Custom headers not working after Postman update — HTTP/2 lowercases header names

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Hi everyone,

I spent a good amount of time debugging an issue that started after a recent Postman update, and I wanted to share the solution in case others are experiencing the same problem.

The Problem

I have an API that requires a custom header called Scope (with a capital “S”) with a value like Chatbot/Test. After a recent Postman update, the API started returning 500 Internal Server Error with the message:

{

"message": "The Scope in the 

    request headers is invalid. 

    Must be a string"

}

The exact same request worked perfectly when executed via curl in the terminal:

curl -X GET https://my-api.example.com/

endpoint \\

-H ‘Scope: Chatbot/Test’ \

-H ‘Authorization: Bearer ’ \

-H 'Content-Type: application/

x-www-form-urlencoded'

Root Cause

Postman now defaults to HTTP/2 (when the HTTP version is set to “auto”). The HTTP/2 protocol automatically converts all header names to lowercase. So Scope becomes scope, Authorization becomes authorization, etc.

This is part of the HTTP/2 specification (RFC 7540, Section 8.1.2), but it causes issues when the backend server performs a case-sensitive lookup on header names — which, while not ideal, is common in many APIs (especially those behind AWS API Gateway, custom middleware, etc.).

Meanwhile, curl uses HTTP/1.1 by default, which preserves the original casing of header names. That’s why the same request works in curl but fails in Postman.

Solution

  1. Open your request in Postman

  2. Go to the Settings tab (within the request, not the global settings)

  3. Change “HTTP Version” from “auto” to “HTTP/1.1”

  4. Send the request again — it should now work :white_check_mark:

Suggestion to the Postman Team

It would be great if Postman could:

  • Display a warning when custom headers with uppercase characters are detected and the request is using HTTP/2

  • Mention this behavior change in the update release notes, since it can silently break existing requests after an update

Hope this helps someone else save a few hours of debugging! :raising_hands:

Environment:

  • Postman Desktop (Windows)

  • HTTP Version: auto (defaults to HTTP/2)

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