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Coverage Delta Gate Workflow

Gate merges on coverage of changed lines rather than a global percentage so new code is tested without blocking on legacy debt.

Overview

Enforce coverage on changed code rather than an absolute global target.

This workflow is reusable across teams and environments; adapt the commands to your tooling while keeping the sequence and checkpoints intact.

Environment and prerequisites

• A coverage tool with diff support
• CI pipeline
• A global coverage baseline

Procedure

1. Measure coverage of changed lines

Compute coverage restricted to the lines changed in the pull request.

Expected state: Changed-line coverage is known.

2. Gate on the delta

Require changed-line coverage above a threshold instead of a global percentage.

Expected state: New code is tested without fighting legacy debt.

3. Allow documented exceptions

Permit justified exclusions for generated or trivial code.

Expected state: Legitimate exclusions are explicit.

4. Track the global trend

Also monitor overall coverage so it does not slide.

Expected state: Global coverage holds or improves.

# diff coverage on changed lines only
diff-cover coverage.xml --compare-branch=origin/main --fail-under=80

Verification

Confirm a change with untested new lines fails the gate and justified exclusions are documented.

Decision points

Global percentage or changed lines?

• Changed-line coverage
• Global percentage

Recommendation: Gate changed lines; global gates block on unrelated legacy debt.

Allow exclusions?

• Allow with justification
• Never allow

Recommendation: Allow documented exclusions for genuinely untestable code.

Common failures

• Global gates block unrelated work.
• Undocumented exclusions hide gaps.
• Ignoring the global trend lets coverage slide.