Prompt Snapshot — Development skill for Claude Code

prompt-snapshot

Vitest-for-prompts. A file-based prompt eval runner with snapshots and assertions. No dashboard, no SaaS, no sign-up — just `.prompt.ts` files in your repo and a CLI that runs them in CI.

[](https://www.npmjs.com/package/@swarmclawai/prompt-snapshot) [](./LICENSE) [](https://github.com/swarmclawai/prompt-snapshot/actions/workflows/ci.yml)

Why this exists

If you ship any LLM feature, you should be testing your prompts the way you test your code — in CI, on every change. The existing options (Langfuse, DeepEval, Phoenix, Promptfoo) are heavy observability platforms shaped for enterprise teams: SDK init, hosted dashboards, sign-ups, YAML configs, whole new abstractions to learn.

A solo dev or small team wants this instead:

• Colocated .prompt.ts files next to the code that uses the prompt.
• A TypeScript-native API with types, autocomplete, and Zod schemas.
• Snapshot testing — run a prompt, capture the output, fail the build if it drifts.
• Deterministic assertions (contains, matchesSchema, …) for the parts that matter.
• One CLI, JSON output, stable exit codes — CI-friendly on day one.

That's what `prompt-snapshot` is.

30-second demo

Create `summarizer.prompt.ts`:

import { defineSuite, contains, shorterThan } from "@swarmclawai/prompt-snapshot";

export default defineSuite({
  name: "summarizer",
  model: { provider: "anthropic", model: "claude-haiku-4-5", temperature: 0 },
  prompt: ({ doc }: { doc: string }) =>
    `Summarize the following in one sentence.\n\n${doc}`,
  cases: [
    {
      name: "short-doc",
      input: { doc: "The cat sat on the mat. The dog watched from across the room." },
      assertions: [contains("cat", { caseInsensitive: true }), shorterThan(300)],
      snapshot: