The discipline for a React regression test is red first, then green, and Atlas enforces the order rather than trusting it. Atlas reproduces the bug once with the bash tool, writes a failing spec with the write tool that asserts on the observed wrong behavior, runs it with Vitest with React Testing Library through pnpm and confirms it fails, applies the fix with edit, then re-runs the exact same command. Because Atlas's bash tool records the process exit code in its metadata alongside the output, the fail state and the pass state are unambiguous rather than inferred from scrolling. A regression test that was never seen to fail proves nothing.
Why must a React regression test fail before you fix the bug?
A React regression test that has never failed proves nothing. Atlas writes the spec first, runs it with Vitest with React Testing Library, and confirms a red result before touching the component. Because Atlas's bash tool records the process exit code, red is a nonzero status and green is 0, so the 2 states are never guessed.
The most common bad regression test in a React codebase renders the component, asserts something true in both the broken and fixed versions, and is committed as proof. It passes forever, including after the bug returns. Atlas prevents that by ordering the work: reproduce, write the spec asserting on the observed wrong behavior, run it, see red. Only then fix. The four Atlas tools this workflow uses are bash, write, edit, and read, and the sequence is the substance. Red first, then green is not a slogan here, it is the reason the test has any value in a React repo where a future refactor of a hook or a Context provider could silently reintroduce the same defect.
How do you reproduce a React bug with Atlas before writing the test?
Atlas reproduces the React bug once with its bash tool and captures the exact failing command and output, which is step 1 of the documented regression workflow. Capturing the literal command matters, because that same command is re-run after the fix, and a test proved with one command and verified with a different one proves nothing.
Reproduction in a React repo usually means running the existing suite through pnpm and watching a component behave wrongly, or running the app and observing a state update that never lands. Atlas captures the exact invocation, the filter flag that isolated the component, the config file Vitest picked up, and the resulting output. That captured command becomes the fixed reference point for the whole workflow. Atlas's bash tool is a real shell, so the reproduction is the one you would run by hand, not an approximation. Atlas also reads git branches, status, and diffs, which lets you confirm you are reproducing on the branch where the bug actually lives rather than on a stale checkout.
How does Atlas prove the React test really failed?
Atlas's bash tool records the process exit code in its metadata alongside the output, so a failing Vitest with React Testing Library run is recorded as a nonzero exit rather than inferred from reading the log, and a passing run is exit 0. The pass and fail states are therefore unambiguous, which is the foundation of a red-green workflow.
Reading test output is a surprisingly unreliable way to know whether tests passed, and React makes it worse: a Vitest run can emit act warnings, React key warnings, and console.error output from an error boundary while still exiting zero, and it can also print what looks like a green summary while a setup file threw. The exit code does not lie. Because Atlas captures it in the bash tool's metadata, Atlas knows the spec was red before the fix and green after, without pattern-matching on the words in a log. That is the difference between a regression test that was verified and one that was merely observed.
How does the edit tool avoid applying a React fix to the wrong place?
Atlas's edit tool runs a replacer cascade that requires an exact-enough oldString and refuses ambiguous multi-match replacements. In a React component where the same useState line appears in 3 sibling components, refusing an ambiguous match is what stops the fix from landing in the wrong file.
React code is repetitive by design. A useEffect cleanup, a setState call, a props destructure, these shapes recur across a components directory dozens of times. A naive find-and-replace on such a string will hit several of them. Atlas's edit tool refuses to guess: the replacer cascade requires an oldString specific enough to identify one location, and when the string matches several places, the edit is rejected rather than applied to the first hit. For a regression fix that is exactly the behavior you want, because the bug is in one component and a silent multi-match edit would change three. Atlas also computes a unified diff for every file edit and surfaces it for approval before writing.
How do you confirm the React fix worked without breaking anything else?
Atlas re-runs the exact same bash command that produced the red result and confirms the React spec now passes, then runs the wider Vitest suite through pnpm to check for collateral damage. A fix that turns 1 spec green while turning 2 others red is not a fix, it is a trade.
