The 403 Forbidden Nightmare: Debugging Freight Rate APIs at 2 AM

I swear, if I see one more {"error": "INVALID_Dimension_UOM"} response, I might just move to a farm and grow potatoes.

Last week, I was tasked with integrating a new freight carrier's rating API into our internal dispatch system. "It's standard REST," they said. "Documentation is comprehensive," they promised.

Spoiler alert: It was not standard, and the documentation was apparently written by someone who had never seen a computer, let alone an API endpoint.

The Setup

We needed to fetch real-time LTL (Less-Than-Truckload) rates. The flow seemed simple enough on the whiteboard:

1. Authenticate via OAuth2 (Client Credentials flow).
2. POST shipment details (weight, class, dims) to /v1/rates/quote.
3. Receive rate.
4. Profit.

I set up the request in Postman, hit Send... and boom. 200 OK.

Great! I copied the generated code into our TypeScript backend, deployed to staging, and went to grab a coffee.

Ten minutes later, Sentry started screaming.

The "Invisible" Character

Every single request from the production environment was failing with 400 Bad Request. But identical payloads from my local machine worked fine.

I logged the raw request body. Identical. I checked the headers. Identical. I validated the token. valid.

I spent 4 hours staring at this:

{
  "code": "VAL_ERR_001",
  "message": "Payload validation failed at line 1"
}

Line 1? The whole thing is line 1! It's minified JSON!

I finally dumped the hex representation of the payload. And there it was. A Zero Width Space (\u200B) right at the beginning of the string.

Turns out, when I copied the "Sample Payload" from their PDF documentation, I picked up a hidden formatting character. My local environment's linter stripped it out automatically on save, but the CI/CD pipeline's build process preserved it in the environment variable config where I had pasted a "template" structure.

Handling "Freight Class" is a Joke

After fixing the invisible character, I hit the next wall: Freight Density logic.

The API demanded freightClass as a string, e.g., "70". But occasionally, for really light items, we have class "92.5".

Sending "92.5" resulted in:

{
  "error": "Type Mismatch. Expected Enum [50, 55, ..., 925, ... 400, 500]"
}

Wait, 925?

Yes. Their legacy system stores class 92.5 as 925. But class 70 is 70. NOT 700.

So I had to write this beautiful monstrosity of a normalizer:

function normalizeFreightClass(fc: string): string {
  // If it's a split class like 77.5 or 92.5, multiply by 10 and stringify.
  // BUT NOT ALWAYS. Because 85 is unchanged.
  if (fc.includes('.')) {
    return fc.replace('.', '');
  }
  return fc;
}

(Don't copy that code, it's terrible, but it works for this specific vendor).

Why This Matters

Building tools for logistics isn't just about clean code. It's about bridging the gap between modern tech stacks (Next.js, Serverless) and Mainframes from 1990 that run the global supply chain.

We sanitize inputs not because users are malicious, but because the upstream systems are fragile.

If you're building in this space:

1. Trust nothing. Validate every response.
2. Log everything. You will need raw payloads to prove to support that their system is down.
3. Buy a comfortable chair. You'll be in it a while.

Happy coding. I'm going to sleep.