Side-by-side benchmark comparison across agentic, coding, multimodal, knowledge, reasoning, and math workflows.
GPT-5.3 Instant is clearly ahead on the aggregate, 87 to 66. The gap is large enough that you do not need to squint at the spreadsheet to see the difference.
GPT-5.3 Instant's sharpest advantage is in coding, where it averages 78.7 against 47.1. The single biggest benchmark swing on the page is SWE-bench Pro, 83 to 49.
GPT-5.3 Instant is also the more expensive model on tokens at $1.75 input / $14.00 output per 1M tokens, versus $0.10 input / $0.30 output per 1M tokens for Step 3.5 Flash. That is roughly 46.7x on output cost alone. GPT-5.3 Instant 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 128K for GPT-5.3 Instant.
Pick GPT-5.3 Instant if you want the stronger benchmark profile. Step 3.5 Flash only becomes the better choice if you want the cheaper token bill or you need the larger 256K context window.
GPT-5.3 Instant
82.9
Step 3.5 Flash
60.2
GPT-5.3 Instant
78.7
Step 3.5 Flash
47.1
GPT-5.3 Instant
95
Step 3.5 Flash
66.7
GPT-5.3 Instant
94.2
Step 3.5 Flash
78.3
GPT-5.3 Instant
80.8
Step 3.5 Flash
60.8
GPT-5.3 Instant
96
Step 3.5 Flash
87
GPT-5.3 Instant
93.4
Step 3.5 Flash
82.8
GPT-5.3 Instant
97.2
Step 3.5 Flash
84.5
GPT-5.3 Instant is ahead overall, 87 to 66. The biggest single separator in this matchup is SWE-bench Pro, where the scores are 83 and 49.
GPT-5.3 Instant has the edge for knowledge tasks in this comparison, averaging 80.8 versus 60.8. Inside this category, HLE is the benchmark that creates the most daylight between them.
GPT-5.3 Instant has the edge for coding in this comparison, averaging 78.7 versus 47.1. Inside this category, SWE-bench Pro is the benchmark that creates the most daylight between them.
GPT-5.3 Instant has the edge for math in this comparison, averaging 97.2 versus 84.5. Inside this category, AIME 2023 is the benchmark that creates the most daylight between them.
GPT-5.3 Instant has the edge for reasoning in this comparison, averaging 94.2 versus 78.3. Inside this category, MRCRv2 is the benchmark that creates the most daylight between them.
GPT-5.3 Instant has the edge for agentic tasks in this comparison, averaging 82.9 versus 60.2. Inside this category, OSWorld-Verified is the benchmark that creates the most daylight between them.
GPT-5.3 Instant has the edge for multimodal and grounded tasks in this comparison, averaging 95 versus 66.7. Inside this category, MMMU-Pro is the benchmark that creates the most daylight between them.
GPT-5.3 Instant has the edge for instruction following in this comparison, averaging 96 versus 87. Inside this category, IFEval is the benchmark that creates the most daylight between them.
GPT-5.3 Instant has the edge for multilingual tasks in this comparison, averaging 93.4 versus 82.8. Inside this category, MMLU-ProX is the benchmark that creates the most daylight between them.
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