How to Mock a REST API in 5 Minutes with APXY

Most mock API setups start with a config file, a YAML schema, or a dependency to install. This tutorial skips all of that. You will have a working mock REST API returning real JSON in about five minutes.

What you need

• macOS or Linux
• A terminal

That is it. No Node.js version pinning. No Docker. No project scaffolding.

Step 1: Install APXY

curl -fsSL https://apxy.dev/install.sh | bash

Verify it worked:

apxy --version

Step 2: Start the proxy

apxy start --port 8080

APXY starts a local HTTPS proxy that intercepts traffic. The Web UI opens at http://localhost:8081 so you can see rules and captured requests in real time.

Step 3: Add your first mock rule

This is the core step. Tell APXY which URL to intercept and what to return:

apxy rules add \
  --match "GET /api/users" \
  --status 200 \
  --body '[{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]' \
  --header "Content-Type: application/json"

Now any GET request to /api/users going through APXY will return that JSON with a 200 status.

Step 4: Test it

Route a request through the proxy and hit your mock:

curl -x http://localhost:8080 http://localhost:3000/api/users

Or route your whole app:

HTTP_PROXY=http://localhost:8080 HTTPS_PROXY=http://localhost:8080 npm run dev

Any request from your app to /api/users now gets the mocked response—no backend required.

Common mock patterns

Mock a 404 for a missing resource

apxy rules add \
  --match "GET /api/users/999" \
  --status 404 \
  --body '{"error":"User not found"}' \
  --header "Content-Type: application/json"

Mock a POST that creates a resource

apxy rules add \
  --match "POST /api/users" \
  --status 201 \
  --body '{"id":3,"name":"Carol","created":true}' \
  --header "Content-Type: application/json"

Simulate a server error

apxy rules add \
  --match "GET /api/products" \
  --status 500 \
  --body '{"error":"Internal Server Error"}'

Add a response delay

apxy rules add \
  --match "GET /api/slow-endpoint" \
  --status 200 \
  --body '{"result":"ok"}' \
  --delay 2000

The --delay flag adds a 2-second pause before the response. Useful for testing loading states and timeout handling.

Use the Web UI instead of the CLI

If you prefer clicking to typing, open http://localhost:8081, go to the Rules tab, and click Add Rule. Fill in the URL pattern, status code, headers, and body in the form. The rule activates immediately.

Use a pre-built template

For common APIs like Stripe, GitHub, and OpenAI, APXY ships with ready-made rule sets so you do not have to write every response yourself:

curl -fsSL https://raw.githubusercontent.com/apxydev/apxy/main/mock-templates/openai/rules.json \
  -o openai-rules.json
apxy rules import openai-rules.json

Browse all available templates at apxy.dev/templates.

What to do next

Once you have a few mock rules running, the natural next step is to capture real traffic and convert it to a mock. That is the Record & Replay pattern: make a real API call through the proxy, see the live response in the traffic log, then save it as a rule. Your mock immediately reflects real API behavior.

See How to Debug a Failing API Call with APXY for the full capture-inspect-replay workflow.