# GPT-OSS 20B vs Mellum2-12B-A2.5B-Thinking

> Side-by-side benchmark comparison for GPT-OSS 20B and Mellum2-12B-A2.5B-Thinking across agentic, coding, multimodal, knowledge, reasoning, multilingual, and math tasks.

- Canonical page: https://benchlm.ai/compare/gpt-oss-20b-vs-mellum2-12b-a2-5b-thinking
- 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-OSS 20B and Mellum2-12B-A2.5B-Thinking.

## Summary

- GPT-OSS 20B and Mellum2-12B-A2.5B-Thinking 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-OSS 20B | Mellum2-12B-A2.5B-Thinking |
|----------|----------|----------|
| Creator | OpenAI | JetBrains |
| Type | Open Weight | Open Weight |
| Reasoning | Non-Reasoning | Reasoning |
| Context | 128K | 128K |
| Independent public score (not head-to-head) | 42.95 | null |
| Benchmarks Covered | 1 | 6 |
| Pricing (input/output) | $0.00 / $0.00 | N/A |

## Category Breakdown

### Agentic

- Winner: Insufficient shared evidence
- GPT-OSS 20B public-lane score: 38.4 (Estimated · #125/151)
- Mellum2-12B-A2.5B-Thinking public-lane score: 48.3 (Estimated · #76/151)

| Benchmark | GPT-OSS 20B | Mellum2-12B-A2.5B-Thinking | Winner |
|-----------|-----------|-----------|--------|
| BFCL v4 | Coming soon | 45.6% | Coming soon |

### Coding

- Winner: Insufficient shared evidence
- GPT-OSS 20B public-lane score: 40.1 (Estimated · #141/183)
- Mellum2-12B-A2.5B-Thinking public-lane score: Coming soon

| Benchmark | GPT-OSS 20B | Mellum2-12B-A2.5B-Thinking | Winner |
|-----------|-----------|-----------|--------|
| React Native Evals | 71% | Coming soon | Coming soon |
| LiveCodeBench v6 | Coming soon | 69.9% | Coming soon |

### Reasoning

- Winner: Insufficient shared evidence
- GPT-OSS 20B public-lane score: 45 (Unranked · 2 rankable rows)
- Mellum2-12B-A2.5B-Thinking public-lane score: Coming soon

### Knowledge

- Winner: Insufficient shared evidence
- GPT-OSS 20B public-lane score: 40.4 (Estimated · #139/181)
- Mellum2-12B-A2.5B-Thinking public-lane score: 48.8 (Estimated · #96/181)

| Benchmark | GPT-OSS 20B | Mellum2-12B-A2.5B-Thinking | Winner |
|-----------|-----------|-----------|--------|
| MMLU-Redux | Coming soon | 86.2% | Coming soon |
| GPQA | Coming soon | 57.6% | Coming soon |
| GPQA-D | Coming soon | 57.6% | Coming soon |

### Instruction Following

- Winner: Insufficient shared evidence
- GPT-OSS 20B public-lane score: 78.9 (#53/120)
- Mellum2-12B-A2.5B-Thinking public-lane score: Coming soon

| Benchmark | GPT-OSS 20B | Mellum2-12B-A2.5B-Thinking | Winner |
|-----------|-----------|-----------|--------|
| IFEval | Coming soon | 76.5% | Coming soon |

## FAQ

### Can I compare GPT-OSS 20B and Mellum2-12B-A2.5B-Thinking 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-OSS 20B vs GPT-OSS 120B](/compare/gpt-oss-120b-vs-gpt-oss-20b)
- [Mellum2-12B-A2.5B-Thinking vs GPT-OSS 120B](/compare/gpt-oss-120b-vs-mellum2-12b-a2-5b-thinking)
- [GPT-OSS 20B vs Mellum2-12B-A2.5B-Instruct](/compare/gpt-oss-20b-vs-mellum2-12b-a2-5b-instruct)
- [Mellum2-12B-A2.5B-Thinking vs Mellum2-12B-A2.5B-Instruct](/compare/mellum2-12b-a2-5b-instruct-vs-mellum2-12b-a2-5b-thinking)

## Explore More

- [GPT-OSS 20B profile](/models/gpt-oss-20b)
- [Mellum2-12B-A2.5B-Thinking profile](/models/mellum2-12b-a2-5b-thinking)
- [Compare Pricing](/llm-pricing)
- [Alternative Finder](/tools/alternative-finder)
- [LLM Selector](/tools/llm-selector)
- [Overall Rankings](/best/overall)