fix: send base64 images without data URL prefix to LM Studio

LM Studio expects raw base64 encoded image data without the data: URL prefix.
This fix adds an optional lmStudioFormat parameter to convertToOpenAiMessages
that strips the prefix when sending images to LM Studio.

Fixes #8827

Author: roomote

src/api/providers/lm-studio.ts

@@ -43,7 +43,7 @@ export class LmStudioHandler extends BaseProvider implements SingleCompletionHan
 	): ApiStream {
 		const openAiMessages: OpenAI.Chat.ChatCompletionMessageParam[] = [
 			{ role: "system", content: systemPrompt },
-			...convertToOpenAiMessages(messages),
+			...convertToOpenAiMessages(messages, { lmStudioFormat: true }),
 		]
 
 		// -------------------------

src/api/transform/__tests__/openai-format.spec.ts

@@ -70,6 +70,52 @@ describe("convertToOpenAiMessages", () => {
 		})
 	})
 
+	it("should handle images with LM Studio format", () => {
+		const anthropicMessages: Anthropic.Messages.MessageParam[] = [
+			{
+				role: "user",
+				content: [
+					{
+						type: "text",
+						text: "Analyze this image",
+					},
+					{
+						type: "image",
+						source: {
+							type: "base64",
+							media_type: "image/png",
+							data: "base64imagedata",
+						},
+					},
+				],
+			},
+		]
+
+		// Test default format (with data URL prefix)
+		const openAiMessages = convertToOpenAiMessages(anthropicMessages)
+		const content = openAiMessages[0].content as Array<{
+			type: string
+			text?: string
+			image_url?: { url: string }
+		}>
+		expect(content[1]).toEqual({
+			type: "image_url",
+			image_url: { url: "data:image/png;base64,base64imagedata" },
+		})
+
+		// Test LM Studio format (without data URL prefix)
+		const lmStudioMessages = convertToOpenAiMessages(anthropicMessages, { lmStudioFormat: true })
+		const lmStudioContent = lmStudioMessages[0].content as Array<{
+			type: string
+			text?: string
+			image_url?: { url: string }
+		}>
+		expect(lmStudioContent[1]).toEqual({
+			type: "image_url",
+			image_url: { url: "base64imagedata" },
+		})
+	})
+
 	it("should handle assistant messages with tool use", () => {
 		const anthropicMessages: Anthropic.Messages.MessageParam[] = [
 			{

src/api/transform/openai-format.ts

@@ -3,6 +3,7 @@ import OpenAI from "openai"
 
 export function convertToOpenAiMessages(
 	anthropicMessages: Anthropic.Messages.MessageParam[],
+	options?: { lmStudioFormat?: boolean },
 ): OpenAI.Chat.ChatCompletionMessageParam[] {
 	const openAiMessages: OpenAI.Chat.ChatCompletionMessageParam[] = []
 
@@ -83,9 +84,13 @@ export function convertToOpenAiMessages(
 						role: "user",
 						content: nonToolMessages.map((part) => {
 							if (part.type === "image") {
+								// LM Studio expects just the base64 data without the data URL prefix
+								const imageUrl = options?.lmStudioFormat
+									? part.source.data
+									: `data:${part.source.media_type};base64,${part.source.data}`
 								return {
 									type: "image_url",
-									image_url: { url: `data:${part.source.media_type};base64,${part.source.data}` },
+									image_url: { url: imageUrl },
 								}
 							}
 							return { type: "text", text: part.text }