# GPT-5.1-Codex vs Grok 4.1 Fast

> Grok 4.1 Fast is the stronger model overall, scoring 40.9 to 38.2 on the Noometry Index.

- Canonical page: https://noometry.com/compare/gpt-5-1-codex-vs-grok-4-1-fast
- Last updated: 2026-10-10
- Shared benchmarks: 3

## Summary

- They share 3 benchmarks with published results for both. GPT-5.1-Codex scores higher in 2 categories and Grok 4.1 Fast in 1 category; 3 gaps are clear of the uncertainty.
- The widest gap is in coding, where GPT-5.1-Codex leads 41.7 to 33.8.
- The biggest single-benchmark swing is ProofBench: 9% for GPT-5.1-Codex and 4% for Grok 4.1 Fast.
- Grok 4.1 Fast is cheaper at $0.20 / $0.50 per million input/output tokens, against $1.25 / $10 for GPT-5.1-Codex.
- GPT-5.1-Codex accepts more context: 400K tokens versus 128K.

## Snapshot

| | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| Provider | OpenAI | xAI |
| Noometry Index | 38.2 | 40.9 |
| Rank | 178 | 131 |
| Context | 400K | 128K |
| Input $/M | $1.25 | $0.20 |
| Output $/M | $10 | $0.50 |
| Weights | Proprietary | Proprietary |

## Coding

- GPT-5.1-Codex: 41.7 (#107)
- Grok 4.1 Fast: 33.8 (#236)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| LMArena WebDev | 1337 | 1241 |
| ALE-Bench | 1,245 | 394.93 |
| SWE-bench Verified (bash only) | 66% | — |
| LMArena Coding | — | 1411 |

## Agentic & Tool Use

- GPT-5.1-Codex: 37.8 (#32)
- Grok 4.1 Fast: 36.1 (#35)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| Terminal-Bench | 60.4% | — |
| Berkeley Function Calling Leaderboard | — | 69.6% |
| τ²-bench Banking | — | 13.1% |
| LMArena Search | — | 1171 |
| METR Time Horizons | 70.8% | — |
| Vending-Bench 2 | — | 1,107 |

## Reasoning

- GPT-5.1-Codex: —
- Grok 4.1 Fast: 42.3 (#45)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| SimpleBench | — | 56% |
| NYT Connections (extended) | — | 87.4% |
| LMArena Hard Prompts | — | 1407 |
| DTBench | — | 87.7% |
| ForecastBench | — | 61 |

## Math

- GPT-5.1-Codex: 29.3 (#218)
- Grok 4.1 Fast: 30.9 (#201)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| ProofBench | 9% | 4% |
| MathArena Final-Answer Competitions | — | 60.9% |
| LMArena Math | — | 1408 |

## Knowledge

- GPT-5.1-Codex: —
- Grok 4.1 Fast: 32.6 (#203)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| Vectara Hallucination Rate | — | 17.8% |
| LMArena Expert | — | 1398 |

## Multimodal

- GPT-5.1-Codex: —
- Grok 4.1 Fast: 36.3 (#71)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| LMArena Vision | — | 1201 |

## Multilingual

- GPT-5.1-Codex: —
- Grok 4.1 Fast: 51.1 (#110)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| LMArena Non-English | — | 1391 |
| LMArena Chinese | — | 1441 |
| LMArena French | — | 1416 |
| LMArena German | — | 1404 |
| LMArena Japanese | — | 1347 |
| LMArena Korean | — | 1361 |
| LMArena Russian | — | 1387 |
| LMArena Spanish | — | 1411 |

## Instruction Following

- GPT-5.1-Codex: —
- Grok 4.1 Fast: 72.8 (#127)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| LMArena Instruction Following | — | 1376 |

## Long Context

- GPT-5.1-Codex: —
- Grok 4.1 Fast: 42.4 (#121)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| LMArena Longer Query | — | 1390 |

## Writing & Preference

- GPT-5.1-Codex: —
- Grok 4.1 Fast: 57.4 (#126)

| Benchmark | GPT-5.1-Codex | Grok 4.1 Fast |
|---|---|---|
| LMArena Text | — | 1408 |
| LMArena Creative Writing | — | 1394 |
| EQ-Bench Creative Writing | — | 1327 |
| LMArena Multi-Turn | — | 1389 |

## FAQ

### Is GPT-5.1-Codex better than Grok 4.1 Fast?

Grok 4.1 Fast is the stronger model overall, scoring 40.9 to 38.2 on the Noometry Index.

### Which is cheaper, GPT-5.1-Codex or Grok 4.1 Fast?

Grok 4.1 Fast is cheaper. It lists at $0.20 per million input tokens and $0.50 per million output tokens; GPT-5.1-Codex lists at $1.25 and $10.

### Is GPT-5.1-Codex or Grok 4.1 Fast better for coding?

GPT-5.1-Codex scores higher on coding benchmarks: 41.7 versus 33.8 in the Noometry coding category.

### Which has the bigger context window?

GPT-5.1-Codex does, with 400K tokens against 128K.

### How many benchmarks do GPT-5.1-Codex and Grok 4.1 Fast share?

3 benchmarks have published results for both models. GPT-5.1-Codex has 6 scored results on Noometry and Grok 4.1 Fast has 32.