How do I troubleshoot webhook deliveries that succeed in Postman but fail in my app?

If a webhook works in Postman but fails in your app, the usual cause is that your app is sending something different from what Postman sends. The fastest way to troubleshoot is to compare them side by side.

1) Compare the exact request

Check these in Postman and in your app:

• URL

  • Same scheme: http vs https
  • Same domain, path, query string
  • No accidental trailing slash differences if your endpoint is strict

• Method

  • Usually POST, but verify

• Headers

  • Content-Type
  • Authorization
  • Any webhook signature headers
  • Custom headers

• Body

  • Raw JSON vs form-data vs x-www-form-urlencoded
  • Field names
  • Data types
  • Encoding
  • Empty/null fields

2) Log the request your app actually sends

Don’t rely on what you think the app is sending. Log or inspect:

• Full request URL
• Method
• Headers
• Body payload
• Any request signature
• Timestamp

If possible, capture the outgoing request with:

• app debug logs
• proxy tools like Charles, Fiddler, mitmproxy
• server logs on the receiving endpoint

3) Check for JSON formatting issues

A very common issue:

• Postman sends valid JSON
• App sends:
  • malformed JSON
  • incorrect escaping
  • nested objects serialized incorrectly
  • wrong Content-Type

Make sure:

• Content-Type: application/json
• body is valid JSON
• strings are properly escaped
• booleans/numbers aren’t accidentally sent as strings if the receiver expects types

4) Check authentication and signing

If the webhook provider expects signed requests:

• Are you computing the signature the same way?
• Is the timestamp included correctly?
• Are you using the right secret?
• Is the signature based on the exact raw body?
  • Some systems fail if the body is altered even slightly

Common pitfalls:

• hashing parsed JSON instead of raw payload
• using the wrong secret for environment
• encoding differences between UTF-8 and another charset
• clock skew affecting timestamp validation

5) Compare network behavior

Postman may succeed because it handles things more flexibly than your app.

Check for:

• TLS/SSL certificate validation problems
• proxy settings
• DNS differences
• firewall restrictions
• timeouts
• redirects
• IPv4/IPv6 issues

6) Inspect the response

Look at:

• HTTP status code
• response body
• response headers
• redirect location if any

A response like 400, 401, 403, 415, or 422 often tells you exactly what’s wrong.

7) Watch for hidden differences in content encoding

Your app might be sending:

• application/x-www-form-urlencoded instead of JSON
• multipart/form-data
• gzip-compressed content
• a charset the server doesn’t accept

Make sure the receiving server expects the same format.

8) Test with a raw HTTP client

If Postman works, try reproducing the app request with:

• curl
• httpie
• a small script using the same library as your app

This helps isolate whether the problem is:

• your app code
• your HTTP library
• the server endpoint

9) Check redirects and retries

Some apps follow redirects differently than Postman.

• A 301/302 might change POST to GET
• Some libraries don’t follow redirects on POST
• Retries can cause duplicate requests or invalid timestamps/signatures

10) Validate against server-side logs

If you control the receiving endpoint, log:

• request headers
• parsed body
• raw body
• auth/signature validation result
• reason for rejection

This is often the quickest way to identify the mismatch.

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Quick checklist

Start here:

• Same URL
• Same HTTP method
• Same headers
• Same raw body
• Valid JSON
• Correct auth/signature
• Correct Content-Type
• No TLS/network issues
• Check response code and server logs

If you want, I can also help you create a side-by-side debugging checklist for Postman vs your app, or review a sample request from both.