Side-by-side benchmark comparison across agentic, coding, multimodal, knowledge, reasoning, and math workflows.
o3 is clearly ahead on the aggregate, 70 to 66. The gap is large enough that you do not need to squint at the spreadsheet to see the difference.
o3's sharpest advantage is in agentic, where it averages 69.9 against 60.2. The single biggest benchmark swing on the page is HLE, 24 to 12. Step 3.5 Flash does hit back in instruction following, so the answer changes if that is the part of the workload you care about most.
o3 is also the more expensive model on tokens at $10.00 input / $40.00 output per 1M tokens, versus $0.10 input / $0.30 output per 1M tokens for Step 3.5 Flash. That is roughly 133.3x on output cost alone. o3 is the reasoning model in the pair, while Step 3.5 Flash 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. Step 3.5 Flash gives you the larger context window at 256K, compared with 200K for o3.
Pick o3 if you want the stronger benchmark profile. Step 3.5 Flash only becomes the better choice if instruction following is the priority or you want the cheaper token bill.
o3
69.9
Step 3.5 Flash
60.2
o3
49.1
Step 3.5 Flash
47.1
o3
72.3
Step 3.5 Flash
66.7
o3
82.6
Step 3.5 Flash
78.3
o3
66
Step 3.5 Flash
60.8
o3
85
Step 3.5 Flash
87
o3
81.1
Step 3.5 Flash
82.8
o3
87.5
Step 3.5 Flash
84.5
o3 is ahead overall, 70 to 66. The biggest single separator in this matchup is HLE, where the scores are 24 and 12.
o3 has the edge for knowledge tasks in this comparison, averaging 66 versus 60.8. Inside this category, HLE is the benchmark that creates the most daylight between them.
o3 has the edge for coding in this comparison, averaging 49.1 versus 47.1. Inside this category, SWE-bench Pro is the benchmark that creates the most daylight between them.
o3 has the edge for math in this comparison, averaging 87.5 versus 84.5. Inside this category, AIME 2023 is the benchmark that creates the most daylight between them.
o3 has the edge for reasoning in this comparison, averaging 82.6 versus 78.3. Inside this category, LongBench v2 is the benchmark that creates the most daylight between them.
o3 has the edge for agentic tasks in this comparison, averaging 69.9 versus 60.2. Inside this category, OSWorld-Verified is the benchmark that creates the most daylight between them.
o3 has the edge for multimodal and grounded tasks in this comparison, averaging 72.3 versus 66.7. Inside this category, MMMU-Pro is the benchmark that creates the most daylight between them.
Step 3.5 Flash has the edge for instruction following in this comparison, averaging 87 versus 85. Inside this category, IFEval is the benchmark that creates the most daylight between them.
Step 3.5 Flash has the edge for multilingual tasks in this comparison, averaging 82.8 versus 81.1. Inside this category, MGSM is the benchmark that creates the most daylight between them.
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