Are HTTP Headers Case Sensitive?

The short answer is no, HTTP headers are case-insensitive at the application level. Yet, we should add more nuance by considering what HTTP headers are and their practical role in data collection. While you can use any case for header names, your web scraper might become easier to detect with uncommon header cases.

What the HTTP Specification Says

The official Hypertext Transfer Protocol (HTTP) specifications are quite precise that header fields are case-insensitive. The original HTTP/1.1 (RFC 2616) established the foundational rule in section 4.2 :

Each header field consists of a name followed by a colon (“:”) and the field value. Field names are case-insensitive.

So, from the web server’s perspective, User-Agent, user-agent, and USER-AGENT are all equivalent and must be accepted. As of June 2022, the current HTTP standard, RFC 9110 , reiterates the same principle.

However, the RFC 9113 standard adds a transmission-layer HTTP/2 requirement that may create confusion. It includes a stricter requirement in section 8.2 :

Field names MUST be converted to lowercase when constructing an HTTP/2 message.

In most cases, this conversion is handled automatically by your HTTP library and transport layer, so you don’t need to do it manually. Whether you write User-Agent or user-agent in your code, compliant HTTP/2 libraries will lowercase header names before transmission.

How Servers and Tools Handle Header Case

For HTTP/1.1 connections, servers will accept any casing. Yet, some anti-bot detection systems may use header casing, together with other factors, as a part of their fingerprinting analysis. What’s important here is that your header name casing is consistent, and most tools already implement that.

Taking Python Requests as an example, the normalization of HTTP headers might look like this:

import requests

# Headers set in lowercase
headers = {'content-type': 'application/json', 'user-agent': 'my-scraper/1.0'}

response = requests.post('https://httpbin.org/post', headers=headers)
print(response.request.headers)

# Output: {'Content-Type': 'application/json', 'User-Agent': 'my-scraper/1.0', ...}

Similarly, Fetch API in Node.js will use lowercase normalization:

import fetch from 'node-fetch';

const response = await fetch('https://httpbin.org/headers', {
  headers: {
    'Content-Type': 'application/json',
    'User-Agent': 'my-scraper/1.0'
  }
});

const data = await response.json();
console.log(data.headers);

Common Issues Developers Face

Even though you’re free to choose your HTTP header casing at the specification level, inconsistencies might create problems. Most often, they appear in web scraping and automation projects or when using middleware solutions.

• Inconsistent header casing detection. Anti-bot systems analyze header casing as part of fingerprinting. A scraper with unusual or inconsistent header casing can be flagged. Popular browsers, such as Chrome, send headers in a specific order and casing which you should follow.
• Header rewriting problems. When requests pass through proxies or middleware, the intermediaries may normalize, rewrite, or strip headers, affecting their casing. This is most common with HTTP proxies or specific middleware, like http-proxy-middleware.
• Debugging challenges. Applications that operate within multiple layers, such as an automation framework, proxy, and server, might face problems when each layer normalizes HTTP headers separately. Debugging issues is much more difficult with no unified approach.

The solution to header casing issues is to maintain consistent conventions throughout your entire stack. Tools like our free proxy headers test will allow you to check whether your output headers match the headers in your code.

Best Practices for Header Usage

Handling HTTP headers consistently can help minimize detection risk and ensure reliability when using proxies or various automation and web scraping tools. Consider following a few best practices when implementing consistency in your header names.

• Use lowercase headers when targeting HTTP/2 services​. Most HTTP/2 libraries handle it automatically, but exceptions might create problems, so it's better to always verify.
• Match the conventions of popular web browsers to blend in better. Real browsers typically use Title-Case and a predictable header order for HTTP/1.1 requests.
• Ensure a matching User-Agent with other header characteristics​. If your User-Agent is “Chrome on Windows”, ensure your other headers and their casing match it.
• Be mindful of header values, not just names, as some may be case-sensitive. For example, cookies or custom authentication tokens might have an expected format.
• Consider using browser automation tools, such as Selenium, Puppeteer, or Playwright, for sites with sophisticated bot detection. These tools help to send headers and manage browser fingerprints like a real user.

Conclusion

Case sensitivity of header names is an easy-to-miss issue that can cause a lot of headaches later. Now that you know the basics, you can think ahead and avoid the tedious troubleshooting that might follow.