StepFun: Step 3.5 Flash
stepfun/step-3.5-flash
Try Step 3.5 Flash for free in your browser, and add it to your app for free with Puter.js AI API.
Try it free Add to your app// 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>
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 256K
tokens
Max Output 256K
tokens
Input Cost $0.1
per million tokens
Output Cost $0.3
per million tokens
Release Date Jan 29, 2026
Try Step 3.5 Flash for free
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 | 24.5% |
| IFBench Instruction following | 66.5% |
| LCR Long-context reasoning | 63.0% |
| 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.
Step 3.5 Flash is free to try with a Puter account. Every account includes a free AI allowance, and you can chat with it in the playground on this page. You can upgrade your account anytime for a larger allowance.
Step 3.5 Flash is free to integrate using the Puter.js AI API. With the User-Pays Model, you can add AI to your app for $0, since users cover their own AI usage through their Puter account.
| Price per 1M tokens | |
|---|---|
| Input | $0.1 |
| Output | $0.3 |
Step 3.5 Flash was created by StepFun and released on Jan 29, 2026.
Step 3.5 Flash supports a context window of 256K tokens. For reference, that is roughly equivalent to 512 pages of text.
Step 3.5 Flash can generate up to 256K tokens in a single response.
Step 3.5 Flash scores 17.0 on the Artificial Analysis Intelligence Index, outperforming 63% of tracked 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.
Add Step 3.5 Flash to your app for free
Developers can integrate Step 3.5 Flash for free using the Puter.js AI API.
With the User-Pays Model, each user covers their own AI usage instead of the developer.