GET /walkers/:id — 404 when missing

The :id is a string until you make it a number

GET /walkers/42 carries 42 in the path — but every path param arrives as the string "42". Prisma's id column is an Int, so you must convert, and you want to reject GET /walkers/banana before it ever reaches the database.

Nest ships exactly that as a pipe: ParseIntPipe. Attach it to the param and Nest converts the string to a number, or throws a 400 Bad Request if it isn't one — so your method always receives a real number.

@Get(":id")
findOne(@Param("id", ParseIntPipe) id: number) {
  return this.walkers.findOne(id);
}