/fast now asks the same question the next turn asks: resolve_fast_mode_overrides
over the route _resolve_session_agent_runtime returns (session /model override,
channel override, persisted override after a restart), with provider and
base_url. Both the Priority gate and the Ultrafast gate added in #128307 read
that one route, so GPT-6 Astra picked with session /model on openai-codex gets
/fast and /fast ultrafast, and a fast-capable default no longer admits a
session route whose requests never carry the tier (gpt-5.4 via OpenRouter).
The tier override is also stored under the topic-recovered session key, the
one the next turn reads (#30479 class; /model and /reasoning already do this),
so on Telegram DM topics a session /fast is no longer dropped.
Route resolution runs off the event loop (it can refresh an OAuth token, as
the turn does); with no usable route at all the configured model is judged
alone, as before.
Tests: the four salvaged cases collapse into two invariants on the real
fast-mode tables (both gate directions + the turn's request_overrides, and
the recovered-topic key); red on origin/main.
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