/tunit

name: tunit
description: "Write, run, or repair .NET tests that use TUnit. Use when a repo uses `TUnit`, `TUnit.Playwright`, `[Test]`, `[Arguments]`, `ClassDataSource`, `SharedType.PerTestSession`, or Microsoft.Testing.Platform-based execution. Preserve TUnit's parallel-by-default model and constrain only destructive shared-state collisions. DO NOT USE FOR: xUnit projects; MSTest projects."

TUnit

Trigger On

• the repo uses TUnit
• you need to add, run, debug, or repair TUnit tests
• the repo uses Microsoft.Testing.Platform-based test execution
• the repo uses `ClassDataSource<...>(Shared = SharedType.PerTestSession)`, `TUnit.Playwright`, or `--treenode-filter`

Do Not Use For

• xUnit projects
• MSTest projects
• generic test strategy with no TUnit-specific mechanics

Inputs

• the nearest `AGENTS.md`
• the test project file and package references
• the repo's current TUnit execution command

Workflow

1. Confirm the project really uses TUnit and not a different MTP-based framework. 2. Read the repo's real `test` command from `AGENTS.md`. If the repo has no explicit command yet, start with `dotnet test PROJECT_OR_SOLUTION`. 3. Keep the TUnit execution model intact:

• tests are source-generated at build time
• tests run in parallel by default
• on .NET 10, test modules also run in parallel by default up to `Environment.ProcessorCount`
• built-in analyzers should remain enabled

4. Choose the fixture level deliberately:

• plain TUnit tests for isolated logic
• shared AppHost/Aspire fixtures for HTTP, SignalR, SSE, or UI flows
• `WebApplicationFactory` layered over shared Aspire infra when tests need Host DI services, `IGrainFactory`, or other runtime internals

5. Reuse expensive fixtures with `ClassDataSource<Fixture>(Shared = SharedType.PerTestSession)` instead of booting distributed infrastructure per test. Fixture reuse does not serialize consumers: keep the fixture concurrency-safe and give each test unique mutable state. 6. Keep tests and test modules parallel. Do not add `--max-parallel-test-modules 1`, `TUNIT_MAX_PARALLEL_TESTS=1`, `[assembly: NotInParallel]`, a class-wide `[NotInParallel]`, or an equivalent global restriction. 7. Use keyed `[NotInParallel("collision-domain")]` only on the smallest tests that perform destructive changes to the same shared state and can corrupt one another. A shared read-only fixture, expensive startup, module boundary, or vague CI-stability concern is not a reason to limit parallelism. 8. Run the narrowest useful scope first with `dotnet test ... --treenode-filter "..."` on .NET 10. Use the older `--` separator only when the repository is pinned to an SDK that requires it. 9. Capture useful failure evidence: host log dumps, focused console output, coverage files, and Playwright screenshots/HTML for UI tests. 10. Use `[Test]`, `[Arguments]`, hooks, and dependencies only when they make the scenario clearer, not because the framework allows it.

Bootstrap When Missing

If `TUnit` is requested but not configured yet:

1. Detect current state:

• `rg -n "TUnit|Microsoft\\.Testing\\.Platform" -g '*.csproj' -g 'Directory.Build.*' .`

2. Add the minimal package set to the test project:

• `dotnet add TEST_PROJECT.csproj package TUnit`
• do not add `Microsoft.NET.Test.Sdk` to a current TUnit project; it selects the VSTest path and conflicts with the normal Microsoft.Testing.Platform setup

3. Keep the runner model explicit in `AGENTS.md` and CI:

• record that the repo uses Microsoft.Testing.Platform-compatible execution for this test project
• record the exact `dotnet test TEST_PROJECT.csproj` command the repo will use

4. Add one small executable test using `[Test]`. 5. Run `dotnet test TEST_PROJECT.csproj` and return `status: configured` or `status: improved`. 6. If the repo intentionally standardizes on xUnit or MSTest, return `status: not_applicable` unless migration is explicitly requested.

Deliver

• TUnit tests that respect source generation and parallel execution
• commands that work in local and CI runs
• framework-specific verification guidance for the repo
• a fixture strategy that matches the actual test scope: logic-only, AppHost/API, Host DI/grains, or Playwright UI

Validate

• the command matches the repo's TUnit runner style
• focused runs use `--treenode-filter` rather than VSTest-style `--filter`
• .NET 10 runs pass MTP options directly without a `--` separator
• no global or assembly-wide single-thread setting has been introduced
• any keyed non-parallel group is limited to tests with a named destructive shared-state collision
• shared distributed fixtures use `SharedType.PerTestSession` or an equivalent reuse pattern
• fixture and shared infrastructure are safe for concurrent consumers; mutable data is isolated per test
• built-in TUnit analyzers remain active
• coverage tooling matches Microsoft.Testing.Platform if coverage is enabled
• UI failures capture artifacts and server-side failures expose enough logs to avoid blind reruns

Test Harness

flowchart LR
  A["TUnit task"] --> B{"What does the test need?"}
  B -->|"Single component only"| C["Plain TUnit test"]
  B -->|"HTTP / SignalR / resource graph"| D["Shared Aspire/AppHost fixture"]
  B -->|"Host DI / grains / runtime services"| E["Shared Aspire/AppHost fixture + WebApplicationFactory"]
  B -->|"Browser automation"| F["Shared Aspire/AppHost fixture + Playwright"]
  C & D & E & F --> G["Run focused with --treenode-filter"]
  G --> H["Capture logs, artifacts, and coverage"]

Load References

• [references/patterns.md](references/patterns.md)
• [references/migration.md](references/migration.md)
• [references/tunit.md](references/tunit.md)
• [references/integration-testing.md](references/integration-testing.md)

Running Tests

TUnit uses Microsoft.Testing.Platform. Use `--treenode-filter` for filtering, not VSTest `--filter`. On .NET 10, pass MTP switches directly; older SDKs may require `--`.

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