Rapid
Rapid is a Go library for property-based testing: it checks user-defined properties against test cases generated automatically.
What Rapid is
Rapid is a Go library for property-based testing: it checks user-defined properties against test cases generated automatically. Its type-safe, generic generators favor small values and edge cases. When a failure occurs, Rapid reduces the failing case before presenting it, and it can save minimized failures and rerun them. It also supports state machine testing, and its MakeFuzz function lets a Rapid test serve as a target for Go’s standard fuzzer. Tests run with `go test`; options can be configured with `-rapid.` flags or matching `RAPID_` environment variables. Rapid has no dependencies beyond the Go standard library and is licensed under the Mozilla Public License Version 2.0. The project says tests should continue to work across future releases within the same major version, subject to API-change exceptions. Rapid describes itself as suited to structured data and state machine tests compared with Go’s `testing.F.Fuzz`, but it does not provide coverage-guided feedback or mutations. The library is free, with a listed plan price of no cost.
Good
- Minimizes failing test cases automatically.
- Can persist and rerun minimized failures.
- Supports state machine testing.
- Has no dependencies beyond Go’s standard library.
- Tests run with `go test`.
Watch out
- Does not provide coverage-guided feedback.
- Does not provide mutations.
Who it suits
Rapid suits Go developers who want generated test cases for checking properties, especially for structured data or state machine tests. It may be less suited to those who need coverage-guided feedback or mutations.
Verdict
Rapid offers property-based testing, failure minimization, and state machine support in a Go library with no external dependencies. Its stated limitation is the lack of coverage-guided feedback and mutations.
Plans and prices
From Rapid's own pricing page, checked 3 Oct 2026.
| Plan | Price | What it gives you |
|---|---|---|
| Rapid | Free | Open-source Go library · No dependencies outside the Go standard library |
The facts
Each one read from Rapid's own pages, with the date it was checked.
What it does
- Automatic minimization
- When a failure is found, Rapid automatically minimizes the failing test case before presenting it.checked 3 Oct 2026
- Comparison limit
- Compared with Go's testing.F.Fuzz, Rapid describes itself as better suited to structured data and state machine tests, while lacking coverage-guided feedback and mutations.checked 3 Oct 2026
- Configuration
- Rapid's options can be set with flags prefixed -rapid. or corresponding RAPID_ environment variables.checked 3 Oct 2026
- Data generation
- Its type-safe generators use generics and bias data generation toward small values and edge cases.checked 3 Oct 2026
- Dependencies
- Rapid has no dependencies outside the Go standard library.checked 3 Oct 2026
- Failure persistence
- Rapid supports persisting minimized failing test cases and automatically rerunning them.checked 3 Oct 2026
- Fuzzing integration
- The MakeFuzz function lets a Rapid test be used as a fuzz target for Go's standard fuzzer.checked 3 Oct 2026
- License
- Rapid is licensed under the Mozilla Public License Version 2.0.checked 3 Oct 2026
- Stability
- The project says tests should continue to work across future releases with the same major version, subject to stated API-change exceptions.checked 3 Oct 2026
- Stateful testing
- Rapid supports state machine testing.checked 3 Oct 2026
- Usage
- Rapid tests run with the standard go test command.checked 3 Oct 2026
- What it does
- Rapid is a Go library for property-based testing that checks user-defined properties against automatically generated test cases.checked 3 Oct 2026
Alternatives to Rapid
- 1
fast-check
Free planOpen sourceWebAPILinuxSelf-hosted7.1Free plan - 2
ScalaCheck
Free planLinuxmacOSWindows6.6Free plan - 3
Gopter
Free planLinuxSelf-hosted6.4Free plan - 4
JSVerify
Free planWebSelf-hosted6.4Free plan - 5
Racket QuickCheck
Free planLinuxmacOSWindows6.4Free plan - 6
FsCheck
Free planLinuxmacOSWindows6.3Free plan