Model comparison
GPT-5.1-Codex vs Mercury 2.5
GPT-5.1-Codex is the stronger model overall, scoring 38.6 to 33.5 on the Noometry Index. Mercury 2.5 costs 51× 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 Mercury 2.5 in 0 categories; 2 gaps are clear of the uncertainty.
- The widest gap is in math, where GPT-5.1-Codex leads 30.3 to 23.3.
- The biggest single-benchmark swing is ProofBench: 9% for GPT-5.1-Codex and 3% for Mercury 2.5.
- Mercury 2.5 is cheaper at $0.04 / $0.15 per million input/output tokens, against $1.25 / $10 for GPT-5.1-Codex.
- GPT-5.1-Codex accepts more context: 400K tokens versus 260K.
Side by side
| GPT-5.1-Codex | Mercury 2.5 | |
|---|---|---|
| Provider | OpenAI | Inception |
| Noometry Index | 38.6 | 33.5 |
| Released | 2025-11-12 | 2026-09-08 |
| Weights | Proprietary | Proprietary |
| Context window | 400K | 260K |
| Max output | 128K | 66K |
| Input $ / M tokens | $1.25 | $0.04 |
| Output $ / M tokens | $10 | $0.15 |
| Results tracked | 6 | 4 |
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Category by category
Coding GPT-5.1-Codex leads
GPT-5.1-Codex: 41.9 (#116), Mercury 2.5: 39.5 (#156)
| Benchmark | GPT-5.1-Codex | Mercury 2.5 |
|---|---|---|
| ALE-Bench | 1,245 | 301.65 |
| SWE-bench Verified (bash only) | 66% | — |
| LMArena WebDev | 1337 | — |
| SciCode | — | 38.5% |
Agentic & Tool Use Not comparable
GPT-5.1-Codex: 38.0 (#33), Mercury 2.5: —
| Benchmark | GPT-5.1-Codex | Mercury 2.5 |
|---|---|---|
| Terminal-Bench | 60.4% | — |
| METR Time Horizons | 70.8% | — |
Reasoning Not comparable
GPT-5.1-Codex: —, Mercury 2.5: 22.4 (#193)
| Benchmark | GPT-5.1-Codex | Mercury 2.5 |
|---|---|---|
| CritPt | — | 0% |
Math GPT-5.1-Codex leads
GPT-5.1-Codex: 30.3 (#235), Mercury 2.5: 23.3 (#272)
| Benchmark | GPT-5.1-Codex | Mercury 2.5 |
|---|---|---|
| ProofBench | 9% | 3% |
Frequently asked questions
Is GPT-5.1-Codex better than Mercury 2.5?
GPT-5.1-Codex is the stronger model overall, scoring 38.6 to 33.5 on the Noometry Index. Mercury 2.5 costs 51× 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 Mercury 2.5?
Mercury 2.5 is cheaper. It lists at $0.04 per million input tokens and $0.15 per million output tokens; GPT-5.1-Codex lists at $1.25 and $10.
Is GPT-5.1-Codex or Mercury 2.5 better for coding?
GPT-5.1-Codex scores higher on coding benchmarks: 41.9 versus 39.5 in the Noometry coding category.
Which has the bigger context window?
GPT-5.1-Codex does, with 400K tokens against 260K.
How many benchmarks do GPT-5.1-Codex and Mercury 2.5 share?
2 benchmarks have published results for both models. GPT-5.1-Codex has 6 scored results on Noometry and Mercury 2.5 has 4.