API design best practices define principles for creating clean, consistent, and developer-friendly interfaces — establishing conventions for endpoints, methods, error handling, versioning, and documentation that make APIs intuitive to use and maintainable long-term, especially important for LLM services where good design impacts developer experience and adoption.

Why API Design Matters

• Developer Experience: Good APIs are easy to understand and use.
• Adoption: Clean APIs encourage integration and usage.
• Maintenance: Consistent patterns reduce support burden.
• Evolution: Good versioning enables growth without breaking users.
• Scale: Well-designed APIs handle traffic and feature growth.

REST API Fundamentals

Resource-Based URLs:

Good:
GET    /users              # List users
GET    /users/{id}         # Get specific user
POST   /users              # Create user
PUT    /users/{id}         # Update user
DELETE /users/{id}         # Delete user

Bad:
GET    /getUsers
POST   /createUser
POST   /deleteUser/{id}

HTTP Methods:

Method  | Purpose         | Idempotent | Safe
--------|-----------------|------------|------
GET     | Read resource   | Yes        | Yes
POST    | Create resource | No         | No
PUT     | Replace/update  | Yes        | No
PATCH   | Partial update  | No*        | No
DELETE  | Remove resource | Yes        | No

Response Codes:

Code | Meaning              | When to Use
-----|----------------------|----------------------------
200  | OK                   | Successful GET, PUT, PATCH
201  | Created              | Successful POST (resource created)
204  | No Content           | Successful DELETE
400  | Bad Request          | Invalid input from client
401  | Unauthorized         | Missing/invalid auth
403  | Forbidden            | Auth valid, but no permission
404  | Not Found            | Resource doesn't exist
429  | Too Many Requests    | Rate limited
500  | Internal Server Error| Server-side failure

LLM API Design Patterns

Chat Completions Pattern (OpenAI-style):

POST /v1/chat/completions
{
    "model": "gpt-4o",
    "messages": [
        {"role": "system", "content": "You are helpful."},
        {"role": "user", "content": "Hello!"}
    ],
    "temperature": 0.7,
    "max_tokens": 1000,
    "stream": false
}

Response:
{
    "id": "chatcmpl-abc123",
    "object": "chat.completion",
    "created": 1677652288,
    "model": "gpt-4o",
    "choices": [{
        "index": 0,
        "message": {
            "role": "assistant",
            "content": "Hello! How can I help you today?"
        },
        "finish_reason": "stop"
    }],
    "usage": {
        "prompt_tokens": 12,
        "completion_tokens": 9,
        "total_tokens": 21
    }
}

Streaming Response (SSE):

POST /v1/chat/completions
{ "stream": true, ... }

Response (text/event-stream):
data: {"id":"abc","choices":[{"delta":{"content":"Hello"}}]}
data: {"id":"abc","choices":[{"delta":{"content":"!"}}]}
data: {"id":"abc","choices":[{"delta":{},"finish_reason":"stop"}]}
data: [DONE]

REST vs. gRPC

Aspect       | REST              | gRPC
-------------|-------------------|-------------------
Format       | JSON (text)       | Protobuf (binary)
Speed        | Good              | 2-10× faster
Browser      | Native support    | Needs proxy
Streaming    | SSE/WebSocket     | Native bidirectional
Tooling      | Ubiquitous        | Growing
Learning     | Easy              | Steeper curve
Best For     | Public APIs       | Internal services

Versioning Strategies

Strategy     | Example                | Pros/Cons
-------------|------------------------|--------------------
URL path     | /v1/users, /v2/users  | Clear, cacheable
Header       | Accept: application/vnd.api+json;v=2 | Clean URLs, harder
Query param  | /users?version=2      | Simple, less RESTful

Error Handling

Standard Error Response:

{
    "error": {
        "code": "invalid_request_error",
        "message": "The 'model' field is required.",
        "type": "invalid_request_error",
        "param": "model"
    }
}

Error Best Practices:

• Use consistent error format across all endpoints.
• Include actionable error messages.
• Log request ID for debugging.
• Don't expose internal details in production.

Pagination

Cursor-Based (preferred for real-time data):

GET /messages?limit=20&after=cursor_abc123

Response:
{
    "data": [...],
    "has_more": true,
    "next_cursor": "cursor_xyz789"
}

Offset-Based (simpler, less efficient):

GET /users?limit=20&offset=40

Response:
{
    "data": [...],
    "total": 1000,
    "limit": 20,
    "offset": 40
}

Rate Limiting

Headers to Include:

X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1677652288
Retry-After: 60

Best Practices

• Be Consistent: Same patterns across all endpoints.
• Be Predictable: Developers should guess correctly.
• Be Complete: Include all needed info in responses.
• Document Everything: OpenAPI/Swagger specs.
• Version Early: Plan for evolution from day one.
• Test Thoroughly: Automated API contract tests.

API design is the user interface for developers — well-designed APIs make integration easy and enjoyable, while poor APIs create friction that slows adoption and increases support burden, making API design a critical skill for building successful developer products.