気象庁の天気予報JSONをAIエージェントで使う：地域コード・予報の構造・利用条件

class20s 1310100 千代田区
  -> class15s 130011 ２３区西部
  -> class10s 130010 東京地方      ← 予報JSONの areas で使うコード
  -> offices  130000 東京都        ← forecast/130000.json

class20s 0822000 つくば市
  -> class15s 080022 県南地域
  -> class10s 080020 南部
  -> offices  080000 茨城県        ← forecast/080000.json

[0] 3日分   publishingOffice, reportDatetime, timeSeries (3)
    timeSeries[0]  timeDefines 3つ  areas: class10  weatherCodes, weathers, winds, waves
    timeSeries[1]  timeDefines 5つ  areas: class10  pops
    timeSeries[2]  timeDefines 2つ  areas: 地点     temps
[1] 週間     publishingOffice, reportDatetime, timeSeries (2), tempAverage, precipAverage
    timeSeries[0]  timeDefines 7つ  areas: 週間区域  weatherCodes, pops, reliabilities
    timeSeries[1]  timeDefines 7つ  areas: 地点     tempsMin, tempsMinUpper, tempsMinLower,
                                                     tempsMax, tempsMaxUpper, tempsMaxLower

{
  "area": { "name": "東京地方", "code": "130010" },
  "weatherCodes": ["200", "302", "302"],
  "weathers": [
    "くもり　所により　夜　雨",
    "雨　時々　くもり　所により　雷　を伴う",
    "雨　時々　くもり"
  ],
  "winds": ["南の風　やや強く", "南の風　２３区西部　では　南の風　やや強く", "北東の風　後　やや強く"],
  "waves": ["１．５メートル", "１メートル　後　０．５メートル", "０．５メートル　後　１メートル"]
}

{ "area": { "name": "東京地方", "code": "130010" }, "pops": ["30", "60", "50", "50", "50"] }

// jma-weather.mjs — 依存なし（Node.js 18 以降の組み込み fetch）
// 使い方: node jma-weather.mjs つくば市
import { readFile, writeFile, mkdir, stat } from 'node:fs/promises';

const UA = 'jma-weather-example/0.1 (+https://codeagent.jp/)';
const BASE = 'https://www.jma.go.jp/bosai';
const AREA_URL = `${BASE}/common/const/area.json`;
const AREA_CACHE = './cache/area.json';
const AREA_TTL_MS = 24 * 60 * 60 * 1000; // 地域コード表は1日キャッシュ

async function getJson(url) {
  const res = await fetch(url, { headers: { 'User-Agent': UA } });
  const type = res.headers.get('content-type') ?? '';
  // 存在しないコードは 404 の HTML が返るので、content-type も確認する
  if (!res.ok || !type.includes('json')) {
    throw new Error(`取得失敗: ${url} -> HTTP ${res.status} (${type})`);
  }
  return res.json();
}

async function loadArea() {
  try {
    const s = await stat(AREA_CACHE);
    if (Date.now() - s.mtimeMs < AREA_TTL_MS) {
      return JSON.parse(await readFile(AREA_CACHE, 'utf8'));
    }
  } catch { /* キャッシュなし */ }
  const body = await getJson(AREA_URL);
  await mkdir('./cache', { recursive: true });
  await writeFile(AREA_CACHE, JSON.stringify(body));
  return body;
}

// 市区町村名（class20s）から候補を返す。複数あっても自動で1件に絞らない
function resolveArea(area, name) {
  const out = [];
  for (const [code, c20] of Object.entries(area.class20s)) {
    if (c20.name !== name) continue;
    const c15 = area.class15s[c20.parent];
    const c10 = area.class10s[c15.parent];
    out.push({
      class20: code, name: c20.name,
      class10: c15.parent, class10Name: c10.name,
      office: c10.parent, officeName: area.offices[c10.parent].name,
    });
  }
  return out;
}

const blank = (v) => (v === '' ? null : v); // 空文字を 0 や "0" にしない

const input = process.argv[2];
const area = await loadArea();
const candidates = resolveArea(area, input);
if (candidates.length !== 1) {
  console.log(JSON.stringify({
    status: candidates.length ? 'ambiguous' : 'not_found', input, candidates,
  }, null, 2));
  process.exit(0);
}

