Part — skyl

Part is deliberately closed — it has an unexported marker method, so only skyl can implement it. That constraint turns a class of runtime bug into a compile error.

Reference#

type Part interface { isPart() }

The four implementations:

Type	Carries
Text	A run of plain text
Image	An image, as inline bytes or a URL
ToolCall	The model's request to invoke a tool
ToolResult	The outcome of a tool invocation

Caveats

You cannot implement it. The marker method is unexported.
An adapter that cannot represent a part returns ErrUnsupported naming the part, before sending anything — rather than dropping it silently.
Support varies by part and by the role of the message carrying it.
Because it is an interface, Message does not round-trip through encoding/json. Persist your own shape.

Why closed#

Deep diveA compile error beats a production surprise

An open interface would let you construct a part no adapter knows how to render. Every adapter's type switch would hit its default and have to either drop it silently or fail at runtime — a bug that appears in production, on one vendor and not another, long after the code was written.

Closed means the compiler rejects it at the point you type it. The cost is that adding a part type requires a change to skyl. The benefit is that every part you can construct is one every adapter has a defined answer for, even if that answer is a clean ErrUnsupported.

If you need something skyl does not model, ProviderOptions is the hatch.

What each adapter accepts#

Part (role)	anthropic	openai / compat	gemini
Text (user, assistant)	mapped	mapped	mapped
Text (tool role)	becomes user content	rejected	becomes user content
Image URL (user)	mapped	mapped	rejected
Image data (user)	mapped	mapped	mapped
Image (assistant)	accepted	rejected	accepted
ToolCall (assistant)	mapped	mapped	mapped
ToolCall (user)	accepted	rejected	accepted
ToolResult (tool role)	mapped	mapped	mapped

Usage#

Switching over parts

goCompiles

for _, part := range resp.Message.Parts {
	switch p := part.(type) {
	case skyl.Text:
		fmt.Print(p.Text)
	case skyl.ToolCall:
		fmt.Printf("[calling %s]", p.Name)
	case skyl.Image:
		fmt.Print("[image]")
	case skyl.ToolResult:
		fmt.Printf("[result for %s]", p.CallID)
	}
}

for _, part := range resp.Message.Parts {
	switch p := part.(type) {
	case skyl.Text:
		fmt.Print(p.Text)
	case skyl.ToolCall:
		fmt.Printf("[calling %s]", p.Name)
	case skyl.Image:
		fmt.Print("[image]")
	case skyl.ToolResult:
		fmt.Printf("[result for %s]", p.CallID)
	}
}

Because the interface is closed, this switch is exhaustive by construction.

Handling an unsupported part

goCompiles

_, err := client.Complete(ctx, req)
if errors.Is(err, skyl.ErrUnsupported) {
	// e.g. "gemini: Gemini requires inline image data, not a URL"
	var e *skyl.Error
	if errors.As(err, &e) {
		return fmt.Errorf("this model (%s) cannot handle that input: %s", e.Provider, e.Message)
	}
}

_, err := client.Complete(ctx, req)
if errors.Is(err, skyl.ErrUnsupported) {
	// e.g. "gemini: Gemini requires inline image data, not a URL"
	var e *skyl.Error
	if errors.As(err, &e) {
		return fmt.Errorf("this model (%s) cannot handle that input: %s", e.Provider, e.Message)
	}
}

Troubleshooting#

I want to add an audio part

You cannot — the interface is closed. Send it through ProviderOptions, which is the hatch for anything skyl does not model. If several vendors support it stably, that is a good issue to open.

ErrUnsupported naming a part

The adapter cannot represent it. The message says exactly which part and why. This is deliberate: a silently dropped image looks like a model that ignored your question.