How to build an AI Vendor Follow-Up Agent in n8n: step-by-step guide with screenshots

Step 1: Go to n8n.io

In the last lesson you built a Custom GPT that writes procedures. That is great for one person. But what if the AI wrote a follow-up for every late vendor order and logged it in a spreadsheet your team can track? That is what we are building now. First, you need an n8n account.

Step 2: Enter your email

This is just practice, so anything works — it won't actually send anything.

Step 3: Sign up

In the real n8n, it sends you a six digit code, then asks for a name, a password, and an account name. Go do that in your own browser later, then come back and we will build.

Step 4: Add credit first

Your agent needs a brain, and that means an API key from OpenAI. Go to platform dot openai dot com — that is the developer side, not ChatGPT, and it bills completely separately. Before you make a key, put some money on the account. A brand new key with a zero balance just returns errors.

Step 5: Ten dollars is plenty

Ten dollars goes a very long way. You are charged per request, not per month, and each batch of follow-ups costs fractions of a cent.

Step 6: Now the key

Balance sorted. Now go to API keys in the left menu.

Step 7: Create a secret key

Step 8: Name the key

Name it after whatever is going to use it. Future you, staring at a list of forty keys, will thank you. I am calling this one n8n agent.

Step 9: Create it

Default project, all permissions — that is fine for this. Create the key.

Step 10: Copy it now

Now pay attention, because this is the only time you will ever see this key. Copy it and paste it into n8n, or into your password manager. If you lose it, you do not get it back — you delete it and make another.

Step 11: Back to n8n

One last warning — this key can spend your money. Never commit it to code, never paste it into a chat, and if it ever shows up on screen while you are recording, revoke it before you publish.

Step 12: Start a new workflow

Now you are inside n8n. This overview page lists every workflow you have. Top right, click create workflow, and you land on an empty canvas.

Step 13: Add the first step

Every workflow needs a trigger — the thing that kicks it off. We want a chat trigger so you can tell the agent which orders are late or pending.

Step 14: Trigger on a chat message

A panel slides in with every way a workflow can start — manually, on a schedule, on a webhook. We want on chat message, so we can have a conversation with the agent.

Step 15: Back to the canvas

n8n opens the node so you can configure it. There is nothing we need to change here, so close it with the X in the top right and get back to the canvas.

Step 16: Add the next node

Now we decide what happens when a message comes in.

Step 17: Add the AI Agent

There it is — AI Agent. This is the piece that can plan and then carry out actions using whatever tools you give it. You give it your open orders, and it can write the follow-ups and save them to a spreadsheet — all in one step.

Step 18: Open the options

Right now this agent has no personality and no orders. It does not know it works for Ridgeline Supply.

Step 19: Add a system message

Pick system message. This is the standing instruction the agent reads before every request. It is where we tell it who Ridgeline Supply is, how the messages should sound, and how to format them.

Step 20: Write the system prompt

Here is the actual prompt. It describes Ridgeline Supply, sets a polite but firm tone, and tells the agent what to return: vendor, order, issue, follow-up message, and due date. It also tells the agent to use the Google Sheets tool when you ask it to save.

You are the purchasing follow-up assistant for Ridgeline Supply Co.

Ridgeline Supply Co. is a small warehouse and distribution business that buys pallets, boxes, stretch wrap and janitorial supplies from vendors and ships them to local businesses.

The audience is vendor sales reps and customer service teams.

The tone should be polite, plain, firm and brief. Sound like an operations manager who values the relationship but needs an answer.

Your job is to write a follow-up message for each late or pending purchase order the user gives you.

For each message:
- Name the PO number and what was ordered
- State the problem in one sentence
- Ask one clear question: a ship date, a tracking number, or a fix
- Give a reply-by date: 1 business day if late, 2 if pending

For each follow-up, return:
- Vendor
- Order
- Issue
- Follow-up Message
- Due Date

When the user asks you to save the follow-ups, use the Google Sheets tool to store each one.

Do not threaten to cancel orders or invent penalties. Keep each message under 80 words.

Step 21: Back to the canvas

That gives the agent everything it needs to know about Ridgeline Supply. Close the node.

Step 22: Give the agent a brain

See that red triangle on the agent? It is complaining because it has no model. An agent can plan, but it cannot think without a language model underneath it.

Step 23: Pick OpenAI

OpenAI chat model. This is where that API key finally gets used.

Step 24: Open the credentials

And here is where that key you copied finally goes.

Step 25: Create a new credential

You want create new credential, right at the bottom.

Step 26: Paste the key

This is the one moment it matters that you copied it — remember, OpenAI never shows it to you again.

Step 27: Save the credential

n8n tests the connection straight away, and from now on this credential is available to every node in every workflow — you only do this once.

