Claude API Compaction API実践ガイド — コンテキスト要約で「無限会話」エージェントを構築する

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

type Message = {
  role: "user" | "assistant";
  content: string;
};

const messages: Message[] = [];

async function chat(userMessage: string): Promise<string> {
  messages.push({ role: "user", content: userMessage });

  const response = await client.beta.messages.create(
    {
      model: "claude-sonnet-4-6",
      max_tokens: 4096,
      system: "あなたは親切なアシスタントです。",
      messages,
      context_management: {
        edits: [
          {
            type: "compact_20260112",
            trigger: { type: "input_tokens", value: 80000 },
            // pause_after_compaction: false, // デフォルト
            // デフォルトではcompactionは透過的に処理され、
            // 通常のレスポンスと同じ形式で結果が返る
          },
        ],
      },
    },
    {
      headers: { "anthropic-beta": "compact-2026-01-12" },
    }
  );

  const assistantMessage =
    response.content[0].type === "text" ? response.content[0].text : "";

  messages.push({ role: "assistant", content: assistantMessage });

  return assistantMessage;
}

const response = await client.beta.messages.create(
  {
    model: "claude-sonnet-4-6",
    max_tokens: 4096,
    messages,
    context_management: {
      edits: [
        {
          type: "compact_20260112",
          trigger: { type: "input_tokens", value: 80000 },
          pause_after_compaction: true,
          // カスタム要約プロンプトで要約品質を制御
          instructions:
            "重要なタスク指示、ユーザーの好み、未完了の作業項目を優先して要約してください。",
        },
      ],
    },
  },
  {
    headers: { "anthropic-beta": "compact-2026-01-12" },
  }
);

if (response.stop_reason === "compaction") {
  // 圧縮後のメッセージで会話履歴を置き換え
  console.log("Compaction完了。圧縮後トークン数を記録...");
  // 必要な処理を挟んでから、再度リクエストを送る
}

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

type ConversationMessage = Anthropic.Beta.Messages.BetaMessageParam;

async function agentLoop(initialPrompt: string): Promise<void> {
  let messages: ConversationMessage[] = [
    { role: "user", content: initialPrompt },
  ];

  const tools: Anthropic.Beta.Messages.BetaTool[] = [
    {
      name: "read_file",
      description: "ファイルを読み取る",
      input_schema: {
        type: "object" as const,
        properties: { path: { type: "string" } },
        required: ["path"],
      },
    },
  ];

  while (true) {
    const response = await client.beta.messages.create(
      {
        model: "claude-sonnet-4-6",
        max_tokens: 4096,
        messages,
        tools,
        context_management: {
          edits: [
            {
              type: "compact_20260112",
              trigger: { type: "input_tokens", value: 120000 },
            },
          ],
        },
      },
      { headers: { "anthropic-beta": "compact-2026-01-12" } }
    );

    // compactionが発動した場合、デフォルト（pause_after_compaction: false）では
    // 透過的に処理されるため、通常のstop_reasonが返る

    messages.push({ role: "assistant", content: response.content });

    if (response.stop_reason === "end_turn") break;

    // ツール呼び出し処理
    if (response.stop_reason === "tool_use") {
      const toolResults: Anthropic.Beta.Messages.BetaToolResultBlockParam[] =
        [];
      for (const block of response.content) {
        if (block.type === "tool_use") {
          const result = await executeToolCall(block.name, block.input);
          toolResults.push({
            type: "tool_result",
            tool_use_id: block.id,
            content: result,
          });
        }
      }
      messages.push({ role: "user", content: toolResults });
    }
  }
}

async function executeToolCall(
  name: string,
  input: unknown
): Promise<string> {
  // 実際のツール実行ロジック
  return `${name} の実行結果`;
}

function calculateTotalUsage(usage: {
  input_tokens: number;
  output_tokens: number;
  iterations?: Array<{
    type: string;
    input_tokens: number;
    output_tokens: number;
  }>;
}): { totalInput: number; totalOutput: number } {
  if (!usage.iterations || usage.iterations.length === 0) {
    // compaction未発動: トップレベルの値がそのまま使える
    return {
      totalInput: usage.input_tokens,
      totalOutput: usage.output_tokens,
    };
  }

  // compaction発動時: iterations全体を合算
  let totalInput = 0;
  let totalOutput = 0;
  for (const iteration of usage.iterations) {
    totalInput += iteration.input_tokens;
    totalOutput += iteration.output_tokens;
    console.log(
      `  [${iteration.type}] in: ${iteration.input_tokens}, out: ${iteration.output_tokens}`
    );
  }
  return { totalInput, totalOutput };
}

// 使用例
// const { totalInput, totalOutput } = calculateTotalUsage(response.usage);
// console.log(`実トークン消費: ${totalInput} input + ${totalOutput} output`);