# GPT-5.1-Codex vs gpt-oss-120b

> GPT-5.1-Codex is the stronger model overall, scoring 38.6 to 36.3 on the Noometry Index. gpt-oss-120b costs 49× less per token, which makes it the better buy when GPT-5.1-Codex's lead doesn't matter for your workload.

- Canonical page: https://noometry.com/compare/gpt-5-1-codex-vs-gpt-oss-120b
- Last updated: 2026-10-12
- Shared benchmarks: 4

## Summary

- They share 4 benchmarks with published results for both. GPT-5.1-Codex scores higher in 2 categories and gpt-oss-120b in 1 category; 3 gaps are clear of the uncertainty.
- The widest gap is in agentic & tool use, where GPT-5.1-Codex leads 38.0 to 12.2.
- The biggest single-benchmark swing is Terminal-Bench: 60.4% for GPT-5.1-Codex and 18.7% for gpt-oss-120b.
- gpt-oss-120b is cheaper at $0.037 / $0.17 per million input/output tokens, against $1.25 / $10 for GPT-5.1-Codex.
- GPT-5.1-Codex accepts more context: 400K tokens versus 131K.
- gpt-oss-120b has downloadable open weights; the other is API-only.

## Snapshot

| | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| Provider | OpenAI | OpenAI |
| Noometry Index | 38.6 | 36.3 |
| Rank | 186 | 217 |
| Context | 400K | 131K |
| Input $/M | $1.25 | $0.037 |
| Output $/M | $10 | $0.17 |
| Weights | Proprietary | Open |

## Coding

- GPT-5.1-Codex: 41.9 (#116)
- gpt-oss-120b: 33.5 (#256)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| SWE-bench Verified (bash only) | 66% | 26% |
| ALE-Bench | 1,245 | 575.62 |
| Aider Polyglot | — | 41.8% |
| LMArena WebDev | 1337 | — |
| SciCode | — | 36% |
| WeirdML | — | 48.2% |
| LMArena Coding | — | 1380 |
| AlgoTune | — | 1.41 |

## Agentic & Tool Use

- GPT-5.1-Codex: 38.0 (#33)
- gpt-oss-120b: 12.2 (#153)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| Terminal-Bench | 60.4% | 18.7% |
| METR Time Horizons | 70.8% | 56.6% |
| APEX-Agents | — | 4.4% |
| Vending-Bench 2 | — | -21.53 |

## Reasoning

- GPT-5.1-Codex: —
- gpt-oss-120b: 20.0 (#245)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| SimpleBench | — | 22.1% |
| Kagi LLM Benchmark | — | 58.6% |
| CritPt | — | 1.1% |
| Chess Puzzles | — | 20% |
| LMArena Hard Prompts | — | 1364 |
| Mystery Game Puzzles | — | 2% |
| DTBench | — | 76.3% |
| LMCA | — | 22.1% |
| Surface Evolver Bench | — | 25% |
| Epoch Capabilities Index | — | 139.93 |

## Math

- GPT-5.1-Codex: 30.3 (#235)
- gpt-oss-120b: 52.5 (#50)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| OTIS Mock AIME 2024-2025 | — | 88.9% |
| ProofBench | 9% | — |
| Omni-MATH | — | 68.8% |
| LMArena Math | — | 1389 |

## Knowledge

- GPT-5.1-Codex: —
- gpt-oss-120b: 42.4 (#96)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| GPQA Diamond | — | 75.8% |
| MMLU-Pro | — | 79.5% |
| Confabulations | — | 15.7% |
| Vectara Hallucination Rate | — | 14.2% |
| GPQA (HELM) | — | 68.4% |
| LMArena Expert | — | 1356 |

## Multilingual

- GPT-5.1-Codex: —
- gpt-oss-120b: 48.0 (#147)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| LMArena Non-English | — | 1351 |
| LMArena Chinese | — | 1385 |
| LMArena French | — | 1369 |
| LMArena German | — | 1353 |
| LMArena Japanese | — | 1331 |
| LMArena Korean | — | 1282 |
| LMArena Russian | — | 1343 |
| LMArena Spanish | — | 1389 |

## Instruction Following

- GPT-5.1-Codex: —
- gpt-oss-120b: 69.3 (#173)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| IFEval | — | 83.6% |
| LMArena Instruction Following | — | 1318 |

## Long Context

- GPT-5.1-Codex: —
- gpt-oss-120b: 31.4 (#278)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| Fiction.LiveBench | — | 44.4% |
| LMArena Longer Query | — | 1319 |

## Writing & Preference

- GPT-5.1-Codex: —
- gpt-oss-120b: 46.5 (#217)

| Benchmark | GPT-5.1-Codex | gpt-oss-120b |
|---|---|---|
| LMArena Text | — | 1365 |
| LMArena Creative Writing | — | 1275 |
| Short-Story Creative Writing | — | 77.1% |
| EQ-Bench Creative Writing | — | 961 |
| WildBench | — | 84.5% |
| LMArena Multi-Turn | — | 1340 |

## FAQ

### Is GPT-5.1-Codex better than gpt-oss-120b?

GPT-5.1-Codex is the stronger model overall, scoring 38.6 to 36.3 on the Noometry Index. gpt-oss-120b costs 49× less per token, which makes it the better buy when GPT-5.1-Codex's lead doesn't matter for your workload.

### Which is cheaper, GPT-5.1-Codex or gpt-oss-120b?

gpt-oss-120b is cheaper. It lists at $0.037 per million input tokens and $0.17 per million output tokens; GPT-5.1-Codex lists at $1.25 and $10.

### Is GPT-5.1-Codex or gpt-oss-120b better for coding?

GPT-5.1-Codex scores higher on coding benchmarks: 41.9 versus 33.5 in the Noometry coding category.

### Which has the bigger context window?

GPT-5.1-Codex does, with 400K tokens against 131K.

### How many benchmarks do GPT-5.1-Codex and gpt-oss-120b share?

4 benchmarks have published results for both models. GPT-5.1-Codex has 6 scored results on Noometry and gpt-oss-120b has 48.