How is Step 3.5 Flash billed?

Input is $0.130 and output is $0.390 per million tokens; cached input is $0.065. The charge for every request is listed in Usage Logs in the console.

How long is the context window?

262K tokens. In a long conversation every turn is billed for the full input, so keep the history under control.

Does it support streaming?

Yes. Add "stream": true to the request to receive server-sent events in the OpenAI format.

StepFun: Step 3.5 Flash

step-3.5-flash

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Step 3.5 Flash costs $0.130 per million input tokens and $0.390 per million output tokens, supports 262K context, and is available through the OpenAI-compatible API.

StepFun's most capable open-source reasoning model with visible reasoning traces. Built on a sparse Mixture-of-Experts architecture with 196B total parameters and only 11B active per token, it achieves frontier-level performance in math, logic, and agentic coding while reaching up to 350 tokens/sec. Supports 256K context. NOTE: This model runs via StepFun, which may log and train on your prompts.

Modalities

Text → Text

Input / output price

$0.130 / $0.390 per 1M

Context

262K · max output 66K

Capabilities

ReasoningLong contextOpen weights

Pricing

Billed per token actually used and deducted from your credits.

Item	USD per 1M tokens
Input	$0.130
Output	$0.390
Cached input	$0.065

Calculator

Drag the sliders to estimate what this model will cost.

Input per request 3K tokensOutput per request 800 tokensRequests per day 500Cached input 0%(higher with long chats or a fixed system prompt)

Per request: $0.00070
Per day: $0.3510
Per month (30 days): $10.53

Step 3.5 Flash: input $0.13/M · output $0.39/M · cached input $0.065/M

Request with these settings

curl https://api.geniusrelaygpt.com/v1/chat/completions \
  -H "Authorization: Bearer $API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model": "step-3.5-flash", "max_tokens": 800,
       "messages": [{"role": "user", "content": "…"}]}'

Code samples

OpenAI-compatible. Set the base URL to https://api.geniusrelaygpt.com/v1 and the model to step-3.5-flash.

curl https://api.geniusrelaygpt.com/v1/chat/completions \
  -H "Authorization: Bearer $API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "step-3.5-flash",
    "stream": true,
    "messages": [{"role": "user", "content": "Hello"}]
  }'

Use it in a client: Cursor · Cline · SillyTavern · Codex

Parameters

Common OpenAI parameters can be passed as-is.

temperaturetop_pmax_tokensstreamstoppresence_penaltyfrequency_penaltyseed

FAQ

How is Step 3.5 Flash billed?: Input is $0.130 and output is $0.390 per million tokens; cached input is $0.065. The charge for every request is listed in Usage Logs in the console.
How long is the context window?: 262K tokens. In a long conversation every turn is billed for the full input, so keep the history under control.
Does it support streaming?: Yes. Add "stream": true to the request to receive server-sent events in the OpenAI format.

Pricing

Calculator

Code samples

Parameters

Related models

FAQ