Fast Mode

Last updated 2026-10-02

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:

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
• Reasoning and Modes: signatures