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    OpenAPI-to-Mock Test Harness Builder

    1

    Turn an OpenAPI spec into an executable local mock server, deterministic fixtures, constraint-derived negative tests, and an endpoint coverage report.

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    OpenAPI-to-Mock Test Harness Builder

    OpenAPI-to-Mock Test Harness Builder

    Example session with this skill installed

    Build a local mock and contract-test harness from:

    openapi.yaml

    Requirements

    • parse the full OpenAPI document;
    • inventory every documented operation;
    • resolve local $ref references;
    • prefer explicit examples already present in the spec;
    • generate deterministic valid request and response fixtures when examples are missing;
    • generate negative cases from required fields, enums, primitive types, string constraints, numeric bounds, array constraints, and required parameters;
    • generate a local mock server for all documented routes;
    • include representative documented 4xx/5xx responses when available;
    • generate positive contract tests;
    • generate concrete negative tests where the schema provides enough information;
    • flag unresolved external references instead of inventing schemas;
    • do not call production APIs;
    • do not embed real credentials;
    • generate an endpoint coverage report;
    • run validation and tests when the local dependencies are available.

    Use the repository's existing API-test/mock stack if one is already configured. Otherwise, generate the default Python/FastAPI/pytest harness.

    • Read your context and instructions
    • Compiled the openapi-to-mock test harness

    OpenAPI mock/test harness complete

    OpenAPI version
    3.1.0

    API
    Commerce API

    Operations discovered
    42

    Success fixtures generated
    42

    Request fixtures generated
    27

    Documented error fixtures generated
    18

    Constraint-derived negative cases
    96

    Mock routes generated
    42

    Positive contract tests
    42

    Concrete negative tests
    71

    Manual-review negative cases
    25

    Unresolved references
    0

    Coverage summary

    Operations with success fixtures
    42 / 42

    Operations with positive tests
    42 / 42

    Operations with at least one negative case:
    34 / 42

    Operations with documented error fixtures
    18 / 42

    Operations requiring manual review
    5

    Harness coverage
    100% route generation
    81% operations with negative coverage

    This is harness coverage, not production code coverage.

    Example endpoint

    POST /customers

    Generated valid request

    {
  "name": "Example Customer",
  "email": "user@example.test",
  "plan": "pro"
}

    Generated success fixture

    {
  "id": "00000000-0000-4000-8000-000000000001",
  "name": "Example Customer",
  "email": "user@example.test",
  "plan": "pro"
}

    Constraint-derived tests

    email:

    • required field missing
    • wrong primitive type
    • invalid email format

    plan:

    • enum violation

    name:

    • below minLength
    • above maxLength

    Generated local server

    Run

    python -m pip install -r requirements.generated.txt
    uvicorn mock_server.app:app --reload --port 4010

    Mock URL

    http://localhost:4010

    Example generated test

    def test_create_customer_happy_path():
    response = client.post(
        "/customers",
        json={
            "name": "Example Customer",
            "email": "user@example.test",
            "plan": "pro"
        },
    )

    assert response.status_code == 201

    Example negative test

    def test_create_customer_missing_email():
    payload = {
        "name": "Example Customer",
        "plan": "pro"
    }

    response = client.post("/customers", json=payload)

assert response.status_code in {400, 422}

    Files generated

    • api_inventory.json
    • fixtures/
    • negative_cases.json
    • mock_server/app.py
    • tests/test_contracts.py
    • requirements.generated.txt
    • coverage_report.json
    • validation_report.md
    • README.generated.md

    Validation
    Fixture JSON parsing: PASS
    Local reference resolution: PASS
    Mock server import: PASS
    Contract tests: PASS

    This harness validates the generated mock contract against the OpenAPI document. It does not prove that the production backend implements the same behavior unless the same contract tests are also run against that backend.

    api_inventory.json

    Connects securely to your tools. The creator never sees your data.

    About this skill

    Manual API testing and mock creation waste hours of developer time, especially when the spec changes. This skill automates the generation of an executable test harness directly from your OpenAPI 3.x definitions. It produces more than just documentation, it builds a functional sandbox for frontend development and a rigorous contract testing suite for the backend.

    What it does

    • Parse specifications Validates and inventories all paths, operations, and schemas from your OpenAPI YAML or JSON files.
    • Generate fixtures Derives deterministic valid data from examples, enums, and schema constraints to use in tests.
    • Derive negative cases Automatically identifies boundary violations like missing required fields, type mismatches, and range errors.
    • Build mock servers Scaffolds a Python FastAPI and Uvicorn server that serves your fixtures locally.
    • Create contract tests Generates a pytest suite to verify that your implementation matches the documented request and response shapes.
    • Report coverage Produces a detailed inventory of operations, fixtures, and test coverage to identify gaps in your API design.

    How it works

    1. Inventory The agent scans your openapi.yaml to map all endpoints, parameters, and response codes.
    2. Fixture derivation It extracts values from example tags or generates them based on minLength, maximum, and other constraints.
    3. Harness generation The agent writes the FastAPI mock server, the pytest files, and the api_inventory.json.
    4. Validation It validates the generated JSON against the original schemas to ensure the harness itself is compliant.

    Frameworks & tools

    Built for Python environments using FastAPI, Uvicorn, pytest, httpx, and jsonschema. It supports OpenAPI 3.x specifications in both YAML and JSON formats.

    Why this beats prompting it yourself

    Generic prompts often produce hallucinated examples or ignore schema constraints like multipleOf or pattern. This skill follows a strict resolution priority and deterministic generation logic to ensure your mocks and tests actually match the contract every time.

    Use cases

    • Accelerate frontend development by providing a local mock server before the backend exists.
    • Detect breaking changes in API updates by running generated contract tests against the new spec.
    • Generate exhaustive negative test cases to improve backend input validation.
    • Audit existing OpenAPI files for missing examples or incomplete schema definitions.

    Known limitations

    Does not support external $ref links that require remote authentication. Requires a Python environment with PyYAML for YAML processing. Does not verify production business logic beyond the schema.

    How to use

    Skills on Agensi run inside your AI agent. Copy the prompt, paste it into your agent, and it connects to Agensi and installs this skill for you. If a payment is needed, your agent shows it to you first.

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