head to head · open source
faiss vs Arize Phoenix
faiss has 40,939 GitHub stars, 4,527 forks, 327 open issues and last shipped yesterday. Arize Phoenix has 11,544 stars, 1,140 forks, 970 open issues and last shipped yesterday. faiss leads on adoption by 255% (40,939 vs 11,544 stars). faiss is written in C++ under MIT; Arize Phoenix is written in Python under a custom or non-standard licence. faiss has attracted 11% as many forks as stars, Arize Phoenix 10%. faiss was the more recently maintained of the two, and both are self-hostable with no licence fee.
Two open source projects, one decision. Both are free and self-hostable — the differences are community size, license terms, language stack and release pace.
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Side by side
| faiss | Arize Phoenix | |
|---|---|---|
| GitHub stars | ★ 41K | ★ 12K |
| License | MIT | Custom / other |
| Written in | C++ | Python |
| Last push | 2026-09-19 | 2026-09-19 |
| Forks | ⑂ 4.5K | ⑂ 1.1K |
| Self-hosting | Yes | Yes |
| Data ownership | Your server | Your server |
pick faiss if
- You weight community size — 41K stars and counting
- You want the MIT license terms
- Your stack matches C++
- You value the larger contributor base for long-term maintenance
pick Arize Phoenix if
- You want the Arize Phoenix feature set and don't need the biggest community
- You prefer the Custom / other license terms
- Your stack matches Python
- You evaluated both and Arize Phoenix fits your workflow better
About faiss
Faiss is an MIT licensed C++ library for efficient similarity search and clustering of dense vectors, with complete Python and NumPy wrappers, developed primarily at Meta's Fundamental AI Research group, and it is intended for engineers and researchers who need nearest neighbour search over vector sets that range from small collections to sets that possibly do not fit in RAM.
read the full faiss overview →
About Arize Phoenix
Arize Phoenix is an open source AI observability platform for experimentation, evaluation, and troubleshooting. It lives in the machine learning infrastructure ecosystem and is written in Python. The project provides tracing, evaluation, datasets, experiments, prompt management, and a playground for LLM applications.
read the full Arize Phoenix overview →
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Frequently asked questions
Is faiss or Arize Phoenix more popular?
faiss has 40,939 GitHub stars and Arize Phoenix has 11,544. faiss has the larger community by that measure.
Are faiss and Arize Phoenix free?
Both are open source. faiss is licensed under MIT, and Arize Phoenix has no licence declared in this registry. Both are free to self-host.
What is the difference between faiss and Arize Phoenix?
faiss is written in C++ and Arize Phoenix in Python. The practical differences are community size, licence terms, language stack and release cadence — all compared in the table above.
Which should I choose, faiss or Arize Phoenix?
Choose faiss if you want the larger community (40,939 stars) or its MIT licence terms. Choose Arize Phoenix if its feature set, stack or Custom / other licence fits better. Both are self-hostable.