how to debug one failing xUnit test without running the whole solution

Atlas runs just the failing test with bash, using xUnit's filter flag through dotnet test so the output is small enough to reason about. A filtered run gives a 15-second feedback loop instead of a multi-minute full-solution rebuild.

how do I find what a C# test is actually calling through dependency injection

Use Atlas's lsp tool goToDefinition operation. Because DI means the concrete type is not visible at the call site, the language server's resolution does real work that reading the code by eye cannot, walking from the xUnit assertion into the implementation in src/.

can an AI agent add temporary logging while debugging C#

Yes. Atlas adds temporary logging with edit and re-runs the filtered dotnet test through bash. Because bash is a real shell, the same levers you would use by hand, Console.WriteLine, ITestOutputHelper, a higher verbosity flag, are all available.

when should I use apply_patch instead of edit in Atlas

When the C# change spans several hunks. Changing a method's return type from Task to Task means updating the signature, the return statements, and the callers. Chaining sequential edits through that is brittle, so Atlas uses apply_patch instead.

how do I stop an AI agent from just changing the assertion to make a test pass

The job is to fix the code, not the assertion. Atlas walks the call path with the lsp tool before editing, so the change lands in the production class in src/ rather than in the xUnit test, and every edit arrives as a unified diff you approve.

does Atlas remove the debug logging it added

Yes. Removing temporary logging is a step in the workflow, not a nicety. Atlas snapshots file changes as git patches so edits can be diffed and rolled back, which makes clearing the hypothesis-loop Console.WriteLine calls straightforward.

what does Atlas need to debug a C# project

Run atlas in a solution with a .csproj or .sln. Atlas reads your namespaces, project references, and NuGet packages, and runs xUnit via dotnet test and dotnet format through the bash tool.

Debug a single failing test in C# with Atlas (2026)

To debug a single failing C# test with Atlas, isolate it before you theorize. Atlas runs just the failing test with bash, using xUnit's filter flag through dotnet test so the output is small enough to reason about, then reads the test and the module it exercises. Atlas walks the call path with the lsp tool's goToDefinition and findReferences operations, which is how a failing Assert.Equal in a test project is traced back to the actual class in src/ that produced the wrong value. Because bash is a real shell, the same debugging levers you would use by hand are all available: add temporary logging with edit, re-run with a verbose flag, or narrow the filter further. Atlas fixes the production code with edit, or apply_patch when the change spans several hunks, then re-runs the single test and the full suite. NuGet resolves the packages and dotnet format cleans the diff. The goal is to fix the code, not the assertion.

How do you run just one failing xUnit test through Atlas?

Atlas runs just the failing test with bash, using the framework's filter flag so the output is small enough to reason about. For xUnit via dotnet test that means a filter narrowing the run to a single fully qualified test method, which turns 4 minutes of solution-wide output into 5 lines of signal in 2026.

Isolation is the first move because a whole-suite run drowns the one failure you care about. A filtered dotnet test run against a single xUnit method rebuilds only what is needed, executes only that test, and prints only its assertion failure and stack trace. The short loop is what makes iterative hypothesis testing bearable: you will run this command a dozen times while debugging, and a 4-minute cycle makes that impossible while a 15-second cycle makes it routine. Every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, so the filtered dotnet test invocation is one you approved, and Atlas will keep using the same command shape rather than silently widening it.

How does Atlas trace a failing C# assertion back to the real bug?

Atlas reads the test and the module it exercises, then walks the call path with 2 lsp operations: goToDefinition and findReferences. A failing Assert.Equal in a C# test project names a class and a method; goToDefinition jumps from the test's call site into the implementation in src/, and findReferences shows who else depends on it.

The assertion tells you the output was wrong. The call path tells you where it went wrong. In a C# service the chain is often long: an xUnit test calls a controller, which calls a service resolved through dependency injection, which calls a repository, which calls a mapper. goToDefinition follows each hop, and because dependency injection means the concrete type is not visible at the call site, the lsp tool's resolution is doing real work that reading the code by eye would not do. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, so each method Atlas lands on comes back whole, with its signature, its attributes, and its async modifier intact.

How do you test a hypothesis about a failing C# test?

Atlas forms a hypothesis and checks it: add temporary logging with edit, or re-run with a verbose flag through bash. In 2026 bash is still a real shell, so the same debugging levers you would use by hand, extra Console.WriteLine calls, an xUnit ITestOutputHelper write, or a higher dotnet test verbosity level, are all available to Atlas.

