fix(tools): write_file handler now rejects missing 'content'/'path' args instead of silently writing zero-byte files (#19096)

Under context pressure, frontier models sometimes emit tool calls with
required fields dropped. Previously _handle_write_file() used
args.get('content', '') which substituted an empty string for the missing
key, returned success with bytes_written=0, and created a zero-byte file
on disk. The model had no way to detect the failure.

Changes:
- Reject calls where 'path' is absent or not a non-empty string
- Reject calls where 'content' key is entirely absent (key-presence check,
  not truthiness) — distinguishing a legitimately empty file from a dropped arg
- Reject calls where 'content' is a non-string type
- All error messages include guidance to re-emit the tool call or switch
  to execute_code with hermes_tools.write_file() for large payloads
- Explicit empty string content (file truncation) continues to work

Regression tests added for all four cases: missing path, missing content,
explicit-empty content, and wrong content type.

Fixes #19096

tests/tools/test_file_tools.py

@@ -104,6 +104,44 @@ class TestWriteFileHandler:
         assert result["error"] == "boom"
         assert any("write_file error" in r.getMessage() for r in caplog.records)
 
+    def test_missing_content_key_returns_error(self):
+        """#19096 — handler must reject tool calls where 'content' key is absent."""
+        from tools.file_tools import _handle_write_file
+
+        result = json.loads(_handle_write_file({"path": "/tmp/oops.md"}))
+        assert "error" in result
+        assert "content" in result["error"]
+        assert "path" not in result.get("error", "").lower() or "missing" not in result.get("error", "").lower() or True  # just check error present
+
+    def test_missing_path_key_returns_error(self):
+        """#19096 — handler must reject tool calls where 'path' key is absent."""
+        from tools.file_tools import _handle_write_file
+
+        result = json.loads(_handle_write_file({"content": "hello"}))
+        assert "error" in result
+
+    def test_explicit_empty_content_is_allowed(self):
+        """#19096 — explicit empty string content (file truncation) must still work."""
+        from tools.file_tools import _handle_write_file
+
+        with patch("tools.file_tools._get_file_ops") as mock_get:
+            mock_ops = MagicMock()
+            result_obj = MagicMock()
+            result_obj.to_dict.return_value = {"status": "ok", "path": "/tmp/empty.txt", "bytes": 0}
+            mock_ops.write_file.return_value = result_obj
+            mock_get.return_value = mock_ops
+
+            result = json.loads(_handle_write_file({"path": "/tmp/empty.txt", "content": ""}))
+            assert result["status"] == "ok"
+
+    def test_non_string_content_returns_error(self):
+        """#19096 — content must be a string, not a dict or list."""
+        from tools.file_tools import _handle_write_file
+
+        result = json.loads(_handle_write_file({"path": "/tmp/x.txt", "content": {"nested": "dict"}}))
+        assert "error" in result
+        assert "string" in result["error"].lower() or "content" in result["error"].lower()
+
 
 class TestPatchHandler:
     @patch("tools.file_tools._get_file_ops")

tools/file_tools.py

@@ -1097,7 +1097,25 @@ def _handle_read_file(args, **kw):
 
 def _handle_write_file(args, **kw):
     tid = kw.get("task_id") or "default"
-    return write_file_tool(path=args.get("path", ""), content=args.get("content", ""), task_id=tid)
+    if not args.get("path") or not isinstance(args.get("path"), str):
+        return tool_error(
+            "write_file: missing required field 'path'. Re-emit the tool call with "
+            "both 'path' and 'content' set."
+        )
+    if "content" not in args:
+        return tool_error(
+            "write_file: missing required field 'content'. The tool call included a "
+            "path but no content argument — this is almost always a dropped-arg bug "
+            "under context pressure. Re-emit the tool call with the full content "
+            "payload, or use execute_code with hermes_tools.write_file() for very "
+            "large files."
+        )
+    if not isinstance(args["content"], str):
+        return tool_error(
+            f"write_file: 'content' must be a string, got "
+            f"{type(args['content']).__name__}."
+        )
+    return write_file_tool(path=args["path"], content=args["content"], task_id=tid)
 
 
 def _handle_patch(args, **kw):