Where to validate parameters sent to an API?

In which place is best practice to validate parameters sent by the user in an API design? By parameter validation I refer to: checking required params are sent, ensure they have correct format and so... Here are a couple of simple examples that validate an id has been sent. It is Python using Flask to illustrate:

A) Add validation logic in the route definition, within the controller.

@api.route('/job', methods=['GET'])
def get_jobs():
    try:
        if params["id"] is None:
            raise Exception("Invalid param ID parameters.")

        job = job_manager.get_job(params["id"])
        return jsonify(job)

B) In the core of the app. This is the business layer, where logic is applied to transform data.

class JobManager:
   def get_job(self, job_id) -> None:
        if job_id is None:
            raise Exception("Invalid param ID parameters.")

In more complex scenarios a validator service or decorators could be used, but the question would be the same: At which point of the code is best practice to validate a user's input.

If the answer is none of the scenarios above (or both), please provide more details on your answer. If possible, try to be language agnostic as I'm looking for a best practice that can be applied anywhere.