The best AI prompt testing tool depends on what you test. Prompt text before any run: PromptEval. Output tests in CI: Promptfoo (free, open source) or Braintrust. Tracing plus evals in production: Langfuse, LangSmith or Arize. Prompt registry for non-engineers: PromptLayer. If you are on OpenAI Evals, move before November 30, 2026, when OpenAI shuts it down.
On October 31, 2026, existing evals in OpenAI Evals become read-only, and on November 30 the Evals dashboard and API shut down, according to OpenAI's deprecations page. The migration guide it links points to Promptfoo, the open-source test runner that agreed to be acquired by OpenAI on March 9, 2026. If you test prompts for a living, your shortlist probably changed this year. This page compares nine prompt testing tools with prices checked on each vendor's pricing page on October 28, 2026.
Disclosure up front: PromptEval publishes this page and is one of the nine. Its row says what it doesn't do, same as the others.
The 9 prompt testing tools at a glance
| Tool | What it tests | Free plan | Paid from | Best for |
|---|---|---|---|---|
| PromptEval | The prompt text (0-100, 4 dimensions), plus BYOK playground and batch A/B | 3 evaluations/month, no card | $9/month (Basic), $19/month (Pro) | Checking prompts before tests; no-YAML CI gate |
| Promptfoo | Outputs against YAML assertions; red teaming | Community: free, open source, all providers | Enterprise only (custom) | Engineers who want tests in CI |
| PromptLayer | Prompt registry, visual editor, batch and regression evals | 10 prompts, 2,500 requests/month, 5 users | $49/month (Pro) | Teams where non-engineers edit prompts |
| Braintrust | Datasets, experiments, code-defined scorers | Starter: 1 GB data, 10k scores, unlimited users | $249/month per org (Pro) | Systematic experiments across models |
| LangSmith | Tracing, datasets, evals, prompt hub | 1 seat, 5k traces/month | $39/seat/month (Plus) | LangChain and LangGraph stacks |
| Langfuse | Tracing, prompt management, evals (open source) | Hobby: 50k units/month, 2 users; or self-host | $29/month (Core) | Tracing-first teams; self-hosting |
| Confident AI / DeepEval | 50+ metrics (DeepEval, Apache-2.0); hosted branches and gates | 2 seats, 1 project, 5 test runs/week | $200/month per org (Starter) | Metric-heavy evaluation with pytest |
| Arize (Phoenix / AX) | Tracing, online and offline evals, experiments | Phoenix open source; AX Free: 25k spans/month | $50/month (AX Pro) | Observability-first teams |
| Maxim AI | Simulation, evaluation, observability | No public free plan for the eval platform | Sales-led (demo) | Large teams with procurement |
If you're leaving OpenAI Evals before November 30
Three OpenAI dates matter, all from the same deprecations page:
| Date | What happens | Where to move |
|---|---|---|
| Oct 31, 2026 | Existing evals become read-only | Export your datasets and grader definitions now |
| Nov 30, 2026 | Evals dashboard and API shut down | Promptfoo (OpenAI's migration guide), or Braintrust, Langfuse, LangSmith for a hosted UI |
| Nov 30, 2026 | v1/prompts API and reusable prompt objects shut down | A prompt registry: PromptLayer, Langfuse prompt management, or PromptEval slug serving |
The two shutdowns are separate problems. Evals held your tests; prompt objects held the prompts your app fetched at runtime. Moving the tests to Promptfoo doesn't give your app a new place to read prompts from, so check which of the two you actually used before picking a replacement. Teams that used both usually end up with one tool per job: a test runner and a registry.
Three kinds of prompt testing, and which tools do which
"Prompt testing" covers three different activities, and every tool above is strong in one or two of them:
| Kind | Question it answers | When it runs | Tools |
|---|---|---|---|
| Structural evaluation | Does the instruction itself have gaps? | Before any input | PromptEval |
| Output testing | Does it give the right answer on these inputs? | Before shipping, in CI | Promptfoo, Braintrust, DeepEval, PromptLayer |
| Production monitoring | Is it still behaving with real traffic? | After shipping | Langfuse, LangSmith, Arize, Confident AI |
Structural evaluation is the cheapest of the three and the one vendor lists skip. Prompt evaluation is the act of scoring the prompt text itself, before you run it, on things like clarity, specificity, structure and edge-case handling. In the State of Prompt Quality report (2026-Q3 edition), only 10.5% of 1,018 prompts reached the 75-point production bar, and robustness was the weakest dimension with an average of 31.5. Those prompts would pass a few happy-path test cases and fail on the first odd input; a structural check flags the missing edge-case handling before you write a single test.
