Tailtest

tailtest -- AI software testing for Claude Code
You build. Claude builds. tailtest makes sure it works.

tailtest is the open-source AI software testing layer for Claude Code. It runs inside the build loop: every time Claude edits a file, tailtest queues the file, generates production-shaped scenarios using the R1-R15 rule layer, runs them with your existing test runner (pytest, jest, go test, etc.), and surfaces failures back to Claude within the same turn. Hook-based. Deterministic. No prompting required.

Open source (MIT), no telemetry, no SaaS account. Includes adversarial mode (R15) that biases scenario generation toward 8 breakage categories -- the mode we've used to find 16 real bugs across 47 open-source Python repositories.

Read more on tailtest.com · Platform overview · Agent-edit testing deep dive · Full docs

Install

Restart Claude Code after install. No other setup required.

Update

Uninstall

How it works

After any Claude-written file lands in your project:

1. tailtest generates scenarios that describe real business behavior -- not function signatures
2. It runs them using your existing test runner (pytest, vitest, jest, go test, and more)
3. Pass: tailtest: N scenarios -- all passed. Fail: one line surfaced, "want me to fix this?"

Supported languages

Python, TypeScript, JavaScript, Go, Ruby, PHP, Java, Kotlin, C#, Rust

Runners are auto-detected from pyproject.toml, package.json, go.mod, build.gradle.kts, *.csproj, *.sln, and other standard manifests.

Quick config

Create .tailtest/config.json in your project root to control depth:

Options: simple (2-3 scenarios), standard (5-8, default), thorough (10-15).

See tailtest.com/docs/config for all options.

Other tailtest variants

Same R1-R15 rule layer, same adversarial test mode, different host…