Custom HTTP client (C) gets 403 from Cloudflare

I’m building a custom web browser in C to interact with specific target URLs. Target frequency is human-level requests (once every 1–3 minutes). I use GET/POST requests and avoid automation or scripting.

I was told that a certain set of features is required for compatibility with Cloudflare.

The client currently supports:

• HTTPS (TLS handshake via OpenSSL)

• Basic HTTP/1.1 requests (GET/POST)

• Header control

• Cookie parsing/storage (basic)

• Redirect handling

Now my application cannot access some Cloudflare-protected sites. I receive either:

• 403 Forbidden, or

• a challenge page that my client cannot process

What should I focus on to make my application compatible with such systems?

Is it realistic to build such an application in 2026 without using a full browser engine?

Please note, its not CloudFlare automation, it does not pass a human mouse click