API Design

Overview

Contract-first API design patterns for defining stable, evolvable interfaces between system components. Protocol-agnostic — applies to REST, gRPC, GraphQL, message-based, or any other interface style.

Constraints

• Define the contract before implementing it; implementation conforms to the contract
• Every endpoint must have an error contract; unspecified failure modes are not acceptable
• Never remove a field or endpoint without a deprecation period and migration path
• Do not expose internal domain models directly through API boundaries

Core Concepts

Contract-First Design

Define the interface before implementing it. The contract is the source of truth; implementation conforms to the contract, not the other way around. Contracts are versioned artifacts.

Resource Modeling

• Entities — individual resources with identity (e.g., /orders/{id})
• Collections — groups of entities with filtering, pagination, sorting
• Relationships — links between resources expressed through references or embedding
• Operations — actions on resources (CRUD and domain-specific operations)

Versioning Strategies

Strategy	Mechanism	Trade-offs
URL path	/v1/resources, /v2/resources	Simple routing, but proliferates endpoints
Header	Accept: application/vnd.api.v2+json	Clean URLs, but harder to test and discover
Content negotiation	Media type with version parameter	Standards-compliant, but adds client complexity

Choose one strategy per API boundary and apply it consistently. URL path versioning is the simplest default.

Error Contracts

Every API defines structured error responses:

Field	Purpose
Error code	Machine-readable identifier (stable across versions)
Message	Human-readable description
Correlation ID	Trace identifier linking request to logs
Details	Optional structured context (field-level validation errors, etc.)

Error contracts are part of the API contract — not an afterthought.

Patterns

API Design Procedure

1. Identify consumers — who calls this API, what do they need, what are their constraints
2. Define resources — model the domain entities and relationships exposed through the interface
3. Specify operations — define actions on each resource with request/response shapes
4. Define error cases — enumerate failure modes with error codes and response structures
5. Document constraints — rate limits, pagination defaults, size limits, authentication requirements
6. Choose version strategy — select and document the versioning approach

Backward Compatibility Checklist

Change Type	Impact	Action
Add optional field	Safe (additive)	No version bump needed
Add new endpoint	Safe (additive)	No version bump needed
Remove field	Breaking	Deprecate first, remove in next major version
Change field type	Breaking	New version required
Change endpoint URL	Breaking	Redirect old URL, new version
Tighten validation	Breaking	New version required
Loosen validation	Safe (relaxing)	No version bump, document change

Contract Validation

Before implementation begins, verify the API contract against:

• BDD scenarios cover all operations and error cases
• Acceptance criteria include non-functional requirements (latency, throughput)
• Consumer needs are met without exposing internal implementation details
• Error codes are unique and documented
• Versioning strategy is declared and consistent

When to Apply

Scenario	Apply?	Depth
New external or public API	Yes	Full procedure
New inter-service boundary	Yes	Full procedure
Internal module boundary	Partially	Resource modeling and error contracts
Private helper functions	No	—
Modifying existing API contract	Yes	Backward compatibility checklist

Guidelines

1. Design for consumers, not implementations. The API shape reflects what callers need, not how the backend is structured.
2. Every endpoint has an error contract. Callers must know what failures look like before they write integration code.
3. Deprecation before removal. Never remove a field or endpoint without a deprecation period and migration path.
4. Contracts are versioned artifacts. Treat API contracts with the same rigor as source code — reviewed, versioned, and tested.
5. Avoid leaking internal models. Domain entities exposed through an API may need a separate representation (DTO/view model) to prevent tight coupling.
6. Pagination is not optional for collections. Unbounded collection responses are a reliability risk.

Output

API contract document covering resources, operations, error cases, versioning strategy, and the backward compatibility checklist results. Be concise — use tables; skip narrative prose.

Integration

• Agent-Assisted Specification — API contracts emerge during the Architecture Specification stage; validate them as part of the consistency gate
• Hexagonal Architecture — API contracts define the ports; adapters implement the protocol-specific translation
• Domain-Driven Design — API boundaries often align with bounded context interfaces; use context maps to identify integration points