Step 28: Open the model list

Now the model dropdown.

Step 29: Choose the model

GPT five mini. Fast, cheap, and plenty for short follow-up messages. Do not reach for the expensive model until you have a reason.

Step 30: Back to the canvas

That is the brain wired in. Close it up.

Step 31: Add memory

Next, memory. Without this, every message starts from nothing — you could not say save those follow-ups, because it would not remember which ones you meant.

Step 32: Simple Memory

Simple memory is fine here — it keeps the recent conversation inside n8n, no database, no extra credentials.

Step 33: Add a tool

And now the good part — tools. This is what separates an agent from a chatbot. A chatbot just talks. An agent does things. We are going to give it Google Sheets so it can log every follow-up in a spreadsheet your team can track.

Step 34: Add Google Sheets

Google Sheets tool. Now the agent can write rows into a real spreadsheet, with columns matching what the system prompt tells it to return. When you say save those follow-ups, each one becomes a row.

Step 35: Back to the canvas

Before we set this up, we need to actually create the spreadsheet. Close this node for now — we are going to hop over to Google Sheets and make one.

Step 36: Open Google Sheets

Switch to Google Sheets. You need to create the spreadsheet that this node will write to.

Step 37: Name the spreadsheet

First, name it so you can find it later.

Step 38: Type the first column header

Now set up the column headers. These need to match what the system prompt tells the agent to return. In A1, type Vendor.

Step 39: Order

B1 — Order.

Step 40: Issue

C1 — Issue.

Step 41: Follow-up Message

D1 — Follow-up Message.

Step 42: Due Date

And E1 — Due Date. Five columns, matching exactly what the system prompt tells the agent to return. Each follow-up becomes a row in this sheet.

Step 43: Back to n8n

The spreadsheet is ready. Switch back to n8n and connect the Google Sheets node to it.

Step 44: Open the Sheets node

Double click the Google Sheets node to open it back up. Now we can connect it to the spreadsheet we just made.

Step 45: Add a credential

Now we need to connect this to a real Google account.

Step 46: Create a new credential

Create new credential — this one uses Google OAuth instead of an API key, so it is going to open a sign-in window.

Step 47: Sign in with Google

This opens the Google account chooser so you can pick which account to connect.

Step 48: Choose your account

Pick the Google account you want n8n to use. This is the account that owns the spreadsheet.

Step 49: Allow Sheets access

Google is asking what n8n is allowed to do. Check the first box — this lets n8n read and write your spreadsheets.

Step 50: Allow Drive access

Second box — n8n needs Drive access so it can find your spreadsheets by name.

Step 51: Allow external service

Last one — this lets Google connect to n8n as an external service. Check it.

Step 52: Sign in

All three checked.

Step 53: Pick the operation

Now we tell the node what to do. By default it reads rows, but we want to add rows.

Step 54: Append Row

Append row. Every follow-up the agent writes gets added as a new row at the bottom of the sheet.

Step 55: Choose the spreadsheet

Now pick the spreadsheet.

Step 56: Vendor Follow-Ups

Vendor Follow-Ups — the sheet we set up with columns for vendor, order, issue, message, and due date.

Step 57: Pick the sheet tab

Now pick which tab inside that spreadsheet.

Step 58: Sheet1

Sheet one — it is the only tab, so this is simple.

Step 59: Map the Vendor column

Now the columns. Each column has a purple button — click it and the AI agent figures out what data goes there.

Step 60: Map the Order column

Order. Same thing — click the purple button.

Step 61: Map the Issue column

Issue.

Step 62: Map the Follow-up Message column

Follow-up message.

Step 63: Map the Due Date column

And due date. Each one is now set to defined automatically by the model — the agent reads your column headers and figures out what goes where.

Step 64: Back to the canvas

The sheets tool is fully wired up — credential, operation, spreadsheet, sheet, and column mapping. Close it and let us test this thing.

Step 65: Open the chat

Everything is connected. Time to see if it works.

Step 66: Ask for follow-ups

Give it your late and pending orders.

Write follow-ups: Tallpine Pallet Co., PO 4471, 5 days late. Corrugo Box Supply, PO 4480, no ship date. Northwind Wrap, PO 4492, 12 rolls short.

Step 67: Send the message

Send it off. The agent reads your system prompt and writes a follow-up for each order in the exact format you asked for.

Step 68: Save to the sheet

Three follow-ups — a late order, a missing ship date, and a short shipment, each with one clear question and a reply-by date. Now tell it to save them. This is where the Google Sheets tool kicks in.

Step 69: Send it

Send it. The agent uses the Google Sheets tool to write each follow-up as a row — vendor, order, issue, message, and due date.