kwargs.setdefault("metadata", {}).update(
    {"model_group": kwargs.get("model", None)}
)  # update model_group used, if fallbacks are done

---

model_list:
  - model_name: gpt-4.1
    litellm_params:
      model: openai/gpt-4.1
      api_base: https://<azure-v1-compat-host>/openai/v1/
      api_key: <primary-key>
  - model_name: gpt-4.1-paid
    litellm_params:
      model: openai/gpt-4.1
      api_base: https://<different-azure-v1-compat-host>/openai/v1/
      api_key: <fallback-key>

router_settings:
  fallbacks:
    - gpt-4.1: [gpt-4.1-paid]
  num_retries: 2
  enable_pre_call_checks: true

---

curl -sS -o /tmp/direct.json -w "STATUS=%{http_code}\n" \
  -H "Authorization: Bearer $LITELLM_KEY" \
  -H "Content-Type: application/json" \
  "$PROXY_URL/v1/responses/compact" \
  -d '{"model":"gpt-4.1-paid","input":[{"role":"user","content":"hi"},{"role":"assistant","content":"hello"},{"role":"user","content":"thanks"},{"role":"assistant","content":"sure"},{"role":"user","content":"bye"},{"role":"assistant","content":"bye"}]}'
# STATUS=200

---

curl -sS -o /tmp/resp.json -w "STATUS=%{http_code}\n" \
  -H "Authorization: Bearer $LITELLM_KEY" \
  -H "Content-Type: application/json" \
  "$PROXY_URL/v1/responses" \
  -d '{"model":"gpt-4.1","input":"Reply with exactly OK","mock_testing_fallbacks":true}'
# STATUS=200, x-litellm-model-api-base points at fallback host

---

curl -sS -o /tmp/compact.json -w "STATUS=%{http_code}\n" \
  -H "Authorization: Bearer $LITELLM_KEY" \
  -H "Content-Type: application/json" \
  "$PROXY_URL/v1/responses/compact" \
  -d '{"model":"gpt-4.1","input":[{"role":"user","content":"hi"},{"role":"assistant","content":"hello"},{"role":"user","content":"thanks"},{"role":"assistant","content":"sure"},{"role":"user","content":"bye"},{"role":"assistant","content":"bye"}],"mock_testing_fallbacks":true}'
# STATUS=500

---

litellm.InternalServerError: This is a mock exception for model=gpt-4.1, to trigger a fallback.
Fallbacks=[{'gpt-4.1': ['gpt-4.1-paid']}].
Received Model Group=gpt-4.1
Available Model Group Fallbacks=['gpt-4.1-paid']
Error doing the fallback: litellm.BadRequestError: OpenAIException - {
  "error": {
    "message": "Unknown parameter: 'metadata'.",
    "type": "invalid_request_error",
    "param": "metadata",
    "code": "unknown_parameter"
  }
}

---

# current (litellm/router_utils/fallback_event_handlers.py:133)
kwargs.setdefault("metadata", {}).update(
    {"model_group": kwargs.get("model", None)}
)

# fixed: pick the right key based on the active responses-API route
from litellm.proxy.litellm_pre_call_utils import LITELLM_METADATA_ROUTES
_metadata_key = (
    "litellm_metadata"
    if kwargs.get("original_function") is not None
       and kwargs["original_function"].__name__ in (
           "acompact_responses",
           "aresponses",
           "aget_responses",
           "adelete_responses",
           "acancel_responses",
           "alist_input_items",
       )
    else "metadata"
)
kwargs.setdefault(_metadata_key, {}).update(
    {"model_group": kwargs.get("model", None)}
)