Conversation is the closed union accepted by AgenticMessages. The unexported marker prevents an external package from pretending that an unrelated value is a conversation turn.

API surface

type Conversation interface{ isMessage() }

func (*UserMessage) isMessage()
func (*AIMessage) isMessage()
func (*SystemMessage) isMessage()
func (*ToolResultMessage) isMessage()

The marker methods are intentionally not callable outside content; the declarations above show the complete set of variants. Handle them with a type switch:

func blocksOf(m content.Conversation) []content.Block {
	switch typed := m.(type) {
	case *content.UserMessage:
		return typed.Blocks
	case *content.AIMessage:
		return typed.Blocks
	case *content.SystemMessage:
		return typed.Blocks
	case *content.ToolResultMessage:
		return typed.Blocks
	default:
		return nil
	}
}

Do not use the role string as the only discriminator. A role is a field and can be populated incorrectly; the concrete message type carries the package’s closed-world guarantee.

Proof

Source: core/content/message.go
Tests: core/content/message_test.go includes compile-time interface assertions for all four variants.