StepFun: Step 3.5 Flash
stepfun/step-3.5-flash
Access Step 3.5 Flash from StepFun using Puter.js AI API.
Get Started// npm install @heyputer/puter.js
import { puter } from '@heyputer/puter.js';
puter.ai.chat("Explain quantum computing in simple terms", {
model: "stepfun/step-3.5-flash"
}).then(response => {
document.body.innerHTML = response.message.content;
});
<html>
<body>
<script src="https://js.puter.com/v2/"></script>
<script>
puter.ai.chat("Explain quantum computing in simple terms", {
model: "stepfun/step-3.5-flash"
}).then(response => {
document.body.innerHTML = response.message.content;
});
</script>
</body>
</html>
# pip install openai
from openai import OpenAI
client = OpenAI(
base_url="https://api.puter.com/puterai/openai/v1/",
api_key="YOUR_PUTER_AUTH_TOKEN",
)
response = client.chat.completions.create(
model="stepfun/step-3.5-flash",
messages=[
{"role": "user", "content": "Explain quantum computing in simple terms"}
],
)
print(response.choices[0].message.content)
curl https://api.puter.com/puterai/openai/v1/chat/completions \
-H "Content-Type: application/json" \
-H "Authorization: Bearer YOUR_PUTER_AUTH_TOKEN" \
-d '{
"model": "stepfun/step-3.5-flash",
"messages": [
{"role": "user", "content": "Explain quantum computing in simple terms"}
]
}'
Model Card
Step 3.5 Flash is an open-source reasoning model from StepFun, built on a sparse Mixture-of-Experts (MoE) architecture with 196B total parameters but only 11B active per token. It supports a 256K-token context window and native tool calling.
The model is purpose-built for agentic and coding workflows, with generation throughput of 100–300 tokens/sec in typical usage. It scores 74.4% on SWE-bench Verified, 97.3 on AIME 2025, 86.4% on LiveCodeBench-V6, and 88.2 on τ²-Bench.
Step 3.5 Flash is a strong choice for developers building AI agents, code assistants, or multi-step reasoning pipelines who need frontier-level intelligence at low per-token cost.
Context Window 262K
tokens
Max Output 66K
tokens
Input Cost $0.1
per million tokens
Output Cost $0.3
per million tokens
Release Date Feb 1, 2026
Output Speed 157
tokens / sec
Latency 0.80s
time to first token
Model Playground
Try Step 3.5 Flash instantly in your browser.
This playground uses the Puter.js AI API — no API keys or setup required.
Benchmarks
How Step 3.5 Flash performs on standard evaluations.
| Benchmark | Score |
|---|---|
| GPQA Diamond Graduate-level science Q&A | 82.6% |
| Humanity's Last Exam Cross-domain reasoning | 22.6% |
| SciCode Scientific programming | 38.5% |
| IFBench Instruction following | 66.5% |
| LCR Long-context reasoning | 54.3% |
| Terminal-Bench Hard Agentic terminal tasks | 32.6% |
| τ²-Bench Tool use / agents | 87.4% |
Scores sourced from Artificial Analysis.
Frequently Asked Questions
You can access Step 3.5 Flash by StepFun through Puter.js AI API. Include the library in your web app or Node.js project and start making calls with just a few lines of JavaScript — no backend and no configuration required. You can also use it with Python or cURL via Puter's OpenAI-compatible API.
Yes, it is free if you're using it through Puter.js. With the User-Pays Model, you can add Step 3.5 Flash to your app at no cost — your users pay for their own AI usage directly, making it completely free for you as a developer.
| Price per 1M tokens | |
|---|---|
| Input | $0.1 |
| Output | $0.3 |
Step 3.5 Flash was created by StepFun and released on Feb 1, 2026.
Step 3.5 Flash supports a context window of 262K tokens. For reference, that is roughly equivalent to 524 pages of text.
Step 3.5 Flash can generate up to 66K tokens in a single response.
Step 3.5 Flash scores 38.5 on the Artificial Analysis Intelligence Index, outperforming 82% of tracked models. On coding, it scores 34.6 (outperforms 79% of models).
Yes — the Step 3.5 Flash API works with any JavaScript framework, Node.js, or plain HTML through Puter.js. Just include the library and start building. See the documentation for more details.
Get started with Puter.js
Add Step 3.5 Flash to your app without worrying about API keys or setup.
Read the Docs View Tutorials