GPT-OSS 20B is a 20.91B-parameter mixture-of-experts model (3.6B active per token). At MXFP4 it needs about 12.9 GB of memory. The MacBook Pro M5 Max 128GB has 96 GB usable, so it fits entirely in memory. Expect about 139 tokens/sec (123–154), with the first token after 0.3 s for a short prompt. This estimate is based on community benchmarks of this exact setup.
| Quantization | File size | Memory | Generation | First token | Source |
|---|---|---|---|---|---|
| MXFP4 | 12.1 GB | Fits | 139 tok/s | 0.3 s | Measured |
All models on the MacBook Pro M5 Max 128GB · GPT-OSS 20B on all hardware
Estimates for a short prompt on a single device. Measured numbers come from community benchmarks (r/LocalLLaMA, localscore.ai and more); others are derived from similar setups or projected from specs. Open source on GitHub.