Zod Contract Mock Forge

npm version npm downloads CI License: MIT

An MCP server that turns Zod schemas into mocks, violations, and contract tests — so your AI agent can reason about API contracts without manually crafting payloads.

The Problem

Zod schemas are runtime code. An AI agent cannot execute them, introspect their constraints, or generate valid/invalid payloads without this layer. The agent also cannot detect when the schema and the OpenAPI docs silently diverged, or whether a schema change breaks existing test fixtures.

Tools

Mock generation

Tool	Arguments	What it returns
generate_valid_mock	schema_code, count?	Valid mock data matching the schema
generate_mock_variants	schema_code, count?, seed?	N structurally valid but value-diverse mocks — for property-based testing

Violation generation

Tool	Arguments	What it returns
generate_boundary_violations	schema_code	Invalid payloads for each constraint: missing fields, type mismatches, min/max, email/uuid/url
generate_exhaustive_union_violations	schema_code	Per-variant violations for every branch of a z.union() or z.discriminatedUnion()

Schema analysis

Tool	Arguments	What it returns
introspect_schema	schema_code	JSON Schema representation — for LLM understanding of the contract
read_schema_from_file	file_path, export_name?	Extracts the Zod schema expression from a TypeScript/JS file
detect_schema_drift	zod_file_path, schema_export_name, openapi_file_path, openapi_schema_name?	Diffs Zod vs OpenAPI — reports missing_in_openapi, missing_in_zod, type_conflict, required_mismatch
evaluate_schema_evolution	schema_file_path, schema_export_name, old_schema_content?	Generates mocks from old schema, validates against new — detects breaking changes before tests run

Contract testing

Tool	Arguments	What it returns
scaffold_api_contract_test	framework, base_url, endpoint, method, schema_code, test_name?	Contract test boilerplate for Playwright, Jest, Vitest, or MSW
suggest_contract_fix	schema_code, payload	Validates a JSON payload and explains each violation with a fix suggestion

Setup

1. Install

npm install -g zod-contract-mock-forge-mcp

2. Add to your editor

Cursor / VS Code (.cursor/mcp.json or .vscode/mcp.json)

{
  "mcpServers": {
    "zod-forge": {
      "command": "zod-contract-mock-forge-mcp"
    }
  }
}

Claude Code

claude mcp add zod-forge zod-contract-mock-forge-mcp

Example usage

My schema file is src/schemas/user.ts, exported as UserSchema.
My OpenAPI spec is docs/openapi.yaml.

1. introspect_schema — what are the constraints on this schema?
2. generate_mock_variants — give me 10 diverse valid payloads (seed: 42) for CI reproducibility
3. generate_exhaustive_union_violations — test every branch of the role discriminated union
4. detect_schema_drift — has the Zod schema diverged from the OpenAPI docs?
5. evaluate_schema_evolution — does my schema change break any existing mock data?

Example output

generate_mock_variants — 3 diverse valid mocks, seeded for CI:

{
  "schema_id": "schema_a1b2c3d4",
  "count": 3,
  "all_valid": true,
  "variants": [
    { "name": "Colleen Rowe", "age": 37 },
    { "name": "Pat Reynolds", "age": 24 },
    { "name": "Veronica Konopelski", "age": 45 }
  ]
}

detect_schema_drift — field missing in OpenAPI, extra field in Zod:

{
  "drift_count": 2,
  "drifts": [
    {
      "field_path": "role",
      "drift_type": "missing_in_openapi",
      "zod_value": "string",
      "openapi_value": null
    },
    {
      "field_path": "email",
      "drift_type": "missing_in_zod",
      "zod_value": null,
      "openapi_value": "string"
    }
  ]
}

evaluate_schema_evolution — new required field breaks existing mocks:

{
  "breaking_change": true,
  "sample_count": 20,
  "invalid_mock_count": 20,
  "failure_reasons": [
    {
      "field_path": "status",
      "zod_code": "invalid_type",
      "expected": "string",
      "received": "undefined",
      "affected_mock_count": 20
    }
  ]
}

Scripts

npm run build        # compile TypeScript → dist/
npm run lint         # ESLint
npm run format       # Prettier --write
npm run format:check # Prettier check (used in CI)
npm test             # Vitest

License

MIT