Claude API Cheat Sheet for TypeScript Developers

import Anthropic from "@anthropic-ai/sdk";
 
const client = new Anthropic(); // reads ANTHROPIC_API_KEY
 
const message = await client.messages.create({
  model: "claude-opus-5-5",
  max_tokens: 4096,
  system: "You are a concise assistant.",
  messages: [{ role: "user", content: "Explain idempotency in one paragraph." }],
});
console.log(message.content.flatMap((b) => (b.type === "text" ? [b.text] : [])).join(""));

import type Anthropic from "@anthropic-ai/sdk";
 
export type NextAction =
  | { kind: "answer"; text: string }
  | { kind: "run_tools"; calls: Anthropic.ToolUseBlock[] }
  | { kind: "continue" }
  | { kind: "truncated"; partial: string }
  | { kind: "refused" }
  | { kind: "context_full" };
 
const textOf = (m: Anthropic.Message) => m.content.flatMap((b) => (b.type === "text" ? [b.text] : [])).join("");
 
export function nextAction(message: Anthropic.Message): NextAction {
  switch (message.stop_reason) {
    case "tool_use":
      return { kind: "run_tools", calls: message.content.filter((b): b is Anthropic.ToolUseBlock => b.type === "tool_use") };
    case "pause_turn":
      return { kind: "continue" };
    case "max_tokens":
      return { kind: "truncated", partial: textOf(message) };
    case "refusal":
      return { kind: "refused" };
    case "model_context_window_exceeded":
      return { kind: "context_full" };
    case "end_turn":
    case "stop_sequence":
    case null:
      return { kind: "answer", text: textOf(message) };
    default: {
      // A new stop reason added in a future API version fails the type check here.
      const unknown: never = message.stop_reason;
      throw new Error(`Unhandled stop_reason: ${String(unknown)}`);
    }
  }
}

import { test } from "node:test";
import assert from "node:assert/strict";
import type Anthropic from "@anthropic-ai/sdk";
import { nextAction } from "./lib/next-action.ts";
 
const msg = (stop_reason: Anthropic.StopReason, content: unknown[]) => ({ stop_reason, content }) as unknown as Anthropic.Message;
 
test("maps stop reasons to actions", () => {
  assert.deepEqual(nextAction(msg("end_turn", [{ type: "text", text: "Hi" }])), { kind: "answer", text: "Hi" });
  assert.equal(nextAction(msg("tool_use", [{ type: "tool_use", id: "t", name: "x", input: {} }])).kind, "run_tools");
  assert.deepEqual(nextAction(msg("max_tokens", [{ type: "text", text: "Part" }])), { kind: "truncated", partial: "Part" });
  assert.equal(nextAction(msg("pause_turn", [])).kind, "continue");
  assert.equal(nextAction(msg("refusal", [])).kind, "refused");
});