> ## Documentation Index
> Fetch the complete documentation index at: https://platform.stepfun.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Step 3.5 Flash

`step-3.5-flash` is StepFun's flagship language reasoning model. It delivers top-tier reasoning quality and fast, reliable execution — decomposing and planning complex tasks, and reliably orchestrating tool calls. Suitable for logical reasoning, math, software engineering, deep research, and other complex workloads. Built on a 196B-parameter / 11B-activation sparse MoE architecture.

## Key facts

> [card] Model type
>
> Sparse MoE architecture\
> 196B total params / 11B activated params

> [card] Context length
>
> 256K tokens

> [card] Best for
>
> High-throughput reasoning + tool calling\
> Optimized for agent and coding workloads

## Core capabilities

> [card] 🚀 High-throughput reasoning
>
> Sparse MoE architecture delivers high throughput and low latency, ideal for real-time agent workflows and high-volume calls.

> [card] 🛠️ Tool calling
>
> Reliable `tools` / `tool_choice` orchestration, supports multi-step task decomposition and plan execution.

> [card] 🧠 Complex reasoning
>
> Handles logical reasoning, math, software engineering, and deep research — a dependable foundation for long-chain agent reasoning.

## Model variants

> [card] step-3.5-flash
>
> **Base version**: general-purpose reasoning and tool calling — well-suited to most Agent and complex-task scenarios.

> [card] step-3.5-flash-2603
>
> **Agent-optimized version**: tuned from `step-3.5-flash` for high-frequency agent scenarios. Better token efficiency and faster inference, with an optional low-reasoning mode that significantly reduces token consumption. Improved compatibility with coding workflows and agent frameworks. Supports the `reasoning_effort` field (`low` / `high`).

## API endpoint

> [card] [Chat Completion](https://platform.stepfun.ai/docs/en/api-reference/chat/chat-completion-create.md)
>
> `POST /v1/chat/completions`\
> OpenAI-compatible, with streaming and tool calling.

## Pricing

| Item | Price (per million tokens) |
| :- | :- |
| Input (cache miss) | \$0.10 |
| Input (cache hit) | \$0.02 |
| Output | \$0.30 |

[See full pricing details →](https://platform.stepfun.ai/docs/en/guides/pricing/details.md)

## Quickstart

> [tab] curl
>
> ```bash theme={"theme":{"light":"light-plus","dark":"dark-plus"}}
> curl https://api.stepfun.ai/v1/chat/completions \
>   -H "Authorization: Bearer $STEP_API_KEY" \
>   -H "Content-Type: application/json" \
>   -d '{
>     "model": "step-3.5-flash",
>     "messages": [
>       {"role": "user", "content": "Hello, please introduce yourself."}
>     ]
>   }'
> ```

> [tab] Python (OpenAI SDK)
>
> ```python theme={"theme":{"light":"light-plus","dark":"dark-plus"}}
> from openai import OpenAI
>
> client = OpenAI(
>     api_key="YOUR_STEP_API_KEY",
>     base_url="https://api.stepfun.ai/v1",
> )
>
> response = client.chat.completions.create(
>     model="step-3.5-flash",
>     messages=[
>         {"role": "user", "content": "Hello, please introduce yourself."}
>     ],
> )
>
> print(response.choices[0].message.content)
> ```

## Related reading

> [card] [Reasoning model guide](https://platform.stepfun.ai/docs/en/guides/developer/reasoning.md)
>
> Recommended usage of reasoning models for complex tasks, tool calling, and long contexts.

> [card] [Step 3.7 Flash — multimodal flagship](https://platform.stepfun.ai/docs/en/guides/models/step-3.7-flash.md)
>
> Building on Step 3.5 Flash with native image and video understanding.


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