StepFun: Step 3.5 Flash (free)

stepfun/step-3.5-flash:free

Released Jan 29, 2026256,000 context

$0/M input tokens$0/M output tokens

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 for Step 3.5 Flash (free)

OpenRouter routes requests to the best providers that are able to handle your prompt size and parameters, with fallbacks to maximize uptime.

Performance for Step 3.5 Flash (free)

Compare different providers across OpenRouter

Effective Pricing for Step 3.5 Flash (free)

Actual cost per million tokens across providers over the past hour

Apps using Step 3.5 Flash (free)

Top public apps this month

Recent activity on Step 3.5 Flash (free)

Total usage per day on OpenRouter

Prompt

194B

Reasoning

Completion

1.14B

Prompt tokens measure input size. Reasoning tokens show internal thinking before a response. Completion tokens reflect total output length.

Uptime stats for Step 3.5 Flash (free)

Uptime stats for Step 3.5 Flash (free) across all providers

Sample code and API for Step 3.5 Flash (free)

OpenRouter normalizes requests and responses across providers for you.

OpenRouter supports reasoning-enabled models that can show their step-by-step thinking process. Use the reasoning parameter in your request to enable reasoning, and access the reasoning_details array in the response to see the model's internal reasoning before the final answer. When continuing a conversation, preserve the complete reasoning_details when passing messages back to the model so it can continue reasoning from where it left off. Learn more about reasoning tokens.

In the examples below, the OpenRouter-specific headers are optional. Setting them allows your app to appear on the OpenRouter leaderboards.

Using third-party SDKs

For information about using third-party SDKs and frameworks with OpenRouter, please see our frameworks documentation.

See the Request docs for all possible fields, and Parameters for explanations of specific sampling parameters.

Step 3.5 Flash (free) - API Pricing & Providers | OpenRouter