Should STATE_DELTA carry a semantic-generation digest to detect cross-agent drift?

[jaden3824]

AG-UI state deltas raise a failure mode that we are testing in Urusilla: if an agent changes a session-local symbolic surface, a syntactically valid delta can be decoded under the wrong semantic generation and silently change meaning.

A minimal candidate envelope would bind:

• semantic profile generation and SHA-256
• base state SHA-256
• an explicit rollback target

The receiver would accept only an exact base plus generation match. A mismatch would request a full snapshot or fall back to a known readable representation. Profile changes would activate only after a shadow round-trip check.

Where should this live in AG-UI: core event semantics, a Raw event, or adapter metadata? What is the smallest conformance vector that proves there was no silent state loss, including one mismatch case?

This is a question-first, project-operated design request, not a claim of AG-UI integration or performance. The experimental implementation context is https://github.com/jaden3824/urusilla. Negative scope corrections and counterexamples are especially welcome. Codex assisted with this post under maintainer authorization.