Head-to-head comparison across 2benchmark categories. Overall scores shown here use BenchLM's provisional ranking lane.
GPT-5.5
91
Mellum2-12B-A2.5B-Thinking
59
Verified leaderboard positions: GPT-5.5 #6 · Mellum2-12B-A2.5B-Thinking unranked
Pick GPT-5.5 if you want the stronger benchmark profile. Mellum2-12B-A2.5B-Thinking only becomes the better choice if coding is the priority.
Coding
+11.3 difference
Knowledge
+8.8 difference
GPT-5.5
Mellum2-12B-A2.5B-Thinking
$5 / $30
N/A
N/A
N/A
N/A
N/A
1M
128K
Pick GPT-5.5 if you want the stronger benchmark profile. Mellum2-12B-A2.5B-Thinking only becomes the better choice if coding is the priority.
GPT-5.5 is clearly ahead on the provisional aggregate, 91 to 59. The gap is large enough that you do not need to squint at the spreadsheet to see the difference.
GPT-5.5's sharpest advantage is in knowledge, where it averages 66.4 against 57.6. The single biggest benchmark swing on the page is GPQA, 93.6% to 57.6%. Mellum2-12B-A2.5B-Thinking does hit back in coding, so the answer changes if that is the part of the workload you care about most.
GPT-5.5 gives you the larger context window at 1M, compared with 128K for Mellum2-12B-A2.5B-Thinking.
GPT-5.5 is ahead on BenchLM's provisional leaderboard, 91 to 59. The biggest single separator in this matchup is GPQA, where the scores are 93.6% and 57.6%.
GPT-5.5 has the edge for knowledge tasks in this comparison, averaging 66.4 versus 57.6. Inside this category, GPQA is the benchmark that creates the most daylight between them.
Mellum2-12B-A2.5B-Thinking has the edge for coding in this comparison, averaging 69.9 versus 58.6. GPT-5.5 stays close enough that the answer can still flip depending on your workload.
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