fix(llm): guard against null arguments in streaming tool-call accumulator (#2923)

src/llm_core.py

@@ -1690,7 +1690,12 @@ async def stream_llm(url: str, model: str, messages: List[Dict], temperature: fl
                                             if func.get("name"):
                                                 _tc_acc[idx]["name"] = func["name"]
                                             if "arguments" in func:
-                                                _tc_acc[idx]["arguments"] += func["arguments"]
+                                                # Guard against a null arguments delta: `func` can be
+                                                # {"arguments": None} (JSON null), and a raw `+= None`
+                                                # raises TypeError that the broad except swallows,
+                                                # silently dropping the rest of the chunk. Matches the
+                                                # Anthropic accumulator (`partial = ... or ""`) above.
+                                                _tc_acc[idx]["arguments"] += func["arguments"] or ""
                                                 # Stream tool arg deltas for doc tools
                                                 if func["arguments"] and _tc_acc[idx].get("name") in ("create_document", "update_document", "edit_document"):
                                                     yield f'data: {json.dumps({"type": "tool_call_delta", "index": idx, "name": _tc_acc[idx]["name"], "arg_delta": func["arguments"]})}\n\n'