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API Documentation
Implementation Layer — Translating specification interfaces into callable contracts
Definition
API documentation describes the programmatic interfaces through which software components interact. It provides developers with the information needed to make requests, handle responses, and integrate with a system.
Relationship to the Specification
API documentation is a direct derivative of the specification’s interface definitions. The specification defines what the interface contract is; API documentation explains how to invoke it.
| Specification Says | API Documentation Provides |
|---|---|
| ”The system shall accept authentication tokens” | Endpoint paths, header formats, token schemas |
| ”Queries must support pagination” | Parameter names, default values, response structures |
| ”Errors shall include diagnostic codes” | Error code enumerations, troubleshooting guidance |
When API documentation diverges from the specification, one of two things has happened:
- The implementation has drifted from the spec (a bug)
- The spec is out of date (a documentation debt)
Core Components
- Endpoints/Methods: The callable entry points and their paths
- Parameters: Required and optional inputs, their types, and constraints
- Request/Response Schemas: Data structures for input and output
- Authentication: How callers establish identity and permissions
- Error Handling: Failure modes, error codes, and recovery guidance
- Examples: Concrete invocations demonstrating typical usage
Dependency Chain
Specification
↓
API Documentation ← derives interface contracts
↓
SDK Guides ← wraps API calls in language bindings
↓
Developer Guides ← teaches patterns using APIs
Why Spec-Grounding Matters
Without specification grounding, API documentation becomes descriptive rather than prescriptive. It tells you what the API does today, not what it should do. This creates problems:
- Breaking changes go unnoticed: If the spec says “pagination is required” but the docs just describe current behavior, removing pagination might seem like a valid choice
- Ambiguity in edge cases: When docs describe only happy paths, undefined behavior becomes implementation-dependent
- Testing gaps: QA can only verify documented behaviors; spec-grounded docs ensure coverage of requirements
Best Practices
- Reference specification section numbers when documenting behaviors derived from requirements
- Flag any documented behavior that extends beyond the specification as “implementation-specific”
- Version API documentation in lockstep with specification versions
- Include machine-readable schemas (OpenAPI, AsyncAPI, etc.) that can be validated against spec definitions