agent: surface _llm_extract failure reason in visit tool_response

When the visit extractor returns None, attach `extractor_error` so the
trace JSON tells us whether the env vars are missing, the OpenAI client
errored, or the model rejected the call. Diagnostic only — strips out
once we are confident the SUMMARY model is wired up.

app.py

@@ -1610,11 +1610,24 @@ def _jina_readpage(url: str) -> Optional[str]:
     return None
 
 
+# Last error from the most recent _llm_extract call (for diagnostics in trace).
+_LAST_EXTRACT_ERR: Optional[str] = None
+
+
 def _llm_extract(webpage_content: str, goal: str) -> Optional[str]:
     """Run the SUMMARY model as the visit extractor. Mirrors
     inference/prompt.py:build_visit_extractor_messages + tool_visit's call."""
+    global _LAST_EXTRACT_ERR
+    _LAST_EXTRACT_ERR = None
+    if not SUMMARY_MODEL_NAME:
+        _LAST_EXTRACT_ERR = "SUMMARY_MODEL_NAME env var not set"
+        return None
+    if not SUMMARY_API_KEY:
+        _LAST_EXTRACT_ERR = "API_KEY / SUMMARY_OPENAI_API_KEY env var not set"
+        return None
     client = _get_openai_client(SUMMARY_API_KEY, SUMMARY_API_BASE)
-    if client is None or not SUMMARY_MODEL_NAME:
+    if client is None:
+        _LAST_EXTRACT_ERR = "openai client could not be constructed (package missing?)"
         return None
     try:
         resp = client.chat.completions.create(
@@ -1630,7 +1643,8 @@ def _llm_extract(webpage_content: str, goal: str) -> Optional[str]:
             timeout=120,
         )
         return (resp.choices[0].message.content or "").strip() or None
-    except Exception:
+    except Exception as exc:
+        _LAST_EXTRACT_ERR = f"{type(exc).__name__}: {str(exc)[:300]}"
         return None
 
 
@@ -1729,6 +1743,8 @@ def _run_visit_single(url: str, max_chars: int, goal: str = "") -> Dict[str, Any
             "content": (extract or jina_md)[:max_chars],
             "extractor": "llm" if extract else "jina-raw",
         }
+        if not extract and _LAST_EXTRACT_ERR:
+            result["extractor_error"] = _LAST_EXTRACT_ERR
         VISIT_CACHE[cache_key] = result
         return result