Build an air-quality agent that picks the right HDB purifier using MCP

Singapore's PSI can swing from 20 to 200 in a single afternoon. Most people buy an air purifier that's the wrong size for their room, or that costs 40% more than they need to.

This post shows how to build a small AI agent that recommends the right air purifier for your specific HDB room and budget — using live pricing data from Singapore merchants.

The agent does three things:

1. Reads your room size (in square meters) and budget (in SGD)
2. Searches the BuyWhere MCP catalog for air purifiers that match
3. Ranks them by CADR-per-dollar and includes live merchant links

The whole thing fits in 70 lines of Python.

Why CADR-per-dollar matters more than brand

Every air purifier advertises a CADR (Clean Air Delivery Rate) measured in m³/h. For HDB rooms, the rule of thumb is:

CADR needed = room area (m²) × 8

A 12 m² bedroom needs a purifier with at least 96 m³/h CADR. A 25 m² living room needs 200 m³/h. Most people don't know this, and end up buying a purifier that's undersized — which does nothing during Haze season.

Two purifiers with the same price can have wildly different CADR. The right comparison is CADR per dollar, not price.

What the agent looks like

The MCP server exposes a search_products tool. Our agent just calls it with the right query and filters the response.

import json
import subprocess
from pathlib import Path

def search_buywhere(query: str, country: str = "SG", limit: int = 30) -> list:
    """Search BuyWhere MCP and return priced products."""
    payload = {
        "jsonrpc": "2.0",
        "id": 1,
        "method": "tools/call",
        "params": {
            "name": "search_products",
            "arguments": {
                "query": query,
                "country": country,
                "limit": limit,
                "min_price": 50,
                "max_price": 2000,
            },
        },
    }
    result = subprocess.run(
        ["curl", "-s", "-X", "POST",
         "https://mcp.buywhere.ai/mcp",
         "-H", "Content-Type: application/json",
         "-d", json.dumps(payload)],
        capture_output=True, text=True, check=True,
    )
    return json.loads(result.stdout)["result"]["data"]

def recommend(room_m2: float, budget_sgd: float) -> dict:
    """Pick the best CADR-per-dollar air purifier for an HDB room."""
    needed_cadr = room_m2 * 8
    products = search_buywhere("air purifier HEPA")
    scored = []
    for p in products:
        cadr = p.get("cadr_m3h") or 0
        price = p["price_sgd"]
        if cadr >= needed_cadr and price <= budget_sgd:
            scored.append({
                "title": p["title"],
                "merchant": p["merchant"],
                "price_sgd": price,
                "cadr_m3h": cadr,
                "cadr_per_dollar": round(cadr / price, 2),
                "link": f"https://buywhere.ai/r/direct/{p['id']}",
            })
    scored.sort(key=lambda x: x["cadr_per_dollar"], reverse=True)
    return {
        "needed_cadr": needed_cadr,
        "recommendations": scored[:5],
    }

if __name__ == "__main__":
    import sys
    room = float(sys.argv[1]) if len(sys.argv) > 1 else 12
    budget = float(sys.argv[2]) if len(sys.argv) > 2 else 800
    print(json.dumps(recommend(room, budget), indent=2))

Save as purifier_agent.py. Run with python purifier_agent.py 12 800 and you'll get the top 5 air purifiers sized for a 12 m² HDB room at S$800 — ranked by CADR per dollar.

Why this works

The agent has three properties that a static comparison page can't:

1. Always-live data. Every call hits the current BuyWhere catalog. Prices, stock, and merchant count update without code changes.
2. Personalised recommendations. Same agent, different rooms → different recommendations. No manual curation.
3. Composable. You can extend it with PSI data, weather APIs, or your existing smart-home agent.

Production notes

• Add a 5-minute cache if you're hitting MCP more than once per user session.
• The BuyWhere MCP server enforces rate limits per API key; sign up at mcp.buywhere.ai to get a key.
• For production, swap subprocess.run(curl) for httpx.AsyncClient — the JSON-RPC schema is identical.

What to ship next

If you're building a real estate or smart home product, you can wrap this agent in a Telegram bot, a Discord slash command, or a Webflow embed. The agent is the backend — the chat surface is up to you.

The point isn't "use AI for shopping." The point is: live, structured product data is what makes AI agents useful for commerce, and the MCP pattern is the simplest way to wire it up.

Try it on your own HDB room and tell me what you get.