snapshot-testing_skill

Overview

This skill integrates Verify-based snapshot testing into .NET projects to approve and track expected outputs. It helps teams detect unintended changes by comparing runtime output against human-approved baseline files. Use it for rendered HTML/emails, public API surfaces, HTTP responses, and serialized objects.

How this skill works

On first run a test produces a .received file containing the actual output. A developer reviews and approves that output, creating a .verified baseline file. Subsequent test runs compare current output to the .verified file and fail when differences are detected, showing a diff for human review. Dynamic values can be scrubbed or replaced so only meaningful changes surface.

When to use it

• Validate rendered HTML or email templates to catch layout and content regressions
• Approve public API surfaces to prevent accidental breaking changes
• Verify HTTP responses including status, headers, and body together
• Confirm serialization and wire-format outputs remain stable
• Test complex object graphs where many assertions would be cumbersome

Best practices

• Use descriptive test names — they become snapshot file names and simplify review
• Scrub timestamps, GUIDs, tokens, and other dynamic values consistently
• Keep .verified files in source control and ignore .received files
• Use file extensions for non-text content (e.g., extension: "html") so snapshots open in appropriate viewers
• Configure ModuleInitializer for project-relative snapshot directories and CI-safe diff behavior

Example use cases

• Render and verify welcome or transactional emails to detect CSS/layout regressions
• Generate and approve a public API surface listing to catch breaking changes before release
• Verify HTTP endpoint responses in integration tests including selected headers
• Snapshot serialized DTOs or generated code to detect accidental formatting or structural changes
• Approve complex report or document output where visual review is required