Preflight scaffold-plugin collisions

agentbridge/scaffold.py

@@ -61,9 +61,15 @@ def scaffold_adapter_plugin(
         ),
     }
 
+    collisions = [path for path in files if path.exists()]
+    if collisions and not force:
+        formatted = ", ".join(str(path) for path in collisions)
+        raise FileExistsError(
+            f"Scaffold target already exists at generated path(s): {formatted}; "
+            "pass force=True to overwrite"
+        )
+
     for path, content in files.items():
-        if path.exists() and not force:
-            raise FileExistsError(f"{path} already exists; pass force=True to overwrite")
         path.parent.mkdir(parents=True, exist_ok=True)
         path.write_text(content, encoding="utf-8")
         created.append(ScaffoldedFile(path=path, created=True))

tests/test_scaffold.py

@@ -62,3 +62,24 @@ def test_scaffold_adapter_plugin_does_not_overwrite_without_force(tmp_path) -> N
         assert "already exists" in str(exc)
     else:  # pragma: no cover - defensive assertion style
         raise AssertionError("Expected existing files to fail without force")
+
+
+def test_scaffold_adapter_plugin_preflights_collisions_before_writing(tmp_path) -> None:
+    target = tmp_path / "plugin"
+    package_dir = target / "agentbridge_custom"
+    package_dir.mkdir(parents=True)
+    (package_dir / "adapter.py").write_text("# existing adapter\n", encoding="utf-8")
+
+    try:
+        scaffold_adapter_plugin(target, backend_name="custom")
+    except FileExistsError as exc:
+        message = str(exc)
+        assert "adapter.py" in message
+        assert "force=True" in message
+    else:  # pragma: no cover - defensive assertion style
+        raise AssertionError("Expected existing adapter.py to fail without force")
+
+    assert not (target / "pyproject.toml").exists()
+    assert not (target / "README.md").exists()
+    assert not (package_dir / "__init__.py").exists()
+    assert not (target / "tests" / "test_adapter.py").exists()