diff --git a/agent/error_classifier.py b/agent/error_classifier.py
index fa6a985041..fcdb8ba676 100644
--- a/agent/error_classifier.py
+++ b/agent/error_classifier.py
@@ -290,7 +290,7 @@ def classify_api_error(
     if isinstance(body, dict):
         _err_obj = body.get("error", {})
         if isinstance(_err_obj, dict):
-            _body_msg = (_err_obj.get("message") or "").lower()
+            _body_msg = str(_err_obj.get("message") or "").lower()
             # Parse metadata.raw for wrapped provider errors
             _metadata = _err_obj.get("metadata", {})
             if isinstance(_metadata, dict):
@@ -302,11 +302,11 @@ def classify_api_error(
                         if isinstance(_inner, dict):
                             _inner_err = _inner.get("error", {})
                             if isinstance(_inner_err, dict):
-                                _metadata_msg = (_inner_err.get("message") or "").lower()
+                                _metadata_msg = str(_inner_err.get("message") or "").lower()
                     except (json.JSONDecodeError, TypeError):
                         pass
         if not _body_msg:
-            _body_msg = (body.get("message") or "").lower()
+            _body_msg = str(body.get("message") or "").lower()
     # Combine all message sources for pattern matching
     parts = [_raw_msg]
     if _body_msg and _body_msg not in _raw_msg:
@@ -606,10 +606,10 @@ def _classify_400(
     if isinstance(body, dict):
         err_obj = body.get("error", {})
         if isinstance(err_obj, dict):
-            err_body_msg = (err_obj.get("message") or "").strip().lower()
+            err_body_msg = str(err_obj.get("message") or "").strip().lower()
         # Responses API (and some providers) use flat body: {"message": "..."}
         if not err_body_msg:
-            err_body_msg = (body.get("message") or "").strip().lower()
+            err_body_msg = str(body.get("message") or "").strip().lower()
     is_generic = len(err_body_msg) < 30 or err_body_msg in ("error", "")
     is_large = approx_tokens > context_length * 0.4 or approx_tokens > 80000 or num_messages > 80
 
diff --git a/tests/agent/test_error_classifier.py b/tests/agent/test_error_classifier.py
index 766c5475f8..dd74249b14 100644
--- a/tests/agent/test_error_classifier.py
+++ b/tests/agent/test_error_classifier.py
@@ -849,3 +849,73 @@ class TestAdversarialEdgeCases:
         )
         result = classify_api_error(e, provider="openrouter")
         assert result.reason == FailoverReason.model_not_found
+
+    # ── Regression: dict-typed message field (Issue #11233) ──
+
+    def test_pydantic_dict_message_no_crash(self):
+        """Pydantic validation errors return message as dict, not string.
+
+        Regression: classify_api_error must not crash when body['message']
+        is a dict (e.g. {"detail": [...]} from FastAPI/Pydantic). The
+        'or ""' fallback only handles None/falsy values — a non-empty
+        dict is truthy and passed to .lower(), causing AttributeError.
+        """
+        e = MockAPIError(
+            "Unprocessable Entity",
+            status_code=422,
+            body={
+                "object": "error",
+                "message": {
+                    "detail": [
+                        {
+                            "type": "extra_forbidden",
+                            "loc": ["body", "think"],
+                            "msg": "Extra inputs are not permitted",
+                        }
+                    ]
+                },
+            },
+        )
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.format_error
+        assert result.status_code == 422
+        assert result.retryable is False
+
+    def test_nested_error_dict_message_no_crash(self):
+        """Nested body['error']['message'] as dict must not crash.
+
+        Some providers wrap Pydantic errors in an 'error' object.
+        """
+        e = MockAPIError(
+            "Validation error",
+            status_code=400,
+            body={
+                "error": {
+                    "message": {
+                        "detail": [
+                            {"type": "missing", "loc": ["body", "required"]}
+                        ]
+                    }
+                }
+            },
+        )
+        result = classify_api_error(e, approx_tokens=1000)
+        assert result.reason == FailoverReason.format_error
+        assert result.status_code == 400
+
+    def test_metadata_raw_dict_message_no_crash(self):
+        """OpenRouter metadata.raw with dict message must not crash."""
+        e = MockAPIError(
+            "Provider error",
+            status_code=400,
+            body={
+                "error": {
+                    "message": "Provider error",
+                    "metadata": {
+                        "raw": '{"error":{"message":{"detail":[{"type":"invalid"}]}}}'
+                    }
+                }
+            },
+        )
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.format_error