Wires `speed: "fast"` and the `fast-mode-2026-02-01` beta into the
Anthropic provider, plumbs a matching `speed` option through
`SimpleStreamOptions` and the Agent, and teaches `/fast` to dispatch
on the active model's API (Anthropic -> speed=fast, OpenAI -> existing
serviceTier=priority path). Server is the authority on which models
support fast mode.
When the server rejects an unsupported model, the provider mirrors the
strict-tools fallback: drops the field, retries the same turn
transparently, persists the disable via `providerSessionState`, and
surfaces the action through the new `AssistantMessage.disabledFeatures`
marker so the session can sync the toggle off and warn the user.
Lucas Szwarcberg
d0efcea5ad