Exception raised when a behaviour callback returns a value the library cannot use.

The library invokes the callbacks you implement (for example the ones in GenMCP.Suite.Tool or GenMCP.Suite.ResourceRepo) and matches their result against the return shapes each callback documents. When an implementation returns something none of those shapes match, the library raises this exception instead of carrying the bad value further. It is a programming error in the callback, not a runtime condition a client can trigger, so it surfaces as a raised exception during development.

The struct carries three fields that identify the offending callback:

• :mfa - the {module, function, args} of the callback that was invoked, where args is the actual argument list (its length gives the arity shown in the message).
• :behaviour - the behaviour module the callback is implementing, for example GenMCP.Suite.Tool.
• :return_value - the value the callback returned that could not be accepted.