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Favicon for stepfun

StepFun: Step 3.5 Flash

stepfun/step-3.5-flash

Model weights
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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.

Modalities
In / out price
$0.10 / $0.30per 1M
Context
262K
Released
Jan 29, 2026
Compare
ProvidersPricingPerformanceUptimeBenchmarksAppsActivityFAQExplore

Providers

This model is hosted by one provider. OpenRouter forwards every request to it directly — no routing decisions to make.

Pricing

The average price customers actually pay for this model, next to the prices providers post. Caching and discounts mean the price actually paid is often well below the listed one.

Performance

Throughput is how fast the model writes (tokens per second — higher is better). Latency is total round-trip time (lower is better). TTFT is time-to-first-token — how long before you see anything appear (lower is better).

Uptime

Uptime is the percentage of the past 3 days that at least one provider was responding to requests. Availability is the percentage of time that inference was successfully served. OpenRouter continuously monitors and uses the next-best provider when one returns an error.

Benchmarks

Scores on standardized evaluations and this model’s own outputs on shared benchmark prompts. Higher percentages are better — and each rank shows where this model lands among the models evaluated on that source.

Benchmark score summary for StepFun: Step 3.5 Flash (Artificial Analysis)
SourceBenchmarkScore
Artificial AnalysisStep 3.5 Flash 2603 GPQA Diamond82.6%
Artificial AnalysisStep 3.5 Flash 2603 HLE24.5%
Artificial AnalysisStep 3.5 Flash 2603 IFBench66.5%
Artificial AnalysisStep 3.5 Flash 2603 τ²-Bench Telecom87.4%
Artificial AnalysisStep 3.5 Flash 2603 AA-LCR63.0%
Artificial AnalysisStep 3.5 Flash 2603 CritPt2.3%
Artificial AnalysisStep 3.5 Flash 2603 Terminal-Bench Hard32.6%
Artificial AnalysisStep 3.5 Flash GPQA Diamond83.1%
Artificial AnalysisStep 3.5 Flash HLE21.1%
Artificial AnalysisStep 3.5 Flash IFBench64.6%
Artificial AnalysisStep 3.5 Flash τ²-Bench Telecom94.4%
Artificial AnalysisStep 3.5 Flash AA-LCR50.0%
Artificial AnalysisStep 3.5 Flash CritPt2.5%
Artificial AnalysisStep 3.5 Flash Terminal-Bench Hard27.3%

Apps

Public apps that send the most traffic to this model. Good signal for what real production workloads look like — and a hint at which use cases this model is best suited for.

Activity

Token volume and request traffic to this model over time.

API

Drop-in code to call this model. OpenRouter's API is OpenAI-compatible — most SDKs work by just swapping the base URL. The only thing that changes between models is the model slug below.

Not currently available through EU in-region routing. In-region routing docs

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Availability over the last 3 days

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Availability over the last 24 hours

OpenRouter Availability
94.34%

When an error occurs in an upstream provider, we can recover by routing to another healthy provider, if your request filters allow it. You can access per-provider uptime data programmatically through the Endpoints API. Learn more about our load balancing and provider routing options.

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Frequently asked questions

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.

Step 3.5 Flash costs $0.10/M input tokens and $0.30/M output tokens.

Step 3.5 Flash has a 262,144 token context window. It supports up to 65,536 completion tokens.

Yes. Step 3.5 Flash accepts tools and tool_choice for function calling. It does not support response_format, so JSON output is not enforced.

Step 5 Preview and Step 3.7 Flash are other text models from StepFun.

Step 3.5 Flash was released on January 29, 2026.

More models from stepfun

Step 5 Preview

Step 5 Preview is StepFun's flagship model for agentic work, built on a sparse Mixture-of-Experts architecture (27B active / 600B total parameters). It performs strongly in software engineering and professional knowledge work, with particular strength in finance.

It is designed for extended tasks that span large codebases and documents, using tools and refining results over multiple steps.

Text1.0M context$1 / $2.70
Step 3.7 Flash

Step 3.7 Flash is StepFun's latest high-efficiency multimodal Mixture-of-Experts model. It pairs a 196B-parameter language backbone with a vision encoder for native image and video understanding, activating roughly 11B parameters per token. The model supports a 256K context window and exposes selectable reasoning levels (high/medium/low), letting callers trade off speed, cost, and depth of reasoning.

Designed for coding, agentic workflows, structured outputs, and long-context productivity tasks.

Text262K context$0.20 / $1.15