Code-first or no-code: the filter that removes most of the list
Before comparing features, decide how you'll work. Code-first tools (Promptfoo, Braintrust, DeepEval, LangSmith) want a repo, test files and a CI step. Setup takes hours; afterwards every prompt change is tested automatically. No-code tools (PromptEval, PromptLayer's editor) work in a browser: paste, run, read. You iterate in seconds, and a non-engineer can read the result.
The common mistake is buying a code-first platform to iterate on one prompt, or expecting a browser tool to replace a regression suite. If your last tool was a shared registry, the same fork is mapped tool by tool in our PromptLayer alternatives breakdown.
The 9 tools, one by one
PromptEval: check the prompt before you test it
Scores prompt text from 0 to 100 on clarity, specificity, structure and robustness, with a BYOK playground (Anthropic or OpenAI), a batch A/B wizard (up to 7 criteria and 10 inputs, LLM judge) and a GitHub Action that fails a pull request when a prompt's score drops or a contradiction appears (Pro). The eval API is open on every plan. Free: 3 evaluations a month. Basic $9, Pro $19, Team $49 a month.
Doesn't do: YAML output assertions, red teaming, or LLM call tracing in production. Pair it with Promptfoo or Langfuse for those.
Promptfoo: the default for tests in CI
Open-source CLI that runs prompts against test cases defined in YAML, with assertions, model comparisons and red teaming (the free Community edition includes up to 10k red-team probes a month). It's the tool OpenAI's Evals migration guide points to. Promptfoo says it will stay open source and multi-provider after the OpenAI deal. Paid tier: Enterprise only, custom pricing.
Doesn't do: a hosted dashboard for non-engineers in the free edition, or structural scoring of the prompt text.
PromptLayer: a registry non-engineers can use
Prompt registry with a visual editor, version history, A/B traffic splits and batch evals on datasets. The free plan covers 5 users, 10 prompts, 2,500 requests and 250 eval cell executions a month; Pro is $49 a month and Team $500. It's also the most direct landing spot for teams losing OpenAI's prompt objects.
Doesn't do: deep tracing of multi-step agents compared to Langfuse or Arize; the free plan's 10-prompt cap is tight.
Braintrust: experiments with code-defined scorers
Datasets, experiments across models, code or LLM scorers, and a UI to compare runs. The Starter plan is $0 with 1 GB of processed data, 10k scores and unlimited users; Pro is $249 a month per organization plus usage ($3 per extra GB, $1.50 per 1,000 scores).
Doesn't do: cheap middle tier; the jump from free to $249 is steep for a two-person team.
LangSmith: if you already run LangChain
Tracing, datasets, evaluations and a prompt hub, built by the LangChain team. Developer plan: one free seat and 5k traces a month. Plus: $39 per seat a month with 10k traces, then pay-as-you-go.
Doesn't do: per-seat pricing gets expensive for larger teams, and the fit is weaker outside LangChain and LangGraph.
Langfuse: open-source tracing with evals
Tracing, prompt management and evaluation in one open-source project you can self-host for free. Cloud Hobby includes 50k units a month and 2 users; Core is $29 a month with 100k units and unlimited users. Bring your own metrics: Langfuse records and runs evaluations but ships fewer ready-made metrics than DeepEval.
Doesn't do: a large built-in metric library or structural prompt scoring.
Confident AI and DeepEval: the metric library
DeepEval is an open-source (Apache-2.0) framework with 50+ evaluation metrics that run as pytest tests. Confident AI is the hosted platform on top: git-style prompt branches, evaluation gates on commit and merge, and production monitoring. Free: 2 seats, 1 project, 5 test runs a week. Starter: $200 a month per organization; Team: $2,000.
Doesn't do: a roomy free plan; 5 test runs a week is a trial, not a workflow.
Arize: Phoenix open source, AX hosted
Phoenix is Arize's open-source, local-first platform for tracing, evaluation and prompt iteration. Arize AX is the hosted version: AX Free has 25k trace spans a month with unlimited evals and users; AX Pro is $50 a month with 50k spans.
Doesn't do: much for a single prompt before it has traffic; it shines once traces exist.
Maxim AI: for procurement-led teams
Agent simulation, evaluation and observability for larger teams. The eval platform has no published price or free plan; access is through a demo. Its public pricing page now covers Bifrost, Maxim's open-source AI gateway, which is a separate product.
