API Contract

The formal declaration of endpoints, methods (GET, POST, PUT, DELETE), and their operation IDs. This is the functional blueprint of the API, often defined in an Interface Definition Language (IDL) like OpenAPI Specification (OAS) or Protocol Buffers. It answers what operations are available.

• Example: An OpenAPI paths object defining /api/v1/users with a POST operation for user creation.
• Purpose: Enables automated client generation, documentation, and discovery.

The strict definition of all request and response data structures using a schema language like JSON Schema. This specifies data types (string, integer, object), validation rules (format, pattern, required fields), and nested object structures.

• Key Elements: type, properties, required, items (for arrays), $ref (for reusability).
• Importance: Guarantees type safety and data integrity, preventing malformed payloads from causing runtime errors. It is the foundation for structured output guarantees in AI tool calling.

The contract must explicitly declare the security schemes required to access its endpoints. This defines the handshake for establishing identity and permissions.

• Common Schemes: API Keys, OAuth 2.0 flows (authorization code, client credentials), Bearer tokens, and mTLS.
• Contractual Role: The securitySchemes and security fields in OpenAPI tell integrators how to authenticate. For AI agents, this dictates the credential management and authentication flows that must be executed before making a call.

A standardized structure for communicating failures, distinct from successful responses. A robust contract defines a consistent error object schema.

• Typical Fields: code (machine-readable string), message (human-readable description), details (optional array), trace_id (for debugging).
• Critical for AI: Enables agents to implement error handling and retry logic by programmatically detecting error types (e.g., rate_limit_exceeded vs. validation_error) and deciding on corrective actions.

The contract is versioned using Semantic Versioning (SemVer) (e.g., v1.2.3) to communicate the nature of changes. It also declares its stability (e.g., experimental, stable, deprecated) and sunset policies.

• MAJOR.MINOR.PATCH: Breaking changes increment MAJOR, new features increment MINOR, bug fixes increment PATCH.
• Purpose: Manages backwards compatibility and allows consumers to plan for upgrades. AI orchestration layers can use this to select compatible API versions.

While often a separate legal document, key operational promises can be codified in or referenced by the technical contract. These set expectations for non-functional requirements.

• Common Metrics: Latency (p95 response time), availability (uptime percentage, e.g., 99.9%), rate limits (requests per second/minute).
• Integration Impact: Informs the orchestration layer design for latency budgets and the need for agent-side caching to stay within rate limits.

The overarching software design pattern that enables a core host application to be extended by modular, independently developed components called plugins. The API contract is the formal specification that defines how these plugins interact with the host.

• Core Principles: Defines the host, the plugin interface, and a discovery/loading mechanism.
• Key Benefit: Enables functional extensibility without modifying the host's core codebase.
• Common Use: Found in IDEs (VS Code), web servers (Apache modules), and graphics software (Photoshop filters).

A specific, well-defined interface or hook within the host application where a plugin can attach itself to contribute functionality. The API contract enumerates and specifies all available extension points.

• Mechanism: Can be a concrete class to inherit from, an abstract method to implement, or an event to subscribe to.
• Example: A TextEditor host might have a SyntaxHighlighter extension point. A plugin implements the highlight(code) method defined in the contract.
• Role: Acts as the integration anchor; a plugin's capabilities are defined by which extension points it implements.

A metadata file (JSON/YAML) that declares a plugin's identity, capabilities, and requirements to the host system. It is the human and machine-readable representation of a plugin's side of the API contract.

• Typical Contents: Plugin ID, version, name, description, entry point, required extension points, dependencies, and configuration schema.
• Function: Enables discovery and validation. The host reads the manifest to verify the plugin is compatible before loading it.
• Analogy: The package.json for a Node.js module or the AndroidManifest.xml for an Android app.

A design pattern where a plugin's required dependencies (services, configurations) are provided ('injected') by the host framework. This inverts control from the plugin to the host, reducing coupling and making plugins easier to test and manage.

• Mechanism: The API contract defines service interfaces. The host's DI container provides concrete implementations to plugins at runtime.
• Benefit: Plugins declare what they need without knowing how to construct it, adhering to the Dependency Inversion Principle.
• Framework Example: The Angular framework uses DI extensively to provide services to components.

A security mechanism that isolates a plugin's execution environment, restricting its access to system resources, memory, and other plugins. It enforces the boundaries implied by the API contract at a system level.

• Purpose: Prevents a faulty or malicious plugin from crashing the host or accessing unauthorized data.
• Techniques: Can use process isolation, virtual machines, WebAssembly (WASM) runtimes, or language-level security managers.
• Trade-off: Increased security often comes with a performance overhead due to context switching or serialization.