fix(provider): normalize DeepSeek non-string message content

nanobot/providers/openai_compat_provider.py

@@ -345,10 +345,25 @@ class OpenAICompatProvider(LLMProvider):
             return json.dumps(arguments, ensure_ascii=False)
         return "{}"
 
+    @staticmethod
+    def _coerce_content_to_string(content: Any) -> str | None:
+        """Coerce block/list content into plain text for strict string-only APIs."""
+        if content is None or isinstance(content, str):
+            return content
+        text = OpenAICompatProvider._extract_text_content(content)
+        if isinstance(text, str) and text:
+            return text
+        try:
+            dumped = json.dumps(content, ensure_ascii=False)
+        except Exception:
+            dumped = str(content)
+        return dumped or "(empty)"
+
     def _sanitize_messages(self, messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
         """Strip non-standard keys, normalize tool_call IDs."""
         sanitized = LLMProvider._sanitize_request_messages(messages, _ALLOWED_MSG_KEYS)
         id_map: dict[str, str] = {}
+        force_string_content = bool(self._spec and self._spec.name == "deepseek")
 
         def map_id(value: Any) -> Any:
             if not isinstance(value, str):
@@ -382,6 +397,11 @@ class OpenAICompatProvider(LLMProvider):
                     clean["content"] = None
             if "tool_call_id" in clean and clean["tool_call_id"]:
                 clean["tool_call_id"] = map_id(clean["tool_call_id"])
+            if (
+                force_string_content
+                and not (clean.get("role") == "assistant" and clean.get("tool_calls"))
+            ):
+                clean["content"] = self._coerce_content_to_string(clean.get("content"))
         return self._enforce_role_alternation(sanitized)
 
     def _drop_deepseek_incomplete_reasoning_history(