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stepfun · Open weights

Step 3.5 Flash

Added Feb 2, 2026Open weights

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Use in the app

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.

  • Reasoning
Input
Text
Output
Text
Discovery tier
Open weights

Pricing

USD per 1M tokens
Input
$0.10
Output
$0.30

Reference prices as published by the model provider. API example

Specifications

Context window
262.1K
Max output
65.5K
Average output (7 days)
2.4K tokens
Longer than 94% of 465 models

Side by side

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TryDeepSeek V3.2 Exp ThinkingGemma 4 26B A4BOpenAI o4-mini highAll 3

Public evaluations

Benchmarks

Independent evaluation results, with how each score ranks among the models in the Noumen catalog.

Intelligence Index

16.6

Better than 43% of 326 scored models

3.8Catalog models by score55.1

Document reasoning

  • AA-LCRv1.1

    50.0%

    Reasoning across long documents

    Better than 29% of 309 scored models

Reasoning and knowledge

  • Humanity’s Last Exam

    21.1%

    Expert-level questions across academic fields

    Better than 49% of 323 scored models

  • GPQA Diamond

    83.1%

    Graduate-level science questions

    Better than 54% of 291 scored models

  • IFBench

    64.6%

    Following precise output instructions

    Better than 64% of 215 scored models

Legacy evaluations

  • Terminal-Bench Hard

    27.3%

    Earlier terminal task suite

    Better than 52% of 212 scored models

  • τ²-Bench Telecom

    94.4%

    Tool use in a customer-support simulation

    Better than 84% of 214 scored models

  • AA-LCR (unversioned)

    50.0%

    Long-context reasoning, earlier grading

    Better than 29% of 309 scored models

Scores from Artificial Analysis, updated Oct 26, 2026. Artificial Analysis

Live routing data

Performance and reliability

Recent averages across requests routed for this model. Your own speed depends on prompt length, reasoning effort and load.

Prompt cache hits
0.0%
Request success (24h)
100.0%
Reliable

Cache hits are the share of input tokens recently served from a prompt cache across all requests. They are not a forecast of your own hit rate.

Hourly request success · last 72 hoursLowest hour 100.0%
  • Healthy (98% or more)
  • Degraded (90–98%)
  • Down (under 90%)
  • No requests

Developers

Use it through the API

Noumen’s API is OpenAI-compatible: any Chat Completions client works with a base URL and a key.

Read the quickstart
curl https://www.noumen.xyz/api/v1/chat/completions \
  -H "Authorization: Bearer $NOUMEN_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "stepfun-ai/step-3.5-flash",
    "messages": [{ "role": "user", "content": "Explain this stack trace." }]
  }'
import os
from openai import OpenAI

client = OpenAI(base_url="https://www.noumen.xyz/api/v1", api_key=os.environ["NOUMEN_API_KEY"])
reply = client.chat.completions.create(
    model="stepfun-ai/step-3.5-flash",
    messages=[{"role": "user", "content": "Explain this stack trace."}],
)
print(reply.choices[0].message.content)

stepfun

Other stepfun models, starting with the same family.