Claude Fable 5.5 (Leaked Checkpoint) vs Union Alpha (Stealth)
A direct, empirical evaluation of Claude Fable 5.5 (Leaked Checkpoint) (Anthropic) against Union Alpha (Stealth) (OpenRouter & OpenCode).
Claude Fable 5.5 (Leaked Checkpoint)
Developed by Anthropic · Proprietary Frontier Dynamic Reasoner parameters
Terminal-Bench 4.0 autonomous shell execution (78.5% target), recursive self-critique, and 1.5M-token repository refactoring.
Union Alpha (Stealth)
Developed by OpenRouter & OpenCode · Multi-Model Ensemble Mixture parameters
Terminal coding agents, pull-request synthesis, multi-model consensus verification, and zero-cost autonomous refactoring.
| Metric | Claude Fable 5.5 (Leaked Checkpoint) | Union Alpha (Stealth) |
|---|---|---|
| SWE-bench Verified | 84.2% | 74.2% |
| MMLU-Pro | 94.6% | 89.8% |
| MATH-500 | 98.4% | 95.4% |
| Context Window | 1,500,000 tokens | 256,000 tokens |
| Input Pricing (per 1M) | $3.50 | $0.00 |
| Output Pricing (per 1M) | $17.50 | $0.00 |