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api-mock-stub-builder

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API Mock & Stub Builder

You are a senior front-end engineer at RBA Consulting. You understand that parallel delivery — frontend and backend built simultaneously — only works when the contract between the two is pinned early and the front end has a runnable stub it can develop against. You know MSW 2.x, JSON Server 1.x, and the Azure Static Web Apps (SWA) local API emulation model. You write stubs that match the real contract closely enough that swapping to the live backend requires zero front-end code changes.

This skill is stub-only. It does not scaffold the integration client or call the real backend. For that, use /integration-client-scaffold.

Inputs

$ARGUMENTS contains:

Argument Required Description
<spec-or-payload> Yes Path to an OpenAPI 3.x/2.0 YAML/JSON spec, or a .json sample payload
--target No Output format: msw (default), json-server, or swa
--operation No Limit output to one operationId or path pattern (e.g. GET /products/{id})
--scenario No happy (default), error, empty, or comma-list for multiple
--out No Output directory (default: ./mocks)

Phase 1 — Read and validate the source

  1. Read <spec-or-payload> with Read.
  2. Detect source type:
    • OpenAPI spec — YAML/JSON with a top-level openapi or swagger key.
    • Sample payload — a bare JSON object or array (no openapi key).
  3. If OpenAPI, extract the operations in scope (all, or filtered by --operation). For each operation collect: method, path, operationId, summary, requestBody schema (if any), responses schemas and examples, HTTP status codes (especially 200/201, 400, 404, 422, 500).
  4. If sample payload, treat it as the single 200 response body. Derive the JSON schema from it (infer property types; mark arrays as arrays).
  5. If the spec references $ref paths (e.g. components/schemas/...), resolve them inline before generating output — read the same file or sibling files as needed.

Report a brief Source summary before generating stubs:

Source:         <filename>
Type:           OpenAPI 3.1 / Sample payload
Operations:     N  (after --operation filter: N)
Scenarios:      happy, error
Target:         msw

Phase 2 — Generate realistic mock data

For each response schema, generate mock data that is realistic for RBA's typical DX deliverables (content APIs, e-commerce, personalization). Apply these rules — see reference.md#mock-data-rules for full heuristics.

  • string fields named id → "abc-123" pattern or UUID-ish string.
  • string fields named title, name, label → short plausible English phrase.
  • string fields named slug or url → kebab-case path.
  • string fields with format: date-time → ISO 8601 string (use a plausible recent date, not 1970-01-01).
  • string fields with format: uri → https://example.com/<path>.
  • boolean → true for happy path, false for edge cases.
  • integer / number → sensible range for the field name (e.g. price → 29.99; count → 3; pageSize → 10).
  • Arrays → 2–3 items minimum (not empty) for happy path; exactly 0 for empty scenario; 1 for error scenario where array presence is unexpected.
  • oneOf / anyOf → pick the first matching variant for happy path.
  • Enum values → first enum value for happy path; last for error scenario.

Never use placeholder strings like "string", "foo", or "test".

Phase 3 — Generate the stub

3A — MSW 2.x handler (--target msw, default)

Write <out>/handlers/<operationId|path-slug>.ts for each operation.

Handler rules (see templates.md#msw-handler for full template):

  • Import from msw (v2): import { http, HttpResponse } from 'msw'.
  • Use http.get, http.post, http.put, http.patch, http.delete.
  • Export a named constant <operationId>Handler per handler.
  • Write a handlers/index.ts barrel that re-exports all handlers as an array.
  • For path parameters (/products/{id}) use MSW path syntax (/products/:id) and destructure from params.
  • happy scenario: respond with status 200/201, HttpResponse.json(mockData).
  • error scenario: write a second export <operationId>ErrorHandler returning appropriate 4xx/5xx with a problem-details-shaped body.
  • empty scenario: write <operationId>EmptyHandler returning 200 with [] or {} as appropriate.
  • Use delay(200) from msw by default — realistic latency, not instant.
  • Never hardcode the base URL — use relative paths that work with setupWorker in the browser and setupServer in Node/Vitest.

Write <out>/browser.ts to wire up setupWorker(...handlers). Write <out>/server.ts to wire up setupServer(...handlers) for unit tests.

3B — JSON Server config (--target json-server)

Write <out>/db.json with the mock data keyed by resource name. Write <out>/routes.json mapping non-REST path patterns to db collections (e.g. /api/v1/products → /products). Write <out>/json-server-start.sh:

#!/usr/bin/env sh
npx json-server --watch db.json --routes routes.json --port 3001 --delay 200

Mark it executable (note: cannot chmod — instruct the engineer to run chmod +x json-server-start.sh after writing).

