FAQs

A collection of answers to frequently asked questions about Kerno

What is Kerno?

Kerno learns how your product works, maps the user flows through your app, and records how they behave in tests that run against your real stack. Every time your code changes, Kerno validates the change inside your coding agent's loop and shows what behavior moved.

Kerno runs every change against your real stack and validates it across functional behavior, error handling, edge cases, security, auth, and response content.

What makes Kerno different from other tools

Most code tools either statically analyze your code or run unit tests in isolation against mocks. Kerno does neither.

It knows the exact blast radius of every change. Kerno indexes your code deterministically, so it can tell you precisely which entry points a change affects, following the call graph rather than returning approximate matches from vector embeddings.

It tests against your real stack. Scenarios run against your actual running application and its real databases and queues, not mocks.

You don't write or maintain the tests. Kerno generates them, and updates them when your entry points change on purpose. When it writes new tests for an entry point, it shows you the plan and waits for your approval first. Updates and batch runs go ahead unless you ask to review the plan.

Does Kerno support my backend stack?

Most likely, yes. Kerno detects routes in TypeScript, JavaScript, Python, Java, Kotlin, Scala, Go, Ruby, PHP, C#, Rust, and Swift, across the common web framework for each.

Your scenarios are always written in TypeScript and reach your application over HTTP, MCP, or the queue a background consumer reads, so the language your service is written in doesn't change how tests run.

Kerno can also connect directly to PostgreSQL, MySQL, MariaDB, MongoDB, Redis, Kafka, RabbitMQ, Amazon SQS, Azure Service Bus, DynamoDB, ClickHouse, Azure Storage and S3-compatible object storage, for setting up and verifying state your API can't express. It also reads Zitadel and Amazon Cognito to obtain test credentials.

The full matrix lives in Supported technologies.

Does Kerno store my code

No. Kerno never stores your source code on its servers. Relevant excerpts pass through Kerno's proxy to LLM providers (Anthropic and OpenAI) configured with zero-data-retention tiers, so nothing is persisted on the provider side either.

Note that the report of every test run your agent starts is uploaded as the run progresses, so your team can view it in the portal. That report includes the request and response payloads your application produced during the run. See Security & Privacy.

Does Kerno use my code for training or fine tuning LLMs?

No. Kerno doesn't train models. The LLM providers Kerno uses run on zero-data-retention tiers, so your code isn't stored, logged, or used for training on their side.

Are there discounts available for startups?

Kerno offers free usage for qualified Open Source Projects and a significant discounts for pre-Series A startups. Drop us a note to check your availability.

Can Kerno test an app that spans several repositories?

Kerno works within one repository at a time. That repository can hold several apps, such as the services of a monorepo, and Kerno tests each of them.

Does Kerno work with my coding agent?

Yes, if your agent supports MCP. Kerno exposes a set of MCP tools your agent calls to connect to your running app, generate tests, and validate changes. Claude Code, Cursor, Windsurf, and Codex are the verified setups.

See the Quickstart to install the CLI and register the MCP server with your agent.

What if I'm not using a coding agent?

Kerno runs through a coding agent that supports MCP, such as Claude Code, Cursor or Codex. The agent connects Kerno to your running app, generates tests for an entry point, and validates your changes. See the Quickstart to set one up. Once your tests are committed, the CI action re-runs them on your pull requests without an agent.

If you encounter issues or have questions, message us on Slack, and we’ll gladly help.