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Most application errors return FastAPI-style JSON with a top-level detail field:
Validation errors use a structured detail array:
Usage-limit responses on billable market endpoints add extra fields:

Status Codes

Error Examples

401 Unauthorized

Returned when the API key is missing or invalid.

400 Bad Request

Returned when required parameters are missing or malformed.

422 Validation Error

Returned when FastAPI rejects the request before endpoint logic runs.

429 Too Many Requests

Returned when the current plan’s monthly market-endpoint allowance has been exhausted.

500 Internal Server Error

Returned when an unexpected server-side error occurs. If this persists, contact founders@entityml.com.