The Best Free LLM Observability Tools in 2026, Ranked by What "Free" Actually Buys You

“Free” is the most abused word in this category. Every tool has a free tier. The question that matters is what survives when you stop paying - do you get the real product self-hosted, or a hobbled core with the good features gated behind a cloud plan? I ranked these four on the free reality, not the free headline. Three of them are genuinely, usefully free. One is free and I am telling you not to start on it.

The axes:

• Self-host license. MIT and Apache-2.0 are OSI open source. Elastic License 2.0 is source-available, which is not the same freedom.
• Feature completeness when free. How much of the real product you keep with no paid plan.
• Free cloud tier. For teams that will not run infrastructure, what the managed free plan gives you.
• Is it a safe bet. Because a frozen tool is a bad foundation, free or not.

Here is the ranked list.

The short version

Tool	Free self-host	License	Free cloud tier	Safe to start?
Langfuse	Yes, near-complete	MIT	50k units/mo	Yes
Opik	Yes, full, no gates	Apache-2.0	25k spans/mo	Yes
Arize Phoenix	Yes, no feature gates	ELv2 server	n/a (OSS-first)	Yes, read license
Helicone	Yes, but frozen	Apache-2.0	10k requests/mo	No - maintenance mode

1. Langfuse - the free default that does not cripple you

Langfuse is the free default for a reason. It is MIT-licensed, and in the self-hosted build only three features are enterprise-gated - Organization Creators, the Instance Management API, and UI customization. Everything you actually need - tracing, evals, prompt management, human annotation, RBAC - is free. That is far more generous than tools where self-host is Enterprise-only. If you would rather not run infrastructure, the free Hobby cloud tier gives you 50k billable units a month with a hard cap, 30-day retention and 2 users. And at scale it is roughly 25x cheaper than LangSmith per its published comparison.

The catch is operational, not commercial. v3 needs Postgres plus ClickHouse, Redis and S3-compatible storage - four services - and the migration to that architecture is where self-hosters get stuck, with GitHub threads reporting memory errors and timeouts. The software is free; the ops time is not. It is also a ClickHouse subsidiary after the January 2026 acquisition. If you can run the stack, nothing else gives you this much for free.

2. Opik - the most permissive free self-host, zero gates

Opik is a hair behind Langfuse and, on the strict definition of free, arguably ahead. Comet’s open-source platform is Apache-2.0 with the full feature set self-hosted - unlimited spans, members and retention, and no feature gates at all. Where Langfuse gates three features, Opik gates none, and it is designed to self-host at Kubernetes scale without losing anything. The free cloud tier is generous too - 25k spans a month, up to 10 members, 60-day retention. It is the fastest-growing project of its peers at roughly 20.8k GitHub stars, and it is backed by Comet, an established company since 2017.

The gotcha only shows up when you pay. The recurring complaint is per-seat pricing scaling poorly as your team grows on the cloud, and a few users report UI slowdown on very large projects. Neither touches the free self-host. If your definition of free is “Apache-2.0, full features, unlimited, no asterisk,” Opik is the cleanest answer in the category - and the only reason it is not first is that Langfuse is the more battle-tested default.

3. Arize Phoenix - free, fast, but read the license

Arize Phoenix is the OSS pick that starts fastest - a working trace UI on your laptop in under a minute - and Phoenix OSS is free with no usage caps and no feature gates on the features themselves. It is genuinely OpenTelemetry-native, and its 50+ pre-built eval metrics include what reviewers call the best RAG evaluation in the category. For a team that wants free tracing plus strong eval and does not want to touch a cloud plan, it is excellent.

The gotcha is the license, and it is the reason it sits at third in a free roundup. Arize markets Phoenix as “fully open source, no feature gates,” but the main server repo is Elastic License 2.0 - source-available, not OSI-approved open source. Only the client and eval subpackages are Apache-2.0. ELv2 forbids offering Phoenix as a hosted service to third parties. For internal use it behaves like open source and the features are not gated - the license is. If OSI-approved open source is your hard requirement, Langfuse and Opik win on license alone. There are also reports of ingest lag before traces appear.

4. Helicone - free, and do not start here

I am including Helicone only to tell you not to build on it. It was a clean, open-source, proxy-based observability tool, Apache-2.0, free to self-host with Docker or Helm. Then Mintlify acquired it on 3 March 2026 and put it in maintenance mode - security patches, bug fixes and new-model support only, no roadmap, and Mintlify is actively helping customers migrate off.

For a new buyer that single fact overrides everything. Self-hosting a frozen codebase whose own creator is pointing people to the exit is a weak bet, free or not. The proxy model was also a structural single point of failure - if Helicone is down, your LLM calls fail even when the provider is healthy, and every proxied call adds latency (Helicone cites ~10ms). If you are already on it, use the runway to plan your migration. If you are evaluating fresh, skip to Langfuse or Opik.

So which one?

• You want the free default that also has a mature managed option - Langfuse, if you can run the four-service self-host.
• You want the most permissive free self-host, zero feature gates - Opik, Apache-2.0 with unlimited everything.
• You want free tracing plus the best RAG eval, fastest to try - Arize Phoenix, as long as you are not reselling it.
• You are on Helicone today - plan your exit. Do not start there fresh.

Every price, license and date above is read from each vendor’s own materials, verified on 23 July 2026, and this category ships breaking changes monthly - we re-verify every 30 days. The full breakdowns live in our tool reviews. Free is real here for three of the four - just make sure “free” means the real product, not a trial with the good parts locked away.