core/tools+config: refuse write_file overwrite unless overwrite=true

tests/test_tools/test_danger.py

@@ -24,14 +24,18 @@ def test_classify_write_overwrite_only(tmp_path: Path) -> None:
     instance = InstanceState(workspace_root=tmp_path.resolve())
     with bind_instance(instance):
         _ = set_workspace_root(tmp_path)
-        # New file: no soft-confirm
+        # New file: no soft-confirm, with or without the overwrite flag
         assert classify_danger("write_file", {"path": "new.txt"}) is None
+        assert classify_danger("write_file", {"path": "new.txt", "overwrite": True}) is None
         # Partial edits: never soft-confirm
         assert classify_danger("edit_replace", {"path": "x.txt"}) is None
         assert classify_danger("edit_lineno", {"path": "x.txt", "start_line": 1, "end_line": 2}) is None
-        # Existing file write: confirm total overwrite
+        # Existing file: only an explicit overwrite asks; an unset/false flag is
+        # refused inside the tool, so no prompt may fire for it.
         (tmp_path / "x.txt").write_text("old", encoding="utf-8")
-        reason = classify_danger("write_file", {"path": "x.txt"})
+        assert classify_danger("write_file", {"path": "x.txt"}) is None
+        assert classify_danger("write_file", {"path": "x.txt", "overwrite": False}) is None
+        reason = classify_danger("write_file", {"path": "x.txt", "overwrite": True})
         assert reason is not None and "overwrite" in reason
         (tmp_path / "dst.txt").write_text("d", encoding="utf-8")
         (tmp_path / "src.txt").write_text("s", encoding="utf-8")

tests/test_tools/test_edit_lineno.py

@@ -63,7 +63,7 @@ def test_edit_lineno_error_suggests_the_read_file_call(workspace: object) -> Non
     out = call_sync(edit_lineno, "s.txt", 15, 15, "X\n")
     assert "read_file(path='s.txt', offset=11, limit=7, with_lineno=true)" in out
     # A stale read gets a fresh call for the same range after an external write.
-    _ = call_sync(write_file, "s.txt", "changed\n")
+    _ = call_sync(write_file, "s.txt", "changed\n", overwrite=True)
     out = call_sync(edit_lineno, "s.txt", 1, 1, "X\n")
     assert "changed since it was read" in out
     assert "read_file(path='s.txt', offset=0, limit=1, with_lineno=true)" in out
@@ -149,7 +149,7 @@ def test_edit_lineno_requires_rerun_after_write(workspace: object) -> None:
     del workspace
     _ = call_sync(write_file, "f.txt", "one\ntwo\nthree\n")
     _ = _read_lineno("f.txt")
-    _ = call_sync(write_file, "f.txt", "changed\n")
+    _ = call_sync(write_file, "f.txt", "changed\n", overwrite=True)
     out = call_sync(edit_lineno, "f.txt", 1, 1, "x\n")
     assert "changed" in out and "re-read" in out
 
@@ -182,7 +182,7 @@ def test_edit_lineno_detects_external_change(workspace: object) -> None:
     _ = call_sync(write_file, "i.txt", "one\ntwo\nthree\n")
     _ = _read_lineno("i.txt")
     # Simulate an external/process-tool mutation: rewrite content + new mtime.
-    _ = call_sync(write_file, "i.txt", "one\ntwo\nthree\nCHANGED\n")
+    _ = call_sync(write_file, "i.txt", "one\ntwo\nthree\nCHANGED\n", overwrite=True)
     os.utime(workspace / "i.txt", (time.time() + 1, time.time() + 1))
     out = call_sync(edit_lineno, "i.txt", 1, 1, "x\n")
     assert "changed since it was read" in out

tests/test_tools/test_file.py

@@ -28,6 +28,15 @@ def test_write_read_listdir_edit(workspace: object) -> None:
     assert call_sync(read_file, "notes/a.txt") == "L1-1\nhello there"
 
 
+def test_write_file_refuses_existing_file_without_overwrite(workspace: Path) -> None:
+    assert "wrote" in call_sync(write_file, "keep.txt", "first")
+    refused = call_sync(write_file, "keep.txt", "second")
+    assert refused == "error: file exists: keep.txt (set overwrite=true to replace it)"
+    assert (workspace / "keep.txt").read_text(encoding="utf-8") == "first"
+    assert "wrote" in call_sync(write_file, "keep.txt", "second", overwrite=True)
+    assert (workspace / "keep.txt").read_text(encoding="utf-8") == "second"
+
+
 def test_read_file_max_chars_embeds_token(workspace: Path) -> None:
     (workspace / "big.txt").write_text("a" * 1000, encoding="utf-8")
     out = call_sync(read_file, "big.txt", max_chars=200)