Step 3.5 Flash (Free) API
About
Build advanced reasoning agents with Step 3.5 Flash (Free), a specialized AI model optimized for complex logic and chain-of-thought tasks. This model delivers available completely free of charge, native tool calling support. Access Step 3.5 Flash (Free) via the ZenMux API with up to 64K output tokens.
Capabilities
Input Modalities
text
Output Modalities
text
Reasoning
Yes
Structured Output
No
Tool Use
Yes
WeightsClosed
Temperature
Adjustable
Attachment
Not Supported
Limits
Context Window
256K
Tokens
Input Limit
256K
Tokens
Max Output
64K
Tokens
Pricing
Standard (per 1M tokens)
Input
Free
Output
Free
Frequently Asked Questions
Step 3.5 Flash (Free) by ZenMux costs Free per 1M input tokens and Free per 1M output tokens.
Model Details
ID
stepfun/step-3.5-flash-free
Provider
ZenMuxFamily
Release Date
Feb 2, 2026
Knowledge Cutoff
Jan 1, 2025
API Integration
NPM Package
@ai-sdk/openai-compatible
Environment Variables
ZENMUX_API_KEY
API Base URL
https://zenmux.ai/api/v1