Zod Testing

Created by Anivar Aravind

An AI agent skill for testing Zod schemas with Jest and Vitest.

The Problem

AI agents often write schema tests that only check the happy path, use parse() instead of safeParse() (crashing instead of failing), test schema internals (.shape, ._def) instead of behavior, or hardcode mock data instead of generating it from the schema. The result: tests that miss regressions and break on harmless refactors.

This Solution

A focused testing skill covering schema correctness testing, error assertion patterns, mock data generation, snapshot testing with z.toJSONSchema(), property-based testing, structural testing, and drift detection — with 14 anti-patterns showing exactly what goes wrong and how to fix it.

Install

npx skills add anivar/zod-testing -g

Or with full URL:

npx skills add https://github.com/anivar/zod-testing

Baseline

• zod ^4.0.0
• Jest or Vitest
• TypeScript ^5.5

What's Inside

Testing Approaches

Approach	Type	Use When
safeParse() result checking	Correctness	Default — always use safeParse in tests
z.flattenError() assertions	Error messages	Verifying specific field errors
z.toJSONSchema() snapshots	Schema shape	Detecting unintended schema changes
Mock data generation	Fixtures	Need valid/randomized test data
Property-based testing	Fuzz testing	Schemas must handle arbitrary valid inputs
Structural testing	Architecture	Verify schemas are only imported at boundaries
Drift detection	Regression	Catch unintended schema changes via JSON Schema snapshots

Anti-Patterns

14 common testing mistakes with BAD/GOOD code examples:

• Testing schema internals instead of behavior
• Not testing error paths (only happy path)
• Using parse() in tests (crashes instead of failing)
• Not testing boundary values (min/max edges)
• Hardcoding mock data instead of generating from schema
• Snapshot testing raw ZodError instead of formatted output
• Not seeding random data generators (flaky tests)
• Not testing at boundaries (schema tests pass but handler doesn't validate)
• No snapshot regression testing (field removal goes unnoticed)
• Testing schema shape but not error observability
• No drift detection workflow (schema changes land without review)

Structure

├── SKILL.md                      # Entry point for AI agents
└── references/
    ├── api-reference.md          # Testing patterns, assertion helpers, mock generation
    └── anti-patterns.md          # Common testing mistakes to avoid

Ecosystem — Skills by Anivar Aravind

Testing Skills

Skill	What it covers	Install
jest-skill	Jest best practices — mock design, async testing, matchers, timers, snapshots	npx skills add anivar/jest-skill -g
msw-skill	MSW 2.0 API mocking — handlers, responses, GraphQL	npx skills add anivar/msw-skill -g
redux-saga-testing	Redux-Saga testing — expectSaga, testSaga, providers	npx skills add anivar/redux-saga-testing -g

Library & Framework Skills

Skill	What it covers	Install
zod-skill	Zod v4 schema validation, parsing, error handling	npx skills add anivar/zod-skill -g
redux-saga-skill	Redux-Saga effects, fork model, channels, RTK	npx skills add anivar/redux-saga-skill -g

Engineering Analysis

Skill	What it covers	Install
contributor-codebase-analyzer	Code analysis, annual reviews, promotion readiness	npx skills add anivar/contributor-codebase-analyzer -g

Author

Anivar Aravind — Building AI agent skills for modern JavaScript/TypeScript development.

License

MIT