Use Claude to Analyse Test Coverage and Fill the Gaps

Most projects have test suites that cover the happy path but miss edge cases, error handling, and boundary conditions. Claude can audit your coverage and write the missing tests.

# Generate a coverage report first
npx vitest run --coverage --reporter=json --outputFile=coverage.json

# Then feed it to Claude
claude "Read coverage.json and identify the files with the lowest branch coverage.
For each of the bottom 5 files, read the source code, understand what's untested,
and write tests that cover the missing branches. Use Vitest with describe/it blocks."

You can also point Claude at a specific file for a deep-dive:

Look at src/services/payment.ts and its existing test file.
Run the tests with coverage for just this file.
Identify every uncovered branch and edge case, then write tests to hit them all.
Aim for 100% branch coverage.

Claude will read both the source and existing tests, understand what's already tested, and generate new test cases that specifically target the gaps — error paths, null checks, boundary values, and race conditions.

For a quick scan without generating a report file:

npx jest --coverage --coverageReporters=text 2>&1 | claude --print "Which files need more tests? List the top 5 priorities."

Coverage reports tell you what's missing — Claude actually writes the tests to fix it.