Claude Sonnet 4 vs Step-3.5-Flash

Claude Sonnet 4

Anthropic

Claude Sonnet 4, released by Anthropic in May 2025, is a large language model from the Claude 4 family that delivers a balance of performance and efficiency for coding, reasoning, and analytical tasks. It features a 200K token context window (extendable to 1M tokens in beta), 64K maximum output tokens, native image understanding, and extended thinking support. Sonnet 4 targets development workflows, document analysis, and applications that benefit from the performance characteristics of the Claude 4 generation.

StepFun

Step-3.5-Flash, released by StepFun on February 2, 2026, is a Mixture-of-Experts large language model with 197 billion total parameters and approximately 11 billion active parameters per inference. It features a 256K token context window using a 3:1 sliding-window-to-full-attention ratio, processing 100–350 tokens per second. Step-3.5-Flash targets agentic tasks, coding workflows, and open-source deployments requiring frontier reasoning capabilities with efficient inference, under an Apache 2.0 license.