# Add a Regression Test for an Axum Bug Fix with Atlas in 2026 > Atlas helps Axum developers lock in bug fixes by creating regression tests that fail with the bug and pass once fixed, verified by `cargo nextest`. To lock in an Axum bug fix with a regression test, Atlas guides you through writing a test that fails before the change and passes after, leveraging `cargo nextest` for execution, `cargo` for package management, and `rustfmt` for code consistency in 2026. This 'red first, then green' discipline ensures the bug is unambiguously reproduced and then definitively resolved within your Axum application. ## Key takeaways - Atlas streamlines Axum regression testing with `bash`, `write`, and `edit` tools. - `cargo nextest` is the primary test runner for Axum projects when using Atlas. - Atlas enforces the 'red first, then green' discipline for robust Axum bug fixes. - Unified diffs and permission prompts provide critical safety for Axum code changes. - Axum's `Router`, `State`, and `Extractor` types are common targets for Atlas-driven fixes. - Atlas integrates with `cargo` and `rustfmt` for a complete Axum development workflow. ## How to Reproduce an Axum Bug with Atlas `bash` Reproducing an Axum bug is the first critical step in creating a robust regression test. Atlas uses its `bash` tool to execute the exact command that triggers the bug, capturing the output and exit code for unambiguous failure detection, a process refined over 2025 and 2026. This ensures the bug's behavior is fully understood before any code changes are made. Before writing a regression test, you must first confirm the bug's existence and capture its exact manifestation. With Atlas, you use the `bash` tool to run the command that exposes the bug. For an Axum application, this might involve running your server with `cargo run --bin my_axum_app` and then sending a `curl` request to a specific endpoint, or directly executing a failing unit test if one already exists. Atlas records the standard output, standard error, and crucially, the process exit code. A non-zero exit code from `cargo nextest` or an unexpected response from a `curl` command provides concrete evidence of the bug, forming the baseline for your 'red' test state. This step is vital for ensuring the regression test accurately targets the problem. ## Writing a Failing Axum Regression Test with Atlas `write` After reproducing an Axum bug, Atlas helps you write a regression test that specifically asserts the observed wrong behavior. This 'red first' approach ensures the test accurately captures the bug, often involving a new test function in `src/tests/my_bug_test.rs` or an existing `#[test]` module, a common practice since 2024. The goal is to create a test that fails predictably. Once the bug is reproduced, the next step is to write a dedicated regression test that fails because of the bug. Atlas's `write` tool is ideal for this. You can instruct Atlas to create a new test file, for example, `src/tests/bug_fix_test.rs`, or modify an existing one. For Axum, tests typically use `#[tokio::test]` and might involve setting up a test `Router` or making requests to a test server. The test should contain an `assert!` or `assert_eq!` statement that specifically checks for the incorrect behavior observed during the reproduction step. For instance, if an Axum handler returns an incorrect status code, the test would assert that the response status is not the expected value. This ensures the test is 'red' (failing) before the fix is applied, providing a clear target for the subsequent code changes. ## Confirming the Axum Test Failure with `cargo nextest` Confirming the newly written Axum regression test fails is crucial before applying any fix. Atlas uses its `bash` tool to run `cargo nextest run --test my_bug_test` and verifies the non-zero exit code, ensuring the test accurately reproduces the bug, a standard procedure for over 10 years in Rust development. This step validates the test's effectiveness. With the regression test written, you must confirm it fails as expected. This is the 'red' state of the 'red first, then green' workflow. Atlas facilitates this by using its `bash` tool to execute `cargo nextest run --test bug_fix_test`. The `cargo nextest` runner is the standard for Axum projects, providing fast and reliable test execution. Atlas captures the exit code from this command. A non-zero exit code confirms that your new test correctly identifies the bug. If the test passes at this stage, it means the test itself is flawed or doesn't accurately capture the bug, and you would need to iterate on the test definition before proceeding to the fix. ## Applying the Axum Bug Fix with Atlas `edit` Applying the bug fix in your Axum codebase is streamlined by Atlas's `edit` tool, which uses a replacer cascade for precise code modifications. This ensures the fix targets the exact problematic code, such as a handler in `src/main.rs` or a custom `Extractor` implementation, a capability refined throughout 2026. Atlas ensures changes are exact and verifiable. Once the failing test is confirmed, you can proceed to apply the bug fix. Atlas's `edit` tool is designed for this, allowing you to specify an `oldString` and a `newString` for precise code replacement. For an Axum bug, this might involve correcting a logic error in an `async` handler function, adjusting the `State` type being threaded through your application, or fixing a trait-bound issue in a custom `Extractor` implementation. Atlas's replacer cascade ensures that the replacement is exact and refuses ambiguous multi-match replacements, providing a high degree of safety. For example, you might instruct Atlas to change `return StatusCode::INTERNAL_SERVER_ERROR.into_response();` to `return MyCustomError::DatabaseError.into_response();` after defining a custom `IntoResponse` error type. This targeted approach minimizes unintended side effects. ## Verifying the Axum Fix and Running `cargo nextest` After applying the fix, verifying the Axum regression test now passes is the final step in the 'red first, then