Question 1

What does this GPT-5.1 vs Step 3.5 Flash comparison cover?

Accepted Answer

It brings together documented inputs and outputs, token limits, deployment terms and available pricing. When both models have source-linked, verified results on a shared benchmark, those results are shown with their individual sources. The comparison is a starting point for evaluating your own workload.

Question 2

Which is cheaper: GPT-5.1 or Step 3.5 Flash?

Accepted Answer

GPT-5.1 lists $1.25 input and $10.00 output per 1M tokens. Step 3.5 Flash lists $0.10 input and $0.30 output. The cheaper workload depends on the mix of input and output. Long-context tiers, caching, batch processing and retries can change the bill.

Question 3

Which has the larger context window: GPT-5.1 or Step 3.5 Flash?

Accepted Answer

GPT-5.1 lists 400K tokens and Step 3.5 Flash lists 256K. GPT-5.1 therefore has the larger recorded context window, but capacity alone is not a quality score.

Question 4

Which is faster: GPT-5.1 or Step 3.5 Flash?

Accepted Answer

There is no matched speed measurement for both models with the same source, host, region and date. Test the same prompts at your expected concurrency and measure time to the first useful response as well as complete task time.

Question 5

What input and output types do GPT-5.1 and Step 3.5 Flash support?

Accepted Answer

GPT-5.1 records text, image input and text output. Step 3.5 Flash records text input and text output. “Not documented” means the current source record is incomplete; it does not prove that a modality is unsupported.

Question 6

Should I choose GPT-5.1 or Step 3.5 Flash?

Accepted Answer

Consider GPT-5.1 if you need larger context. Consider Step 3.5 Flash if you need larger maximum output or a lower listed input rate or a lower listed output rate. Step 3.5 Flash offers downloadable weights; GPT-5.1 is listed as a proprietary model. Compare licence terms and operating costs before choosing a deployment. Test both on the same representative examples and judge the output against your acceptance criteria. Include tool failures, retries and the time to a completed task.

Specification	GPT-5.1	Step 3.5 Flash
Provider	OpenAI	StepFun
Listed pricing route	OpenAI	StepFun
Model family	gpt	—
Accepted inputs	text, image	text
Produced outputs	text	text
Context window	400KHigher	256K
Max output	128K	256KHigher
Input price / 1M	$1.25	$0.10Lower
Output price / 1M	$10.00	$0.30Lower
Reasoning mode	Documented	Documented
Tool calling	Documented	Documented
Image input	Documented	Not documented
Audio input	Not documented	Not documented
Structured output	Documented	Not documented
License	Proprietary	Open weights

GPT-5.1 vs Step 3.5 Flash

GPT-5.1

Step 3.5 Flash

Key differences

Pricing, limits and capabilities

Which model should you choose?

Documentation and model profiles

Questions people ask

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