Next cookies() not being set on Mutation in App Dir (T3 Turbo)

export const postRouter = createTRPCRouter({
  all: publicProcedure.query(({ ctx }) => {
    cookies().set("Cookies work on QUERY", "yes")
    return [{id:0,
      title: "query test",
      content: "a test for trpc query",
      createdAt: Date.now(),
      updatedAt: Date.now()}]
  }),

  create: publicProcedure
    .input(
      z.object({
        title: z.string().min(1),
        content: z.string().min(1),
      }),
    )
    .mutation(({ ctx, input }) => {
      cookies().set("Cookies work on MUTATION", "No!")
      console.log(cookies().getAll())
      return 200;
    }),
});

@acme/nextjs:dev: [
@acme/nextjs:dev:   { name: 'Cookies work on QUERY', value: 'yes', path: '/' },
@acme/nextjs:dev:   { name: 'Cookies work on MUTATION', value: 'No!', path: '/' }
@acme/nextjs:dev: ]