Step 3.5 Flash (Free)

Reasoning Tool Calling Open Weights

Step 3.5 Flash is StepFun's most capable open-source foundation model. Built on a sparse Mixture of Experts (MoE) architecture, it selectively activates only 11B of its 196B parameters per token. It is a reasoning model that is incredibly speed efficient even at long contexts.

Providers 2

Released Jan 29, 2026

Input Modalities text

Output Modalities text

Tarsk Use coding

Available Providers (2)

Provider	Model ID	Input Cost	Output Cost	Context	Max Output	Docs
ZenMux	`stepfun/step-3.5-flash-free`	$0.00/MTok	$0.00/MTok	256K	64K
OpenRouter	`stepfun/step-3.5-flash:free`	$0.00/MTok	$0.00/MTok	256K	256K

Capabilities

Reasoning

Tool Calling

Attachments

Open Weights

Structured Output