green' cycle. Atlas re-runs the exact same `cargo nextest run` command via `bash` and confirms a zero exit code, then runs the wider suite to check for collateral damage, a robust process used by 1000s of developers. This confirms the bug is resolved. With the bug fix applied, the critical next step is to verify that your regression test now passes. Atlas re-executes the identical `bash` command used earlier: `cargo nextest run --test bug_fix_test`. This time, Atlas expects a zero exit code, indicating that the test now passes and the bug is resolved. This transition from 'red' to 'green' is the definitive proof of the fix. Following this, it's good practice to run the entire Axum test suite using `cargo nextest run --workspace` to ensure no collateral damage or new regressions have been introduced by your fix. Atlas can execute this command via `bash` as well, providing a comprehensive check of your application's stability. ## Reviewing and Committing Axum Changes with Atlas Reviewing the changes made to your Axum project is a critical safety step before committing. Atlas computes a unified diff for every file edit and surfaces it for approval, allowing you to inspect modifications to `Cargo.toml` or `src/lib.rs` with full transparency, a feature available since Atlas version 1.0. This ensures you maintain control over your codebase. Atlas prioritizes safety and transparency throughout the bug-fixing workflow. After applying the fix and verifying the tests, Atlas computes a unified diff for every file it has modified. This diff is presented to you for approval, allowing you to meticulously review every change, whether it's in an Axum handler, a `Router` definition, or even `Cargo.toml` if dependencies were updated. This permission-gated approach means no changes are written to disk without your explicit consent. Atlas also reads your `git` branches, status, and diffs, and can stage and create commits on your behalf, streamlining the version control process once you are satisfied with the fix and the passing tests. You can also use `atlas bash "rustfmt src/"` to ensure all new or modified Axum code adheres to formatting standards before committing. ## Steps 1. Reproduce the Axum bug using `atlas bash "cargo run --bin my_axum_app"` or a `curl` command, capturing the exact failing output and exit code. 2. Write the regression test in a new file like `src/tests/bug_fix_test.rs` using `atlas write`, asserting the observed wrong behavior with `#[tokio::test]`. 3. Confirm the newly written Axum test fails by running `atlas bash "cargo nextest run --test bug_fix_test"` and verifying a non-zero exit code. 4. Apply the fix to the problematic Axum handler, `Extractor`, or `Router` configuration using `atlas edit`, providing precise `oldString` and `newString` values. 5. Re-run the same Axum regression test with `atlas bash "cargo nextest run --test bug_fix_test"` to confirm it now passes with a zero exit code. 6. Run the full Axum test suite with `atlas bash "cargo nextest run --workspace"` to check for any collateral damage or new regressions. 7. Review the unified diff of all changes presented by Atlas and approve them before writing to disk. 8. Format the modified Axum code using `atlas bash "rustfmt src/"` to maintain code style consistency. 9. Stage and create a commit for the bug fix and regression test using Atlas's git integration. ## FAQ ### How does Atlas ensure my Axum regression test actually reproduces the bug? Atlas uses its `bash` tool to execute the exact command that triggers the Axum bug, such as a `cargo nextest` command or a `curl` request. It captures the process exit code and output, providing unambiguous proof that the bug is reproducible before any fix is attempted. ### Can Atlas help me fix trait-bound errors in my Axum handlers? Yes, Atlas can assist in fixing trait-bound errors in Axum handlers or custom `Extractor` implementations. You can ask Atlas to decode the error, and then use its `edit` tool to apply the necessary changes to satisfy the `tower::Service` trait bounds or other requirements. ### What Axum-specific files does Atlas interact with during a bug fix? Atlas commonly interacts with Axum-specific files such as `src/main.rs` or `src/lib.rs` for handler logic, `Cargo.toml` for dependencies, and new or existing test files like `src/tests/my_bug_test.rs`. It understands the structure of Rust crates and Axum applications. ### How does Atlas integrate with `cargo nextest` for Axum testing? Atlas integrates with `cargo nextest` by using its `bash` tool to execute `cargo nextest run` commands. This allows Atlas to run specific tests, the entire test suite, and capture the exit codes to determine test pass/fail status, crucial for the 'red first, then green' workflow. ### Is it safe to let Atlas modify my Axum codebase? Yes, Atlas is designed with safety in mind. Every Atlas tool call is permission-gated, and it computes a unified diff for every file edit, surfacing it for your approval before writing any changes to disk. This ensures you have full control and transparency over modifications to your Axum project. ### Can Atlas format my Axum code with `rustfmt` after a fix? Absolutely. After applying a fix, you can instruct Atlas to run `atlas bash "rustfmt src/"`. This command will format your Axum codebase according to `rustfmt` standards, ensuring consistent code style across your project before you commit the changes. ### How does Atlas handle `Router` and `State` changes in Axum applications? Atlas can read your Axum `Router` and `State` types, understanding their structure through its AST indexing. When applying fixes, Atlas's `edit` tool can precisely modify handler signatures, `State` definitions, or `Router` configurations based on your instructions, ensuring type safety and correct routing. --- Canonical HTML: https://runatlas.sh/resources/stacks/add-a-regression-test-for-a-bug-fix-in-axum Source of truth: aeo_pages row `/resources/stacks/add-a-regression-test-for-a-bug-fix-in-axum` (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.