/dart-test-coverage

name: dart-test-coverage
description: |-
  Understand and improve test coverage in a Dart package.
  Helps agents run coverage, interpret results, and identify missed lines.
key_features:
  - LCOV report collection
  - Missed line identification
  - Coverage analysis & improvement

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.

When NOT to use (Abstention Guardrails)

Do NOT collect or demand test coverage for:

• **Generated Code**: Code generated by builders (`*.g.dart`, `*.freezed.dart`,

protobufs).

• **Platform-Specific FFI / OS Entrypoints**: Low-level platform bindings or

entrypoints that require specific physical hardware or OS services not available in the execution environment.

• **Example Applications & Tool Scripts**: Standalone sample code under

`example/` or developer maintenance scripts under `tool/` unless the package explicitly mandates 100% repository-wide coverage.

• **Failing Test Suites**: If existing tests are failing under `dart test`, fix

the broken tests first before collecting or reporting coverage.

Discovery

To find areas lacking test coverage:

Run Coverage Analysis

Follow the workflow to generate and interpret coverage data:

1. **Run Tests with Coverage**: `dart test --coverage=.dart_tool/coverage` 2. **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)

1. Ensure tests pass by running `dart test`. 2. Collect coverage by running `dart test --coverage=.dart_tool/coverage`. 3. Interpret the results using the provided script or standard tools. 4. 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:

1. **State the high-level percentage first** to give immediate context. 2. **Identify specific files and missed lines** clearly. 3. **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"). 4. **Propose concrete fixes**: Provide example test code that the user can immediately apply to cover the missed lines. 5. **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.