Claude の『このモデルは現在利用できません』が出る時に効くモデル選択フォールバックの設計

type ModelTier = "frontier" | "balanced" | "fast";
 
interface ModelFallbackChain {
  primary: string;
  fallbacks: string[];
}
 
const FALLBACK_CHAINS: Record<ModelTier, ModelFallbackChain> = {
  frontier: {
    primary: "claude-opus-4-6",
    fallbacks: ["claude-sonnet-4-6", "claude-sonnet-4-5"],
  },
  balanced: {
    primary: "claude-sonnet-4-6",
    fallbacks: ["claude-sonnet-4-5", "claude-haiku-4-5-20251001"],
  },
  fast: {
    primary: "claude-haiku-4-5-20251001",
    fallbacks: ["claude-haiku-4-5", "claude-haiku-4"],
  },
};

import Anthropic from "@anthropic-ai/sdk";
 
async function invokeWithFallback(
  client: Anthropic,
  tier: ModelTier,
  buildRequest: (modelId: string) => Anthropic.Messages.MessageCreateParams
): Promise<Anthropic.Messages.Message> {
  const chain = FALLBACK_CHAINS[tier];
  const candidates = [chain.primary, ...chain.fallbacks];
  let lastError: unknown;
 
  for (const modelId of candidates) {
    try {
      const req = buildRequest(modelId);
      return await client.messages.create(req);
    } catch (err) {
      if (isModelUnavailable(err)) {
        console.warn(`model ${modelId} unavailable, trying next`);
        lastError = err;
        continue;
      }
      // 別種のエラーは即座に投げ返す。フォールバックの対象ではない
      throw err;
    }
  }
  throw lastError;
}
 
function isModelUnavailable(err: unknown): boolean {
  if (!err || typeof err !== "object") return false;
  const e = err as { status?: number; error?: { type?: string; message?: string } };
  if (e.status !== 400 && e.status !== 404) return false;
  const msg = e.error?.message?.toLowerCase() ?? "";
  return (
    msg.includes("currently unavailable") ||
    msg.includes("model is not available") ||
    msg.includes("model_not_available") ||
    e.error?.type === "model_not_found_error"
  );
}

interface ModelCapabilities {
  modelId: string;
  extendedThinking: boolean;
  parallelToolUse: boolean;
  computerUse: boolean;
  // ... 他の機能フラグ
}
 
const CAPABILITY_MATRIX: Record<string, ModelCapabilities> = {
  "claude-opus-4-6": {
    modelId: "claude-opus-4-6",
    extendedThinking: true,
    parallelToolUse: true,
    computerUse: true,
  },
  "claude-sonnet-4-6": {
    modelId: "claude-sonnet-4-6",
    extendedThinking: true,
    parallelToolUse: true,
    computerUse: false,
  },
  "claude-sonnet-4-5": {
    modelId: "claude-sonnet-4-5",
    extendedThinking: true,
    parallelToolUse: true,
    computerUse: false,
  },
  "claude-haiku-4-5-20251001": {
    modelId: "claude-haiku-4-5-20251001",
    extendedThinking: false,
    parallelToolUse: true,
    computerUse: false,
  },
};
 
function buildRequestForModel(
  modelId: string,
  basePrompt: string,
  tools: Anthropic.Messages.Tool[]
): Anthropic.Messages.MessageCreateParams {
  const caps = CAPABILITY_MATRIX[modelId];
  return {
    model: modelId,
    max_tokens: 4096,
    messages: [{ role: "user", content: basePrompt }],
    tools,
    ...(caps.extendedThinking ? { thinking: { type: "enabled", budget_tokens: 16384 } } : {}),
  };
}

function isModelUnavailable(err: unknown): boolean {
  // 同上
}
 
function isAccountTierViolation(err: unknown): boolean {
  if (!err || typeof err !== "object") return false;
  const e = err as { error?: { message?: string } };
  const msg = e.error?.message?.toLowerCase() ?? "";
  return (
    msg.includes("not authorized") ||
    msg.includes("tier") ||
    msg.includes("upgrade your account")
  );
}

interface ModelInvocationLog {
  timestamp: string;
  intendedModel: string;
  actualModel: string;
  fallbackDepth: number;       // 0 = primary, 1 = first fallback...
  taskType: string;
  outcome: "success" | "all_failed";
}
 
async function loggedInvoke(/* ... */) {
  // invokeWithFallback の中で、最終的にどのモデルで成功したかをログに残す
  const log: ModelInvocationLog = { /* ... */ };
  await appendInvocationLog(log);
}