Some Bugs Aren't in Your Code. They're Hidden in Your Dependencies.

The Mystery

A few developers on our team started reporting inconsistent behavior while testing the same feature. Developers working on one group of VMs couldn't reproduce the issue. Developers using another group of VMs could reproduce it every single time. Naturally, we started comparing everything.

• Were we using different builds?
• Different configuration files?
• Different database versions?
• Different operating system patches?
• Different .NET runtimes?

Nothing stood out. The environments were supposed to be identical.

Chasing the Wrong Suspects

When a bug only appears in one environment, it's tempting to blame infrastructure. We spent hours checking VM configurations, network settings, installed software, and deployment differences. Every comparison ended with the same conclusion: Everything looked correct. The more we investigated, the less the infrastructure looked guilty.

So we changed our approach. Instead of asking: "What's different about these VMs?" We started asking: "What exactly happens between sending a request and receiving a response?" That shifted our attention from the environment to the application's HTTP layer.

Looking Beyond Business Logic

We started comparing requests and responses generated from both environments. Headers looked normal. Payloads looked normal. The API wasn't returning errors. Business logic wasn't failing. At first, nothing explained why identical requests were producing different behavior.

Then we started looking at something nobody had questioned: The HTTP client itself. Our project was still using an older version of RestSharp. Digging deeper revealed something interesting. Older versions of RestSharp relied on SimpleJson as their built-in serializer. That wasn't something we'd consciously chosen-it had simply existed in the project for years. Until now, nobody had questioned it.

The Hidden Dependency

The breakthrough came when we focused on serialization instead of infrastructure. Serialization sits in an awkward place in an application. You rarely think about it. You trust it to convert objects into JSON and JSON back into objects. When it works, it's invisible. When it doesn't... Nothing makes sense.

The issue wasn't our business logic. It wasn't the API. It wasn't the database. It was a legacy serialization layer quietly sitting underneath everything.

While researching the dependency, we discovered that this wasn't unique to our project. When RestSharp introduced version 107, the maintainers removed SimpleJson completely and migrated to System.Text.Json, citing legacy architecture and serialization quirks as reasons for the redesign. That discovery completely changed how we looked at the problem. Instead of trying to patch symptoms, we focused on eliminating the outdated dependency.

The Fix

Rather than adding workarounds around the failing behavior, we updated our serialization approach by moving away from the older RestSharp implementation that depended on SimpleJson. After validating the changes across both VM environments, the inconsistent behavior disappeared. The application behaved the same everywhere. Not because we changed our business logic. Not because we reconfigured the VMs. But because we removed a piece of technical debt that had quietly become part of the application's foundation.

What This Bug Taught Me

This incident completely changed the way I investigate environment-specific issues. When code behaves differently across environments, it's easy to blame the environment itself. Sometimes that's true. But sometimes the real culprit is hidden several layers beneath your own code. A dependency you haven't updated in years. A serializer you didn't even know your project was using. A library that everyone forgot existed because it "just worked." Until it didn't.

Takeaways

• Don't stop at your own code-inspect the libraries underneath it.
• Environment-specific bugs aren't always infrastructure bugs.
• Legacy dependencies can introduce behavior that's incredibly difficult to diagnose.
• If a dependency has been completely redesigned by its maintainers, it's worth understanding why.

The biggest lesson from this bug wasn't about RestSharp. It was about curiosity. The answer wasn't in our application code. It was hiding inside a dependency that most of us had never looked at. Sometimes, the hardest bugs aren't the ones you wrote. They're the ones you've been shipping for years without even realizing it.

restsharp #dotnet #api-development #serializer #deserializer