# GPT-5.1-Codex vs Step 3.5 Flash

> Step 3.5 Flash is the stronger model overall, scoring 41.7 to 38.2 on the Noometry Index.

- Canonical page: https://noometry.com/compare/gpt-5-1-codex-vs-step-3-5-flash
- Last updated: 2026-10-10
- Shared benchmarks: 0

## Summary

- The widest gap is in math, where Step 3.5 Flash leads 40.6 to 29.3.
- Step 3.5 Flash is cheaper at $0.10 / $0.30 per million input/output tokens, against $1.25 / $10 for GPT-5.1-Codex.
- GPT-5.1-Codex accepts more context: 400K tokens versus 256K.
- Step 3.5 Flash has downloadable open weights; the other is API-only.

## Snapshot

| | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| Provider | OpenAI | StepFun |
| Noometry Index | 38.2 | 41.7 |
| Rank | 178 | 116 |
| Context | 400K | 256K |
| Input $/M | $1.25 | $0.10 |
| Output $/M | $10 | $0.30 |
| Weights | Proprietary | Open |

## Coding

- GPT-5.1-Codex: 41.7 (#107)
- Step 3.5 Flash: 41.7 (#106)

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| SWE-bench Verified (bash only) | 66% | — |
| LMArena WebDev | 1337 | — |
| LMArena Coding | — | 1436 |
| ALE-Bench | 1,245 | — |

## Agentic & Tool Use

- GPT-5.1-Codex: 37.8 (#32)
- Step 3.5 Flash: —

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| Terminal-Bench | 60.4% | — |
| METR Time Horizons | 70.8% | — |

## Reasoning

- GPT-5.1-Codex: —
- Step 3.5 Flash: 21.2 (#202)

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| NYT Connections (extended) | — | 28.4% |
| LMArena Hard Prompts | — | 1411 |

## Math

- GPT-5.1-Codex: 29.3 (#218)
- Step 3.5 Flash: 40.6 (#77)

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| MathArena Final-Answer Competitions | — | 66.8% |
| ProofBench | 9% | — |
| LMArena Math | — | 1408 |

## Knowledge

- GPT-5.1-Codex: —
- Step 3.5 Flash: 39.0 (#127)

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| LMArena Expert | — | 1422 |

## Multilingual

- GPT-5.1-Codex: —
- Step 3.5 Flash: 50.7 (#115)

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| LMArena Non-English | — | 1386 |
| LMArena Chinese | — | 1448 |
| LMArena French | — | 1421 |
| LMArena German | — | 1404 |
| LMArena Japanese | — | 1353 |
| LMArena Korean | — | 1350 |
| LMArena Russian | — | 1385 |
| LMArena Spanish | — | 1419 |

## Instruction Following

- GPT-5.1-Codex: —
- Step 3.5 Flash: 73.3 (#119)

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| LMArena Instruction Following | — | 1385 |

## Long Context

- GPT-5.1-Codex: —
- Step 3.5 Flash: 42.8 (#112)

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| LMArena Longer Query | — | 1402 |

## Writing & Preference

- GPT-5.1-Codex: —
- Step 3.5 Flash: 59.0 (#108)

| Benchmark | GPT-5.1-Codex | Step 3.5 Flash |
|---|---|---|
| LMArena Text | — | 1403 |
| LMArena Creative Writing | — | 1357 |
| LMArena Multi-Turn | — | 1405 |

## FAQ

### Is GPT-5.1-Codex better than Step 3.5 Flash?

Step 3.5 Flash is the stronger model overall, scoring 41.7 to 38.2 on the Noometry Index.

### Which is cheaper, GPT-5.1-Codex or Step 3.5 Flash?

Step 3.5 Flash is cheaper. It lists at $0.10 per million input tokens and $0.30 per million output tokens; GPT-5.1-Codex lists at $1.25 and $10.

### Is GPT-5.1-Codex or Step 3.5 Flash better for coding?

They score almost the same on coding (41.7 vs 41.7); test both on your own repository before choosing.

### Which has the bigger context window?

GPT-5.1-Codex does, with 400K tokens against 256K.

### How many benchmarks do GPT-5.1-Codex and Step 3.5 Flash share?

0 benchmarks have published results for both models. GPT-5.1-Codex has 6 scored results on Noometry and Step 3.5 Flash has 19.