# Debug a Single Failing Test in Fiber with Atlas in 2026

> Atlas helps Fiber developers debug a single failing test by running `go test` in isolation, analyzing the call graph with LSP, and proposing precise code fixes.

To debug a single failing test in Fiber with Atlas, you will run the specific test using `go test (app.Test)` via Atlas's `bash` tool, then work through the call graph with `lsp`, and finally fix the production code using `edit` or `apply_patch`. This process ensures you address the root cause within your Fiber application, not just the assertion, leveraging the familiar `go mod` and `gofmt` toolchain.

## Key takeaways

- Atlas uses `bash` to run `go test` with specific filters, isolating failing Fiber tests.
- The `lsp` tool in Atlas navigates Fiber's `fiber.Ctx` handlers and middleware call graphs.
- Atlas's `edit` and `apply_patch` tools fix Fiber code, respecting `fasthttp` buffer lifecycles.
- All Atlas changes are presented as diffs for approval, followed by automatic `gofmt` execution.
- Atlas ensures a clean codebase by removing temporary logging and re-running full Fiber test suites.

## How Atlas Runs a Single Fiber Test for Debugging

Atlas runs a single failing Fiber test by invoking `go test` directly through its `bash` tool, allowing you to specify a precise test filter like `^TestMyFailingHandler$` to isolate the issue. This approach ensures that only the relevant test executes, providing a focused output for debugging in 2026, often reducing execution time by 90% or more compared to a full suite.

When a specific test within your Fiber application, such as `TestUserRegistrationHandler` in `handlers_test.go`, begins to fail, Atlas can isolate it. Instead of running the entire test suite with `go test ./...`, Atlas uses its `bash` tool to execute `go test -run ^TestUserRegistrationHandler$ ./handlers_test.go`. This command targets only the specified test function, minimizing output and allowing you to focus on the relevant code path. Because Atlas's `bash` tool is a real shell, you can also add standard Go test flags like `-v` for verbose output or `-count=1` to prevent caching, just as you would manually. This direct interaction with the `go test` runner ensures that Atlas operates within the familiar Fiber testing environment, respecting your existing `go.mod` dependencies and project structure.

## Navigating Fiber Code and Call Graphs with Atlas's LSP

After identifying a failing Fiber test, Atlas uses its `lsp` tool to work through the application's call graph, starting from the test and tracing into the `fiber.Ctx` handlers and middleware. This allows Atlas to understand the flow of data and control, crucial for debugging issues related to `fasthttp` buffer lifetimes, a common gotcha in Fiber v3 applications.

Once a single Fiber test fails, Atlas employs its `lsp` tool to perform `goToDefinition` and `findReferences` operations. For instance, if `TestUserRegistrationHandler` fails, Atlas will first `read` the test file, then use `lsp goToDefinition` on the `app.Test()` call or the specific handler function being tested, like `userRegistrationHandler`. This allows Atlas to jump directly to the definition of `userRegistrationHandler` within your `app.Group` routes. From there, Atlas can trace the execution path, examining how `fiber.Ctx` values are used and passed, and identifying any middleware registered before `app.Listen`. This deep understanding of the call graph is vital for diagnosing problems, especially those involving `fasthttp`'s buffer reuse, where `fiber.Ctx` values retained past the handler can lead to unexpected behavior. Atlas's indexing by AST declarations, rather than blind line windows, ensures precise navigation through your Fiber codebase.

## Forming Hypotheses and Fixing Fiber Application Code

With a clear understanding of the failing Fiber test and its call graph, Atlas forms a hypothesis and checks it by adding temporary logging or re-running tests with verbose flags. Atlas then fixes the production code using its `edit` tool, ensuring changes are precise and respect Fiber's idioms, such as handling `fasthttp` buffer lifecycles correctly in 2026.

