Building your first Claude Code agent: an Australian small business case study

Most “first agent” tutorials are toys. This one is the agent we actually run for an Australian Shopify skincare brand every night at 23:00 AEST. It pulls inventory state from Shopify, runs sanity checks (stock at zero, prices changed, products with no image), and posts a summary to a Slack channel.

End-to-end build below. You can adapt this template to any nightly-audit workflow.

What the agent does

Every night at 23:00 AEST:

1. Connects to the Shopify Admin API via MCP
2. Pulls every product + inventory level (cached from Shopify, fast)
3. Runs three checks:

• Products with stock at zero or below
• Products with no image
• Products whose price changed in the last 24 hours

4. Posts a summary to Slack channel #overnight-inventory
5. If anything urgent (stock zero for a top-10 SKU), DMs the operations lead

If the run is clean, it posts a one-line “All clear” so we know it ran. If it failed entirely (Shopify down, Anthropic API hiccup), we get a separate alert via UptimeRobot watching the Hetzner-side log file.

Prerequisites

• Claude Code installed (we run on a Hetzner Ubuntu 24.04 VPS, same as our Telegram bot)
• Anthropic API key with at least $20 USD credit and a budget alert at $40
• Shopify custom app with read scopes for products, orders, inventory
• A Slack incoming webhook URL
• 90 minutes

Step 1: directory + Claude config

mkdir -p /opt/agents/skincare-inventory
cd /opt/agents/skincare-inventory

CLAUDE.md:

# Agent: nightly inventory audit

You are an unattended agent. You run once at 23:00 AEST every night.
You have no human in the loop. Speak in clear, concise prose suitable for Slack.

## Mission
1. Pull the client's current inventory state via Shopify MCP.
2. Run the three checks defined in checks.md.
3. Post a Slack summary via the slack_webhook tool.
4. Exit cleanly.

## Constraints
- Australian English.
- AUD pricing.
- Do not make changes to Shopify (read-only).
- If you cannot reach Shopify or Slack, write the error to error.log and exit 1.
- Do not call any tool more than 3 times for the same purpose.

## Output format
Slack message:
**🌙 client overnight audit, DD/MM/YYYY**
- Out of stock: <list of SKUs or "none">
- No image: <list or "none">
- Price changed in last 24h: <list with old → new or "none">

End with "All checks complete." or "Issues found, investigate."

checks.md:

# Inventory audit checks

## Check 1: Out of stock
For every product in the store, find any variant where inventory_quantity <= 0 AND track_inventory is true. List SKU + product title.

## Check 2: No image
For every active product, check that images.length > 0. List any with zero images.

## Check 3: Price changed
For every product, compare current price to the price recorded in shopify-prices-yesterday.json (created by the previous run). List any that changed, with the previous and new price in AUD.

After audit, write today's prices to shopify-prices-today.json (the wrapper script renames it to -yesterday.json).

Step 2: MCP config

.mcp.json:

{
 "mcpServers": {
 "shopify": {
 "command": "npx",
 "args": ["-y", "@shopify/mcp-server-storefront@latest"],
 "env": {
 "SHOPIFY_STORE_DOMAIN": "your-store.myshopify.com",
 "SHOPIFY_ACCESS_TOKEN": "shpat_REDACTED"
 }
 },
 "slack": {
 "command": "npx",
 "args": ["-y", "@modelcontextprotocol/server-slack-webhook"],
 "env": {
 "SLACK_WEBHOOK_URL": "https://hooks.slack.com/services/..."
 }
 }
 }
}

Step 3: settings + permissions

.claude/settings.json:

{
 "permissions": {
 "allow": [
 "mcp:shopify:*",
 "mcp:slack:post",
 "Read",
 "Write(*.json)",
 "Bash(date:*)"
 ],
 "deny": [
 "Bash(rm:*)",
 "Bash(curl:*)",
 "WebFetch"
 ]
 }
}

Only the tools the agent needs. Explicit deny on dangerous shell commands and on WebFetch (everything goes through MCP).

Step 4: the wrapper script

run.sh:

#!/usr/bin/env bash
set -euo pipefail
cd /opt/agents/skincare-inventory

# Rotate prices file (today → yesterday) for the next run's diff
[ -f shopify-prices-today.json ] && mv shopify-prices-today.json shopify-prices-yesterday.json

# Cap turns + tokens to prevent runaway
export ANTHROPIC_MAX_TOKENS=120000

# Run headless. --print exits after one cycle of agentic work.
claude --print --max-turns 25 "$(cat <<'EOF'
Run the nightly inventory audit per CLAUDE.md. Use the checks defined in checks.md.
Post the result to Slack. Write today's prices to shopify-prices-today.json.
EOF
)" > last-run.log 2>&1 || {
 echo "Inventory audit failed at $(date)" >> error.log
 curl -s -X POST $SLACK_WEBHOOK_URL -H 'Content-Type: application/json' \
 -d '{"text":"⚠️ the nightly inventory audit FAILED, check /opt/agents/skincare-inventory/error.log"}'
 exit 1
}

chmod +x run.sh.

Step 5: cron entry

crontab -e

Add:

# the nightly inventory audit, 23:00 AEST nightly (13:00 UTC during AEDT, 13:00 UTC during AEST too if server is in UTC)
0 13 * * * /opt/agents/skincare-inventory/run.sh

The server is in UTC. 23:00 Melbourne time is 13:00 UTC during AEDT and 13:00 UTC during AEST (Melbourne uses AEDT October-April, AEST otherwise). For client deployments we accept the one-hour drift twice a year; if you need precision, use a wrapper that calculates the local-time offset.

Step 6: smoke test

./run.sh

You should see a Slack message in #overnight-inventory within 60-90 seconds. If you don’t, check last-run.log, it’ll show what Claude actually did.

Step 7: budget guardrails

In Anthropic Console:

• Set a hard spending cap of $30 USD/month on the API key this agent uses.
• Email alert at 50% and 80%.
• Use a dedicated API key for this agent so spend is attributable.

What it costs to run

Each nightly run does about 15-30 tool calls, ~40k input tokens, ~5k output. With caching, that’s roughly $0.08 AUD per run. 30 runs/month = $2.40 AUD. Real bills: we average $4-6 AUD/month including a few re-runs and occasional poking.

Common failures + fixes

• Shopify rate limit. Reduce concurrent product reads in your prompt. Tell Claude to fetch in batches of 50.
• Slack webhook silently rejected. Check the webhook URL is valid; Slack rotates these without warning if compromised.
• Claude hits the turn cap. Increase --max-turns cautiously. Or tighten the prompt so the work fits.
• Output drifts in format. Reinforce the format requirement in CLAUDE.md and add an example.

Where to take this next

Once the inventory agent is reliable, the natural follow-ons are:

• A morning agent that summarises overnight orders + flags any suspicious ones
• A weekly agent that runs a cohort revenue analysis and writes the result to a Notion page
• A monthly agent that drafts the marketing report from Meta Ads + Shopify data

Same template each time: CLAUDE.md → MCP → settings → wrapper → cron. The first one is the hardest; subsequent agents take 30-45 minutes.

If you want us to build the first one for you and then teach you the pattern, that’s literally the Quick Start package.