# microsoft/coyote

Coyote is a library and tool for testing concurrent C# code and deterministically reproducing bugs.

Repository: https://github.com/microsoft/coyote
Canonical: https://ross.abutalabs.com/products/coyote
Homepage: https://microsoft.github.io/coyote/
Language: C#
License: NOASSERTION
License Family: other
Topics: systematic-testing, software-reliability, dotnet, coyote, testing, testing-tools, fuzzing
Last push: 2026-08-24T16:07:43+00:00

## Health v2 (maintenance only)
Score: 77/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 99, release rhythm 35, longevity 100
- inputs: {"age_days": 2576, "days_push": 9, "days_rel": null, "gap_med": null, "n_releases_24m": 0}
- flags: no_releases, no_license
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 1592, forks 91 (observed 2026-08-28T04:05:08.806078+00:00)

## What it is
Coyote is a .NET library and CLI tool from Microsoft Research for systematically testing concurrent C# code and deterministically reproducing concurrency bugs. It uses IL binary rewriting to control scheduling of tasks, actors, and other nondeterminism, exploring many interleavings within ordinary unit tests.

## Use cases
- find race conditions in C# code
- test concurrent ASP.NET controllers deterministically
- reproduce flaky concurrency bugs reliably
- systematically explore interleavings of tasks and actors
- test distributed systems logic on a laptop without production environment
- add concurrency unit tests to xUnit test suites

## When to choose
- you write concurrent or async C# code and suffer from flaky or hidden race-condition bugs
- you want systematic exploration of schedules instead of stress testing
- you build actor or state-machine based services and want reliable testing
- you need deterministic reproduction of nondeterministic bugs

## When to avoid
- your codebase is not on .NET/C#
- you need formal verification or correctness proofs rather than testing
- you only need simple unit tests without concurrency
- you want a language-agnostic fuzzing tool

## Facets
- artifact type: library
- maturity: active
- function: testing, fuzzing, cli, concurrency
- domain: testing, developer-tools, microservices
- platform: dotnet, cross-platform, cli
- tags: systematic-testing, concurrency-unit-testing, binary-rewriting, actor-model, race-condition-detection, csharp

## Member repositories
- microsoft/coyote (main) score 77

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:05:08.806078+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T03:54:41.073416+00:00, confidence not recorded.
  - readme: https://github.com/microsoft/coyote (fetched 2026-08-28T04:05:08.806078+00:00, sha 923013044313)
  - homepage: https://microsoft.github.io/coyote/ (fetched 2026-08-29T11:25:09.485905+00:00, sha c161b65de648)
- Data as of 2026-08-30T08:39:29.467469+00:00.