Grok 4 Fast Reasoning
SpaceXSI
Key facts · verified Oct 29, 2026
Grok 4 Fast Reasoning is a SpaceXSI model. It was retired on 15 May 2026. Its last listed API price was $0.20 per million input tokens and $0.50 per million output tokens. Its context window is 2M tokens. It ranks #159 on LMArena's text leaderboard (Elo 1405, read 1 Nov 2026). Source: docs.x.ai.
Grok 4 Fast Reasoning is a SpaceXSI model that was available on the xAI API as grok-4-fast-reasoning until 15 May 2026, when xAI retired it and began redirecting requests to Grok 4.3 at low reasoning effort. It was listed at $0.20 per million input tokens and $0.50 per million output tokens, with a 2M-token context window. On LMArena's text leaderboard: 1405 Elo (rank 159, reasoning effort).
Pricing & specs checkedSource: vendor documentation · checked Oct 29, 2026LMArena: 1405 Elo (rank 159, reasoning effort) · source · retrieved Nov 1, 2026
Pricing
$0.20 / $0.50 per million tokens
Context
2M tokens
LMArena Elo
LMArena rank
Full Specifications
$0.20 / $0.50 per million tokens | |
|---|---|
| Identity | |
| Developer | SpaceXSI |
| Retired | 15 May 2026 |
| Status | Retired. xAI retired grok-4-fast-reasoning on 15 May 2026 at 12:00 PM PT; requests redirect to grok-4.3 (low reasoning effort) and are billed at grok-4.3 rates. |
| Licence | Proprietary |
| Self-hostable | No |
| Cost | |
| Blended $/1M tokensinput × 0.75 + output × 0.25 | $0.275 / 1M tokens |
| Input price | $0.200 / 1M tokens |
| Output price | $0.500 / 1M tokens |
| Capacity | |
| Context window | 2M tokens |
| Measured quality | |
| LMArena Elo | 1405 (checked Nov 2026) |
| LMArena rank | Rank 159 |
| Elo per dollarLMArena Elo ÷ blended $/1M tokens | 5109 |