"But detective stories don't work if your world isn't grounded in something real."
No, it just has to be grounded in something consistent and if the book starts by explaining the mechanics of the world(or is in a world with known mechanics), a detective story very well can work.
It can’t be too different from what people are familiar with. So it has to be consistent with itself and with reality.”It’s like our reality except …” but the except part (faster than light travel, mind reading, time travel, dragons, magic, etc) can’t get arbitrarily complex or unreasonable or the reader will be lost or confused.
"can’t get arbitrarily complex or unreasonable or the reader will be lost or confused."
Indeed, but that is always the challenge of a writer. But if you introduce magic in your story, and you do it in a way to show the limitations and what is roughly possible, then you can give hints later, if there was a murder with the help of a magic trick. But I actually think I only ever read one detective fantasy story
It mostly works, but in one instance there was a surprise fantasy ability, that I could have not guessed before. But the actual murderer in the end, there were enough hints that gave them away despite magic, as the skills were explained and then you could combine.
No, it just has to be grounded in something consistent and if the book starts by explaining the mechanics of the world(or is in a world with known mechanics), a detective story very well can work.