Uppercase Path Handling — when /V1 quietly ruins your entire day

This is one of those bugs that feels embarrassing to admit — not because it’s rare, but because everyone has seen it, and almost nobody tests for it.

Someone writes /V1/users instead of /v1/users. The API responds with 400 Bad Request, 404, or something equally unhelpful. And suddenly, half a day disappears into debugging.

What was tested

Rentgen takes a known, valid API request and mutates only the path — everything after the domain is converted to uppercase.

• TEST: Uppercase Path Test
• Mutation: URL path converted to uppercase
• Expected:
  • 404 Not Found — resource does not exist
  • or 2xx — API explicitly treats paths as case-insensitive
• Anything else: misleading behavior

Why this matters

Unlike domain names, URL paths are usually treated as case-sensitive. That means /v1 and /V1 are different resources. And that’s fine — as long as the API communicates that clearly.

The problem starts when the API responds with something vague or misleading. A 400 Bad Request suggests the payload is wrong. A 403 Forbidden suggests permissions. A 500 suggests the server broke.

None of those point to the real issue: the resource simply does not exist.

The debugging trap

This is how it usually plays out:

• One developer types /V1 instead of /v1
• The API returns 400
• They inspect the payload
• They compare headers
• They re-run the request multiple times
• They compare it with a colleague’s request that “works”

Everything looks identical. Same endpoint. Same body. Same auth.

The difference hides in plain sight: one character. One uppercase letter.

When it’s finally spotted, the reaction is always the same: “Wait… seriously?”

What Rentgen found

In a correct implementation, one of two behaviors is acceptable:

• Strict routing: return 404 Not Found because /V1 is not a valid resource
• Normalized routing: treat /v1 and /V1 as equivalent and return 2xx

Both are defensible. What’s not defensible is returning something that sends developers looking in the wrong direction.

Why this bug survives

Because it sits in an awkward gap:

• too small to be a feature bug
• too inconsistent to reproduce easily
• too embarrassing to write a test for manually

Testers rarely try random casing in paths. Developers assume URLs are copied, not typed. And tooling often hides the actual string being sent.

Real-world impact

This issue doesn’t bring systems down. It does something worse: it quietly burns time.

• hours lost debugging payloads
• support tickets that can’t be reproduced
• “works on my machine” arguments
• unnecessary blame on auth, data, or infrastructure

All because the API didn’t clearly say: “this resource does not exist”.

How to fix it

Pick a behavior and make it explicit:

• If paths are case-sensitive, return a clean 404
• If you normalize paths, do it consistently and document it
• Never turn path casing into a 400 or 500

The goal is not flexibility. The goal is predictability.

Why this check exists in Rentgen

Because this bug is too small for people to think about — and too expensive to ignore when it hits.

Rentgen checks it automatically so you don’t have to discover it during a debugging session that ends with: “Oh… you used a capital letter.”

Final thoughts

APIs don’t fail only on complex edge cases. Sometimes they fail on typography.

If your API handles uppercase paths predictably, you remove an entire class of pointless debugging. And that’s a bigger win than it sounds.