fix(agent): JSON-serialize non-string tool results to prevent API 400

agent/agent_runtime_helpers.py

@@ -1671,6 +1671,14 @@ def sanitize_api_messages(messages: List[Dict[str, Any]]) -> List[Dict[str, Any]
         filtered.append(msg)
     messages = filtered
 
+    # 0. Coerce non-string tool message content to strings so the API never
+    # receives a dict/list in ``content`` (which OpenAI rejects with 400).
+    for msg in messages:
+        if msg.get("role") == "tool":
+            content = msg.get("content")
+            if not isinstance(content, str):
+                msg["content"] = json.dumps(content, ensure_ascii=False, default=str)
+
     surviving_call_ids: set = set()
     for msg in messages:
         if msg.get("role") == "assistant":

run_agent.py

@@ -3346,6 +3346,13 @@ def _tool_result_content_for_active_model(self, tool_name: str, result: Any) ->
         the agent has a chance to recover.
         """
         if not _is_multimodal_tool_result(result):
+            # Non-multimodal results must be string-safe for the API.  MCP tools
+            # (and a few native helpers) can return Python dicts/lists that the
+            # OpenAI SDK serializes as nested objects, causing HTTP 400
+            # "invalid message content type: map[string]interface {}" because
+            # the API expects `content` to be a string or a content-parts list.
+            if not isinstance(result, str):
+                return json.dumps(result, ensure_ascii=False, default=str)
             return result
 
         content = result.get("content") or []
@@ -3658,6 +3665,10 @@ def _needs_kimi_tool_reasoning(self) -> bool:
             or base_url_host_matches(self.base_url, "api.kimi.com")
             or base_url_host_matches(self.base_url, "moonshot.ai")
             or base_url_host_matches(self.base_url, "moonshot.cn")
+            or (
+                "kimi" in (self.model or "").lower()
+                and self.provider == "ollama-cloud"
+            )
         )
 
     def _needs_deepseek_tool_reasoning(self) -> bool: