Fix tool-call parsing to ignore non-tool JSON (fixes #511)

gemma/gm/tools/_manager.py

@@ -20,7 +20,7 @@
 import dataclasses
 import functools
 import json
-import re
+from typing import Any
 
 from etils import epy
 from gemma.gm.tools import _tools
@@ -100,7 +100,10 @@ def maybe_execute_tool(self, model_output: str) -> _tools.ToolOutput | None:
     if not tool_kwargs:
       return None
 
-    tool_name = tool_kwargs.pop('tool_name')
+    tool_name = tool_kwargs.pop('tool_name', None)
+    # If the model output contained JSON but it wasn't a tool call, ignore it.
+    if not isinstance(tool_name, str) or not tool_name:
+      return None
     if tool_name not in self.name_to_tool:
       return _tools.ToolOutput(
           text=f'Unknown (or unregistered) tool: {tool_name}.'
@@ -122,18 +125,26 @@ def maybe_execute_tool(self, model_output: str) -> _tools.ToolOutput | None:
     return tool_result
 
 
-def _parse_tool_call(model_output: str) -> dict[str, str] | None:
-  """Parses the tool call from the model output."""
-  # This regex finds the first '{' and the last '}'
-  match = re.search(r'\{.*\}', model_output)
-
-  if not match:
-    return None
-  json_string = match.group(0)
-  try:
-    return json.loads(json_string)
-  except json.JSONDecodeError:
-    return None
+def _parse_tool_call(model_output: str) -> dict[str, Any] | None:
+  """Parses a tool call dict from the model output.
+
+  The model may emit arbitrary JSON (e.g. when asked to output structured data).
+  We should only treat JSON as a tool call if it looks like a tool call (i.e.
+  it is a JSON object containing a `tool_name` field).
+  """
+  # Avoid regex-based extraction: it is brittle (greedy brace matching).
+  decoder = json.JSONDecoder()
+  for i, ch in enumerate(model_output):
+    if ch != '{':
+      continue
+    try:
+      obj, _ = decoder.raw_decode(model_output, i)
+    except json.JSONDecodeError:
+      continue
+    # Only consider JSON objects that look like a tool call.
+    if isinstance(obj, dict) and isinstance(obj.get('tool_name'), str):
+      return obj
+  return None
 
 
 def _format_tool_example(