Model comparison
GPT-5.1-Codex vs gpt-oss-20b
GPT-5.1-Codex is the stronger model overall, scoring 38.6 to 32.5 on the Noometry Index. gpt-oss-20b costs 95× less per token, which makes it the better buy when GPT-5.1-Codex's lead doesn't matter for your workload.
Last verified . 2 shared benchmarks.
Summary
- They share 2 benchmarks with published results for both. GPT-5.1-Codex scores higher in 2 categories and gpt-oss-20b 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 9.3.
- The biggest single-benchmark swing is Terminal-Bench: 60.4% for GPT-5.1-Codex and 3.4% for gpt-oss-20b.
- gpt-oss-20b is cheaper at $0.018 / $0.09 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-20b has downloadable open weights; the other is API-only.
Side by side
| GPT-5.1-Codex | gpt-oss-20b | |
|---|---|---|
| Provider | OpenAI | OpenAI |
| Noometry Index | 38.6 | 32.5 |
| Released | 2025-11-12 | 2025-08-05 |
| Weights | Proprietary | Open |
| Context window | 400K | 131K |
| Max output | 128K | 16K |
| Input $ / M tokens | $1.25 | $0.018 |
| Output $ / M tokens | $10 | $0.09 |
| Results tracked | 6 | 34 |
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Category by category
Coding GPT-5.1-Codex leads
GPT-5.1-Codex: 41.9 (#116), gpt-oss-20b: 37.6 (#192)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| ALE-Bench | 1,245 | 566.05 |
| SWE-bench Verified (bash only) | 66% | — |
| LMArena WebDev | 1337 | — |
| SciCode | — | 34.4% |
| WeirdML | — | 40.9% |
| LMArena Coding | — | 1306 |
Agentic & Tool Use GPT-5.1-Codex leads
GPT-5.1-Codex: 38.0 (#33), gpt-oss-20b: 9.3 (#154)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| Terminal-Bench | 60.4% | 3.4% |
| METR Time Horizons | 70.8% | — |
Reasoning Not comparable
GPT-5.1-Codex: —, gpt-oss-20b: 19.3 (#261)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| Kagi LLM Benchmark | — | 53.2% |
| CritPt | — | 1.4% |
| Chess Puzzles | — | 4% |
| LMArena Hard Prompts | — | 1274 |
| DTBench | — | 68% |
| LMCA | — | 14.5% |
| Epoch Capabilities Index | — | 137.82 |
Math gpt-oss-20b leads
GPT-5.1-Codex: 30.3 (#235), gpt-oss-20b: 39.4 (#103)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| OTIS Mock AIME 2024-2025 | — | 65.3% |
| ProofBench | 9% | — |
| Omni-MATH | — | 56.5% |
| LMArena Math | — | 1317 |
Knowledge Not comparable
GPT-5.1-Codex: —, gpt-oss-20b: 34.6 (#195)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| GPQA Diamond | — | 60.8% |
| MMLU-Pro | — | 74% |
| GPQA (HELM) | — | 59.4% |
| LMArena Expert | — | 1258 |
Multilingual Not comparable
GPT-5.1-Codex: —, gpt-oss-20b: 42.2 (#197)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| LMArena Non-English | — | 1268 |
| LMArena Chinese | — | 1314 |
| LMArena German | — | 1255 |
| LMArena Japanese | — | 1244 |
| LMArena Korean | — | 1236 |
| LMArena Russian | — | 1278 |
| LMArena Spanish | — | 1267 |
Instruction Following Not comparable
GPT-5.1-Codex: —, gpt-oss-20b: 61.8 (#240)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| IFEval | — | 73.2% |
| LMArena Instruction Following | — | 1236 |
Long Context Not comparable
GPT-5.1-Codex: —, gpt-oss-20b: 37.9 (#209)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| LMArena Longer Query | — | 1250 |
Writing & Preference Not comparable
GPT-5.1-Codex: —, gpt-oss-20b: 35.5 (#265)
| Benchmark | GPT-5.1-Codex | gpt-oss-20b |
|---|---|---|
| LMArena Text | — | 1287 |
| LMArena Creative Writing | — | 1201 |
| EQ-Bench Creative Writing | — | 666 |
| WildBench | — | 73.7% |
| LMArena Multi-Turn | — | 1268 |
Frequently asked questions
Is GPT-5.1-Codex better than gpt-oss-20b?
GPT-5.1-Codex is the stronger model overall, scoring 38.6 to 32.5 on the Noometry Index. gpt-oss-20b costs 95× 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-20b?
gpt-oss-20b is cheaper. It lists at $0.018 per million input tokens and $0.09 per million output tokens; GPT-5.1-Codex lists at $1.25 and $10.
Is GPT-5.1-Codex or gpt-oss-20b better for coding?
GPT-5.1-Codex scores higher on coding benchmarks: 41.9 versus 37.6 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-20b share?
2 benchmarks have published results for both models. GPT-5.1-Codex has 6 scored results on Noometry and gpt-oss-20b has 34.