UltraGPT
StepFun: Step 3.5 Flash
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....
Intelligence overview
262.1K tok
Context window
65.5K tok
Max output
What StepFun: Step 3.5 Flash can do.
Reasoning
Extended reasoning
Works through problems step by step before answering, trading latency for accuracy on harder tasks.
Development
Tool calling
Calls external functions mid-response, then continues reasoning from the result.
Streaming
Streams tokens as they're generated instead of waiting on the full response.
Pay for what you use.
Every model in UltraGPT runs behind one flat subscription — this is the underlying per-token cost the catalog tracks.
$0.1 / 1M tok
$0.3 / 1M tok
Built for serious work.
Build
Generate and refactor code, with the ability to run and verify tool calls.
Think
Works through multi-step problems with extended reasoning before it answers.
Research
Holds up to 262.1K tokens of source material in context without losing the thread.
Automate
Chains tool calls across multi-step workflows, reasoning between each one.
Ask StepFun: Step 3.5 Flash anything.
A preview of the real UltraGPT composer — the model you see here is the model you get.
Continues in the UltraGPT app — one login, every model.
How StepFun: Step 3.5 Flash stacks up.
The closest models in the catalog by provider, capability and score — real entries, pulled from the same directory.
Technical details.
Specification
- Context window
- 262.1K tokens
- Max output
- 65.5K tokens
- Input modalities
- Text
- Output modalities
- Text
- Knowledge cutoff
- Jan 1, 2025
- Release date
- Jan 29, 2026
- Streaming
- Supported
- Tool calling
- Supported
- JSON mode
- Not supported
More from StepFun, and everything else.
This model is one entry in the full UltraGPT catalog — browse the rest, compare pricing and context windows, or filter by capability.