UserImage — skyl

The convenience constructor for the common multimodal case: a picture plus a question about it.

Reference#

func UserImage(mediaType string, data []byte, caption string) Message

Parameters

mediaType — the IANA media type, e.g. "image/png". Required.
data — the raw image bytes. Not base64-encoded.
caption — optional text. Omitted from the message entirely when empty.

Returns

A user message with an Image part, followed by a Text part when caption is non-empty.

Caveats

It always produces the inline-bytes form, which is the portable one — Gemini rejects URL images.
The image comes first, then the caption. Build the message directly if you need the other order or several images.
data is raw. Encoding it yourself produces double-encoded content.

Usage#

An image with a question

goCompiles

data, err := os.ReadFile("chart.png")
if err != nil {
	return err
}

resp, err := client.Complete(ctx, &skyl.Request{
	Model:     "claude-opus-5",
	MaxTokens: 1024,
	Messages:  []skyl.Message{skyl.UserImage("image/png", data, "What trend does this show?")},
})

data, err := os.ReadFile("chart.png")
if err != nil {
	return err
}

resp, err := client.Complete(ctx, &skyl.Request{
	Model:     "claude-opus-5",
	MaxTokens: 1024,
	Messages:  []skyl.Message{skyl.UserImage("image/png", data, "What trend does this show?")},
})

Several images, built directly

skyl.Message{
	Role: skyl.RoleUser,
	Parts: []skyl.Part{
		skyl.Text{Text: "Compare these:"},
		skyl.Image{MediaType: "image/png", Data: before},
		skyl.Image{MediaType: "image/png", Data: after},
	},
}

skyl.Message{
	Role: skyl.RoleUser,
	Parts: []skyl.Part{
		skyl.Text{Text: "Compare these:"},
		skyl.Image{MediaType: "image/png", Data: before},
		skyl.Image{MediaType: "image/png", Data: after},
	},
}

Detecting the media type

goCompiles

part := skyl.UserImage(http.DetectContentType(data), data, "Describe this.")

part := skyl.UserImage(http.DetectContentType(data), data, "Describe this.")

Troubleshooting#

ErrBadRequest: image data needs a media type

You passed an empty mediaType. Use http.DetectContentType(data) if you do not know it.

I want to send a URL instead

Build the message directly with skyl.Image{URL: ...} — but note Gemini rejects that form. Fetching the bytes yourself is portable.