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Model comparison

Claude Opus 4.5 vs Mellum2-12B-A2.5B-Thinking

Data verified

Head-to-head evidence from 3 shared benchmark results across 2 categories. Overall scores shown here use the public BenchAlign v5 ranking lane.

64.22/100
No comparison
N/A
1 category wins1 category wins

Public leaderboard positions: Claude Opus 4.5 #34 (Supported); Mellum2-12B-A2.5B-Thinking unranked (Not scored). Intervals and evidence labels describe ranking uncertainty, not a guarantee for a specific workload.

Evidence parity. Claude Opus 4.5 and Mellum2-12B-A2.5B-Thinking share 3 comparable benchmark results. 2 of 8 categories are comparable. 56 results are unique to Claude Opus 4.5; 2 to Mellum2-12B-A2.5B-Thinking.

Updated July 23, 2026
Shared results
3
Claude Opus 4.5 only
56
Mellum2-12B-A2.5B-Thinking only
2
Comparable categories
2 / 8

Treat this as a split decision. Claude Opus 4.5 makes more sense if knowledge is the priority or you need the larger 200K context window; Mellum2-12B-A2.5B-Thinking is the better fit if instruction following is the priority or you want the stronger reasoning-first profile.

Confidence note. This is a partial-evidence comparison with 3 shared benchmark results across 2 evidence categories; 2 of 8 categories currently have scoreable aggregates for both models. Treat the verdict as directional until coverage is more balanced.

Why this result

Claude Opus 4.5 and Mellum2-12B-A2.5B-Thinking finish on the same BenchAlign overall score, so this is less about a single winner and more about where the edge shows up. The BenchAlign headline says tie; the benchmark table is where the real choice happens.

Mellum2-12B-A2.5B-Thinking is the reasoning model in the pair, while Claude Opus 4.5 is not. That usually helps on harder chain-of-thought-heavy tests, but it can also mean more latency and more token spend in real use. Claude Opus 4.5 gives you the larger context window at 200K, compared with 128K for Mellum2-12B-A2.5B-Thinking.

Category breakdown

Exact category averages are shown below. Not measured means BenchLM does not have enough sourced public coverage for that model and category.

Category scores and score margins for Claude Opus 4.5 and Mellum2-12B-A2.5B-Thinking
CategoryClaude Opus 4.5ΔMellum2-12B-A2.5B-Thinking
Inst. FollowingClaude Opus 4.569.5Margin→ 7.0Mellum2-12B-A2.5B-Thinking76.5
KnowledgeClaude Opus 4.558.1Margin← 0.5Mellum2-12B-A2.5B-Thinking57.6
AgenticClaude Opus 4.562.6MarginNo overlapMellum2-12B-A2.5B-ThinkingNot measured
CodingClaude Opus 4.571.7MarginNo overlapMellum2-12B-A2.5B-ThinkingNot measured
ReasoningClaude Opus 4.564.4MarginNo overlapMellum2-12B-A2.5B-ThinkingNot measured
MathClaude Opus 4.557.5MarginNo overlapMellum2-12B-A2.5B-ThinkingNot measured
MultilingualClaude Opus 4.585.7MarginNo overlapMellum2-12B-A2.5B-ThinkingNot measured
MultimodalClaude Opus 4.569.9MarginNo overlapMellum2-12B-A2.5B-ThinkingNot measured

Decisive benchmark drivers

The largest measured benchmark gaps in this matchup, with exact reported values.

More
A · Claude Opus 4.5B · Mellum2-12B-A2.5B-Thinking

  1. GPQA

    Knowledge
    Source ↗
    A 87%B 57.6%
    Winner: Claude Opus 4.5Δ 29.4
    GPQA: Claude Opus 4.5 scored 87%; Mellum2-12B-A2.5B-Thinking scored 57.6%. Claude Opus 4.5 wins this benchmark.

  2. IFEval

    Inst. Following
    Source ↗
    A 90.9%B 76.5%
    Winner: Claude Opus 4.5Δ 14.4
    IFEval: Claude Opus 4.5 scored 90.9%; Mellum2-12B-A2.5B-Thinking scored 76.5%. Claude Opus 4.5 wins this benchmark.

Operational comparison

Runtime and commercial metrics are compared only when both models have a complete sourced value.

MetricClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingComparison
Input / output priceUSD per 1M tokensClaude Opus 4.5$5 input / $25 outputMellum2-12B-A2.5B-ThinkingNot availableA complete price comparison is not available.
Generation speedtokens per secondClaude Opus 4.546 tok/sMellum2-12B-A2.5B-ThinkingNot availableA complete speed comparison is not available.
First-answer latencyseconds to first tokenClaude Opus 4.51.01 sMellum2-12B-A2.5B-ThinkingNot availableA complete latency comparison is not available.
Context windowmaximum listed tokensClaude Opus 4.5200KMellum2-12B-A2.5B-Thinking128KClaude Opus 4.5 lists the larger context window.