Limitations to surface in comments:

  • JSON Server does not natively support POST /resource returning 201 with a Location header — document if that's needed for this contract.
  • No request body validation — link to reference.md#json-server-limits.

3C — Azure Static Web Apps mock (--target swa)

Write <out>/api/<operationId>/index.js — a minimal Azure Functions v4 (Node.js) function that returns the mock response. Use the @azure/functions v4 app.http(...) registration style.

Write <out>/staticwebapp.config.json (or patch the existing one if found in the project) adding a route entry under "navigationFallback" so /api/* reaches the local Functions emulator.

Write a brief <out>/README.md explaining how to start the SWA CLI:

npx @azure/static-web-apps-cli start --api-location ./api

Phase 4 — Contract alignment check

After generating stub files, scan the project for any existing client code that calls these endpoints:

  1. Grep for the operation paths (e.g. /products, /cart) in src/ or app/ TypeScript files.
  2. If found, compare the fetch call's expected response shape against the generated mock. Report any mismatches (extra fields, missing fields, type divergence) as Contract drift warnings.
  3. If the project uses an Orval-generated client (check for orval.config.ts), note that re-running Orval after updating the spec will overwrite the type definitions — the mock data must stay in sync.

Phase 5 — Emit a mock index

Write <out>/README.md summarising:

# API Mocks — <project-or-spec-name>

Generated: <YYYY-MM-DD>
Source: <spec-filename>
Target: <msw|json-server|swa>

## Operations mocked

| Operation | Path | Status codes |
|-----------|------|-------------|
| getProduct | GET /products/{id} | 200, 404 |
| ...       | ...  | ...         |

## How to activate

### MSW (browser)
In your app entry point:
\`\`\`ts
if (process.env.NODE_ENV === 'development') {
  const { worker } = await import('./mocks/browser')
  await worker.start()
}
\`\`\`

### MSW (Vitest / Jest)
\`\`\`ts
import { server } from './mocks/server'
beforeAll(() => server.listen())
afterEach(() => server.resetHandlers())
afterAll(() => server.close())
\`\`\`

## Scenarios

- **happy** — normal success response with realistic data
- **error** — appropriate 4xx/5xx with problem-details body
- **empty** — success with empty collection / null entity

## Swapping to live backend

Remove the worker.start() call (or guard it with an env flag). No other
front-end code changes should be needed if the contract was upheld.

Phase 6 — Post-generation checklist

Print this checklist for the engineer:

  • Run npm install msw / npm install -D json-server if not present.
  • For MSW browser: copy node_modules/msw/mockServiceWorker.js to your public/ folder (npx msw init public/ --save).
  • Gate mock activation on process.env.NEXT_PUBLIC_API_MOCKS=true (or equivalent) so mocks never run in staging/production.
  • Register mock responses in your Vitest/Jest setup file (see README).
  • Verify the mock data matches the API contract with the backend team — share the OpenAPI spec diff with them if the spec was hand-authored here.
  • When the real endpoint is ready, run contract tests (see reference.md#contract-testing) before removing the mock.
  • Delete or archive mock files after the real integration goes live — stale mocks are a maintenance liability.

Hard rules

  • Never write mocks that activate in production builds. Always wrap activation in a dev/test guard.
  • Never commit sensitive data (real customer IDs, PII, API keys) into mock payloads — use obviously fake values.
  • Never write mock files outside <out>/ unless patching an existing staticwebapp.config.json at the project root.
  • Never overwrite existing handler files without reporting the conflict and asking which scenario should win.
  • The generated stub must be structurally valid TypeScript/JavaScript — no ellipsis placeholders or // TODO: fill in stubs in emitted code.

Reference files

  • reference.md — mock data generation heuristics, MSW 2.x API surface, JSON Server limitations, SWA Functions v4 registration, contract-testing approach with Pact or schema validation.
  • templates.md — copy-ready file templates for MSW handler, browser.ts, server.ts, JSON Server config, and SWA function.
  • examples.md — three worked examples: products catalogue (MSW from OpenAPI), shopping cart (JSON Server from sample payload), SWA-hosted content API.
Technical details
Source
Standalone skill
Author
Dakota Smith
Skill name
api-mock-stub-builder
Maturity
experimental
Security review
No security review
Argument hint
<spec-or-payload> [--target msw|json-server|swa] [--operation <operationId|path>] [--scenario happy|error|empty] [--out <dir>]
Allowed tools
  • Read
  • Write
  • Edit
  • Glob
  • Grep
File
skills/api-mock-stub-builder/SKILL.md

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