StepFun: Step 3.5 Flash

stepfun/step-3.5-flash

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 i...

tools

MODALITIES

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INPUT PRICE

$0.10per 1M

OUTPUT PRICE

$0.30per 1M

CONTEXT

262.1K

RELEASED

Jan 30, 2026

Provider				Cache	Uptime	Chat
FastAPI	—	$0.1	$0.3	Cache read$0.02222

Capabilities

Input modalities

text

Output modalities

text

Features

frequency_penaltyinclude_reasoningmax_tokensreasoningtemperaturetool_choicetoolstop_ktop_p

Get your API key

Create an API key from the Keys page, then set it as an environment variable:

export ONLIST_API_KEY=sk-...

Make your first request

Endpoints

POSThttps://onlist.io/v1/chat/completions

OpenAI Chat Completions format

Request Headers

Authorization:Bearer $ONLIST_API_KEY

Content-Type:application/json

Model:stepfun/step-3.5-flash

POSThttps://onlist.io/v1/responses

OpenAI Responses format

Request Headers

Authorization:Bearer $ONLIST_API_KEY

Content-Type:application/json

Model:stepfun/step-3.5-flash

POSThttps://onlist.io/v1/messages

Anthropic Messages format

Request Headers

Authorization:Bearer $ONLIST_API_KEY

Content-Type:application/json

Model:stepfun/step-3.5-flash

Code samples

curl https://onlist.io/v1/chat/completions \
  -H "Authorization: Bearer $ONLIST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
       "model": "stepfun/step-3.5-flash",
       "messages": [
         {
           "role": "user",
           "content": "Explain quantum entanglement in one paragraph."
         }
       ]
     }'

Replace $ONLIST_API_KEY with the API key from your Keys page.

Authentication

All requests must include an Authorization: Bearer <TOKEN> header. Generate keys from the Keys page; keys can be scoped to specific models, groups, IP ranges, and rate limits.

Enable streaming

Add "stream": true to receive partial responses as server-sent events in real time.

Streaming example

curl https://onlist.io/v1/chat/completions \
  -H "Authorization: Bearer $ONLIST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
       "model": "stepfun/step-3.5-flash",
       "messages": [
         {
           "role": "user",
           "content": "Write a haiku about recursion."
         }
       ],
       "stream": true
     }'

Supported parameters

Name	Type	Description
`frequency_penalty`	number	Penalizes new tokens based on their existing frequency in the text (-2.0 to 2.0).
`include_reasoning`
`max_tokens`	integer	Maximum number of tokens to generate in the completion.
`reasoning`
`temperature`	number	Sampling temperature between 0 and 2. Higher values make output more random.
`tool_choice`	string \| object	Controls which tool is called. "auto", "none", "required", or a specific function.
`tools`	array	A list of tools the model may call. Currently supports functions.
`top_k`	integer	Limits token selection to the k most likely candidates at each step.
`top_p`	number	Nucleus sampling. The model considers tokens with top_p probability mass.

These are the request parameters this model accepts. Parameter semantics follow the OpenAI Chat Completions specification.