Model comparison
GPT-5.1-Codex vs Mercury 2
GPT-5.1-Codex and Mercury 2 score almost the same on the Noometry Index (38.2 vs 38.8), so choose on price, context window or the category you care about most.
Last verified . 2 shared benchmarks.
Summary
- They share 2 benchmarks with published results for both. GPT-5.1-Codex scores higher in 1 category and Mercury 2 in 0 categories; one gap is clear of the uncertainty.
- The widest gap is in coding, where GPT-5.1-Codex leads 41.7 to 33.2.
- Mercury 2 is cheaper at $0.25 / $0.75 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.
Side by side
| GPT-5.1-Codex | Mercury 2 | |
|---|---|---|
| Provider | OpenAI | Inception |
| Noometry Index | 38.2 | 38.8 |
| Released | 2025-11-12 | 2026-02-20 |
| Weights | Proprietary | Proprietary |
| Context window | 400K | 128K |
| Max output | 128K | 50K |
| Input $ / M tokens | $1.25 | $0.25 |
| Output $ / M tokens | $10 | $0.75 |
| Results tracked | 6 | 17 |
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Category by category
Coding GPT-5.1-Codex leads
GPT-5.1-Codex: 41.7 (#107), Mercury 2: 33.2 (#250)
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| LMArena WebDev | 1337 | 1170 |
| ALE-Bench | 1,245 | 785.58 |
| SWE-bench Verified (bash only) | 66% | — |
| SciCode | — | 38.7% |
| WeirdML | — | 43.2% |
| LMArena Coding | — | 1391 |
Agentic & Tool Use Not comparable
GPT-5.1-Codex: 37.8 (#32), Mercury 2: —
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| Terminal-Bench | 60.4% | — |
| METR Time Horizons | 70.8% | — |
Reasoning Not comparable
GPT-5.1-Codex: —, Mercury 2: 23.0 (#170)
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| CritPt | — | 0.8% |
| LMArena Hard Prompts | — | 1362 |
Math Not comparable
GPT-5.1-Codex: 29.3 (#218), Mercury 2: —
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| ProofBench | 9% | — |
Knowledge Not comparable
GPT-5.1-Codex: —, Mercury 2: 35.6 (#170)
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| Vectara Hallucination Rate | — | 12.3% |
| LMArena Expert | — | 1358 |
Multilingual Not comparable
GPT-5.1-Codex: —, Mercury 2: 46.8 (#153)
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| LMArena Non-English | — | 1331 |
| LMArena Chinese | — | 1416 |
| LMArena Russian | — | 1303 |
Instruction Following Not comparable
GPT-5.1-Codex: —, Mercury 2: 70.4 (#161)
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| LMArena Instruction Following | — | 1328 |
Long Context Not comparable
GPT-5.1-Codex: —, Mercury 2: 40.4 (#149)
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| LMArena Longer Query | — | 1329 |
Writing & Preference Not comparable
GPT-5.1-Codex: —, Mercury 2: 54.0 (#151)
| Benchmark | GPT-5.1-Codex | Mercury 2 |
|---|---|---|
| LMArena Text | — | 1355 |
| LMArena Creative Writing | — | 1289 |
| LMArena Multi-Turn | — | 1358 |
Frequently asked questions
Is GPT-5.1-Codex better than Mercury 2?
GPT-5.1-Codex and Mercury 2 score almost the same on the Noometry Index (38.2 vs 38.8), so choose on price, context window or the category you care about most.
Which is cheaper, GPT-5.1-Codex or Mercury 2?
Mercury 2 is cheaper. It lists at $0.25 per million input tokens and $0.75 per million output tokens; GPT-5.1-Codex lists at $1.25 and $10.
Is GPT-5.1-Codex or Mercury 2 better for coding?
GPT-5.1-Codex scores higher on coding benchmarks: 41.7 versus 33.2 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 Mercury 2 share?
2 benchmarks have published results for both models. GPT-5.1-Codex has 6 scored results on Noometry and Mercury 2 has 17.