A read-only interface should expose retrieval operations without permitting creation, modification or deletion. The restriction must be enforced by the underlying service, not merely communicated through a label or visual design.
For agent systems, every available request path must respect the same restriction. If an ordinary web request can still write data, the interface is not effectively read-only from the agent's perspective.
ELI5
A read-only interface is like a display case in a museum. You can look at what is inside, but the barrier should stop you from changing it.
For example, a wiki viewer may show pages without an edit button. It is only truly read-only if the server also rejects direct requests that try to save changes.
