You deploy with confidence.
Tests pass.
The build is green. ✅
Minutes later…
🔥 Production is broken because an API field changed.
If this has already happened to you (or will), this post is for you.
Recently, we’ve started adopting contract testing at the company where I work, integrating it into the set of validations that already run in our CI pipeline. The company has a strong testing culture and well-established quality standards, and the goal here was to complement what was already in place by adding more confidence to service-to-service communication. In this post, I want to share what we’ve learned so far and my practical impressions of using contract testing in day-to-day development, keeping things straightforward and free from unnecessary theory.
❌ The problem traditional tests don’t solve
In modern architectures, this is common:
Unit tests pass
Integration tests pass
Swagger is up to date
But… the API consumer breaks at runtime
Why?
Because traditional tests validate implementations, not agreements between systems.
And that’s where the real problem lives.
🤝 What Contract Tests are (no academic definition)
Contract Testing is basically this:
A formal agreement between API consumers and providers.
It ensures both sides agree on:
Payload structure
Field types
Status codes
Implicit rules (required fields, formats, etc.)
If someone breaks that agreement…
🚫 the build fails before reaching production.
💥 A simple (and painful) example
The consumer expects this:
{
"id": 1,
"name": "Patrick"
}
The provider decides to “improve” the API:
{
"id": 1,
"fullName": "Patrick Bastos"
}
✔️ Backend works
✔️ Swagger updated
✔️ Tests pass
❌ Frontend breaks
❌ App crashes
❌ The user finds out first
This bug is not about code — it’s about communication.
🛡️ Where Contract Tests come in
With Contract Testing, the flow changes:
Consumer defines expectations
↓
Contract is generated
↓
Provider validates the contract
↓
Deploy only happens if the contract is respected
In other words:
Whoever changes the API without warning… breaks their own pipeline.
And that’s beautiful ❤️
🧰 The most used tool in .NET: Pact
In the .NET ecosystem, the most mature and widely adopted tool is Pact, using PactNet.
Why Pact works so well
Consumer-Driven Contract Testing (CDC)
Tests written in C#
Versioned contracts
Automatic provider verification
Easy CI/CD integration
🧪 How this works in practice (very short version)
On the Consumer side
You write a test saying:
“When I call /customers/1…”
“I expect this response…”
That test generates a contract file.
On the Provider side
The backend runs a test validating:
“Does my API still respect this contract?”
If not:
❌ build fails
❌ deploy blocked
No production surprises.
⚠️ Contract Testing is NOT a silver bullet
Important to be clear:
❌ It doesn’t replace integration tests
❌ It doesn’t test business rules
❌ It doesn’t guarantee bug-free code
✅ It guarantees communication stability
✅ It prevents breaking changes
✅ It reduces silent incidents
🟢 When it’s REALLY worth it
Microservices
Multiple teams
Public APIs
Independent deployments
Frequently evolving endpoints
🟡 Maybe not worth it (for now)
Simple monoliths
Small teams
Joint deployments
Low complexity
✅ Quick checklist (real-world lessons)
❌ Don’t rely only on Swagger
✅ Version your contracts
✅ Let the consumer define the contract
❌ Don’t couple contracts to implementation
✅ Run contract tests in CI
✅ Fail fast
🎯 Conclusion
Contract Tests don’t prevent bugs.
They prevent surprises.
And in production, surprise usually means incident.
If your API changes without fear,
your consumer suffers without warning.
💬 Let’s talk
Have you ever broken production because of a contract?
Have you used Pact or another tool?
Are you still relying only on Swagger?
Drop a comment 👇
These pains are collective 😄
Top comments (0)