Benchmark Deep Dive

Agentic
BenchmarkClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingResult
Terminal-Bench 2.0Source 59.3%—Not comparable
OSWorld-VerifiedSource 66.3%—Not comparable
OSWorldSource 66.3%—Not comparable
Claw-EvalSource 59.6%—Not comparable
QwenClawBenchSource 52.3%—Not comparable
τ³-bench resultsSource 70.2%—Not comparable
VITA-BenchSource 23.3%—Not comparable
DeepPlanningSource 26.4%—Not comparable
ToolathlonSource 43.5%—Not comparable
MCP AtlasSource 42.3%—Not comparable
MCP-TasksSource 71.8%—Not comparable
WideResearchSource 76.4%—Not comparable
CyberGymSource 50.6%—Not comparable
τ²-bench resultsSource 86.3%—Not comparable
Gert LabsSource 64.23%—Not comparable
JobBenchSource 32.3%—Not comparable
BFCL v4Source —45.6%Not comparable
Coding
BenchmarkClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingResult
SWE-bench VerifiedSource 80.9%—Not comparable
LiveCodeBench v6Source 84.8%—Not comparable
SWE-bench ProSource 57.1%—Not comparable
SWE MultilingualSource 77.5%—Not comparable
NL2RepoSource 43.2%—Not comparable
AA-SciCodeSource 47.0%—Not comparable
Reasoning
BenchmarkClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingResult
LongBench v2Source 64.4%—Not comparable
AI-NeedleSource 74%—Not comparable
AA-LCRSource 65.3%—Not comparable
CritPtSource 0.3%—Not comparable
KnowledgeClaude Opus 4.5 wins
BenchmarkClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingResult
GPQASource 87%57.6%Claude Opus 4.5 leads
SuperGPQASource 70.6%—Not comparable
MMLU-ProSource 89.5%—Not comparable
MMLU-ReduxSource 96.6%86.2%Claude Opus 4.5 leads
C-EvalSource 92.2%—Not comparable
HLESource 30.8%—Not comparable
Artificial Analysis Intelligence IndexSource 34.7%—Not comparable
AA-GPQA DiamondSource 81.0%—Not comparable
AA-HLESource 12.9%—Not comparable
AA-Omniscience IndexSource -3.9%—Not comparable
AA-Omniscience AccuracySource 40.7%—Not comparable
AA-Omniscience Hallucination RateSource 75.4%—Not comparable
AA MMLU-ProSource 88.9%—Not comparable
GPQA-DSource —57.6%Not comparable
Math
BenchmarkClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingResult
AIME26Source 95.1%—Not comparable
HMMT Feb 2025Source 92.9%—Not comparable
HMMT Nov 2025Source 93.3%—Not comparable
HMMT Feb 2026Source 85.3%—Not comparable
MMAnswerBenchSource 84.0%—Not comparable
FrontierMath v2 (Tiers 1-3)Source 20.690%—Not comparable
FrontierMath v2 (Tier 4)Source 4.167%—Not comparable
Multilingual
BenchmarkClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingResult
MMLU-ProXSource 85.7%—Not comparable
NOVA-63Source 56.7%—Not comparable
Multimodal
BenchmarkClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingResult
MMMU-ProSource 70.6%—Not comparable
MathVisionSource 74.3%—Not comparable
CharXivSource 68.5%—Not comparable
VideoMMMUSource 84.4%—Not comparable
ScreenSpot ProSource 45.7%—Not comparable
V*Source 67.0%—Not comparable
AA-MMMU-ProSource 71.2%—Not comparable
Design Arena WebsiteSource 1277—Not comparable
Inst. FollowingMellum2-12B-A2.5B-Thinking wins
BenchmarkClaude Opus 4.5Mellum2-12B-A2.5B-ThinkingResult
IFEvalSource 90.9%76.5%Claude Opus 4.5 leads
IFBenchSource 58%—Not comparable
AA-IFBenchSource 43.0%—Not comparable
Frequently Asked Questions (3)

Which is better, Claude Opus 4.5 or Mellum2-12B-A2.5B-Thinking?

Claude Opus 4.5 and Mellum2-12B-A2.5B-Thinking are tied on the BenchAlign overall score, so the right pick depends on which category matters most for your use case.

Which is better for knowledge tasks, Claude Opus 4.5 or Mellum2-12B-A2.5B-Thinking?

Claude Opus 4.5 has the edge for knowledge tasks in this comparison, averaging 58.1 versus 57.6. Inside this category, GPQA is the benchmark that creates the most daylight between them.

Which is better for instruction following, Claude Opus 4.5 or Mellum2-12B-A2.5B-Thinking?

Mellum2-12B-A2.5B-Thinking has the edge for instruction following in this comparison, averaging 76.5 versus 69.5. Inside this category, IFEval is the benchmark that creates the most daylight between them.

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