Native V4 admission and Session adoption reject every request/header.header.system field because system prompts belong to system/message events. EpochHeader omitted that key, so the generated schema classified adding optional system text as a same-version optional-property addition even though released readers would reject it.
Declare system?: never with the shared persistenceReserved marker so the schema records the existing prohibition. Widening the field to string now changes a forbidden value set and requires a version bump. Keep both runtime guards and all unknown-metadata handling unchanged, and document the reservation alongside the current header type and V4 acknowledgement.
The regression extracts the actual event:request/header schema, checks the optional-never field, widens only that property, and verifies the version-bump diagnostic and native refusal. It fails without marker extraction and passes with the shared tooling support. The focused reservation, retired-syntax, and Session surface suites pass 65 tests; all 436 type-equiv blocks match their source. Generic marker extraction belongs to the foundational tooling change.
Yixiang Tu