There is a quiet clause in the 1 September 2026 AI Max migration that catches the advertisers who thought they were being conservative. If a Search campaign uses automatically created assets (ACA) — the feature that writes extra headlines and descriptions for you — Google upgrades it to AI Max with search term matching switched on by default. Search term matching is AI Max's query expansion: it reaches searches your keyword list does not contain. So a campaign you configured only to generate ad copy comes out of the migration matching on queries you never signed up for, and the first sign is a search terms report full of terms no keyword of yours would have triggered.
This is not the broad-match story everyone has been bracing for. Plenty of accounts deliberately avoided campaign-level broad match and assumed that kept them clear of the AI Max reach changes. ACA is the second, less-discussed trigger — and it flips on the same expansion behaviour through a different door. This post is about that specific door: why enabling only automated assets now means inheriting query expansion, where the resulting leak shows up, and how to decide whether to keep the setting or turn it off. It sits alongside the AI Max migration checklist, but narrows to the one behaviour change that surprises people most.
Two independent triggers, one upgrade
The September auto-upgrade fires on either of two settings: campaign-level broad match, or standalone automatically created assets. They are independent. A campaign needs only one of them to be in scope, which is why "I do not use broad match" is not the same as "I am not affected." A campaign built around exact and phrase keywords, with ACA switched on to pad the ad copy, is upgraded to AI Max exactly like a broad-match campaign is — and Google's own migration guidance is explicit that ACA campaigns are converted with search term matching enabled by default.
That default is the whole problem. The advertiser opted into a copywriting convenience and, at migration, receives a reach change bundled with it. The two features have nothing to do with each other in terms of risk: one decides what your ad says, the other decides who sees it. Yet the upgrade treats a yes on the first as a yes on the second. If you run a portfolio of accounts, assume every campaign with ACA attached is on this path, and check the ones you least expect — the tidy, exact-match campaigns you would never have described as "expanded" are the ones most likely to behave differently after the flip.
What search term matching actually does
Search term matching is AI Max's name for keywordless query expansion. Google describes it as using AI, including broad-match and keywordless technology, to find relevant queries beyond your existing keywords, learning from your keywords, creatives and landing-page URLs. In practice it behaves like a broad-match layer that sits on top of whatever keywords you have, widening the set of queries that can trigger your ads. For a campaign that was previously matching only on exact and phrase keywords, that is a categorical change in traffic composition, not a minor tuning knob.
The consequence for search-term hygiene is direct: your account starts serving on intent it never served on before. Some of that new reach may be genuinely incremental — queries you would have wanted and never thought to add. Much of it is the usual expansion tax: adjacent products, research intent, wrong audiences, and the close-variant drift that already eats a chunk of most budgets. The difference is that you did not opt into it as a targeting decision; it arrived as a side effect of a copy setting. If you manage to a tight cost-per-acquisition, the new queries can move your blended numbers before you have looked at a single report.
Where the leak shows up
The leak surfaces in exactly one place you already know: the search terms report. The good news about AI Max — and the reason it is easier to control than Performance Max — is that it retains full search terms report visibility, so every query search term matching reaches is auditable. Nothing hides. After a campaign migrates, set the report's date range to the days since the upgrade and scan for queries that do not map to any keyword you added. Those unmatched rows are the query-expansion footprint, and they are where the new spend is going.
Read that footprint the way you read any waste: sort by cost descending, filter to zero conversions, and look for themes rather than one-off strings. The queries expansion pulls in tend to cluster — a wrong-product word, a free-tier word, a job-seeker word — which is why grouping them into patterns beats chasing individual terms. If you had a pre-migration baseline of what your campaign normally served on, the contrast makes the new intent obvious; if you did not, the unmatched-to-keyword filter is the fastest proxy for "this is what expansion added."
Deciding whether to keep it or switch it off
Search term matching is a campaign setting, and it can be turned off after migration. Whether you should is an account-level judgement, not a blanket rule. The setting is defensible when you have a clean, well-structured negative list already doing its job and you want the incremental reach — AI Max plus strong negatives is a workable combination precisely because the report stays visible. It is a liability when your account is already leaking on close variants and adjacent intent, because expansion adds a new stream of exactly that, and you will spend the following weeks negating queries you never wanted the campaign to see.
If you disable it, the campaign reverts to matching on your keywords and their close variants — the behaviour you had before, minus the expansion. If you keep it, treat the decision as a commitment to a tighter review cadence, because you have widened the mouth of the funnel and the negatives are now doing more work. The wrong move is to do neither: to let the default ride unexamined, discover the extra spend a month later, and mistake an inherited setting for a targeting problem. Make the call on purpose, then hold the setting accountable in the report.
Before and after the flip
Before your campaigns migrate, inventory which ones have ACA attached — those are the ones that will inherit search term matching, independent of their keyword match types. Confirm your account and shared negative lists are actually applied to them, because a widened match surface with no negative floor is the worst-case version of this change. The pre-launch discipline of building the negative list before the traffic arrives applies here too: the expansion is coming on a known date, so the defensive list should be in place before it does, not assembled reactively from the wreckage.
After the flip, schedule a report review for the days immediately following each campaign's upgrade rather than waiting for your normal monthly pass. This is the same reasoning behind the post-migration negative audit: the first week after expansion turns on is when the new, un-negated intent spends fastest and cheapest to fix. Catch the wasteful themes early, add them at the right level, and the campaign settles into the familiar containment loop — read the report, negate the junk, repeat — just with a wider net to manage than the one you originally built.
The bottom line
The one-sentence version: opting into automated assets now means opting into query expansion at migration, whether or not you ever touched broad match. The two features do different jobs — one writes copy, one widens reach — but the September upgrade bundles them, and the reach half is the one that changes your search terms report. If you do nothing else before 1 September, list your ACA campaigns and decide, per campaign, whether search term matching earns its place.
None of this makes AI Max a black box. Unlike Performance Max, it keeps the report intact, so the expansion is fully visible and fully controllable — the risk is not opacity, it is inattention. The advertisers who get hurt are the ones who assumed a copy setting was a copy setting and never looked. Treat the migration as a prompt to re-audit your negatives and your match surface together, using the search terms report as the source of truth, and the inherited default becomes a deliberate choice instead of a surprise on next month's invoice.