How can I access the trpc context in zod's refine function?

To do something like

create: myProcedure
    .input(myInputSchema.refine(async ({ slug }, ctx) => 
      slugDoesNotAlreadyExist(ctx.db, slug),
      {
        message: 'Slug already exists',
        path: ['slug'],
    }))
    .mutation(async ({ ctx, input }) => {
       return ctx.db.insert(...)
    }),

Edit:

It would allow for easier UX because I can use the existing validation code in the frontend to show the user the errors at the correct field.

You can’t, validators validate data in place and are generally 3rd party libs. You probably want a middleware after the input to then take the ID and check it exists

Okay, thank you. I guess I'll do it on field change in the frontend then. I think that's better for UX anyway.