Debugging Test Failures

Systematic approach to diagnosing and fixing test failures using MCP tools.

Step 1: Classify the Failure

Use test_history to understand the failure pattern:

• New failure: First time this test failed. Likely caused by a recent change
• Persistent failure: Failed multiple runs in a row. Likely a real bug or environment issue
• Flaky test: Intermittent pass/fail. Could be timing, ordering, or shared state

Step 2: Examine the Error

Use test_errors to search for the specific error:

• Search by error name to find similar failures across tests
• Check if the same error appears in other projects
• Look at the diff (expected vs received) for assertion failures

Step 3: Find Related Tests

Use test (action: for_file) to find all tests that cover the file containing the failure. This helps you:

• Understand the test surface for the affected code
• Find tests that might be affected by the same bug
• Identify missing test coverage

Step 4: Check Coverage Context

Use test_coverage to see:

• Which lines in the failing file are uncovered
• Whether the failure is in a well-tested or poorly-tested area
• Coverage trends (is coverage improving or degrading?)

Step 5: Document Findings

Use note (action: create) to record:

• Root cause analysis
• Steps to reproduce
• Fix strategy
• Any workarounds applied

Pin important notes with note (action: update) so they persist across sessions.

Step 6: Verify the Fix

1. Run the specific failing test: run_tests({ files: ["path/to/failing.test.ts"] })
2. Run the full suite to check for regressions: run_tests({})
3. Check test_trends to confirm coverage direction

Always use the run_tests MCP tool instead of running vitest via Bash. It uses Vitest's programmatic API, so results persist to the database and all query tools reflect the latest run immediately.