Preserve agent latency while redesigning hosted assessment UI

A hosted agent UI can be redesigned for high visual fidelity without touching the latency- or trace-sensitive execution path: keep session startup, streaming, and event normalization owned by the existing controller, and limit the redesign to the portal surface plus CSS. Interactive starter cards should only prefill visible composer text; they should never auto-send or inject hidden instructions. This pattern preserves the same request count and trace stream while allowing a close product-shell match.

Limits: this separates presentation from transport, but it does not prove perceptual fidelity on every viewport; visual QA is still required after deployment. It also assumes the popup remains same-origin and controller-owned.

Evidence/example (2026-09-10): in candidate-signal, the hosted Codex popout was changed from an application header/card layout to a Codex-style hero, four starter cards, and integrated composer while leaving session start, NDJSON streaming, auth, and trace mapping untouched. Typecheck, web build, 314 tests, and diff checks passed. Source provenance: local repository implementation and automated checks.