# Fast Mode

How `ModeFast` maps onto each vendor's priority tier and how a silent downgrade is surfaced.

## Checking support

Support depends on the model generation, so check before sending:

```go
mode := llmrouter.ModeDefault
if llmrouter.SupportFast("claude", "claude-opus-5") {
	mode = llmrouter.ModeFast
}
```

## Downgrade warning

When the served tier is neither `fast` nor `priority`, `WarnFastDowngrade` logs a warning — a request billed as fast but downgraded silently would leave both cost and latency unaccounted for.

## Related

- [Reasoning Levels](/core-concepts-reasoning)
- [Reasoning and Modes](/api-reference-reasoning): signatures