Dart Test Coverage

Dart Test Coverage

Guidelines for running and interpreting test coverage in Dart packages.

When to use this skill

• When asked to "check test coverage" or "improve coverage".
• When you need to identify which parts of a library are untested.

Discovery

To find areas lacking test coverage:

Run Coverage Analysis

Follow the workflow to generate and interpret coverage data:

• Run Tests with Coverage: dart test --coverage=.dart_tool/coverage
• Interpret Results: Use the script or format_coverage as described in

the Interpreting Results section to identify specific files and missed

lines.

How to use this skill (The Workflow)

• Ensure tests pass by running dart test.
• Collect coverage by running dart test --coverage=.dart_tool/coverage.
• Interpret the results using the provided script or standard tools.
• Add tests to cover missed lines.

Running Coverage

Run the following command to collect coverage in JSON format:

dart test --coverage=.dart_tool/coverage

> [!NOTE]

> We use .dart_tool/coverage as the output directory because .dart_tool

> is typically already ignored in .gitignore files.

> [!TIP]

> For projects with complex conditional logic, you can pass the

> --branch-coverage flag to dart test to collect branch-level coverage.

Interpreting Results

Option 1: Use the custom interpreter script

This repository includes a zero-dependency script that parses the raw JSON

output and provides a summary of covered percentage and missed lines.

Run it from the project root (adjust path to script as needed):

dart run skills/dart-test-coverage/scripts/interpret_coverage.dart .dart_tool/coverage <package_name>

Replace <package_name> with the name from pubspec.yaml.

Example Output:

package:my_pkg/src/file.dart: 50.0% (2/4 lines)
  Missed lines: 3, 4

Option 2: Use package:coverage

If package:test is installed, package:coverage is likely available as a

transitive dependency. You can use its format_coverage tool.

To get a human-readable "pretty print" of the coverage:

dart run coverage:format_coverage --in=.dart_tool/coverage --out=stdout --pretty-print --report-on=lib

This will output the file content with hit counts on the left (e.g., 0| for

missed lines).

Best Practices for Reporting Results

When presenting coverage results to the user, follow these guidelines:

• State the high-level percentage first to give immediate context.
• Identify specific files and missed lines clearly.
• Translate line numbers to code: Don't just say "lines 3-6 are missed".

Look at the source file and tell the user which functions or blocks are

untested (e.g., "The divide function is missing coverage").

• Propose concrete fixes: Provide example test code that the user can

immediately apply to cover the missed lines.

• Use tables for multi-file summaries: When reporting on multiple files,

use a markdown table with columns for File, Coverage %, and Missed Lines

to make the summary easy to scan.

Constraints

• ALWAYS verify that tests pass before collecting coverage.
• DO NOT commit the .dart_tool/coverage directory.
• Focus coverage improvements on lib/ files, not test/ or generated files.