By default, Schemathesis is strict on Open API spec interpretation, but other 3rd-party tools often are more flexible and not always comply with the spec.

Using FastAPI

FastAPI uses pydantic for JSON Schema generation, and it produces Draft 7 compatible schemas. But Open API 2 / 3.0.x use earlier versions of JSON Schema (Draft 4 and Wright Draft 00 respectively), which leads to incompatibilities when Schemathesis parses input schema.

It is a known issue on the FastAPI side, and Schemathesis provides a way to handle such schemas. The idea is to convert Draft 7 keywords syntax to Draft 4.

To use it, you need to add this code before you load your schema with Schemathesis:

import schemathesis

# will install all available compatibility fixups.
# You can provide a list of fixup names as the first argument
# schemathesis.fixups.install(["fast_api"])

If you use the Command Line Interface, then you can utilize the --fixups=all option.


Some web servers may prefix JSON with UTF-8 Byte Order Mark - \ufeff, which is explicitly forbidden by RFC 7159 (JSON). Enabling the utf8_bom fixup allows Schemathesis to process such responses by removing this mark from responses.