Debugging is hypothesis testing, and the loop must be cheap. If the hypothesis is that a nullable reference is arriving null from the DI container, Atlas adds a temporary log line with edit, re-runs the filtered dotnet test, and reads the output. If the hypothesis is that the async method is being awaited on the wrong context, Atlas re-runs with a higher verbosity so the full exception chain, including the inner exception xUnit sometimes hides, is visible. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so even a temporary Console.WriteLine is a change you saw. And because Atlas snapshots file changes as git patches, the temporary logging is trivially removable at the end.

When should Atlas use apply_patch instead of edit in C#?

Atlas fixes the production code with edit, but if the change spans several hunks, it uses apply_patch instead of chaining brittle edits. A C# fix that adds a null check, changes a method signature, and updates the DI registration in Program.cs touches 3 places, and 3 sequential edits shift the line numbers under each other.

The single-hunk fix belongs to edit, which is precise and shows a clean diff. The multi-hunk fix belongs to apply_patch, which applies the whole set as a unit. In C# a real fix often is multi-hunk: changing a method's return type from Task<T> to Task<T?> means updating the signature, the return statements inside it, and the callers that no longer compile. Chaining edits through that sequence is brittle. apply_patch handles the whole structural change at once. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so a multi-file C# fix arrives as a reviewable set of diffs across each .cs file rather than as an opaque bulk write.

How do you finish a C# debugging session cleanly?

Atlas re-runs the single test, then the full suite, and removes any temporary logging it added. A filtered xUnit via dotnet test run proves the specific fix; the full solution run proves no other test project regressed. Cleaning out the temporary Console.WriteLine calls is a required step, not a nicety, in 2026.

The end of a C# debugging session has three checks. First, the filtered dotnet test against the originally failing xUnit method must pass. Second, the full dotnet test across the .sln must pass, which restores every NuGet package and rebuilds every .csproj, catching the case where the fix broke a caller in another project. Third, every temporary log line added during the hypothesis loop must be removed, which Atlas does with edit or by rolling back the git patch snapshot. Run dotnet format so the changed .cs files match the solution's style, then let Atlas stage and create the commit, since Atlas reads git branches, status, and diffs and can commit on your behalf.

Step by step

01Run atlas in a solution with a .csproj or .sln so Atlas can read your namespaces, project references, and NuGet packages.
02Run just the failing test with bash, using xUnit's filter flag through dotnet test so the output is small enough to reason about.
03Read the test and the module it exercises, then use the lsp tool's goToDefinition and findReferences operations to walk the call path from the xUnit assertion into the class in src/.
04Form a hypothesis and check it: add temporary logging with edit, or re-run with a higher dotnet test verbosity through bash so xUnit's inner exception is visible.
05Fix the production code with edit; if the change spans several hunks, for example a signature change plus a DI registration in Program.cs, use apply_patch instead of chaining brittle edits.
06Re-run the single filtered xUnit via dotnet test to confirm the specific fix landed.
07Run the full dotnet test across the .sln to confirm no other test project regressed.
08Remove any temporary logging you added, run dotnet format, and let NuGet resolve before dotnet build and commit.

Frequently asked questions

how to debug one failing xUnit test without running the whole solution: Atlas runs just the failing test with bash, using xUnit's filter flag through dotnet test so the output is small enough to reason about. A filtered run gives a 15-second feedback loop instead of a multi-minute full-solution rebuild.
how do I find what a C# test is actually calling through dependency injection: Use Atlas's lsp tool goToDefinition operation. Because DI means the concrete type is not visible at the call site, the language server's resolution does real work that reading the code by eye cannot, walking from the xUnit assertion into the implementation in src/.
can an AI agent add temporary logging while debugging C#: Yes. Atlas adds temporary logging with edit and re-runs the filtered dotnet test through bash. Because bash is a real shell, the same levers you would use by hand, Console.WriteLine, ITestOutputHelper, a higher verbosity flag, are all available.
when should I use apply_patch instead of edit in Atlas: When the C# change spans several hunks. Changing a method's return type from Task<T> to Task<T?> means updating the signature, the return statements, and the callers. Chaining sequential edits through that is brittle, so Atlas uses apply_patch instead.
how do I stop an AI agent from just changing the assertion to make a test pass: The job is to fix the code, not the assertion. Atlas walks the call path with the lsp tool before editing, so the change lands in the production class in src/ rather than in the xUnit test, and every edit arrives as a unified diff you approve.
does Atlas remove the debug logging it added: Yes. Removing temporary logging is a step in the workflow, not a nicety. Atlas snapshots file changes as git patches so edits can be diffed and rolled back, which makes clearing the hypothesis-loop Console.WriteLine calls straightforward.
what does Atlas need to debug a C# project: Run atlas in a solution with a .csproj or .sln. Atlas reads your namespaces, project references, and NuGet packages, and runs xUnit via dotnet test and dotnet format through the bash tool.

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