fix(anthropic): route usage-limit 400 through rate-limit detection

The helper change alone was not on the Anthropic exception path:
_map_anthropic_exception maps the invalid_request_error (400) usage-limit
response to BadRequestError before any rate-limit check runs. Call
is_error_str_rate_limit inside the Anthropic mapper, ahead of the 400
branches, so it surfaces as RateLimitError (429) as documented. Add an
end-to-end test driving exception_type for the reported payload.

litellm/litellm_core_utils/exception_mapping_utils.py

@@ -598,6 +598,17 @@ def _map_anthropic_exception(
             model=model,
             llm_provider="anthropic",
         )
+    # Anthropic returns usage/spend-limit exhaustion typed as invalid_request_error (400),
+    # but per the exception-mapping docs it should surface as a RateLimitError (429). Check
+    # before the generic 400 branches below, which would otherwise map it to BadRequestError.
+    if ExceptionCheckers.is_error_str_rate_limit(
+        error_str, getattr(original_exception, "status_code", None)
+    ):
+        raise RateLimitError(
+            message=f"AnthropicError - {error_str}",
+            model=model,
+            llm_provider="anthropic",
+        )
     if "Client error '400 Bad Request'" in error_str:
         raise BadRequestError(
             message=f"AnthropicError - {error_str}",

tests/unit/litellm_core_utils/test_exception_mapping_utils.py

@@ -218,6 +218,31 @@ class TestExceptionCheckers:
         )
         assert ExceptionCheckers.is_error_str_rate_limit(error_str, status_code=400) is True
 
+    def test_anthropic_usage_limit_maps_to_rate_limit_error_end_to_end(self):
+        """Production path: an Anthropic usage-limit error is typed invalid_request_error
+        (HTTP 400). Driving it through exception_type must surface a RateLimitError (429),
+        not a BadRequestError (#37599) — the helper alone is not on the Anthropic mapper.
+        """
+        message = (
+            "AnthropicException - "
+            '{"type":"error","error":{"type":"invalid_request_error",'
+            '"message":"You have reached your specified API usage limits. '
+            'You will regain access on 2026-09-01 at 00:00 UTC."},'
+            '"request_id":"req_011CeDfeyYs7N6neZrLwFxPJ"}'
+        )
+        original_exception = httpx.HTTPStatusError(
+            message=message,
+            request=httpx.Request("POST", "https://api.anthropic.com/v1/messages"),
+            response=httpx.Response(400),
+        )
+
+        with pytest.raises(litellm.RateLimitError):
+            exception_type(
+                model="claude-haiku-4-5-20251001",
+                original_exception=original_exception,
+                custom_llm_provider="anthropic",
+            )
+
     def test_is_azure_content_policy_violation_error_with_policy_violation_text(self):
         """Test detection of Azure content policy violation with explicit policy violation text"""