Slack Bot を LLM で作る ─ FastAPI + OpenAI で社内ツール化

import os, hmac, hashlib, time
from fastapi import FastAPI, Request, HTTPException
from openai import OpenAI
import httpx

app = FastAPI()
openai = OpenAI()

SLACK_TOKEN = os.environ['SLACK_BOT_TOKEN']
SLACK_SECRET = os.environ['SLACK_SIGNING_SECRET']

async def verify_slack(req: Request, body: bytes):
    ts = req.headers.get('X-Slack-Request-Timestamp', '')
    if abs(time.time() - int(ts)) > 60 * 5:
        raise HTTPException(403, 'Stale timestamp')
    sig_basestring = f'v0:{ts}:{body.decode()}'.encode()
    expected = 'v0=' + hmac.new(SLACK_SECRET.encode(), sig_basestring, hashlib.sha256).hexdigest()
    if not hmac.compare_digest(expected, req.headers.get('X-Slack-Signature', '')):
        raise HTTPException(403, 'Bad signature')

@app.post('/slack/events')
async def events(req: Request):
    body = await req.body()
    await verify_slack(req, body)
    payload = await req.json()

    # Slack の URL 検証
    if payload.get('type') == 'url_verification':
        return {'challenge': payload['challenge']}

    if payload.get('event', {}).get('type') == 'app_mention':
        text = payload['event']['text']
        channel = payload['event']['channel']

        # LLM 呼び出し
        resp = openai.chat.completions.create(
            model='gpt-4o-mini',
            messages=[{'role': 'user', 'content': text}],
        )
        reply = resp.choices[0].message.content

        # Slack に投稿
        async with httpx.AsyncClient() as c:
            await c.post(
                'https://slack.com/api/chat.postMessage',
                headers={'Authorization': f'Bearer {SLACK_TOKEN}'},
                json={'channel': channel, 'text': reply},
            )

    return {'ok': True}