Why: Masks must not be the only copy of a key, and personal instructions, lore hits, and candidate replies have to reach the model without rewriting the system prefix or deleting history.
Evidence: go test ./internal/store ./internal/ctxasm ./internal/settings ./internal/runtime ./internal/server. Migration v16 upgrades and rolls back. Startup snapshots settings before moving raw keys.
Impact: One labeled key is active per service. Excluding a message hides its whole run from the model. Regenerating appends another candidate and rewinds files to the user turn. User slash commands expand to prompt text and do not fire on their own.
Refs: docs/records/2026-10-04-sillytavern-review.md
Why: workspace writes had no checkpoint, and a 5xx before any output stopped the turn instead of trying the next configured model.
Evidence: GOWORK=off go test ./internal/runtime/ ./internal/runmgr/ ./internal/tools/ ./internal/settings/ ./internal/server/ ./internal/app/
Impact: models.chat.fallbacks defaults to empty, so existing runs keep their model. Rewind restores only tracked writes and appends a system note. Usage records the model that served.
Refs: feature/durable-rewind
Co-authored-by: Cursor <cursoragent@cursor.com>