reproduce-and-fix-bug

name	reproduce-and-fix-bug
title	Reproduce and Fix Bug
description	Turn a bug report into an automated failing test that reproduces it, then make that test pass with the smallest correct fix. Use when the user reports a bug, pastes a stack trace or error message, or says "reproduce this", "write a failing test", "fix this bug", "why does X break", or "this returns the wrong result".
category	debug-understand
tools	["read_file","glob","grep","write_file","edit_file","Bash(git diff *)","Bash(git log *)","Bash(pytest *)","Bash(npm test *)","Bash(go test *)","Bash(cargo test *)"]

Reproduce and Fix Bug

Reproduce the bug as a failing test first, then fix it. A test that fails before and passes after is the proof — never skip it.

Step 1: Extract the reproduction

From the report, pin down:

• Exact inputs / steps and the environment.
• Expected behavior vs actual (error, wrong value, crash).
• Any stack trace — note the failing file, function, and line. If critical details are missing, state your assumptions explicitly and proceed with the most likely case.

Step 2: Locate the code path

• grep -rn the error message, symbols, and frames from the trace to find the involved functions.
• Read the implicated code and its callers; form a hypothesis about the fault.

Step 3: Learn the test harness

• glob for existing tests (test_*, *_test.*, *.spec.*) near the affected code.
• Read one to copy conventions (fixtures, naming, assertions).
• Confirm how tests run and that the suite is green now: pytest -q / npm test / go test ./... / cargo test.

Step 4: Write a failing test

• Add a focused test that encodes the expected behavior for the reported input.
• Run just that test and confirm it FAILS — and fails for the real reason (asserts on the bug), not a setup/import error.
• If it passes unexpectedly, your reproduction is wrong; refine inputs until it fails as reported.

Step 5: Diagnose the root cause

• Trace from the failing assertion to the origin; distinguish the root cause from its symptom.
• Avoid fixing at the surface if the defect is upstream.

Step 6: Apply the minimal fix

• Use edit_file for the smallest change that corrects the root cause.
• Do not reformat unrelated code or bundle in extra changes.

Step 7: Verify

• Re-run the new test — it must PASS.
• Run the surrounding suite to catch regressions: full module or pytest -q / npm test / go test ./....
• Consider adjacent edge cases (empty, null, boundary, negative) and add a case if warranted.

Step 8: Report

• Show git diff of the fix plus the new test.
• State the root cause in one sentence and why the change resolves it.
• Note any remaining risks or follow-ups. Do not commit unless asked.