Step-3.5 Flash

by stepfun · family step

Facts

Release date	2026-01-29
Retired date	—
Knowledge cutoff	—
Context (max)	262,144
Max output	—
Modalities	text → text
Aliases	—
Open weights	yes (stepfun-ai/Step-3.5-Flash)
Description	196B/11B MoE, 256K ctx (3:1 sliding:full attention), MTP-3 decoding, Parallel Thinking reasoning.

Unknown values are shown as —.

Intelligence Index v4.1.1: 27 · estimated · Source: Artificial Analysis (accessed 2026-08-20)

Offerings

Provider	Wire ID	Variant	Endpoint	Protocol	Status	Price (USD / Mtok)	Reasoning
OpenRouter	`stepfun/step-3.5-flash`	flash	chat-completions	openai-chat	ga	0.10 / 0.30	effort · default on · mandatory · returns reasoning_content · round-trips reasoning_content
StepFun	`step-3.5-flash`	flash	chat-completions	openai-chat	ga	0.10 / 0.30	adaptive · default on · returns reasoning_content

Reasoning controls

OpenRouter· stepfun/step-3.5-flash· chat-completions effort

reasoning.effort minimallowmedium●highxhigh

effort · default on · mandatory · returns reasoning_content · round-trips reasoning_content

StepFun· step-3.5-flash· chat-completions adaptive

adaptive · default on · returns reasoning_content

Example requests

OpenRouter · openai-chat · stepfun/step-3.5-flash

curl 'https://openrouter.ai/api/v1/chat/completions' \
  -H 'Authorization: Bearer <YOUR_API_KEY>' \
  -H 'Content-Type: application/json' \
  -d '{
  "model": "stepfun/step-3.5-flash",
  "messages": [
    {
      "role": "user",
      "content": "Tell me about the weather."
    }
  ],
  "reasoning": {
    "effort": "medium"
  }
}'

Python (requests)

import requests

resp = requests.post(
    "https://openrouter.ai/api/v1/chat/completions",
    headers={
      "Authorization": "Bearer <YOUR_API_KEY>",
      "Content-Type": "application/json"
    },
    json={
      "model": "stepfun/step-3.5-flash",
      "messages": [
        {
          "role": "user",
          "content": "Tell me about the weather."
        }
      ],
      "reasoning": {
        "effort": "medium"
      }
    },
)
print(resp.json())

TypeScript (fetch)

const resp = await fetch("https://openrouter.ai/api/v1/chat/completions", {
  method: "POST",
  headers: {
    "Authorization": "Bearer <YOUR_API_KEY>",
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "model": "stepfun/step-3.5-flash",
    "messages": [
      {
        "role": "user",
        "content": "Tell me about the weather."
      }
    ],
    "reasoning": {
      "effort": "medium"
    }
  }),
})
console.log(await resp.json())

⚠ Responses include reasoning_content artifacts — pass them back unmodified on subsequent turns (dropping them triggers 400s or quality loss).

StepFun · openai-chat · step-3.5-flash

curl 'https://api.stepfun.ai/v1/chat/completions' \
  -H 'Authorization: Bearer <YOUR_API_KEY>' \
  -H 'Content-Type: application/json' \
  -d '{
  "model": "step-3.5-flash",
  "messages": [
    {
      "role": "user",
      "content": "Tell me about the weather."
    }
  ]
}'

Python (requests)

import requests

resp = requests.post(
    "https://api.stepfun.ai/v1/chat/completions",
    headers={
      "Authorization": "Bearer <YOUR_API_KEY>",
      "Content-Type": "application/json"
    },
    json={
      "model": "step-3.5-flash",
      "messages": [
        {
          "role": "user",
          "content": "Tell me about the weather."
        }
      ]
    },
)
print(resp.json())

TypeScript (fetch)

const resp = await fetch("https://api.stepfun.ai/v1/chat/completions", {
  method: "POST",
  headers: {
    "Authorization": "Bearer <YOUR_API_KEY>",
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "model": "step-3.5-flash",
    "messages": [
      {
        "role": "user",
        "content": "Tell me about the weather."
      }
    ]
  }),
})
console.log(await resp.json())