
+
+import pygit2 as pg
+import pytest
+from result import Err, Ok
+
+from src.cmds.add.args_add import AddArgs
+from src.cmds.add.cmd_add import add_worktree
+from src.errors.not_bare_repo_err import NotBareRepoErr
+
+
+def _make_bare_repo_with_main_worktree(tmp_path: Path) -> tuple[pg.Repository, Path]:
+    bare = tmp_path / "bare"
+    bare.mkdir()
+    repo = pg.init_repository(str(bare), bare=True)
+
+    sig = pg.Signature("Test", "test@test.com")
+    tree_id = repo.TreeBuilder().write()
+    repo.create_commit("refs/heads/main", sig, sig, "Initial commit", tree_id, [])
+    repo.set_head("refs/heads/main")
+
+    main_path = bare / "main"
+    branch_ref: pg.Reference = repo.lookup_reference("refs/heads/main")
+    repo.add_worktree("main", str(main_path), branch_ref)
+
+    return repo, bare
+
+
+class TestAddWorktreeNotBareRepo:
+    def test_plain_directory_returns_err(self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+        monkeypatch.chdir(str(tmp_path))
+
+        args = AddArgs(
+            new_branch_name="feat-x",
+            derive_from_branch="main",
+            should_nest_dirs=False,
+            exclude=[],
+            _force=False,
+        )
+        result = add_worktree(args)
+
+        assert isinstance(result, Err)
+        assert isinstance(result.err(), NotBareRepoErr)
+
+
+class TestAddWorktreeUpToDate:
+    def test_up_to_date_base_creates_worktree(self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+        repo, bare = _make_bare_repo_with_main_worktree(tmp_path)
+        monkeypatch.chdir(str(bare))
+
+        args = AddArgs(
+            new_branch_name="feat-x",
+            derive_from_branch="main",
+            should_nest_dirs=False,
+            exclude=[],
+            _force=False,
+        )
+        result = add_worktree(args)
+
+        assert isinstance(result, Ok)
+        assert (bare / "feat-x").exists()
+
+
+class TestAddWorktreeFastForward:
+    def test_fast_forward_base_does_not_crash(self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+        repo, bare = _make_bare_repo_with_main_worktree(tmp_path)
+
+        # Add a second commit to main so it is ahead of HEAD
+        sig = pg.Signature("Test", "test@test.com")
+        blob_id = repo.create_blob(b"content")
+        tb = repo.TreeBuilder()
+        tb.insert("file.txt", blob_id, pg.GIT_FILEMODE_BLOB)
+        tree_b = tb.write()
+        parent: pg.Commit = repo.lookup_reference("refs/heads/main").peel(pg.Commit)
+        repo.create_commit("refs/heads/main", sig, sig, "second commit", tree_b, [parent.id])
+
+        # Detach HEAD at the first commit so main is one ahead (FASTFORWARD scenario)
+        # pygit2 1.19.2 has no set_head_detached; write the HEAD file directly
+        with open(os.path.join(str(bare), "HEAD"), "w") as f:
+            f.write(str(parent.id) + "\n")
+
+        # Capture target OID (tip of main after second commit) before calling add_worktree
+        target_oid: pg.Oid = repo.lookup_reference("refs/heads/main").peel(pg.Commit).id
+
+        monkeypatch.chdir(str(bare))
+
+        args = AddArgs(
+            new_branch_name="feat-y",
+            derive_from_branch="main",
+            should_nest_dirs=False,
+            exclude=[],
+            _force=False,
+        )
+        result = add_worktree(args)
+
+        # Must succeed — old code raised GitError from checkout_tree on bare repo
+        assert isinstance(result, Ok)
+        assert (bare / "feat-y").exists()
+        # Non-crash check — set_target is a no-op until fetch is wired (see TODO in cmd_add.py FF block)
+        assert repo.lookup_reference("refs/heads/main").peel(pg.Commit).id == target_oid