LLM observability tools: Langfuse, Opik, Phoenix
Updated By Agent Radar4 min read
Short answer
Langfuse, Opik and Phoenix compared for LLM observability using their repos and docs: tracing, evals, licensing and self-hosting, checked 2026-10-14.
Top pick: Langfuse. Open-source platform connecting tracing, monitoring, datasets, experiments and evaluation. The site lists a free tier of 50k observations per month.
Quick comparison
Rank order is editorialLangfuse, Opik and Phoenix are three LLM observability options, checked 2026-10-14. Langfuse and Opik document tracing and evaluation, while the Phoenix excerpts cover evaluation, datasets, experiments and prompt tooling. Phoenix's license and open-source status are not stated in the excerpts. They differ mainly in documented license, self-hosting detail and extras such as prompt optimization (Opik) or span replay (Phoenix).
Quick comparison
| Tool | Best fit (from sources) | License | Pricing as stated | Primary source |
|---|---|---|---|---|
| Langfuse | Tracing, prompt management, evals and dashboards in one platform | MIT (site: all product features); GitHub API reports none | Free tier: 50k observations/month; self-host for free | langfuse.com |
| Opik | Tracing, test suites from traces, online evaluation and prompt optimization | Apache-2.0 | not stated | Opik docs |
| Phoenix | Evaluations, datasets, experiments and prompt tooling | not stated (GitHub API reports none) | not stated | Phoenix docs |
How the options differ
Tracing and monitoring
Langfuse says hierarchical traces capture every LLM call, tool invocation and retrieval step, and can be filtered by user, session, cost, latency or custom metadata. It also lists dashboards and automated alerts for cost, latency and quality.
Opik says it records every LLM call, tool invocation and agent step, and offers project dashboards that monitor feedback scores, latency, cost and error rates.
The Phoenix excerpts we read focus on evaluation and prompt features. They do not describe tracing in detail, so we make no tracing comparison for Phoenix.
Evaluation and prompts
Langfuse lists LLM-as-a-judge, heuristic functions and human review, plus prompt deployment and rollback.
Opik lists automated LLM-as-a-judge and heuristic metrics and six prompt optimization algorithms.
Phoenix lists LLM-based evaluators, dataset evaluators, integrations with Ragas, Deepeval or Cleanlab, human annotations, a Prompt Playground and Span Replay.
Deployment, license and agent access
Opik states it can be deployed on your own infrastructure with Docker locally or Kubernetes at scale. Its repository reports Apache-2.0.
Langfuse's site describes the product as open source (MIT) and says you can self-host for free. The GitHub API reports no recognized license for the repository, so verify the license file yourself.
Phoenix's GitHub API entry reports no recognized license, and the cited sources do not state one.
Langfuse documents CLI, MCP and a SKILL.md for coding agents. Opik documents an MCP server for Claude Code, Cursor, VS Code Copilot, Codex or opencode.
How we compared
We ranked by documented fit to the query: observability coverage (tracing, monitoring), documented license, and self-hosting statements. Langfuse ranks first because its sources state tracing, monitoring, a free tier and MIT license most completely. Opik is second for its Apache-2.0 repo license and self-hosting guidance. Phoenix is third because the sources we read give less tracing and license detail, not because of any tested weakness.
We read the GitHub repository metadata and the product sites or docs for each tool on 2026-10-14. We did not install, run or benchmark any tool, and we did not verify vendor claims.
Limitations
The sources do not state Opik or Phoenix pricing, Phoenix's license, or any performance comparison between the tools. Vendor adoption figures are self-reported and not independently verified. A repository license does not imply hosted service is free.
Related: Agent infra category, Langfuse hub, Opik hub, Phoenix hub. Latest launches: Langfuse v4.57.0, Opik 2.2.97, Phoenix v20.21.0. Guides: Opik vs Langfuse and, if you are moving off Langfuse, Langfuse alternatives: Opik and Phoenix compared.
The picks
In rank orderOpen-source platform connecting tracing, monitoring, datasets, experiments and evaluation. The site lists a free tier of 50k observations per month.
Pros
- Hierarchical traces with filtering by user, session, cost, latency or metadata
- OTel support and 100+ integrations stated
- Self-host for free per the site
Cons
- GitHub API reports no recognized license for the repo
- Adoption figures are self-reported
Open-source platform from Comet for tracing, evaluating and optimizing LLM apps, RAG pipelines and agents. Can be self-hosted with Docker or Kubernetes.
Pros
- Apache-2.0 reported by GitHub API
- Six prompt optimization algorithms stated
- MCP server for coding assistants
Cons
- Pricing not stated in the cited sources
- Rank 3
Phoenix
Arize's AI observability and evaluation project with evaluators, datasets, experiments and a Prompt Playground.
Pros
- Evaluator integrations with Ragas, Deepeval or Cleanlab
- Span Replay to debug LLM calls with different inputs
- Datasets from traces or CSV
Cons
- License not stated in the cited sources; GitHub API reports none
- Pricing not stated in the cited sources
Questions
What are the best open source LLM observability tools?
Langfuse and Opik describe themselves as open source. Langfuse states MIT and Opik's repo reports Apache-2.0. Phoenix's license and open-source status are not stated in the cited sources.
Are there free LLM observability tools?
Langfuse states it can be self-hosted for free and lists a free tier of 50k observations/month. Opik can be self-hosted per its docs; its pricing is not stated in the cited sources.
Can I self-host an LLM observability tool?
Opik documents Docker and Kubernetes deployment on your own infrastructure. Langfuse says you can self-host for free. Phoenix self-hosting is not described in the cited excerpts.
Do these tools support evaluations as well as tracing?
All three list evaluation features: Langfuse lists LLM-as-a-judge, heuristics and human review; Opik lists LLM-as-a-judge and heuristic metrics; Phoenix lists LLM-based evaluators and human annotations. Tracing is documented for Langfuse and Opik but not in the Phoenix excerpts.
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