When the user turns Fast mode on, Claude Code can report
`fast_mode_state: "off"` on the next turn's `system`/init. The adapter
mirrored that into the option but explained it only for reasons listed
in `FAST_MODE_UNAVAILABLE_EXPLANATIONS`. `preference`, `unknown` and any
newer code were dropped, so the toggle snapped off with no reason. This
happens on every turn behind an API gateway that authenticates with
`ANTHROPIC_AUTH_TOKEN` only: Claude Code treats Fast mode as disabled by
the organization (`preference`) without asking the server.
Changes:
- `preference` ("not enabled for your organization or API gateway") and
`unknown` ("currently unavailable") are explained. A reason code the SDK
adds later gets a generic `not available (<code>)`.
- Only `sdk_opt_in_required` (every SDK session starts there; the toggle
is the opt-in) and `pending` (eligibility still resolving) stay
unexplained.
- The notice rule is unchanged: Fast mode was on, then off with an
explained reason. Off without a reason stays quiet, because a `result`
can carry the state from the start of the turn.
- One shared function in `session-fast-mode.ts` serves v1 and v2. In
`src/v1/acp-agent.ts` only comments change.
Limitation: the adapter still offers the toggle up front. Until a
session opts in, Claude Code reports `sdk_opt_in_required` and hides the
account or organization blocker, so the adapter can't tell in advance
that Fast mode will be refused.
Tests: unit tests for each reason, plus v1 and v2 flow tests (set `fast`
on, prompt, init reports off/`preference`). The flow tests fail without
the fix.
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