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Haystack

Haystack gives your coding agent hundreds of forked copies of your app, each seeded like a real customer journey learned from production traffic, and runs every affected case with and without the change.

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Introduction

Haystack lets your coding agent see what it really changed. It reads the diff and works out every case the change can affect, using customers’ real journeys learned from production traffic, narrowed to those the change can reach. In one worked example, a permissions refactor touching two files reaches 3 services and 27 code paths and yields 14 cases, mapped in about five seconds.

It then snapshots your services and data, from Postgres and Redis to queues and object storage, and forks that snapshot per case: fourteen copies of production in 26 seconds, isolated so nothing reaches prod. Each environment is seeded into a different state and runs twice, with and without your change, so differences are caused by the change. Failures come with before-and-after screens, changed network calls and events.

Launch and background: Y Combinator’s company directory lists Haystack in its Summer 2024 batch, with the one-liner “Replay real customer journeys against every change before you ship”. Haystack was also posted to Hacker News as a Show HN on 10 September 2025.

Features

  1. Blast-radius analysis: Cases derived from the diff.
  2. Real journeys: Learned from production traffic.
  3. Forked environments: Isolated copies per case.
  4. A/B runs: With and without the change.
  5. Rich failure evidence: Screens, network calls and events.

Pros

  1. Realistic state: Not invented scenarios.

Cons

  1. Price: Team plan at $1,000 a month.
  2. Infrastructure integration: Needs access to services and data snapshots.
  1. Connect your services and traffic source.
  2. Let Haystack map cases on a PR.
  3. Review differences between runs before merging.

Resources:

Pricing

Haystack’s Team plan is listed at $1,000 a month.

  1. SaaS teams whose bugs depend on customer state.

Conclusion

Haystack turns production behaviour into per-PR regression checks.

Weigh the cost against your current staging setup.

On CheckOps, Haystack is filed under the Integration testing, End-to-end & functional and Release & CI gates stages. Related tools listed here include Greptile, CodeRabbit, Speedscale, Keploy, Meticulous and Cursor Bugbot. Before standardising on one, compare recent maintenance activity, the pricing model as your usage grows, and the integrations you rely on today. Information on this page was verified against the official website, documentation and repository on 11 November 2026.

Alternatives

Other tools on CheckOps that cover similar ground. Open a page, or compare one with Haystack side by side.