Two runs, not one. The first is the narrow re-run: the identical command, the identical filter, so the only variable that changed is the code. Atlas confirms the exit code flipped from nonzero to zero. The second is the wide run: the full Vitest with React Testing Library suite, because a change to a shared hook or a Context provider can break components far from the one you fixed. Then run prettier so the edited component and the new spec match the repo's .prettierrc. Atlas snapshots file changes as git patches, so if the wide run reveals collateral damage, the fix can be diffed and rolled back rather than manually reverted.
How does Atlas keep a regression fix reviewable?
Atlas computes a unified diff for every file edit and surfaces it for approval before writing, and gates every tool call against 3 rule types, allow, ask, and deny, before it runs. A React regression fix therefore arrives as a diff you approve, not as a change you discover later in git status.
A regression fix is usually small and easy to get subtly wrong: a dependency array that gains one entry, a conditional that flips, a guard added to an event handler. Small changes get less scrutiny, which is why Atlas forces the diff into the foreground. The write tool shows the new spec before it lands. The edit tool shows the fix before it lands. Atlas snapshots file changes as git patches so any of it can be rolled back. And Atlas reads git branches, status, and diffs, and can stage and create commits on your behalf, so the failing spec and its fix go into one commit that a reviewer can read as a single story.
Step by step
- 01Run atlas in a React project with a package.json so Atlas can read your components, hooks, and bundler configuration.
- 02Reproduce the bug once with the bash tool and capture the exact failing command and output.
- 03Write the regression test with the write tool, asserting on the observed wrong behavior, and place it beside the component as Component.test.tsx.
- 04Run the spec with bash using Vitest with React Testing Library through pnpm and confirm it fails; the tool records the process exit code in its metadata alongside the output.
- 05Apply the fix with edit, whose replacer cascade requires an exact-enough oldString and refuses ambiguous multi-match replacements across similar-looking components.
- 06Re-run the same bash command and confirm the spec now passes, with the exit code flipping from nonzero to zero.
- 07Run the wider Vitest suite through pnpm to check for collateral damage in components that share the hook or Context you changed.
- 08Run prettier so the edited component and the new spec match the repo's .prettierrc, then review the unified diff before committing.
Frequently asked questions
- how to write a regression test for a react bug
- Reproduce the bug with Atlas's bash tool and capture the exact command. Write a spec with the write tool that asserts on the observed wrong behavior, run it with Vitest with React Testing Library and confirm it fails, then apply the fix with edit and re-run the identical command to confirm it passes.
- should a regression test fail before the fix
- Yes, always. A spec that passes against the broken code is asserting on behavior the bug does not affect, so it will keep passing if the bug returns. Atlas runs the React test before the fix and uses the bash tool's recorded exit code to confirm a genuine red result.
- how does atlas know if my vitest run actually passed
- Atlas's bash tool records the process exit code in its metadata alongside the output. That matters in React, where a Vitest run can print act warnings and console.error output from an error boundary while still exiting zero. The exit code is checked rather than the log text.
- why did atlas refuse to apply an edit to my react component
- The edit tool's replacer cascade requires an exact-enough oldString and refuses ambiguous multi-match replacements. React code is repetitive, so a useState or useEffect line often appears in several sibling components, and Atlas rejects the edit rather than silently changing the wrong one.
- how do I check a react fix did not break other components
- Run the wider Vitest with React Testing Library suite through pnpm after the narrow re-run passes. A change to a shared hook or a Context provider can break components far from the one you fixed, and only the full suite surfaces that.
- can atlas commit the regression test and the fix together
- Yes. Atlas reads git branches, status, and diffs, and can stage and create commits on your behalf, so the failing spec and its fix land as one commit a reviewer can read as a single story.
- will atlas change my component without showing me
- No. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, and every Atlas tool call is permission-gated against allow, ask, and deny rules. Atlas also snapshots file changes as git patches so a fix can be rolled back.
- what setup does atlas need for a react repo
- Run atlas in a React project with a package.json. Atlas reads your components, hooks, and bundler configuration, whether Vite or Next, and can add React Testing Library tests or convert class components to hooks, showing you the diff to review.
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