---
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. Follow `Test Output Budget`: show bounded root-error evidence and link coverage, Playwright screenshots, and HTML artifacts; never dump host logs.
10. Use `[Test]`, `[Arguments]`, hooks, and dependencies only when they make the scenario clearer, not because the framework allows it.

## Test Output Budget

- Keep native runner progress and ANSI enabled (`--progress on --ansi on` for supported MTP/TUnit runners); use a PTY locally. Do not replay progress redraws into model context. Use the detected runner's flags, not MTP switches on VSTest.
- Show warnings and errors plus one final summary (counts, duration, exit code). Configure test-owned console logging at `Warning`; keep Information/Debug/Trace, successful-test output, and expected negative-test noise out of context. Quiet build verbosity alone does not filter application logs.
- On failure, crash, startup error, or timeout, show the failing test/resource, root exception, and relevant stack frames. Deduplicate; cap each diagnostic tool response at 80 lines / 8 KiB, whichever comes first. Never automatically dump stdout/stderr, host logs, browser console history, DOM/HTML, TRX, or crash artifacts.
- Keep necessary diagnostics in size-bounded or rotating artifacts and link them. Search by exact failure/correlation; read bounded excerpts, never whole logs. Capture/filter noisy output before tool delivery, preserve the real exit code, and disclose truncation. Monitor actual activity; silence alone does not prove a hang.

## 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 link artifacts and server-side failures expose bounded root-error excerpts to avoid blind reruns

## Test Harness

```mermaid
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["Show bounded errors and link artifacts"]
```

## 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 `--`.

```bash
# Run all tests
dotnet test --solution MySolution.sln

# Run one test project
dotnet test --project tests/MyProject.Tests/MyProject.Tests.csproj

# Filter by class
dotnet test --project tests/MyProject.Tests/MyProject.Tests.csproj --treenode-filter "/*/*/CalculatorTests/*"

# Filter by category
dotnet test --project tests/MyProject.Tests/MyProject.Tests.csproj --treenode-filter "/*/*/*/*[Category=Integration]"

# Coverage on Microsoft.Testing.Platform
dotnet test --solution MySolution.sln --coverage --coverage-output coverage.cobertura.xml --coverage-output-format cobertura

# Raw runner help when the repo needs direct TUnit app switches
dotnet run --project tests/MyProject.Tests/MyProject.Tests.csproj -- --help
```

Filter syntax: `/<Assembly>/<Namespace>/<Class>/<Test>` with `*` wildcards. See [references/patterns.md](references/patterns.md) for full examples.

## Example Requests

- "Run this TUnit project correctly."
- "Fix our TUnit CI command."
- "Add a regression test in TUnit without breaking parallelism."