zod-contract-mock-forge-mcp

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