Inference
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JetBrains

Mellum2.1-12B-A2.5B-Thinking

Mellum2.1-12B-A2.5B-Thinking is a reasoning model from JetBrains in the Mellum2.1 12B-A2.5B family. It has 7 scored results in 4 categories. It has no index.

  • Open weights
  • Reasoning
  • Input: text
  • Released 2026-10
UnrankedEnough results for an index.
IndexAll categories, out of 100.UnrankedMedian 51.0
CoverageIndex weight that has results.60%
SpeedOutput tokens per second.—Median 67/s
First tokenSeconds to the first output token.—Median 1.3 s
Input / 1MUS dollars per 1M input tokens.—Median $0.60
Output / 1MUS dollars per 1M output tokens.—Median $2.55
ContextMaximum tokens in one request.128KMedian 262K
EloLMArena rating and rank.—Median 1422

Capabilities

Index per category, out of 100.
Out of 100
AgenticMulti-step tasks with tools.1 counted31.3—
CodingCode writing and repair.3 counted40.1—
ReasoningLogic problems and puzzles.None counted——
MultimodalTasks with images and text.None counted——
KnowledgeFacts and expert knowledge.2 counted37.8—
MultilingualTasks in many languages.None counted——
InstructionTasks with strict prompt rules.1 counted45.6—
MathMath problems.None counted——

Results

7 counted
BenchmarkThe benchmark name.ResultThe score from the source.PlacePlace among results on this benchmark.
Instruction-Following EvalInstruction90.6%26 / 70
LiveCodeBench v6Coding82.0%26 / 59
MMLU-ReduxKnowledge87.8%14 / 19
GPQA Diamond · BenchLMKnowledge64.6%75 / 84
Software Engineering Benchmark VerifiedCoding47.0%102 / 114
SWE-bench ProCoding28.0%81 / 83
Terminal-Bench 2.1 · BenchLMAgentic17.4%84 / 86

Sources

BenchLMCC BY-NC 4.0

Questions

What is the context window of Mellum2.1-12B-A2.5B-Thinking?
128K tokens.
Is Mellum2.1-12B-A2.5B-Thinking open source?
Yes. Its weights are published.
When was Mellum2.1-12B-A2.5B-Thinking released?
October 2026.

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