Agree On The Interface First
Contract first design means you write the API definition, the schema, before either side writes implementation code. The contract becomes the shared source of truth.
How The Workflow Runs
The contract drives everything downstream.
- Teams agree on a definition such as a protobuf file or an OpenAPI document.
- Tooling generates client and server stubs from it.
- Both sides build against the generated types in parallel.
- The contract can be linted for breaking changes in continuous integration.
Why It Helps
Starting from the contract changes how teams coordinate.
- Frontend and backend work in parallel instead of waiting.
- Generated types remove a whole class of mismatch bugs.
- The contract is versioned, so changes are reviewed deliberately.
Handling Change Safely
The hard part is evolving the contract without breaking callers.
- Prefer additive changes that old clients ignore.
- Never reuse retired field numbers or change a field meaning.
- Run automated compatibility checks so a breaking change is caught before release.
The opposite approach, letting the implementation define the contract, drifts easily and surprises clients. Contract first trades some upfront ceremony for fewer integration failures later.
Key idea
Contract first design treats the API definition as the shared source of truth that generates stubs and is checked for breaking changes, letting teams build in parallel and catching mismatches before clients are surprised.