diff --git a/nanobot/providers/custom_provider.py b/nanobot/providers/custom_provider.py
index 35c5e71..3daa0cc 100644
--- a/nanobot/providers/custom_provider.py
+++ b/nanobot/providers/custom_provider.py
@@ -51,12 +51,12 @@ class CustomProvider(LLMProvider):
         try:
             return self._parse(await self._client.chat.completions.create(**kwargs))
         except Exception as e:
-            # Extract raw response body from non-JSON API errors.
-            # JSONDecodeError.doc contains the original text (e.g. "unsupported model: xxx");
-            # OpenAI APIError may carry it in response.text.
+            # JSONDecodeError.doc / APIError.response.text may carry the raw body
+            # (e.g. "unsupported model: xxx") which is far more useful than the
+            # generic "Expecting value …" message.  Truncate to avoid huge HTML pages.
             body = getattr(e, "doc", None) or getattr(getattr(e, "response", None), "text", None)
             if body and body.strip():
-                return LLMResponse(content=f"Error: {body.strip()}", finish_reason="error")
+                return LLMResponse(content=f"Error: {body.strip()[:500]}", finish_reason="error")
             return LLMResponse(content=f"Error: {e}", finish_reason="error")
 
     def _parse(self, response: Any) -> LLMResponse: