{"schema":"witness.a2a.evidence-extension@1","uri":"https://chat.witnessstandard.com/a2a/extensions/witness-evidence/v1","a2aProtocolVersion":"1.0","status":"ACTIVE","purpose":"Allow an A2A client to request an existing governed WITNESS capability and receive the exact application result as structured artifact data.","activation":{"header":"A2A-Extensions","messageExtensions":true,"requirement":"The client must request https://chat.witnessstandard.com/a2a/extensions/witness-evidence/v1 before extension metadata is interpreted or returned."},"requestMetadata":{"location":"message.metadata[\"https://chat.witnessstandard.com/a2a/extensions/witness-evidence/v1\"]","schema":{"type":"object","required":["operation"],"properties":{"operation":{"enum":["witness.resolve_subject","witness.ask","witness.get_subject","witness.get_claim","witness.get_evaluation","witness.get_provenance","witness.start_investigation","witness.get_investigation","witness.watch.create","witness.get_record","witness.get_record_history","witness.verify_record"]},"arguments":{"type":"object"},"idempotencyKey":{"type":"string","minLength":8,"maxLength":200}}}},"responseMetadata":{"location":"task.metadata[\"https://chat.witnessstandard.com/a2a/extensions/witness-evidence/v1\"] and artifact.metadata[\"https://chat.witnessstandard.com/a2a/extensions/witness-evidence/v1\"]","fields":["operation","epistemicPolicy","claim_id","evaluation_id","evaluation_state","record_id","evidence_refs","competing_explanations","contradictions","unknowns","source_hashes","signatures","anchor_receipts","as_of","policy_version"],"note":"Fields are copied only when present in the governed application result. The transport never fabricates absent evidence metadata or relabels alternatives as contradictions."},"operations":["witness.resolve_subject","witness.ask","witness.get_subject","witness.get_claim","witness.get_evaluation","witness.get_provenance","witness.start_investigation","witness.get_investigation","witness.watch.create","witness.get_record","witness.get_record_history","witness.verify_record"],"invariants":["UNKNOWN is a valid result.","Model output cannot directly create ESTABLISHED truth.","A2A is a transport projection; it does not own evidence, Evaluation, or Record semantics.","No hidden chain-of-thought or internal scratchpad is returned."]}