Doesn't do: self-serve trial of the eval platform at the time of writing.
Which tool for which team
| Team | Start with | Add when | Skip |
|---|---|---|---|
| Solo developer | PromptEval Free for the prompt, Promptfoo for tests | You ship weekly: PromptEval Pro for the CI gate | Per-org plans ($200+) |
| Small team (2-5), no DevOps | PromptEval Pro + Promptfoo | Real traffic: Langfuse Core ($29) | Maxim, Confident AI Team |
| Non-engineers edit prompts | PromptLayer | Prompts grow past 10: PromptLayer Pro ($49) | YAML-only tools |
| LangChain shop | LangSmith | Metric depth: DeepEval in CI | A second tracing tool |
| Leaving OpenAI Evals | Promptfoo (tests) + a registry if you used prompt objects | Hosted UI: Braintrust or Langfuse | Waiting past Nov 30 |
| Enterprise with procurement | Arize AX, Braintrust, Confident AI or Maxim | Structural review of new prompts: PromptEval Team | Free tiers in production |
What vendor-written lists leave out
The three pages ranking above this one for "best AI prompt testing tools" in late October 2026 are written by Braintrust, Confident AI and PromptLayer, and each puts its own product first. That's normal for the format. What they skip is more useful to know:
- None of the three mentions the OpenAI Evals shutdown, the most time-sensitive reason to pick a testing tool this quarter.
- Each list covers a different slice. Braintrust's covers prompt management, Confident AI's leaves out Braintrust and Promptfoo, PromptLayer's leaves out Promptfoo. Only PromptLayer appears in all three.
- None checks the prompt text itself. They test outputs and monitor traffic, which catches a vague instruction only after it has produced bad answers.
Free plans are also more generous than they used to be. Eight of the nine tools here have a free tier or a free open-source edition, so the real filter is the jump to paid: $29 (Langfuse), $39 per seat (LangSmith), $49 (PromptLayer), $50 (Arize AX), then $200 and $249 per organization (Confident AI, Braintrust). For broader scoring tools rather than testing workflows, see our prompt evaluation tools comparison; for the step-by-step loop that connects these tools, the guide to testing and iterating AI prompts covers each phase, and you can try a single comparison in the free A/B prompt tester.
Frequently asked questions
What is the best free AI prompt testing tool?
It depends on what you test. For output tests in CI, Promptfoo's Community edition is free and open source with all model providers. For hosted experiments, Braintrust, Langfuse, Arize AX, PromptLayer and LangSmith all have free plans with usage caps (as of October 2026). For checking the prompt text itself before any test run, PromptEval scores a prompt from 0 to 100 on four dimensions with 3 free evaluations a month and no credit card.
What is the difference between prompt evaluation and prompt testing?
Prompt evaluation checks the prompt text itself (clarity, specificity, structure, edge-case handling) before you run any inputs. Prompt testing runs the prompt against real inputs and checks whether the outputs are correct. Most tools in this comparison only do testing; PromptEval does the structural evaluation and adds a BYOK playground and batch A/B tests for outputs.
What should I use instead of OpenAI Evals?
OpenAI's deprecations page says existing evals become read-only on October 31, 2026 and the Evals dashboard and API shut down on November 30, 2026, and it links a guide to moving to Promptfoo. Promptfoo is the closest match for code-defined evals in CI. If you want a hosted UI with datasets and experiments, Braintrust, Langfuse and LangSmith are the usual landing spots. If you also used the v1/prompts prompt objects, which shut down the same day, you need a prompt registry such as PromptLayer, Langfuse prompt management or PromptEval slug serving.
Does Promptfoo still support other model providers after the OpenAI deal?
Yes, according to Promptfoo. Its March 9, 2026 announcement that it agreed to be acquired by OpenAI says Promptfoo will remain open source and keep supporting a diverse range of providers and models, and its pricing page still lists all model providers in the free Community edition.
Which prompt testing tool is best for a small team?
For 2 to 5 people without a DevOps budget, pair a structural check with one output-testing tool. PromptEval Pro ($19/month) covers structural evaluation, batch A/B tests and a CI regression gate through a GitHub Action. Add Promptfoo for YAML output assertions and red teaming, or Langfuse ($29/month Core plan) if you also need tracing. Skip per-organization plans in the $200 to $500 range until you have the traffic to justify them.