Role — skyl

Role identifies the author of a Message. skyl has exactly three, and pointedly not a fourth.

Reference#

type Role string

Constant	Value	Meaning
`RoleUser`	user	Input from your application or its users.
`RoleAssistant`	assistant	What the model produced. Append it to replay prior turns.
`RoleTool`	tool	The result of a tool the model asked you to run.

func (r Role) Valid() bool

Caveats

There is no system role. System prompts live on Request.System, because providers place them in three different locations.
An invalid role is rejected by Validate with message N has invalid role "x".
skyl does not validate ordering — two user turns in a row, or a leading assistant turn, are your provider's business rather than skyl's.
Do not prefill an assistant turn to steer the next response. Several current models reject a trailing assistant message.

Why no system role#

Deep diveTwo reasons

Placement. Anthropic has one top-level system parameter, not a message stream; Gemini has systemInstruction. If system were a role, skyl would have to decide what a system message in the middle of a conversation means — and either concatenate it into the top-level field (surprising) or reject it (arbitrary).

Ordering. A role can appear anywhere, and "a system message after three user turns" behaves differently on every vendor. A field has exactly one meaning.

The cost is that you cannot express OpenAI's developer role separately from system. ProviderOptions reaches it if you need to.

Usage#

Building a turn by hand

skyl.Message{Role: skyl.RoleUser, Parts: []skyl.Part{skyl.Text{Text: "hello"}}}

skyl.Message{Role: skyl.RoleUser, Parts: []skyl.Part{skyl.Text{Text: "hello"}}}

Filtering a conversation by role

goCompiles

func userTurns(msgs []skyl.Message) []skyl.Message {
	var out []skyl.Message
	for _, m := range msgs {
		if m.Role == skyl.RoleUser {
			out = append(out, m)
		}
	}
	return out
}

func userTurns(msgs []skyl.Message) []skyl.Message {
	var out []skyl.Message
	for _, m := range msgs {
		if m.Role == skyl.RoleUser {
			out = append(out, m)
		}
	}
	return out
}

Troubleshooting#

Where do I put the system prompt?

Request.System. It is a field, not a role.

ErrBadRequest: invalid role “system”

You constructed skyl.Role("system"). Use Request.System instead.

My prefilled assistant turn worked on one provider and 400ed on another

Prefilling is not supported. AssistantText exists to replay prior turns; steer with the system prompt instead.