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

src/plyngent/config/models.py

@@ -46,6 +46,8 @@ DEFAULT_TOOL_DIRECTIVES = """\
 ### Files
 - Prefer file tools over shell. `edit_replace` fails usually means a bad match — \
 fix `old_string` or `max_replaces`; shell workarounds rarely help.
+- `write_file` refuses to replace an existing file (read it, or change it with \
+`edit_replace` / `edit_lineno`); a full replacement needs `overwrite=true`, which asks the human.
 - `edit_replace` defaults to the first match; if remaining matches are reported, \
 raise `max_replaces` or narrow `old_string`.
 - `edit_lineno` edits only lines you read via `read_file(path, with_lineno=true)`; \

src/plyngent/tools/danger.py

@@ -87,7 +87,13 @@ def _command_reason(argv: Sequence[str], *, via: str) -> str | None:
 
 
 def _write_file_reason(args: Mapping[str, object]) -> str | None:
-    """Confirm only when write_file would replace an existing file."""
+    """Confirm only when write_file was asked to replace an existing file.
+
+    An unset/``false`` ``overwrite`` never reaches this prompt: the tool itself
+    refuses an existing file, so there is nothing to ask about.
+    """
+    if not bool(args.get("overwrite", False)):
+        return None
     path = args.get("path")
     if not isinstance(path, str) or not path:
         return None

src/plyngent/tools/file/write.py

@@ -5,9 +5,15 @@ from plyngent.tools.workspace import AccessMode, resolve_path
 
 
 @tool(tags=ToolTag.LOCAL | ToolTag.INSTANCE_STATE | ToolTag.YOLO)
-async def write_file(path: str, content: str) -> str:
-    """Write text content to a file under the workspace (creates parents)."""
+async def write_file(path: str, content: str, *, overwrite: bool = False) -> str:
+    """Write text content to a file under the workspace (creates parents).
+
+    Refuses to replace an existing file: pass ``overwrite=true`` to replace it
+    (the replacement is then confirmed by the human).
+    """
     target = resolve_path(path, required=AccessMode.WRITE)
+    if target.is_file() and not overwrite:
+        return f"error: file exists: {path} (set overwrite=true to replace it)"
     target.parent.mkdir(parents=True, exist_ok=True)
     _ = target.write_text(content, encoding="utf-8")
     # The write changes the file's mtime, so edit_lineno's freshness check