ADR-0128: Test coverage is measured on every pull request and reported, not gated

• Status: accepted
• Date: 2026-09-29
• Deciders: Mahdi Amirabdollahi

Context

Studio has about 1,300 backend tests and a Vitest suite for the UI, and nothing says what they exercise. A reviewer cannot tell whether a change is tested except by reading the tests, and a module that quietly lost its tests does not show up anywhere.

Decision

We will measure line and branch coverage for the backend and the UI on every pull request that runs their tests, and report it in the job summary and as a downloadable HTML report, with no threshold that fails a build.

• Backend: JaCoCo 0.8.15's agent records every Surefire run into target/jacoco.exec, and verify writes the report to target/site/jacoco. In CI each of the three shards uploads its exec file, and a coverage job merges them with the JaCoCo CLI (resolved through Maven) into one report. The job is part of ci-ok, so broken coverage wiring is caught.
• UI: Vitest's V8 provider (@vitest/coverage-v8, pinned to Vitest's version) through npm run test:coverage, over src/, excluding tests, the test harness and generated types.
• Both summaries go to the job summary; both HTML reports are artifacts kept for 14 days.
• No external coverage service. The numbers stay in GitHub, with no token or third-party app.

Consequences

• Every PR shows what its tests cover, and a drop is visible in review.
• The agent and V8 add a few percent to test time.
• Without a threshold, coverage can fall without failing anything. Once the numbers have settled, a floor per side is a one-line change (coverage.thresholds in Vitest, the JaCoCo check goal).

Alternatives considered

• A hard threshold now. Rejected until a baseline exists: a number picked before measuring either fails at once or is too low to matter.
• Codecov or Coveralls. Rejected for now: a third-party service and token for what the job summary already shows. PR comments and trend graphs are what it would add.
• Coverage per shard only. Rejected: three partial reports that no one can add up.