Something like this?

https://www.screwfix.com/p/jg-speedfit-plastic-push-fit-straight-tap-connectors-15mm-x-1-2-2-pack/29175

Interesting, I'll have to look more into this

There is usually a large, tapered rubber washer inside that big nut to seal it off.

https://www.acehardware.com/departments/plumbing/supply-lines/toilet-supply-lines/42205

So the end that connects to the toilet is flared, but it's hard PEX, so it doesn't make a real good seal

Get a replacement hose? I like the braided ones, never had any issues.

I'm noticing you aren't mentioning a shut-off valve.
That's a problem. There should definitely be a shut-off valve between a supply line and a toilet. If you haven't got one, then you need to put one there. There will be a purpose-specific connector line between the shut-off valve and the toilet that will seal perfectly on both ends.

I actually switched to this after the braided hose leaked, partly because it was cheaper, and partly because I could have a line that was the exact right length

I do have a shutoff valve, no issues with leaking there

In that case you might need a new fill valve. The valve body could be cracked from over tightening as they're plastic nowadays.

Good point, I'll look at that

Did you check the making surface of the connector to the toilet before you coupled the line to it?

I once paid extra for a toilet flapper rebuild kit with the new seat because it was still leaking after I replaced the old one. It turns out there was just a hard water deposit stuck there that caused the new flapper to not be able to seal. I picked it off with a fingernail and cleaned it with white vinegar and it worked perfectly.

A quick scrub with a descaling cleaner like lime-away or CLR on a soft cloth could remove a nearly invisible deposit that is preventing a watertight seal.

I like this idea, I'll give it a try, the toilet was in use before

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Ball cocks. Sealing off big nuts.

Plumbers are fuckin hilarious

It's not that big, but thanks anyway.