api-and-interface-design

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# API and Interface Design

## Overview

Design stable, well-documented interfaces that are hard to misuse. Good interfaces make the right thing easy and the wrong thing hard. This applies to REST APIs, GraphQL schemas, module boundaries, component props, function signatures, and any surface where one piece of code talks to another.

This skill ships with **TypeScript and Python examples side-by-side**. Pick the one that matches the codebase you are working in; the principles are identical.

## When to Use

- Designing new API endpoints
- Defining module boundaries or contracts between teams
- Creating component prop interfaces or function signatures
- Establishing database schema that informs API shape
- Changing existing public interfaces

## Core Principles

### Hyrum's Law

> With a sufficient number of users of an API, all observable behaviors of your system will be depended on by somebody, regardless of what you promise in the contract.

Every observable behavior — including undocumented quirks, error message text, response ordering, and timing — becomes a de facto contract once users depend on it. Design implications:

- **Be intentional about what you expose.** Every observable behavior is a potential commitment.
- **Don't leak implementation details.** If users can observe it, they will depend on it.
- **Plan for deprecation at design time.** Versioning, sunset headers, and migration paths are part of the original design, not a later cleanup.
- **Tests are not enough.** Even with perfect contract tests, "safe" changes can break real users who depend on undocumented behavior.

### The One-Version Rule

Avoid forcing consumers to choose between multiple versions of the same dependency or API. Diamond dependency problems arise when different consumers need different versions of the same thing. Design for a world where only one version exists at a time — extend rather than fork.

### 1. Contract First

Define the interface before implementing it. The contract is the spec; implementation follows.

**TypeScript:**

```typescript
// Define the contract first
interface TaskAPI {
  // Creates a task and returns the created task with server-generated fields
  createTask(input: CreateTaskInput): Promise<Task>;

  // Returns paginated tasks matching filters
  listTasks(params: ListTasksParams): Promise<PaginatedResult<Task>>;

  // Returns a single task or throws NotFoundError
  getTask(id: TaskId): Promise<Task>;

  // Partial update — only provided fields change
  updateTask(id: TaskId, input: UpdateTaskInput): Promise<Task>;

  // Idempotent delete — succeeds even if already deleted
  deleteTask(id: TaskId): Promise<void>;
}
```

**Python (Protocol-style contract):**

```python
from typing import Protocol
from .models import Task, TaskId, CreateTaskInput, UpdateTaskInput, ListTasksParams, Page

class TaskAPI(Protocol):
    async def create_task(self, payload: CreateTaskInput) -> Task: ...
    async def list_tasks(self, params: ListTasksParams) -> Page[Task]: ...
    async def get_task(self, task_id: TaskId) -> Task: ...
    async def update_task(self, task_id: TaskId, patch: UpdateTaskInput) -> Task: ...
    async def delete_task(self, task_id: TaskId) -> None: ...
```

A `Protocol` documents the contract independently of any concrete implementation, and both ASGI handlers and in-process callers can satisfy the same shape.

### 2. Consistent Error Semantics

Pick one error strategy and apply it everywhere.

**TypeScript:**

```typescript
interface APIError {
  error: {
    code: string;        // Machine-readable: "VALIDATION_ERROR"
    message: string;     // Human-readable: "Email is required"
    details?: unknown;   // Additional context when helpful
  };
}
```

**Python (FastAPI exception handler):**

```python
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from pydantic import BaseModel

class APIErrorBody(BaseModel):
    code: str
    message: str
    details: dict | None = None

class APIError(Exception):
    def __init__(self, status: int, code: str, message: str, details: dict | None = None):
        self.status = status
        self.body = APIErrorBody(code=code, message=message, details=details)

app = FastAPI()

@app.exception_handler(APIError)
async def api_error_handler(_: Request, exc: APIError) -> JSONResponse:
    return JSONResponse(status_code=exc.status, content={"error": exc.body.model_dump()})
```

HTTP status mapping (same in any language):

| Status | Meaning |
|---|---|

From jankneumann/agentic-coding-tools, MIT.

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