HTTP 400
invalid_filter_valueInvalid filter value
A category, language, or level does not belong to the current canonical vocabulary.
Resolution
Use GET /api/v1/filter-options or the MCP get_filter_options tool.
Response handling
Keep the response's request_id for diagnostics. Retry only transient 429 and 503 responses, using the rate-limit headers and exponential backoff with jitter. Correct 400, 401, and 403 requests before retrying.