diff --git a/acp.py b/acp.py
index baeff3b..5b09015 100755
--- a/acp.py
+++ b/acp.py
@@ -656,6 +656,14 @@ def create_pr(
     if verbose:
         print(f"Current branch: '{original_branch}'")
 
+    if original_branch.startswith("acp/"):
+        print(
+            f"Error: Already on an ACP branch '{original_branch}'. "
+            "Switch to your base branch before creating a new PR.",
+            file=sys.stderr,
+        )
+        sys.exit(1)
+
     if run_check(["git", "diff", "--cached", "--quiet"]):
         print("Error: No staged changes. Run 'git add' first.", file=sys.stderr)
         sys.exit(1)
diff --git a/test_acp.py b/test_acp.py
index 5975fef..17395ae 100644
--- a/test_acp.py
+++ b/test_acp.py
@@ -38,6 +38,16 @@ def test_run_check_failure(self):
 
 
 class TestCreatePR:
+    @mock.patch("acp.run")
+    @mock.patch("acp.run_check")
+    def test_create_pr_on_acp_branch(self, mock_run_check, mock_run, capsys):
+        mock_run.return_value = "acp/testuser/1234567890123456"
+
+        with pytest.raises(SystemExit) as exc:
+            acp.create_pr("test commit", verbose=False, body="")
+        assert exc.value.code == 1
+        assert "Already on an ACP branch" in capsys.readouterr().err
+
     @mock.patch("acp.run")
     @mock.patch("acp.run_check")
     def test_create_pr_no_staged_changes(self, mock_run_check, mock_run):