The Smallest Stack Your Agent Needs

# --- Pick your provider (set the matching API key env var) ---
# Anthropic:  from anthropic import Anthropic; client = Anthropic()        # ANTHROPIC_API_KEY
# OpenAI:     from openai import OpenAI;       client = OpenAI()           # OPENAI_API_KEY
# Gemini:     from google import genai;        client = genai.Client()     # GEMINI_API_KEY

# main.py — single-file agentic service
import os
from fastapi import FastAPI, Request
from anthropic import Anthropic
import redis, psycopg

app = FastAPI()
client = Anthropic()
r = redis.from_url(os.environ["REDIS_URL"])
db = psycopg.connect(os.environ["DATABASE_URL"])

TOOLS = [{"name": "search_docs", "description": "...", "input_schema": {...}}]

@app.post("/api/chat")
async def chat(req: Request):
    body = await req.json()
    user_id, message = body["user_id"], body["message"]

    # rate limit
    if not allow(user_id, limit=60, window=60):
        return {"error": "rate_limited"}, 429

    # run agent
    messages = [{"role": "user", "content": message}]
    for _ in range(4):
        resp = client.messages.create(model="claude-sonnet-4-6", max_tokens=800, tools=TOOLS, messages=messages)
        messages.append({"role": "assistant", "content": resp.content})
        if resp.stop_reason == "end_turn":
            answer = resp.content[0].text
            break
        # ... tool dispatch
        ...

    # audit
    with db.cursor() as cur:
        cur.execute("INSERT INTO traces (user_id, query, answer) VALUES (%s, %s, %s)", (user_id, message, answer))
    db.commit()

    return {"answer": answer}

# Deployed on Fly.io with one command:  fly launch
# Postgres via Supabase, Redis via Upstash, free tiers on both.
# That's the Day 0 stack. Don't add until you must.