- Introduce 2.1.258 dynamic betas: thinking-display-updates-2026-08-18, server-side-fallback-2026-06-01, and fallback-credit-2026-03-24. - Prune effort-2025-11-24 on Haiku models, probes, and when thinking is disabled. - Prune thinking-display-updates on disabled thinking and probe/helper turns. - Identify Fable 5.1 / Mythos 5.1 models with boundary checks and inject default fallbacks and adaptive thinking display. - Strictly pair 1h cache control with extended-cache-ttl-2025-04-11, stripping TTL on probes and subagents.