01
One schema, four engines
The request schema in the conformance suite is the source of truth. An engine either implements a field or rejects it with unsupported_feature. Reading a request and answering confidently without honouring it is the one behaviour the contract forbids.
02
What a capability requires
A field counts as a capability only when it is in the schema, implemented or explicitly rejected by every engine, exercised by a named fixture, listed in the public field matrix, recomputed by the independent validator and documented. Anything short of that is a prototype.
03
Refusals are structured
Every unpacked item carries a reason, human-readable details and a proof whose level is proven, observed, inferred or unknown_due_to_limit. A refusal that cannot be explained is treated as a defect.