# GPT-5.3 Codex vs Mellum2-12B-A2.5B-Instruct

> Side-by-side benchmark comparison for GPT-5.3 Codex and Mellum2-12B-A2.5B-Instruct across agentic, coding, multimodal, knowledge, reasoning, multilingual, and math tasks.

- Canonical page: https://benchlm.ai/compare/gpt-5-3-codex-vs-mellum2-12b-a2-5b-instruct
- Last updated: September 4, 2026

- Shared sourced benchmarks: 0
- HTML indexing: indexable
- Ranking lane: BenchAlign v5

## Quick Verdict

No benchmark verdict yet: there is no shared sourced benchmark row for GPT-5.3 Codex and Mellum2-12B-A2.5B-Instruct.

## Summary

- GPT-5.3 Codex and Mellum2-12B-A2.5B-Instruct do not currently share a sourced benchmark row. The mirror therefore compares metadata, pricing, context windows, and each model's separately reported coverage without naming a benchmark winner.

## Model Snapshot

| Property | GPT-5.3 Codex | Mellum2-12B-A2.5B-Instruct |
|----------|----------|----------|
| Creator | OpenAI | JetBrains |
| Type | Proprietary | Open Weight |
| Reasoning | Reasoning | Non-Reasoning |
| Context | 400K | 128K |
| Independent public score (not head-to-head) | 65.22 | null |
| Benchmarks Covered | 8 | 6 |
| Pricing (input/output) | $1.75 / $14.00 | N/A |

## Category Breakdown

### Agentic

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 62.3 (Estimated · #19/151)
- Mellum2-12B-A2.5B-Instruct public-lane score: 47.8 (Estimated · #82/151)

| Benchmark | GPT-5.3 Codex | Mellum2-12B-A2.5B-Instruct | Winner |
|-----------|-----------|-----------|--------|
| Terminal-Bench 2.0 | 77.3% | Coming soon | Coming soon |
| OSWorld-Verified | 64.7% | Coming soon | Coming soon |
| Gert Labs | 57.47% | Coming soon | Coming soon |
| JobBench | 33.7% | Coming soon | Coming soon |
| BFCL v4 | Coming soon | 44.2% | Coming soon |

### Coding

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 62.7 (Supported · #16/183)
- Mellum2-12B-A2.5B-Instruct public-lane score: Coming soon

| Benchmark | GPT-5.3 Codex | Mellum2-12B-A2.5B-Instruct | Winner |
|-----------|-----------|-----------|--------|
| SWE-bench Verified | 85% | Coming soon | Coming soon |
| SWE-bench Pro | 56.8% | Coming soon | Coming soon |
| SWE-Rebench | 58.2% | Coming soon | Coming soon |
| Vibe Code Bench | 61.77% | Coming soon | Coming soon |
| LiveCodeBench v6 | Coming soon | 37.2% | Coming soon |

### Multimodal & Grounded

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 75.6 (Unranked · 1 rankable row)
- Mellum2-12B-A2.5B-Instruct public-lane score: Coming soon

### Reasoning

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 77.3 (Unranked · 2 rankable rows)
- Mellum2-12B-A2.5B-Instruct public-lane score: Coming soon

### Knowledge

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 66.3 (Estimated · #22/181)
- Mellum2-12B-A2.5B-Instruct public-lane score: 48.1 (Estimated · #103/181)

| Benchmark | GPT-5.3 Codex | Mellum2-12B-A2.5B-Instruct | Winner |
|-----------|-----------|-----------|--------|
| MMLU-Redux | Coming soon | 78.1% | Coming soon |
| GPQA | Coming soon | 40.9% | Coming soon |
| GPQA-D | Coming soon | 40.9% | Coming soon |

### Instruction Following

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 92.3 (#15/120)
- Mellum2-12B-A2.5B-Instruct public-lane score: Coming soon

| Benchmark | GPT-5.3 Codex | Mellum2-12B-A2.5B-Instruct | Winner |
|-----------|-----------|-----------|--------|
| IFEval | Coming soon | 75.8% | Coming soon |

## FAQ

### Can I compare GPT-5.3 Codex and Mellum2-12B-A2.5B-Instruct on BenchLM yet?

Not fully yet. BenchLM is tracking both models, but sourced benchmark coverage is still incomplete for a fair score-level comparison.

### Why does this page show "coming soon" values?

BenchLM only calls winners when public benchmark coverage is available on both sides of the comparison.

## Related Comparisons

- [GPT-5.3 Codex vs GPT-5.3-Codex-Spark](/compare/gpt-5-3-codex-vs-gpt-5-3-codex-spark)
- [Mellum2-12B-A2.5B-Instruct vs GPT-5.3-Codex-Spark](/compare/gpt-5-3-codex-spark-vs-mellum2-12b-a2-5b-instruct)
- [GPT-5.3 Codex vs Mellum2-12B-A2.5B-Thinking](/compare/gpt-5-3-codex-vs-mellum2-12b-a2-5b-thinking)
- [Mellum2-12B-A2.5B-Instruct vs Mellum2-12B-A2.5B-Thinking](/compare/mellum2-12b-a2-5b-instruct-vs-mellum2-12b-a2-5b-thinking)

## Explore More

- [GPT-5.3 Codex profile](/models/gpt-5-3-codex)
- [Mellum2-12B-A2.5B-Instruct profile](/models/mellum2-12b-a2-5b-instruct)
- [Compare Pricing](/llm-pricing)
- [Alternative Finder](/tools/alternative-finder)
- [LLM Selector](/tools/llm-selector)
- [Overall Rankings](/best/overall)