Step 3.5 Flash 2603
StepFun
- Released
- April 2, 2026
- Data date
- September 26, 2026
Step 3.5 Flash 2603 is StepFun’s large sparse Mixture-of-Experts foundation model for reasoning, coding, and agentic work. Its official card lists 196.81 billion total parameters and about 11 billion active per token. MTP-3 and a 256,000-token context are intended to combine fast generation with long workflows. This checkpoint is for text-to-text open Apache-2.0 weights, not a multimodal Step configuration.
StepFun currently recommends a nightly vLLM path and notes that full MTP-3 support is not yet available there. SGLang, Transformers, and llama.cpp are also documented. API access through StepFun and OpenRouter is listed, while local serving has to account for the 196.81 billion total parameters and the backend support state.
Specifications and access
| Specification | Value and source |
|---|---|
| Weights | Open weightsSource |
| Access | APISource |
| Parameters | 196.81 billionSource |
| Active parameters | 11 billionSource |
| Context window | 256,000 tokensSource |
| Maximum output | 256,000 tokensSource |
| Knowledge Cutoff | January 2025Source |
| License | Apache 2.0Source |
| Inputs | TextSource |
| Outputs | TextSource |
| Reasoning | YesSource |
| Tool calling | YesSource |
Popularity
17.9 / 100
Sources and data date
| Type | Evidence and data date |
|---|---|
| Additional source | models.dev catalog (retrieved September 26, 2026) |
| Additional source | Hugging Face · Step 3.5 Flash model card · Editorial description reviewed October 2, 2026 |
| Additional source | Step 3.5 Flash technical paper · Editorial description reviewed October 2, 2026 |
| Additional source | OpenRouter: Popularity index (derived), CC BY 4.0 |
| Additional source | Vercel AI Gateway: Popularity index (derived), CC BY 4.0 |
| Additional source | Creative Commons Attribution 4.0 |