Configuration

mutation-config.json

Place a mutation-config.json next to your solution (or point at one with --config <file>). Any CLI flag overrides the matching config value, and relative paths resolve against the config file’s directory.

{
  "project":      "src/Polly.Core/Polly.Core.csproj",
  "testsProject": "test/Polly.Core.Tests/Polly.Core.Tests.csproj",
  "filter":       "Polly.Retry.RetryHelper",   // type / namespace prefix to mutate
  "workers":      4,
  "timeout":      10,                            // per-mutant timeout, seconds
  "json":         "report.json",
  "html":         "report.html",
  "thresholds":   { "high": 80, "break": 60 }    // displayed today; exit-code
                                                 // gating is on the roadmap
}

mutato init writes this file for you after discovering the target/test pair.

Common flags

Flag	Purpose
--project <csproj>	The project to mutate (code under test).
--tests-project <csproj>	The test project to build and run.
--tests <dll>	Use a prebuilt test assembly and skip the build.
--filter <prefix>	Restrict mutation to a type or namespace prefix.
--test-class <fqn>	Run only one test class (handy to dodge flaky baselines).
--workers <n>	Number of warm worker hosts.
--timeout <seconds>	Per-mutant timeout ceiling.
--json <path>	Write a Stryker-schema JSON report.
--html <path>	Write an HTML report.
--config <file>	Read a specific config file.

Start with a narrow --filter on a single type; widen it once the baseline is green and the score looks sane.

Fast TUnit host (--host tunit-direct)

For TUnit suites, --host tunit-direct drives the source-generated test delegates directly and skips the per-run Microsoft Testing Platform pipeline (~85 ms/run → sub-millisecond), behind the same warm-pool, coverage, and timeout machinery.

It’s a large win on fast unit suites and a smaller one on slow or async suites where test time dominates. It handles parameterless and [Arguments] tests plus any TUnit data source — [MethodDataSource] and custom generators are expanded through TUnit’s own GetDataRowsAsync. For anything it can’t host faithfully (constructor/property injection, lifecycle hooks) it falls back to the MTP host for that run, so verdicts are never wrong.