Step 3.5 Flash
By StepFun · China
Updated 2026-07-13
Overview
StepFun's 196B MoE with 11B active parameters delivers 100 tokens/sec at 128K context. Ranks #3 by free-tier volume on OpenRouter under Apache 2.0.
When to pick this model
- High-throughput chat backends
- Long-context workloads needing fast inference
- Apache-licensed commercial deployments
- Cost-sensitive production at scale
- Workloads where latency matters more than top quality
VRAM requirements by quantization
| Quantization | VRAM required |
|---|---|
| Q4_K_M (recommended) | 118 GB |
| Q5_K_M | 141 GB |
| Q8_0 | 210 GB |
| FP16 (no quantization) | 392 GB |
VRAM figures include model weights plus a typical 8k KV cache and ~600 MB runtime overhead (Ollama / llama.cpp baseline). Add headroom for higher context lengths.
In practice, Step 3.5 Flash is server-class even at Q4_K_M (118 GB). Stepping up to Q8_0 nearly doubles the footprint to 210 GB, and unquantized FP16 weights take 392 GB — plan your GPU around the Q4 or Q5 figure unless you specifically need the higher fidelity.
Without a GPU, Step 3.5 Flash needs roughly 140 GB of system RAM to run on CPU via llama.cpp or Ollama — workable for background jobs, but far slower than GPU inference. Throughput estimates from our compatibility engine: around 2 tokens/sec on entry-level GPUs, on the order of 10 tokens/sec on a mid-range card, and up to 25 tokens/sec on high-end hardware — assuming the chosen quantization fully fits in VRAM.
What hardware do you need
The table below matches Step 3.5 Flash to common GPU memory tiers, using the highest-fidelity quantization that fully fits each card class. Spilling layers to system RAM works but costs most of the speed, so size your card to the quantization you actually want to run.
| GPU memory | Example cards | Best fit for Step 3.5 Flash |
|---|---|---|
| 8 GB | RTX 5070 Laptop, RTX 5060, RTX 5060 Ti 8GB | Does not fit — needs 118 GB at Q4_K_M |
| 12 GB | RTX 5070, RTX 5070 Ti Laptop, RTX 4080 Laptop | Does not fit — needs 118 GB at Q4_K_M |
| 16 GB | RTX 5080, RTX 4080 Super, Radeon RX 9070 XT | Does not fit — needs 118 GB at Q4_K_M |
| 24 GB | RTX 4090, Radeon RX 7900 XTX, RTX 5090 Laptop | Does not fit — needs 118 GB at Q4_K_M |
| 32 GB | RTX 5090 | Does not fit — needs 118 GB at Q4_K_M |
Which GPU should you buy to run Step 3.5 Flash?
To run Step 3.5 Flash locally at Q4, you need ~118 GB of VRAM. The best value for this is a Apple Mac Studio (64+ GB unified memory).
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Strengths
- 100 tokens/sec sustained at 128K context
- 256K maximum context window
- Only 11B active parameters
- Apache 2.0 license
Limitations
- 118GB+ in Q4 needs a multi-GPU server
- Brand awareness still low outside Asia
- Trails top open models on hardest benchmarks
Typical workloads
In our catalog grid, Step 3.5 Flash is filed under Fast MoE, Long Context — the use cases where its size/quality trade-off makes the most sense. Its tags translate to concrete workloads: everyday chat, drafting and summarization.
The 250k-token context window is large enough to hold entire codebases' worth of files or long reports in a single prompt, which is what makes local RAG and document analysis practical. The Apache 2.0 license is permissive, so shipping it inside a commercial product raises no special legal questions.
Architecture & training
Architecture: MoE 196B/11B active · 256k ctx
Training: StepFun. 100 tok/s at 128k ctx.
A fast, permissively licensed MoE that punches well above its name recognition.
Quick start
# HuggingFace : stepfun-ai/step-3.5-flashOr use the open-source MCP server to query this model from Claude Desktop, Cursor, or any MCP-compatible client.
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Frequently asked questions
How much VRAM does Step 3.5 Flash need?
At the recommended Q4_K_M quantization, Step 3.5 Flash needs about 118 GB of VRAM. Q8_0 takes 210 GB, and unquantized FP16 weights take 392 GB.
Can Step 3.5 Flash run without a GPU?
Yes — with roughly 140 GB of system RAM it runs CPU-only through llama.cpp or Ollama. Expect a fraction of GPU speed, which is fine for background or batch jobs but slow for interactive chat.
What context window does Step 3.5 Flash support?
Step 3.5 Flash supports a 250k-token context window (256,000 tokens).
Can I use Step 3.5 Flash commercially?
Yes. Step 3.5 Flash is released under Apache 2.0, a permissive open-source license that allows commercial use, modification and redistribution.
How fast is Step 3.5 Flash on consumer hardware?
Our compatibility engine estimates on the order of 10 tokens/sec on a mid-range GPU and up to 25 tokens/sec on high-end cards, assuming the quantization fully fits in VRAM.
Which quantization of Step 3.5 Flash should I download first?
Start with Q4_K_M (118 GB) — the standard size/quality sweet spot. Step up to Q5_K_M or Q8_0 only if you have VRAM headroom. It does not fit a single 24 GB consumer card — plan for multi-GPU or server hardware.