Oasdiff expects valid OpenAPI 3 specs as input.
The specs can be written in JSON or YAML.
Oasdiff may return an error when given invalid specs, for example:

Error: failed to load base spec from "spec.yaml": error converting YAML to JSON: yaml: line 2: mapping values are not allowed in this context

The reason for this error is that the underlying library, kin-openapi, converts YAML specs to JSON before parsing them.

When a spec fails to load, oasdiff exits with code 102 (101 for invalid flags, 103 when a glob matches no specs).

diff, breaking, changelog, and summary fail only on specs they can't load or parse (the error above). They're otherwise lenient: a spec that loads but violates the OpenAPI or JSON Schema rules is still diffed.

To check a single spec for compliance (invalid types, missing required fields, bad regex, unresolved $refs, and similar), use oasdiff validate. It reports each violation with a stable rule ID, a severity, and a source location, and exits 102 when the spec can't be loaded at all.