diff --git a/nanobot/providers/base.py b/nanobot/providers/base.py
index d95180c42..062dbbd93 100644
--- a/nanobot/providers/base.py
+++ b/nanobot/providers/base.py
@@ -987,14 +987,16 @@ class LLMProvider(ABC):
         except (TypeError, ValueError):
             error_status_code = None
 
+        raw_error_type = getattr(exc, "error_type", None)
+        raw_error_code = getattr(exc, "error_code", None)
         detail = str(exc).strip() or type(exc).__name__
         return LLMResponse(
             content=f"Error calling LLM: {detail}",
             finish_reason="error",
             error_status_code=error_status_code,
             error_kind=error_kind,
-            error_type=getattr(exc, "error_type", None),
-            error_code=getattr(exc, "error_code", None),
+            error_type=str(raw_error_type) if raw_error_type is not None else None,
+            error_code=str(raw_error_code) if raw_error_code is not None else None,
             error_should_retry=error_should_retry,
         )
 
diff --git a/tests/agent/test_runner_fallback.py b/tests/agent/test_runner_fallback.py
index 15231a244..5cff85679 100644
--- a/tests/agent/test_runner_fallback.py
+++ b/tests/agent/test_runner_fallback.py
@@ -460,6 +460,28 @@ class TestFallbackWhenPrimaryRaises:
         assert exception is primary._exc
         assert response.error_kind == "authentication"
 
+    @pytest.mark.asyncio
+    async def test_non_string_exception_metadata_is_normalized_before_fallback(self) -> None:
+        """Provider exception metadata must be string-like before fallback consumes it."""
+
+        class NumericMetadataError(Exception):
+            error_type = 429
+            error_code = 429
+            status_code = 429
+
+        primary = _RaisingProvider("primary", NumericMetadataError("rate limited"))
+
+        response, exception = await FallbackProvider._call_provider(
+            lambda provider, kwargs: provider.chat(**kwargs),
+            primary,
+            {},
+        )
+
+        assert exception is primary._exc
+        assert response.error_type == "429"
+        assert response.error_code == "429"
+        assert FallbackProvider._should_fallback(response) is True
+
     @pytest.mark.parametrize(
         "exc",
         [