Twilio API resilience testing

Test how your app handles Twilio API errors, rate limiting, and slow responses using Dev Proxy

Summary

Test how your app handles Twilio API errors, rate limiting, and slow responses using Dev Proxy. This sample complements the twilio-api-mocks sample by adding chaos engineering capabilities that help you build more resilient communication integrations.

Compatibility

Contributors

• Waldek Mastykarz

Version history

Version	Date	Comments
1.0	January 19, 2026	Initial release

Minimal path to awesome

• Get the sample:

  • Download just this sample:

    npx gitload-cli https://github.com/pnp/proxy-samples/tree/main/samples/twilio-resilience-testing

    or

  • Download as a .ZIP file and unzip it, or

  • Clone this repository

• Start Dev Proxy:

  devproxy

• Make requests to the Twilio API through the proxy to see random errors:

  # Try sending a message - may get authentication errors, invalid phone, etc.
  curl -ikx http://127.0.0.1:8000 -X POST https://api.twilio.com/2010-04-01/Accounts/AC00000000000000000000000000000000/Messages.json \
    -d 'To=+15559876543' \
    -d 'From=+15551234567' \
    -d 'Body=Hello!'
  
  # Try making a call - may get various errors
  curl -ikx http://127.0.0.1:8000 -X POST https://api.twilio.com/2010-04-01/Accounts/AC00000000000000000000000000000000/Calls.json \
    -d 'To=+15559876543' \
    -d 'From=+15551234567' \
    -d 'Url=http://example.com/twiml'
  
  # Run multiple requests quickly to trigger rate limiting
  for i in {1..150}; do curl -ikx http://127.0.0.1:8000 https://api.twilio.com/2010-04-01/Accounts/AC00000000000000000000000000000000.json; done

Features

This sample simulates realistic failure scenarios for the Twilio API:

Latency simulation

All requests are delayed between 200ms and 1500ms to simulate real-world network conditions and slow API responses.

Rate limiting

The proxy simulates Twilio’s rate limiting behavior:

• Limit: 100 requests per second
• Returns 429 Too Many Requests with proper Twilio error format when exceeded
• Includes Retry-After header

Random error injection (50% failure rate)

Authentication errors (401)

• 20003 - Authentication required

Request errors (400)

• 21211 - Invalid phone number
• 21602 - Message body required
• 21608 - From phone number not verified
• 30004 - Message blocked

Authorization errors (403)

• 20005 - Account not active

Not found errors (404)

• 20404 - Resource not found

Server errors (500, 503)

• 20500 - Internal server error
• 20503 - Service unavailable (with Retry-After header)

Configuration

Adjusting the failure rate

Change the error rate in .devproxy/devproxyrc.json:

"twilioErrors": {
  "rate": 50  // Percentage of requests that will fail (0-100)
}

Adjusting latency

Modify the latency range:

"latencyPlugin": {
  "minMs": 200,
  "maxMs": 1500
}

Adjusting rate limits

Configure rate limiting behavior:

"rateLimitingTwilio": {
  "rateLimit": 100,             // Requests allowed per window
  "resetTimeWindowSeconds": 1   // Window duration
}

Use cases

This sample is useful for:

• Building resilient communication flows - Test retry logic, error handling, and user-facing error messages
• CI/CD pipeline testing - Verify your app handles Twilio failures gracefully
• Load testing - Simulate rate limiting before hitting production limits
• Chaos engineering - Randomly inject failures to find weak points
• Developer training - Help developers understand Twilio error responses

Complementing twilio-api-mocks

The twilio-api-mocks sample is great for:

• ✅ Happy path testing with mock responses
• ✅ Offline development without Twilio credentials
• ✅ Consistent, predictable API responses

This Dev Proxy sample adds:

• ✅ Random error injection
• ✅ Latency simulation
• ✅ Rate limiting simulation
• ✅ Chaos engineering scenarios

Use both together for comprehensive Twilio integration testing!

Help

We do not support samples, but this community is always willing to help, and we want to improve these samples. We use GitHub to track issues, which makes it easy for community members to volunteer their time and help resolve issues.

You can try looking at issues related to this sample to see if anybody else is having the same issues.

If you encounter any issues using this sample, create a new issue.

Finally, if you have an idea for improvement, make a suggestion.

Disclaimer

THIS CODE IS PROVIDED AS IS WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING ANY IMPLIED WARRANTIES OF FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, OR NON-INFRINGEMENT.