GAS×Gemini API実践｜メール返信下書きを自動生成する方法

function processNewEmails() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var settingsSheet = ss.getSheetByName("設定");
  var lastRunRaw = settingsSheet.getRange("B1").getValue();
  var labelName = settingsSheet.getRange("B2").getValue() || "AI下書き";
  var myEmail = Session.getActiveUser().getEmail();
  var lastRun;

  if (lastRunRaw) {
    lastRun = new Date(lastRunRaw);
  } else {
    lastRun = new Date(Date.now() - 24 * 60 * 60 * 1000);
  }

  var excludeSheet = ss.getSheetByName("除外");
  var excludeData = excludeSheet.getDataRange().getValues();
  excludeData.shift();

  var patternSheet = ss.getSheetByName("パターン");
  var patternData = patternSheet.getDataRange().getValues();
  patternData.shift();

  var logSheet = ss.getSheetByName("ログ");
  if (!logSheet) {
    logSheet = ss.insertSheet("ログ");
    logSheet.appendRow(["実行日時", "件名", "送信者", "受信タイプ", "処理区分", "分類結果", "ドラフトURL"]);
  }

  var label = GmailApp.getUserLabelByName(labelName);
  if (!label) { label = GmailApp.createLabel(labelName); }

  var afterDate = Utilities.formatDate(lastRun, Session.getScriptTimeZone(), "yyyy/MM/dd");
  var threads = GmailApp.search("after:" + afterDate + " is:inbox");

  var processedCount = 0;
  var excludedCount = 0;
  var draftedCount = 0;
  var skippedCount = 0;
  var now = new Date();

  for (var i = 0; i < threads.length; i++) {
    var messages = threads[i].getMessages();
    var lastMsg = messages[messages.length - 1];
    if (lastMsg.getDate() <= lastRun) continue;

    var subject = lastMsg.getSubject();
    var from = lastMsg.getFrom();
    var body = lastMsg.getPlainBody();
    var to = lastMsg.getTo() || "";
    var cc = lastMsg.getCc() || "";
    var receiveType = getReceiveType(myEmail, to, cc, lastMsg);
    processedCount++;

    if (isExcluded(subject, from, receiveType, excludeData)) {
      logSheet.appendRow([now, subject, from, receiveType, "除外", "-", "-"]);
      excludedCount++;
      continue;
    }

    var matchedPattern = matchPattern(subject, receiveType, patternData);
    if (matchedPattern) {
      var prompt = matchedPattern.prompt + "\n\n---\n【件名】" + subject + "\n【送信者】" + from + "\n【本文】\n" + body;
      var replyText = callGemini(prompt);
      if (replyText) {
        createReplyDraft(lastMsg, replyText, label);
        logSheet.appendRow([now, subject, from, receiveType, "生成", matchedPattern.name, "下書き作成済"]);
        draftedCount++;
      }
      continue;
    }

    var geminiResult = classifyAndDraftWithGemini(subject, from, body, receiveType);
    if (geminiResult.needsReply && geminiResult.replyText) {
      createReplyDraft(lastMsg, geminiResult.replyText, label);
      logSheet.appendRow([now, subject, from, receiveType, "生成(Gemini分類)", geminiResult.category, "下書き作成済"]);
      draftedCount++;
    } else {
      logSheet.appendRow([now, subject, from, receiveType, "スキップ", geminiResult.category || "返信不要", "-"]);
      skippedCount++;
    }
  }

  settingsSheet.getRange("B1").setValue(now);
  Logger.log("完了: 処理=" + processedCount + " 生成=" + draftedCount + " 除外=" + excludedCount + " スキップ=" + skippedCount);
}

function getReceiveType(myEmail, to, cc, message) {
  var myEmailLower = myEmail.toLowerCase();
  var toLower = to.toLowerCase();
  var ccLower = cc.toLowerCase();

  if (ccLower.indexOf(myEmailLower) !== -1 && toLower.indexOf(myEmailLower) === -1) return "cc";
  if (toLower.indexOf(myEmailLower) === -1 && ccLower.indexOf(myEmailLower) === -1) return "bcc";
  if (toLower.indexOf(myEmailLower) !== -1) {
    var toAddresses = toLower.split(",").map(function(addr) {
      return addr.trim().replace(/.*</, "").replace(/>.*/, "");
    }).filter(function(addr) { return addr.indexOf("@") !== -1; });
    return toAddresses.length === 1 ? "to-only" : "to-multi";
  }
  return "unknown";
}

