Construct messages

There are no message constructors in core/content. Construction is deliberately visible: embed a Message and set the matching Role constant, then add non-nil block pointers in order.

API surface

type Role string

const (
	RoleUser      Role = "user"
	RoleAssistant Role = "assistant"
	RoleSystem    Role = "system"
	RoleTool      Role = "tool"
)

package main

import "github.com/looprig/core/content"

func main() {
	user := &content.UserMessage{Message: content.Message{
		Role: content.RoleUser,
		Blocks: []content.Block{
			&content.TextBlock{Text: "What changed?"},
			&content.ImageBlock{
				MediaType: content.MediaTypeImagePNG,
				Source: content.ImageSource{URL: "https://example.test/diff.png"},
			},
		},
	}}
	_ = user
}

Embedded fields are promoted, so user.Role and user.Blocks work after construction. A nil block interface or typed-nil payload is not a valid value for JSON encoding; MarshalBlock fails closed instead of emitting a misleading empty block.

Role and type must agree

The message codecs preserve the Role field exactly; they do not infer it from the Go type. Set RoleAssistant on AIMessage, RoleTool on ToolResultMessage, and so on. Tests exercise mixed messages and empty or nil block slices.

Proof

• Source: core/content/message.go, core/content/block.go
• Tests: core/content/message_test.go, core/content/block_json_test.go

Related: UserMessage, Content blocks.