DotNetDevMCP

Affected tests

dotnet_test_affected answers “which tests can my change break?” with the compiler, then runs only those.

How it decides

sequenceDiagram
    participant Agent
    participant Tool as dotnet_test_affected
    participant Roslyn
    participant CLI as dotnet test
    Agent->>Tool: changed files (or git)
    Tool->>Roslyn: symbols declared in those files
    loop up to maxDepth hops, within maxSelectionSeconds
        Tool->>Roslyn: FindReferences(symbol)
        Roslyn-->>Tool: the members that use it
    end
    Tool->>Tool: which of them are test methods?
    alt finished, and at most maxSelectedFraction of all tests
        Tool->>CLI: one run per test project, filtered to those tests
    else out of budget, or too many tests
        Tool->>Tool: which test projects reference the changed projects?
        Tool->>CLI: only those test projects (or the whole solution if that's all of them)
    end
    CLI-->>Tool: TRX results
    Tool-->>Agent: results, the reason for each selected test (via), and what was run

1. Changed files: from changedFiles, or from git (uncommitted changes, or gitBase: "main" for a branch).
2. Symbols: everything the files declare that can run: methods, properties, constructors. A type or field hops through its constructors (field initializers run there) and, for fields, through everything that reads them. That’s how xUnit [MemberData] theories are found.
3. Reference walk: Roslyn finds every member that uses those symbols, then every member using those, up to maxDepth hops (default 8).
4. Test methods: members with [Fact], [Theory], [Test], [TestCase], [TestCaseSource], [TestMethod] or [DataTestMethod].

When it doesn’t filter by test name

Situation	Why	What runs
The walk didn’t finish within maxSelectionSeconds (10)	The change touches code nearly everything depends on; tracing it costs more than running tests	The test projects that reference the changed projects
The selection is more than maxSelectedFraction (20%) of all test methods	A filtered run that large is no faster than running those projects whole (measured on Polly)	Same
A changed file isn’t compiled C# (.csproj, .razor, appsettings.json, resources, a deleted .cs file)	The reference walk can’t see it, but it can still break tests	The test projects that reference the project folder holding it; the file is listed in untracedFiles
A build-wide file changed (.props, .targets, global.json, nuget.config, .editorconfig)	It can affect every project	The whole solution
Every test project is reachable	Nothing to narrow	The whole solution, in one run

The response says what ran and why: ranScope is selection, projects or solution, testProjectsRun lists the projects, and note explains the reason. A partial selection is never presented as complete. The project step follows project references only: a test project that uses the changed code through a NuGet package reference isn’t found. On a solution where every test project uses the changed library (Polly and Polly.Core), this still runs everything; it narrows when modules are independent.

Parameters

Parameter	Default	Meaning
changedFiles	git working tree	Files to start from
gitBase	-	Diff against this ref instead, e.g. main
dryRun	false	Only list the tests and why
maxDepth	8	Reference hops. 3 narrows more changes but misses tests reached through long call chains
maxSelectionSeconds	10	Time budget for the walk
maxSelectedFraction	0.2	Above this share of all tests, run everything
framework	all	Run one target framework, e.g. net10.0
noBuild	false	Skip building the affected test projects
timeoutSeconds	600	Kill the run if a test hangs; the response names the modules that never finished

What it can’t see

• Reflection (Activator.CreateInstance, GetMethod, BindingFlags), string-keyed lookups, and DI by convention (assembly scanning). A test that reaches your code only that way won’t be selected. On Polly this was the one miss out of 112 broken tests.
• Documentation and files outside every project (*.md, images, CI workflows) are ignored: they can’t break a test.
• Calls through an interface or base class are followed: changing OrderService.Submit selects a test that only calls IOrderService.Submit.
• Hanging tests are killed after timeoutSeconds and reported by module, not by test name (VSTest runs do name the test).
• The first selection of a session on busy code can hit the time budget and run everything; Roslyn caches what it learned, so the same selection is fast the next time.

Test frameworks

VSTest (dotnet test classic) and Microsoft.Testing.Platform ("test": { "runner": "Microsoft.Testing.Platform" } in global.json) are both supported, with xUnit v3 detected automatically and MSTest/NUnit handled through --filter. DotNetDevMCP runs dotnet from your project’s directory, so your global.json (SDK version, test runner) applies.

Numbers: Benchmarks.

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