# Reasoning Levels and Fast Mode

The shared reasoning scale, how each session sets its level, and the fast-mode toggle.

## Reasoning levels

`go-llm-router` normalizes reasoning onto one scale — `none`, `low`, `medium` (default), `high`, `xhigh`, `max` — and maps it per provider (Claude thinking budgets, Gemini thinking budgets, OpenAI effort, ...). Aliases `minimal`, `extra`, and `ultra` map to `low`, `xhigh`, and `max`. Levels outside a model's supported range are clamped rather than rejected.

The level is passed explicitly through each `Send` call; there is no global reasoning setting in `config.json`. Each session has its own level — `auto` (the default) or a fixed one — set with `/model` → `reasoning` or cycled with `Shift+A` / `Shift+D` in the TUI, `auto` included. With `auto`, the level comes from the selector's work kind (the LLM dispatcher or TypeSafe, whichever runs), falling back to `medium` when it returned none; this also applies to a session pinned to a model. A level passed with the request (for example `reasoning_effort`) wins over both.

**Auto reasoning** (`auto_reasoning`, v1.0.18 to v1.0.27) was a global TypeSafe-only toggle; it was removed in v1.1.0 and replaced by the per-session `auto` level above. The `auto_reasoning` field of `POST /v1/model` and `GET /v1/session/:id` is gone.

## Fast mode

`Shift+F` toggles fast mode, which passes `provider.ModeFast` through the router so supported backends request a faster service tier. Support is model-specific (`core.SupportFast`) — for example recent OpenAI generations, Claude Opus 4.8 / Opus 5, most Grok models, and selected Gemini families. Unsupported models silently fall back to the default tier. Fast mode is process-local and not persisted.