Can personal AI assistants read your brand’s marketing emails? We tested it

Somewhere right now, someone is asking an AI assistant, “Did any of my favorite brands send me a deal this week?” The assistant opens their Gmail, reads a few dozen promotional emails, and answers. Your beautifully designed hero banner may not be part of that answer.

Agents are becoming the inbox’s second reader. They don’t scroll, they don’t admire your typography, and they don’t all read the same parts of an email. So we set out to learn exactly what they do read.

We tested this a little obsessively

We built 37 retail emails for a fictional home brand, Magical Casa. Each one hid the same kind of member offer (a discount plus a code) in a different place. Then we connected four agents to Gmail – Muse, Grok, Claude and ChatGPT – and asked each one to find the deal.

The hiding spots ranged from obvious to absurd:

• The subject line, the body, and the HTML vs. plain-text versions of the same email
• Text styled to be invisible: display:none, zero-size fonts, pushed off screen, buried in an HTML comment
• Image alt text, link tooltips, accessibility labels, page metadata and structured data
• Offers printed inside images, or attached as .txt, .json and .png files
• Offers behind a link, behind two links, behind a redirect, or tucked into a URL parameter
• Control emails with no offer at all, to see who would invent one

We scored a typical human reader against the same emails for comparison. For a few link tests, we went further: unique single-use URLs and server logs, to prove which agents actually fetched the offer rather than guessing.

One honest caveat: this isn’t a benchmark. Each agent reached Gmail through its own tools, some with a browser, so a miss can be a plumbing problem as much as a reading problem. It is, however, a very good map of where offers get lost.

What the agents actually saw

Across 35 offer placements, the most thorough agent found 32. A typical human would reliably catch about 8. Only four placements reached all four agents.

Our 37-email Gmail test, Oct 7-8 2026 · 35 offer placements (2 no-offer controls not shown) · Partial finds counted as misses · Human = likely to notice in normal reading (estimate, not a tested panel)

• Agents read what humans can’t. Invisible text, alt text, accessibility labels, metadata, structured data: two of the agents pulled offers out of nearly all of it. Your “hidden” text is not hidden anymore.
• Humans see what some agents can’t. An offer printed in a banner image is the most human-friendly placement there is, and two of four agents never saw one. Linked offer pages were similar: easy for a person to click, skipped entirely by one agent.
• The universal placements are the subject line and the visible body. Plus, amusingly, text pushed off screen or shrunk to zero size, which every agent read and no human would. (Not a strategy. More on that below.)
• Nobody reads custom email headers. Zero for four.
• HTML and plain text are two different emails. One agent missed an offer that lived only in the HTML; another missed one that lived only in plain text. When the two versions carried different codes, only two agents caught both.
• The good news: nobody hallucinated a deal. When there was no offer, every agent said so.

How to talk to an agent (spoiler: just talk)

Here’s the cheeky part. After hiding offers in aria-labels, JSON-LD and CSS tricks, the most reliable way to reach every agent turned out to be… writing it down. In the email. In words.

So address the new reader directly. Put one short, plain line near the top of the email, visible to everyone:

The short version, for humans and their agents: Members get 27% off everything with code CASA27 through October 31. Free shipping over $50. Details at the link below.

That one line does more for agent visibility than any amount of clever markup. It lands in both the HTML and plain-text versions, it states the code, the terms and the deadline, and the human skimming on their phone benefits just as much.

What not to do: whisper to the agent in hidden text. Yes, most agents will read it. But invisible text aimed at an AI looks exactly like a prompt-injection attack, and the people building agents are working hard to make them ignore it. It also opens a gap between what your customer sees and what their agent tells them, which is a trust problem waiting to happen. Talk to the agent the way you talk to the customer: out loud.

The agent-ready email checklist

1. Put the offer in live text in the body. Not only in the hero image. Live body copy was the one placement every agent and every human found.
2. Say it in the subject line too, if it fits. All four agents read the subject.
3. Keep the HTML and plain-text versions in sync. Same offer, same code, in both. Agents read one, the other, or both, and mismatched versions cause confusion.
4. Write real alt text on offer images. Three of four agents read alt text; only two read the pixels. “Banner” is not alt text. “27% off with code CASA27” is.
5. Don’t make the deal a click away. If you must link, link to one plain, readable web page. No redirects, no chains, no data files. One agent hit safety blocks on redirects; another rarely followed links at all.
6. Keep anything important out of attachments and custom headers. Attachments were hit or miss. Custom headers were missed by everyone.
7. Put the offer near the top. Gmail clips long messages (around 102KB), and at least one agent hit limits reading a long email.
8. Audit what you’ve hidden. Preheaders, display:none blocks, HTML comments and metadata are all fair game for agents. Old codes, internal notes and test copy can and will surface.
9. State the full terms in one sentence. Amount, code, expiry, who qualifies. Agents pass along what they find, so give them a complete answer to pass along.
10. Treat structured data as a bonus, not a substitute. Two agents read it. Humans never see it.

Your email now has two audiences

The good news is that writing for agents mostly means writing clearly for people. An offer in live text, near the top, in one complete sentence works for the customer skimming on a phone and for the assistant summarizing their inbox at 7am.

The bigger shift is that more of your customers will hear about your offer secondhand, from an agent that read the email for them. Design for that reader too, or it will answer without you.

Have you run your own tests, or found a clever way to talk to agents in email? Tell me in the comments.