Claude apps gateway の発表を読んで、個人開発の管制面を作り直した話

// gateway.ts — 最小の管制面プロキシ
interface AppPolicy {
  token: string;           // アプリ別の内部トークン
  allowedModels: string[]; // モデル許可リスト
  monthlyCapUsd: number;   // 月次支出上限
}
 
// 方針はコードに直書きせず環境変数 APP_POLICIES(JSON) から読む
// 価格表（USD / 1M tokens）。改定時はここだけ更新する
const PRICING: Record<string, { in: number; out: number }> = {
  "claude-sonnet-5": { in: 2, out: 10 },   // 導入価格・2026-08-31 まで
  "claude-opus-4-8": { in: 5, out: 25 },
  "claude-haiku-4-5": { in: 1, out: 5 },
};
 
export default {
  async fetch(req: Request, env: Env): Promise<Response> {
    const policies: AppPolicy[] = JSON.parse(env.APP_POLICIES);
    const token = req.headers.get("x-app-token") ?? "";
    const app = policies.find((p) => p.token === token);
    if (!app) return new Response("unknown app", { status: 401 });
 
    const body = await req.json<{ model: string }>();
    if (!app.allowedModels.includes(body.model)) {
      return new Response("model not allowed for this app", { status: 403 });
    }
 
    // 支出上限チェック（fail-closed: 台帳が読めなければ通さない）
    const ledger = env.LEDGER.get(env.LEDGER.idFromName(ledgerKey(app.token)));
    const spent = await ledger.fetch("https://ledger/get").then((r) => r.json<number>());
    if (spent >= app.monthlyCapUsd * 0.9) {
      return new Response("monthly cap reached", { status: 429 });
    }
 
    // 実キーはここでだけ付与する
    const upstream = await fetch("https://api.anthropic.com/v1/messages", {
      method: "POST",
      headers: {
        "x-api-key": env.ANTHROPIC_API_KEY, // Workers secret
        "anthropic-version": "2023-06-01",
        "content-type": "application/json",
      },
      body: JSON.stringify(body),
    });
 
    // 事後計上: usage からコストを計算して台帳に加算
    const json = await upstream.json<any>();
    if (json.usage && PRICING[body.model]) {
      const p = PRICING[body.model];
      const cost =
        (json.usage.input_tokens / 1e6) * p.in +
        (json.usage.output_tokens / 1e6) * p.out;
      await ledger.fetch("https://ledger/add", {
        method: "POST",
        body: JSON.stringify({ cost, requestId: json.id }),
      });
    }
    return new Response(JSON.stringify(json), {
      status: upstream.status,
      headers: { "content-type": "application/json" },
    });
  },
};
 
function ledgerKey(token: string): string {
  const m = new Date().toISOString().slice(0, 7); // YYYY-MM
  return `${token}:${m}`;
}

// ledger.ts — 月次台帳（Durable Object）
export class SpendLedger {
  constructor(private state: DurableObjectState) {}
 
  async fetch(req: Request): Promise<Response> {
    const url = new URL(req.url);
    if (url.pathname === "/get") {
      const v = (await this.state.storage.get<number>("spent")) ?? 0;
      return Response.json(v);
    }
    if (url.pathname === "/add") {
      const { cost, requestId } = await req.json<any>();
      // リトライで同じレスポンスを二重計上しないよう request id で冪等化
      const seen = await this.state.storage.get<boolean>(`seen:${requestId}`);
      if (!seen) {
        const v = (await this.state.storage.get<number>("spent")) ?? 0;
        await this.state.storage.put("spent", v + cost);
        await this.state.storage.put(`seen:${requestId}`, true);
      }
      return Response.json(true);
    }
    return new Response("not found", { status: 404 });
  }
}