Handling of $_GET parameters

I've inherited a huge, complex, badly written website that has tens of thousands of users. It uses Varnish, and has ESI for most blocks.

There is a situation where an article page is displayed but the content of a block is controlled by the previous page providing a "context". Like this:

/article/99999/article-title?context="AAA:BBB"

This is because the article could be linked from various sources and we need to know which one in order to decide what to put in the block.

Which worked fine before we implemented ESI -and now doesn't.

As far as I can tell, with ESI, the $_GET parameters are not provided - in other words this cannot work.

Is that right?