Claude Mythos Preview vs StepFun Step 3.5 Flash
Which one should you pick? Here's the full breakdown.
Claude Mythos Preview
Anthropic's most capable model -- a gated research preview via Project Glasswing, cybersecurity-specialized. 73% success on expert CTF tasks, 32-step autonomous network attacks. Not generally available.
StepFun Step 3.5 Flash
StepFun's (China) agent-focused open-weight model -- Step 3.5 Flash launched 2026-02-01. 196B sparse MoE, ~11B active. Benchmarks slightly ahead of DeepSeek V3.2 at over 3x smaller total size. Step 3 (321B / 38B active, Apache 2.0) and Step3-VL-10B multimodal also in the family
| Category | Claude Mythos Preview | StepFun Step 3.5 Flash |
|---|---|---|
| Ease of Use | 2.0 | 6.0 |
| Output Quality | 10.0 | 8.0 |
| Value | 5.0 | 9.0 |
| Features | 9.0 | 8.0 |
| Overall | 6.5 | 7.8 |
Pricing Comparison
| Feature | Claude Mythos Preview | StepFun Step 3.5 Flash |
|---|---|---|
| Free Tier | No | Yes |
| Starting Price | Invite only | $0 |
Which Should You Pick?
Pick Claude Mythos Preview if...
- ✓Higher output quality (10 vs 8)
- ✓More features (9 vs 8)
Partner organizations in Project Glasswing doing cybersecurity research, defensive red-teaming, threat intelligence, or large-scale vulnerability triage. If your use case is legitimate cybersecurity and you have enterprise Anthropic contact, ask about Glasswing admission.
Visit Claude Mythos PreviewPick StepFun Step 3.5 Flash if...
- ✓Easier to use (6 vs 2)
- ✓Better value for money (9/10)
- ✓Has a free tier
Teams building agent systems on Chinese open-weight foundations who want something other than DeepSeek or Qwen, especially if agentic tool-use is the primary workload. Also good for Chinese-market products where StepFun's domestic tuning advantages matter. And for anyone looking to add diversity to their open-weight evaluation matrix beyond the top-3 Chinese labs.
Visit StepFun Step 3.5 FlashOur Verdict
StepFun Step 3.5 Flash is the clear winner here with 7.8/10 vs 6.5/10. Claude Mythos Preview isn't bad, but StepFun Step 3.5 Flash outperforms it across the board. Pick Claude Mythos Preview only if partner organizations in project glasswing doing cybersecurity research, defensive red-teaming, threat intelligence, or large-scale vulnerability triage.