Question 1

What is Step 3.5 Flash?

Accepted Answer

Step 3.5 Flash is a model from StepFun, released on Feb 1, 2026. 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 is a reasoning model that is incredibly speed efficient even at long contexts.

Question 2

How much does Step 3.5 Flash cost?

Accepted Answer

On the Respan gateway, input starts at $0.10 per 1M tokens on OpenRouter, and output starts at $0.30 per 1M tokens on every provider, after gateway discounts. Averaged across 3 providers, it costs $0.10 per 1M input tokens and $0.30 per 1M output tokens.

Question 3

Which providers serve Step 3.5 Flash on Respan?

Accepted Answer

3 providers serve Step 3.5 Flash: OpenRouter and EmpirioLabs.

Question 4

What is Step 3.5 Flash's context window?

Accepted Answer

Step 3.5 Flash accepts up to 262K tokens of context on its largest provider route.

Question 5

How do I use Step 3.5 Flash with the Respan gateway?

Accepted Answer

Point any OpenAI-compatible SDK at https://api.respan.ai/api/ with your Respan API key and set the model to "step-3.5-flash". To set it up with a coding agent, copy the agent setup prompt at the top of this page.

Question 6

How fast is Step 3.5 Flash?

Accepted Answer

Over the last 30 days of requests through the Respan gateway, the fastest provider was OpenRouter at 32 tokens per second, and the lowest response latency was 2.0 seconds on OpenRouter. Speed varies with request size and provider.

Question 7

Where does Step 3.5 Flash rank on the Respan Index?

Accepted Answer

Step 3.5 Flash scores 25.0 out of 100 on the Respan Index v1.1, ranking #81 of 138 models. By category: Coding 11.0, Agents 25.6, Reasoning 25.7, Knowledge & language 26.0 (estimated), and Human preference 45.3. The Respan Index combines independent and lab-reported benchmark results into one score, using benchmark data collected by llmmetric.com.

Question 8

How does Step 3.5 Flash score on benchmarks?

Accepted Answer

Step 3.5 Flash has 18 published benchmark scores, including LiveCodeBench v6 86.4%, Tau2-Bench 88.2%, and AIME 2025 98.3%. Each score links to where it was published; the data is collected by llmmetric.com.

Provider	Context	Input	Output
OpenRouter`openrouter/stepfun/step-3.5-flash`	262K	$0.10/M	$0.30/M
EmpirioLabs`empiriolabs/step-3-5-flash`	256K	$0.10/M	$0.30/M
EmpirioLabs`empiriolabs/step-3-5-flash-2603`	256K	$0.10/M	$0.30/M

Step 3.5 Flash

Providers

Certified where it counts.

Pricing

Cost by volume

Throughput and latency

Output speed

Response latency

Benchmarks

Respan Indexv1.1

Coding4

Apps

Step 3.5 Flash FAQ

Works with your entire stack

Built for AI agents.
Break less.
Ship more.

Agents9

Math4

Arena1