head to head · open source
faiss vs Arize Phoenix
faiss has 41,256 GitHub stars, 4,586 forks, 327 open issues and last shipped 2 days ago. Arize Phoenix has 12,128 stars, 1,261 forks, 970 open issues and last shipped yesterday. faiss leads on adoption by 240% (41,256 vs 12,128 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%. Arize Phoenix 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 | 2026-11-03 | 2026-11-04 |
| Forks | ⑂ 4.6K | ⑂ 1.3K |
| 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 Faiss is a library for efficient similarity search and clustering of dense vectors. It contains algorithms that search in sets of vectors of any size, up to ones that possibly do not fit in RAM. It also contains supporting code for evaluation and parameter tuning. Faiss is written in C++ with complete wrappers for Python/numpy. Some of the most useful algorithms are implemented on the GPU. It is developed primarily at Meta's Fundamental AI Research group. News See CHANGELOG.md for detailed information about latest features. Introduction Faiss contains several methods for similarity search. It assumes that…
read the full faiss overview →
About Arize Phoenix
English · 简体中文 Arize Phoenix is Arize's open source AI observability platform designed for experimentation, evaluation, and troubleshooting. For managed production workflows, Arize also offers Arize AX. Phoenix provides: Tracing Trace your LLM application's runtime using OpenTelemetry based instrumentation. Evaluation Leverage LLMs to benchmark your application's performance using response and retrieval evals. Datasets Create versioned datasets of examples for experimentation, evaluation, and fine tuning. Experiments Track and evaluate changes to prompts, LLMs, and retrieval. Playground Optimize prompts, compare…
read the full Arize Phoenix overview →
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Frequently asked questions
Is faiss or Arize Phoenix more popular?
faiss has 41,256 GitHub stars and Arize Phoenix has 12,128. 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 (41,256 stars) or its MIT licence terms. Choose Arize Phoenix if its feature set, stack or Custom / other licence fits better. Both are self-hostable.