Patterns

Outfitter Dev Kit Patterns

Primary reference for @outfitter/* package conventions.

Handler Contract

Handlers are pure functions that:

• Accept typed input and context
• Return Result<TOutput, TError>
• Know nothing about transport (CLI flags, HTTP headers, MCP tool schemas)

type Handler<TInput, TOutput, TError extends OutfitterError> = (
  input: TInput,
  ctx: HandlerContext
) => Promise<Result<TOutput, TError>>;

Example

import { Result, NotFoundError, type Handler } from "@outfitter/contracts";

export const getUser: Handler<{ id: string }, User, NotFoundError> = async (
  input,
  ctx
) => {
  ctx.logger.debug("Fetching user", { userId: input.id });
  const user = await db.users.findById(input.id);

  if (!user) {
    return Result.err(NotFoundError.create("user", input.id));
  }
  return Result.ok(user);
};

Why? Testability (just call the function), reusability (same handler for CLI/MCP/HTTP), type safety (explicit types), composability (handlers wrap handlers).

Result Types

Uses Result<T, E> from better-result for explicit error handling.

import { NotFoundError, Result } from "@outfitter/contracts";

// Create
const ok = Result.ok({ name: "Alice" });
const err = Result.err(NotFoundError.create("user", "123"));

// Check
if (result.isOk()) {
  console.log(result.value); // TypeScript knows T
} else {
  console.log(result.error); // TypeScript knows E
}

// Pattern match
const message = result.match({
  ok: (user) => `Found ${user.name}`,
  err: (error) => `Error: ${error.message}`,
});

// Combine
const combined = combine2(result1, result2); // tuple or first error

Error Taxonomy

Ten categories map to exit codes and HTTP status:

import {
  ValidationError,
  NotFoundError,
  getExitCode,
} from "@outfitter/contracts";

ValidationError.create("email", "invalid");
NotFoundError.create("user", "user-123");

getExitCode(error.category); // 2 for not_found
getStatusCode(error.category); // 404 for not_found

Validation

Use Zod with createValidator for type-safe validation returning Results:

import { createValidator } from "@outfitter/contracts";
import { z } from "zod";

const InputSchema = z.object({
  name: z.string().min(1),
  email: z.string().email(),
});

const validateInput = createValidator(InputSchema);

// In handler
const inputResult = validateInput(rawInput);
if (inputResult.isErr()) return inputResult;
const input = inputResult.value; // typed as z.infer<typeof InputSchema>

Context

HandlerContext carries cross-cutting concerns:

import { createContext } from "@outfitter/contracts";

const ctx = createContext({
  logger: myLogger, // structured logger
  config: resolvedConfig, // merged configuration
  signal: controller.signal, // cancellation
  workspaceRoot: "/project",
});

// ctx.requestId is auto-generated UUIDv7 for tracing

Bun-First APIs

Prefer Bun-native APIs:

References

• Handler Contract
• Error Taxonomy
• Result Utilities