How App URLs Are Generated

You might notice that your app’s subdomain could be different from the name you chose. For example, if you name your app “Hello World!!!”, the URL might look like hello-world.fastapicloud.dev instead of hello-world!!!.fastapicloud.dev.

This is because we convert your app name into a valid DNS label to comply with DNS hostname rules (RFC 1035) and ensure it works everywhere.

The Problem

DNS has strict rules for labels. Your creative app names might not fit these rules (and could even break the internet). Here are some examples of names that don’t comply with DNS label requirements:

❌ "My Awesome App!"                  → Invalid characters
❌ "42isTheAnswer"                    → Can't start with number
❌ "Llanfair­pwllgwyngyll­gogery­chwyrn­drobwll­llan­tysilio­gogo­goch" → Too long

The Solution

We automatically convert your app name to a valid DNS label that becomes part of your app’s hostname:

✅ "My Awesome App!"     → my-awesome-app
✅ "_super_app_"         → super-app
✅ "123-numbers"         → a123-numbers

How It Works

1. Convert to lowercase - DNS labels are case-insensitive.
2. Remove special characters - Only letters, digits, and hyphens allowed.
3. Replace spaces/underscores - Converted to hyphens.
4. Fix invalid starts - Labels must start with a letter (prepend ‘a’ if needed).
5. Trim to 63 chars - Maximum DNS label length.
6. Collapse multiple hyphens - -- becomes -.
7. Ensure uniqueness - Add hash suffix if name already exists.

DNS Label Rules (RFC 1035)

According to RFC 1035, every DNS label (the parts between dots in a hostname) must:

• Be 1-63 characters long.
• Start with a letter (a-z).
• End with a letter or digit.
• Use only lowercase letters a-z, digits 0-9, and hyphens -.
• Not start or end with a hyphen.
• No spaces, underscores, or special characters.

Examples

Your App Name	Generated URL
Hello World!	hello-world.fastapicloud.dev
My_API_v2	my-api-v2.fastapicloud.dev
Test!!!	test.fastapicloud.dev

Why Not Restrict Input?

We want you to focus on building your app, not worrying about DNS naming rules. You type what makes sense to you, and we handle the technical requirements. Everyone’s happy!

Custom Domains

Custom domains will be available soon, allowing you to use your own URLs. Stay tuned for updates!