X_CLIENT_SPEC_INVALID
Means
x client was handed a document that is not OpenAPI 3.0 or 3.1
Typical cause
the file or URL given to --from is not JSON, or is JSON without an openapi field of 3.0.x or 3.1.x
Fix
x help client --json — then point --from at the OpenAPI 3.0 or 3.1 document itself