const target = candidates[0];
const forecastUrl = `${BASE}/forecast/data/forecast/${target.office}.json`;
const overviewUrl = `${BASE}/forecast/data/overview_forecast/${target.office}.json`;
const retrievedAt = new Date().toISOString();
const [short, weekly] = await getJson(forecastUrl);
const overview = await getJson(overviewUrl);

const [weatherTs, popTs] = short.timeSeries;
const w = weatherTs.areas.find((a) => a.area.code === target.class10);
const p = popTs.areas.find((a) => a.area.code === target.class10);
if (!w || !p) throw new Error(`予報JSONに ${target.class10} が見つからない`);
const wk = weekly?.timeSeries[0].areas.find((a) => a.area.code === target.class10);

console.log(JSON.stringify({
  status: 'ok',
  input,
  resolved: target,
  data: {
    area: w.area,
    weather: weatherTs.timeDefines.map((time, i) => ({ time, code: w.weatherCodes[i], text: w.weathers[i] })),
    pops: popTs.timeDefines.map((time, i) => ({ time, pop: blank(p.pops[i]) })),
    weekly: wk
      ? { matched: true, days: weekly.timeSeries[0].timeDefines.map((time, i) => ({ time, code: wk.weatherCodes[i], pop: blank(wk.pops[i]), reliability: blank(wk.reliabilities[i]) })) }
      : { matched: false, weeklyAreas: weekly?.timeSeries[0].areas.map((a) => a.area) ?? [] },
    overviewHead: overview.text.trim().slice(0, 40),
  },
  provenance: {
    publisher: short.publishingOffice,
    reportDatetime: short.reportDatetime,
    overviewReportDatetime: overview.reportDatetime,
    retrievedAt,
    sources: [forecastUrl, overviewUrl, AREA_URL],
    attribution: '出典：気象庁ホームページ（https://www.jma.go.jp/bosai/forecast/）を加工して作成',
    warnings: ['気象庁ホームページが内部で使うJSON。API仕様としては公開されておらず、予告なく変わりうる'],
  },
}, null, 2));

$ node jma-weather.mjs 府中市
{ "status": "ambiguous", "input": "府中市",
  "candidates": [
    { "class20": "1320600", "name": "府中市", "class10": "130010", "class10Name": "東京地方", "office": "130000", "officeName": "東京都" },
    { "class20": "3420800", "name": "府中市", "class10": "340010", "class10Name": "南部", "office": "340000", "officeName": "広島県" } ] }

$ node jma-weather.mjs つくば市
"resolved": { "class20": "0822000", "class10": "080020", "class10Name": "南部", "office": "080000", "officeName": "茨城県" }
"weather": [
  { "time": "2026-09-14T17:00:00+09:00", "code": "111", "text": "晴れ　夜　くもり　所により　夜遅く　雨" },
  { "time": "2026-09-15T00:00:00+09:00", "code": "300", "text": "雨　所により　雷　を伴う" },
  { "time": "2026-09-16T00:00:00+09:00", "code": "302", "text": "雨　時々　くもり" } ]
"pops": [ { "time": "2026-09-14T18:00:00+09:00", "pop": "20" }, … { "time": "2026-09-15T18:00:00+09:00", "pop": "70" } ]
"weekly": { "matched": false, "weeklyAreas": [ { "name": "茨城県", "code": "080000" } ] }
"provenance": {
  "publisher": "水戸地方気象台",
  "reportDatetime": "2026-09-14T17:00:00+09:00",
  "overviewReportDatetime": "2026-09-14T16:36:00+09:00",
  "retrievedAt": "2026-09-14T12:39:52.415Z",
  "sources": [ ".../forecast/data/forecast/080000.json", ".../overview_forecast/080000.json", ".../common/const/area.json" ] }

- 天気の回答は get_jma_forecast の結果だけを根拠にし、reportDatetime と出典を併記する
- 地域の候補が複数なら候補を示して止まる（推測で選ばない）
- 値が null の日は「発表なし」と書き、前後の日から補わない
- 取得結果にない予想や、独自の警戒の呼びかけを作らない
- 出典は「出典：気象庁ホームページ（URL）を加工して作成」の形で残す