One of the latest mitigations is to make sure that a URL an agent visits has been indexed by a search engine crawler. At least that is what OpenAI does now in ChatGPT.
That makes sure that not a large amount of private data is leaked in one request. Assuming that if a URL is indexed, it is public data. However, there are still bypasses with using many requests to leak information, like a request per character of pre-indexed URLs.
I have some demos of doing that on my blog, but it makes it more involved for an attacker. And that could also be detected. Still not perfect, but a solid improvement, for a generic agent like ChatGPT.
There is paper OpenAI wrote a few months ago that explains how they do it:
https://embracethered.com/blog/posts/2026/data-exfiltration-...
It's not a 100% bullet proof approach either, but pretty good.
Regarding the point on using URLs returned from trusted tool calls. That is similar to using pre-indexed URLs: If a "trusted tool" includes things like read a document, read an email,... an attacker can return a large list of afterwards "safe" urls, like 26 to cover A-Z. And then an attack can render many requests, e.g. character by character. But, again, similar to the pre-indexing, things are getting more a lot more expensive for an attacker that way. However, still not impossible.
For agents that have a specific purpose simple domain allow-listing is also a pretty effective idea in to prevent attacker controlled endpoints.