After tracing the call graph, Atlas will formulate a hypothesis about the cause of the failure. To validate this, Atlas can use its `edit` tool to insert temporary logging, such as `fmt.Println("DEBUG: value is %v", ctx.Query("param"))` within a `fiber.Ctx` handler or a middleware function. Alternatively, Atlas can re-run the single test with `bash` and a verbose flag, like `go test -run ^TestMyFailingHandler$ -v`. Once the hypothesis is confirmed, Atlas proceeds to fix the production code. For small, localized changes, the `edit` tool is used to modify specific lines or hunks. If the fix involves changes spanning multiple files or requires a more complex refactoring, Atlas can use `apply_patch` to ensure a cohesive and atomic update. Atlas is specifically instructed to consider Fiber's unique aspects, such as copying any `ctx` value retained past the handler to avoid `fasthttp` buffer reuse issues, ensuring robust and idiomatic fixes.

## Reviewing and Applying Code Changes in Fiber with Atlas

Atlas ensures every proposed code change to your Fiber application is transparent and reviewable. Before writing any edits, Atlas computes a unified diff and presents it for your approval, allowing you to accept, modify, or reject changes. This process includes automatically running `gofmt` on touched files and re-running the full test suite to confirm the fix in 2026.

Before Atlas writes any changes to your Fiber application's files, it computes a unified diff for every proposed edit. This diff is surfaced for your approval, giving you complete control over the modifications. You can review the changes, ask Atlas to refine them, or deny them entirely. Once approved, Atlas applies the changes and then automatically runs `gofmt` on all touched files to maintain code consistency and adhere to Go's standard formatting. After the production code is fixed and formatted, Atlas re-runs the single failing test to confirm the immediate fix. Subsequently, it runs the full test suite with `go test ./...` to ensure no regressions have been introduced. Any temporary logging added during the debugging process is then removed by Atlas using the `edit` tool, and these cleanup changes are also presented for your approval, ensuring a clean and stable codebase.

## Steps

1. Run just the failing Fiber test with Atlas's `bash` tool, using `go test -run ^TestMyFailingHandler$` to focus output.
2. Read the Fiber test and the `fiber.Ctx` handler it exercises, then use Atlas's `lsp goToDefinition` and `findReferences` to walk the call path.
3. Form a hypothesis and check it: add temporary logging with Atlas's `edit` tool, or re-run the test with `bash` and a verbose flag like `go test -v`.
4. Fix the production Fiber code with Atlas's `edit` tool; if the change spans several hunks or files, use `apply_patch` instead.
5. Re-run the single Fiber test with `bash` to confirm the fix, then run the full suite with `go test ./...` to check for regressions.
6. Remove any temporary logging you added using Atlas's `edit` tool, and approve the `gofmt` changes applied by Atlas.

## FAQ

### How does Atlas handle `fasthttp` buffer reuse issues in Fiber?

Atlas is aware of `fasthttp`'s buffer reuse and will prompt you to copy any `fiber.Ctx` value you intend to retain past the handler's execution. When fixing code, Atlas will suggest or implement explicit copying to prevent data corruption.

### Can Atlas debug Fiber middleware failures?

Yes, Atlas can debug Fiber middleware failures. By tracing the call graph with `lsp` from the failing test, Atlas identifies the middleware in the execution path and can insert logging or propose fixes within the middleware's logic using `edit`.

### Does Atlas use `go test` directly for Fiber applications?

Absolutely. Atlas uses its `bash` tool to invoke `go test` directly, allowing it to leverage all standard `go test` flags and features, including running `app.Test()` functions and specifying test filters like `-run`.

### How does Atlas ensure code quality after a fix in Fiber?

After applying a fix, Atlas automatically runs `gofmt` on all touched files to ensure formatting consistency. It also re-runs the full `go test ./...` suite to verify that the fix hasn't introduced any regressions into your Fiber application.

### What if a Fiber test fix requires changes across multiple files?

For fixes spanning multiple files or requiring more complex refactoring in your Fiber application, Atlas uses its `apply_patch` tool. This allows for a unified, atomic change across several hunks, ensuring consistency and simplifying review.

### Can I review Atlas's proposed changes to my Fiber code?

Yes, every single file edit proposed by Atlas is presented as a unified diff for your explicit approval. You have full control to accept, modify, or reject any changes before they are written to your Fiber codebase.

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Canonical HTML: https://runatlas.sh/resources/stacks/debug-a-failing-test-in-fiber
Source of truth: aeo_pages row `/resources/stacks/debug-a-failing-test-in-fiber` (segment: Stacks) (this file is generated from it, never hand-edited).
Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.