Why is CompletionKit so much cheaper than Braintrust?

CompletionKit's first paid plan is $29 and the team plan is $99; Braintrust's first paid plan is $249. You're paying CompletionKit for the focused improvement loop, not a full observability platform with deep tracing and monitoring. If you need that platform, Braintrust earns its price. If you mainly need to make a prompt better and ship it, you're paying for surface you won't use.

Can I self-host CompletionKit the way I'd want to with Braintrust?

More easily. Braintrust's core platform is proprietary and self-hosting is an enterprise-only option. CompletionKit's engine is source-available under BSL 1.1 and you can self-host it in production for free on any plan, so your eval data and prompts can stay on your own infrastructure.

Does CompletionKit have observability and tracing like Braintrust?

No, and that's a genuine Braintrust strength. Braintrust gives you deep production tracing, logs, and monitoring on top of evals. CompletionKit is focused on the evaluate, improve, and ship loop. Pair them if you want both, or pick CompletionKit if the loop is what you actually need.

What about Braintrust's Loop?

Loop reads your eval failures and suggests prompt revisions, and it's good. CompletionKit does the same, and adds a computed judge trust score: Braintrust shows the human and LLM-judge scores side by side and leaves you to compare them, while CompletionKit computes the agreement (a Wilson interval and weighted kappa) so you know how far to trust the judge.

CompletionKit vs Braintrust

Braintrust is the full hosted observability and eval platform, built to ship AI at scale. CompletionKit is the focused prompt-improvement loop: source-available, self-hostable, and a fraction of the price.

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The short version

Braintrust is a hosted platform for shipping AI at scale: production tracing and observability, experiments and evals, prompt management and serving, human review, and an AI optimizer (Loop) that suggests prompt edits. It's mature and well-resourced. The core platform is proprietary, self-hosting is enterprise-only, and the first paid plan is $249 a month.

CompletionKit does the improvement loop and not much else on purpose: an LLM judge with a computed trust score, suggested next revisions, prompt versioning and serving. It's source-available under BSL 1.1, self-hostable in production on any plan, and starts at $29. It is not an observability platform, and isn't trying to be.

Side by side

	Braintrust	CompletionKit
Interface	Code SDK + hosted UI	Hosted UI + REST API
Multi-provider (OpenAI, Anthropic, local)
LLM-as-a-judge scoring
Judge trust score, native	Side by side
AI-suggested prompt revisions
Versioned prompts served to your app
Non-coder prompt editing in a UI
Production observability + tracing		—
Red teaming + security scans	—	—
Self-hostable	Enterprise only
License	Proprietary core	BSL 1.1 (source-available)
First paid plan	$249	$29

Both serve prompts to your app, let non-coders edit them, ship an MCP server, and suggest revisions from your evals. The real differences are price, self-hosting, the computed trust score, and how much platform you want around the loop.

Which one fits

Reach for Braintrust if you want the full platform: deep production observability and tracing, evals, and prompt management in one mature, well-resourced product, and the $249-a-month entry and SaaS-first model suit you. It is the stronger choice when observability is a first-class need.

Reach for CompletionKit if you want the improvement loop without the platform around it: a trust-scored judge, suggested revisions, and prompt serving, source-available and self-hostable so your data stays yours, at $29 to $99 instead of $249. Less to learn, less to pay, less you won't use.

They are not either-or. Some teams run Braintrust for production observability and CompletionKit for the tight prompt-improvement loop.

Questions

Why is CompletionKit so much cheaper than Braintrust?: CompletionKit's first paid plan is $29 and the team plan is $99; Braintrust's first paid plan is $249. You're paying CompletionKit for the focused improvement loop, not a full observability platform with deep tracing and monitoring. If you need that platform, Braintrust earns its price. If you mainly need to make a prompt better and ship it, you're paying for surface you won't use.
Can I self-host CompletionKit the way I'd want to with Braintrust?: More easily. Braintrust's core platform is proprietary and self-hosting is an enterprise-only option. CompletionKit's engine is source-available under BSL 1.1 and you can self-host it in production for free on any plan, so your eval data and prompts can stay on your own infrastructure.
Does CompletionKit have observability and tracing like Braintrust?: No, and that's a genuine Braintrust strength. Braintrust gives you deep production tracing, logs, and monitoring on top of evals. CompletionKit is focused on the evaluate, improve, and ship loop. Pair them if you want both, or pick CompletionKit if the loop is what you actually need.
What about Braintrust's Loop?: Loop reads your eval failures and suggests prompt revisions, and it's good. CompletionKit does the same, and adds a computed judge trust score: Braintrust shows the human and LLM-judge scores side by side and leaves you to compare them, while CompletionKit computes the agreement (a Wilson interval and weighted kappa) so you know how far to trust the judge.

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