# GPT-5.3 Codex vs Mellum2.1-12B-A2.5B-Thinking

> Side-by-side benchmark comparison for GPT-5.3 Codex and Mellum2.1-12B-A2.5B-Thinking across agentic, coding, multimodal, knowledge, reasoning, multilingual, and math tasks.

- Canonical page: https://benchlm.ai/compare/gpt-5-3-codex-vs-mellum2-1-12b-a2-5b-thinking
- Last updated: October 8, 2026

- Shared sourced benchmarks: 2
- HTML indexing: indexable
- Ranking lane: BenchAlign v5.8

## Quick Verdict

Evidence is too thin for a benchmark verdict: GPT-5.3 Codex and Mellum2.1-12B-A2.5B-Thinking share only 2 sourced benchmark rows.

## Summary

- Only 2 shared sourced benchmark rows are available. That is not enough for a broad capability verdict, so this mirror does not name an overall winner.
- Mellum2.1-12B-A2.5B-Thinking has the lower published output-token price.
- GPT-5.3 Codex has the larger published context window at 400K.

## Model Snapshot

| Property | GPT-5.3 Codex | Mellum2.1-12B-A2.5B-Thinking |
|----------|----------|----------|
| Creator | OpenAI | JetBrains |
| Type | Proprietary | Open Weight |
| Reasoning | Reasoning | Reasoning |
| Context | 400K | 128K |
| Independent public score (not head-to-head) | 60.84 | null |
| Benchmarks Covered | 10 | 7 |
| Pricing (input/output) | $1.75 / $14.00 | $0.00 / $0.00 |

## Category Breakdown

### Agentic

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 55.6 (Estimated · #35/122)
- Mellum2.1-12B-A2.5B-Thinking public-lane score: Coming soon

| Benchmark | GPT-5.3 Codex | Mellum2.1-12B-A2.5B-Thinking | 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 |
| Terminal-Bench 2.1 | Coming soon | 17.4% | Coming soon |

### Coding

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 55.3 (Supported · #30/146)
- Mellum2.1-12B-A2.5B-Thinking public-lane score: Coming soon

| Benchmark | GPT-5.3 Codex | Mellum2.1-12B-A2.5B-Thinking | Winner |
|-----------|-----------|-----------|--------|
| SWE-bench Verified | 85% | 47% | GPT-5.3 Codex |
| SWE-bench Pro | 56.8% | 28% | GPT-5.3 Codex |
| SWE-Rebench | 58.2% | Coming soon | Coming soon |
| Vibe Code Bench | 61.77% | Coming soon | Coming soon |
| LiveCodeBench (Vals) | 87.3% | Coming soon | Coming soon |
| SWE-bench (Vals) | 78.0% | Coming soon | Coming soon |
| LiveCodeBench v6 | Coming soon | 82.0% | Coming soon |

### Reasoning

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 79.6 (Unranked · 2 rankable rows)
- Mellum2.1-12B-A2.5B-Thinking public-lane score: Coming soon

### Multimodal & Grounded

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 76.7 (Unranked · 1 rankable row)
- Mellum2.1-12B-A2.5B-Thinking public-lane score: Coming soon

### Knowledge

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 64.3 (Estimated · #27/174)
- Mellum2.1-12B-A2.5B-Thinking public-lane score: Coming soon

| Benchmark | GPT-5.3 Codex | Mellum2.1-12B-A2.5B-Thinking | Winner |
|-----------|-----------|-----------|--------|
| MMLU-Redux | Coming soon | 87.8% | Coming soon |
| GPQA-D | Coming soon | 64.6% | Coming soon |

### Instruction Following

- Winner: Insufficient shared evidence
- GPT-5.3 Codex public-lane score: 91.2 (#14/125)
- Mellum2.1-12B-A2.5B-Thinking public-lane score: Coming soon

| Benchmark | GPT-5.3 Codex | Mellum2.1-12B-A2.5B-Thinking | Winner |
|-----------|-----------|-----------|--------|
| IFEval | Coming soon | 90.6% | Coming soon |

## FAQ

### Is there enough evidence to pick between GPT-5.3 Codex and Mellum2.1-12B-A2.5B-Thinking?

Not yet. The models share only 2 sourced benchmark rows, so this mirror reports the available measurements and metadata without naming a broad winner.

### What can I compare safely on this page?

Published pricing, context windows, model type, and individual shared benchmark rows are directly comparable. Independent overall scores and non-overlapping benchmark coverage are labeled separately and should not be read as a controlled head-to-head result.

## Related Comparisons

- [GPT-5.3 Codex vs GPT-5.3-Codex-Spark](/compare/gpt-5-3-codex-vs-gpt-5-3-codex-spark)
- [Mellum2.1-12B-A2.5B-Thinking vs GPT-5.3-Codex-Spark](/compare/gpt-5-3-codex-spark-vs-mellum2-1-12b-a2-5b-thinking)
- [GPT-5.3 Codex vs Claude Opus 5.5](/compare/claude-opus-5-5-vs-gpt-5-3-codex)
- [Mellum2.1-12B-A2.5B-Thinking vs Claude Opus 5.5](/compare/claude-opus-5-5-vs-mellum2-1-12b-a2-5b-thinking)

## Explore More

- [GPT-5.3 Codex profile](/models/gpt-5-3-codex)
- [Mellum2.1-12B-A2.5B-Thinking profile](/models/mellum2-1-12b-a2-5b-thinking)
- [Compare Pricing](/llm-pricing)
- [Alternative Finder](/tools/alternative-finder)
- [LLM Selector](/tools/llm-selector)
- [Overall Rankings](/best/overall)