function isExcluded(subject, from, receiveType, excludeData) {
  for (var i = 0; i < excludeData.length; i++) {
    var condType = String(excludeData[i][1]).trim();
    var condValue = String(excludeData[i][2]).trim();
    if (!condType || !condValue) continue;
    switch (condType) {
      case "送信者": if (from.toLowerCase().indexOf(condValue.toLowerCase()) !== -1) return true; break;
      case "件名": if (subject.indexOf(condValue) !== -1) return true; break;
      case "受信タイプ": if (receiveType === condValue) return true; break;
    }
  }
  return false;
}

function matchPattern(subject, receiveType, patternData) {
  for (var i = 0; i < patternData.length; i++) {
    var name = String(patternData[i][0]).trim();
    var condType = String(patternData[i][1]).trim();
    var condValue = String(patternData[i][2]).trim();
    var allowedTypes = String(patternData[i][3]).trim();
    var prompt = String(patternData[i][4]).trim();
    if (!name || !condType || !condValue || !prompt) continue;
    if (condType === "件名" && subject.indexOf(condValue) === -1) continue;
    if (allowedTypes) {
      var types = allowedTypes.split(",").map(function(t) { return t.trim(); });
      if (types.indexOf(receiveType) === -1) continue;
    }
    return { name: name, prompt: prompt };
  }
  return null;
}

function classifyAndDraftWithGemini(subject, from, body, receiveType) {
  var prompt = "あなたはIT企業のプロジェクトマネージャーのメールアシスタントです。\n"
    + "以下のメールを分析し、JSON形式で回答してください。\n\n"
    + "【判断基準】\n"
    + "- 自分宛て（to-only, to-multi）の業務メールで返信が期待されている場合は返信要\n"
    + "- 通知・報告のみで返信不要と判断できる場合は返信不要\n"
    + "- 迷った場合は返信要として短い受領確認を作成\n\n"
    + "【出力JSON形式】\n"
    + "{\"needsReply\": true/false, \"category\": \"分類名\", \"replyText\": \"返信文\"}\n\n"
    + "【注意】\n- 返信文はビジネスメールとして適切な敬語で作成\n- JSON以外のテキストは出力しないでください\n\n"
    + "---\n【件名】" + subject + "\n【送信者】" + from + "\n【受信タイプ】" + receiveType + "\n【本文】\n" + body;

  var responseText = callGemini(prompt);
  if (!responseText) return { needsReply: false, category: "APIエラー", replyText: "" };

  try {
    var jsonMatch = responseText.match(/\{[\s\S]*\}/);
    if (jsonMatch) {
      var result = JSON.parse(jsonMatch[0]);
      return {
        needsReply: result.needsReply || false,
        category: result.category || "不明",
        replyText: result.replyText || ""
      };
    }
  } catch (e) {
    Logger.log("JSON解析エラー: " + e.message);
  }
  return { needsReply: false, category: "解析エラー", replyText: "" };
}

function callGemini(prompt) {
  var apiKey = PropertiesService.getScriptProperties().getProperty("GEMINI_API_KEY");
  if (!apiKey) {
    Logger.log("エラー: GEMINI_API_KEYが設定されていません");
    return null;
  }

  var url = "https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:generateContent";
  var payload = { contents: [{ parts: [{ text: prompt }] }] };
  var options = {
    method: "POST",
    contentType: "application/json",
    headers: { "x-goog-api-key": apiKey },
    payload: JSON.stringify(payload),
    muteHttpExceptions: true
  };

  var maxRetries = 3;
  for (var attempt = 1; attempt <= maxRetries; attempt++) {
    try {
      var response = UrlFetchApp.fetch(url, options);
      var statusCode = response.getResponseCode();

      if (statusCode === 200) {
        var data = JSON.parse(response.getContentText());
        return data.candidates[0].content.parts[0].text;
      }

      if (statusCode === 429) {
        var retryMatch = response.getContentText().match(/"retryDelay":\s*"(\d+)s"/);
        var waitSec = retryMatch ? parseInt(retryMatch[1]) + 5 : 60;
        Logger.log("レート制限(試行" + attempt + "/" + maxRetries + ")：" + waitSec + "秒待機");
        Utilities.sleep(waitSec * 1000);
        continue;
      }

      Logger.log("APIエラー: " + statusCode + " " + response.getContentText());
      return null;
    } catch (e) {
      Logger.log("API呼び出しエラー(試行" + attempt + "): " + e.message);
      if (attempt < maxRetries) Utilities.sleep(30 * 1000);
    }
  }

  Logger.log("リトライ上限到達。スキップします。");
  return null;
}

function createReplyDraft(message, replyText, label) {
  var draft = message.createDraftReplyAll(replyText);
  var thread = message.getThread();
  thread.addLabel(label);
  return draft;
}

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