Contract-First vs. Code-First Development: Why API Contracts Matter From Day One

Contract-first development is frequently misunderstood, both in concept and implementation. This guide clarifies this powerful approach to API development and explains why it matters—even for early-stage products and small teams.

Understanding the Two Approaches #

What is Contract-First Development? #

Contract-first development is an approach to API design where the API contract (specification) is designed before any implementation code is written. In this methodology, interface definitions—such as OpenAPI specifications, GraphQL schemas, JSON schemas, Avro schemas, or Protocol Buffers—are created and agreed upon by stakeholders first, serving as the single source of truth for both providers and consumers of the API.

Development begins with these contract definitions, from which stubs, Data Transfer Objects (DTOs), interfaces, and clients and documentation are automatically generated. Frameworks then connect these generated APIs with handwritten business logic implementations.

This approach ensures that the implementation matches the defined API contracts, at least in terms of API shape, though it doesn’t guarantee semantic correctness. The semantic verification is handled through integration tests that use the generated clients to validate that other generated clients can successfully use the service.

What is Code-First Development? #

The code-first development approach prioritizes implementation over specification. Developers write application or service code based on business requirements first, then generate API specifications, documentation, and contracts afterward from the existing codebase.

This methodology establishes the code as the primary source of truth, with API descriptions, data schemas, and interface specifications becoming derived artifacts rather than guiding documents. Code-first is common in frameworks like FastAPI, Spring, .NET, and NestJS. These frameworks leverage annotations and decorators that tools can process to generate documentation.

Comparing the Workflows #

The Contract-First Workflow #

1. Define the contract specification first
2. Auto-generate code (stubs, DTOs, interfaces)
3. Implement business logic to fulfill the contract
4. Auto-generate test clients
5. Verify system behavior with integration tests using auto-generated clients
6. When changes are needed, start by modifying the contract. Review contract changes as pool requests (PRs) before implementation begins

The Code-First Workflow (Often Implicit) #

1. Write implementation code based on business requirements
2. Add annotations or decorators to code
3. Generate API documentation from the code
4. Create clients based on implementation or generated specs
5. When changes are needed, modify the code, which may inadvertently change the API

Problems with Code-First Development #

In the code-first approach, API shapes are not clearly defined and may change unexpectedly since they’re automatically generated from implementation code. For example, simply adding a new field to a Plain Old Java Object (POJO) might “magically” expose it in your API without explicit intention.

When internal service contracts aren’t clearly defined, enforcing consistency becomes nearly impossible, leading to integration issues.

For external contracts exposed through proxies, manual synchronization with internal specifications becomes a necessary but error-prone process when appropriate tooling is missing.

Documentation drift presents another significant challenge, where API documentation gradually diverges from actual implementation. As developers modify code, the corresponding documentation often fails to keep pace, leaving teams and integration partners relying on outdated or incorrect information. This silent form of technical debt compounds over time, forcing developers to rely on source code rather than documentation.

Moreover, when complex logic is poorly implemented in the code (e.g., violating SOLID principles), the eventually re-engineered contract might turn out to be poorly defined. Typical issues include:

• Too generic input payloads with many optional fields
• Polymorphic request payloads—a clear indication of violating the Single Responsibility Principle
• No clear purpose for each endpoint
• Lack of clear purpose for the APIs—an Interface Segregation Principle violation

With a contract-first approach, such design issues would be visible at an early stage, enabling correction before implementation begins.

Addressing Common Objections #

Some might argue: “You’re thinking from your classic world of development. Following a contract-first approach when launching an alpha-stage product would be pure overhead for a small team to worry about.”

This perspective misses the point. Contract-first development isn’t about bureaucracy—it’s about clarity and confidence, especially in early-stage products. Defining contracts upfront with code generation and integration testing confirms your system works correctly and prevents future integration problems with service clients.

Converting from code-first to contract-first later in the product lifecycle is typically much more challenging than starting with contracts.

Even small teams or solo developers can benefit from the discipline and clarity that contract-first brings.

When to Use Each Approach #

Consider Code-First When: #

• Requirements are highly fluid and exploratory
• You’re working on a throwaway prototype
• The API will have very few or no external consumers
• You need the absolute fastest path to an initial implementation

Consider Contract-First When: #

• Other teams or services will interact with the API
• The API will be public-facing
• Consistency and stability of the API are important
• You want to enable parallel development of client and server

Minimum Viable Contract Management #

At the very least, teams should:

1. Maintain and publish accurate API specifications (OpenAPI, GraphQL schema, etc.)
2. Ensure the specification matches the implementation
3. Generate and share up-to-date API documentation

Responsibility for this lies with the team owning the service, as they drive and implement changes.

Benefits for Early-Stage Products #

Contract-first development offers several advantages for alpha-stage products:

• Faster iteration when interfaces are clearly defined
• Easier onboarding for new team members
• Better tracking of API evolution
• Simplified testing and validation
• Clearer communication between frontend and backend teams
• Reduced refactoring costs as the product matures

Tools Supporting Contract-First Development #

To make contract-first development practical, teams can leverage tools like:

• Swagger/OpenAPI editors and generators
• GraphQL schema design tools
• gRPC/Protobuf tooling
• Code generator frameworks specific to various languages

Conclusion #

Contract-first development isn’t just for enterprise-scale systems—it’s a practice that builds quality and clarity into products from day one. While it may require a slight shift in mindset and workflow, the benefits in terms of API quality, team coordination, and future maintenance make it worth considering even for small teams and early-stage products.

By starting with clear, well-defined contracts, teams can build more reliable services with fewer integration headaches, setting themselves up for success as their products grow and evolve.