fix: coerce Mistral image_url parts from object to string (#3177)

* fix: coerce Mistral image_url parts from object to string

Mistral's API expects image_url content parts to be a plain string URL,
not the OpenAI-style { url: string } object that process_input_messages
produces. Without this coercion, sending an image to any Mistral model
that supports vision (mistral-small, mistral-medium, mistral-large,
ministral-*) results in a request error from the Mistral SDK.

Add #coerceImageUrls() as a private method on MistralAIProvider that
maps { type: 'image_url', image_url: { url } } -> { type: 'image_url',
image_url: url } for every content part in every message. Messages with
plain string content are left untouched, as are parts whose image_url is
already a string.

Add four unit tests covering: object-to-string coercion, already-flat
strings, plain string message content, and mixed text+image content.

* style: use plain ASCII dashes in comment section divider

src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.test.ts

@@ -655,6 +655,123 @@ describe('MistralAIProvider.complete streaming', () => {
     });
 });
 
+// -- Mistral image_url coercion --
+
+describe('MistralAIProvider image_url coercion', () => {
+    const baseCompletion = {
+        choices: [
+            {
+                message: { content: 'ok', role: 'assistant' },
+                finishReason: 'stop',
+            },
+        ],
+        usage: { promptTokens: 1, completionTokens: 1 },
+    };
+
+    it('flattens image_url objects to plain strings before sending to Mistral', async () => {
+        const { provider } = makeProvider();
+        completeMock.mockResolvedValueOnce(baseCompletion);
+
+        await withTestActor(() =>
+            provider.complete({
+                model: 'mistral-small-2603',
+                messages: [
+                    {
+                        role: 'user',
+                        content: [
+                            { type: 'text', text: 'describe this image' },
+                            {
+                                type: 'image_url',
+                                image_url: { url: 'https://example.com/img.png' },
+                            },
+                        ],
+                    },
+                ],
+            }),
+        );
+
+        const [args] = completeMock.mock.calls[0]!;
+        const imagePart = (args.messages[0].content as unknown[]).find(
+            (p: unknown) => (p as { type: string }).type === 'image_url',
+        ) as { type: string; image_url: unknown };
+        // Mistral expects a plain string, not { url: string }.
+        expect(imagePart.image_url).toBe('https://example.com/img.png');
+    });
+
+    it('leaves image_url parts that are already plain strings unchanged', async () => {
+        const { provider } = makeProvider();
+        completeMock.mockResolvedValueOnce(baseCompletion);
+
+        await withTestActor(() =>
+            provider.complete({
+                model: 'mistral-small-2603',
+                messages: [
+                    {
+                        role: 'user',
+                        content: [
+                            {
+                                type: 'image_url',
+                                image_url: 'https://example.com/already-flat.png',
+                            },
+                        ],
+                    },
+                ],
+            }),
+        );
+
+        const [args] = completeMock.mock.calls[0]!;
+        const imagePart = (args.messages[0].content as unknown[]).find(
+            (p: unknown) => (p as { type: string }).type === 'image_url',
+        ) as { type: string; image_url: unknown };
+        expect(imagePart.image_url).toBe('https://example.com/already-flat.png');
+    });
+
+    it('leaves messages with plain string content untouched', async () => {
+        const { provider } = makeProvider();
+        completeMock.mockResolvedValueOnce(baseCompletion);
+
+        await withTestActor(() =>
+            provider.complete({
+                model: 'mistral-small-2603',
+                messages: [{ role: 'user', content: 'plain text message' }],
+            }),
+        );
+
+        const [args] = completeMock.mock.calls[0]!;
+        expect(args.messages[0].content).toBe('plain text message');
+    });
+
+    it('coerces only image_url parts and leaves other content parts intact', async () => {
+        const { provider } = makeProvider();
+        completeMock.mockResolvedValueOnce(baseCompletion);
+
+        await withTestActor(() =>
+            provider.complete({
+                model: 'mistral-small-2603',
+                messages: [
+                    {
+                        role: 'user',
+                        content: [
+                            { type: 'text', text: 'what is in this image?' },
+                            {
+                                type: 'image_url',
+                                image_url: { url: 'https://example.com/photo.jpg' },
+                            },
+                        ],
+                    },
+                ],
+            }),
+        );
+
+        const [args] = completeMock.mock.calls[0]!;
+        const parts = args.messages[0].content as { type: string; text?: string; image_url?: unknown }[];
+        const textPart = parts.find((p) => p.type === 'text');
+        const imgPart = parts.find((p) => p.type === 'image_url');
+        expect(textPart?.text).toBe('what is in this image?');
+        expect(imgPart?.image_url).toBe('https://example.com/photo.jpg');
+    });
+});
+
 // ── Error mapping ───────────────────────────────────────────────────
 
 describe('MistralAIProvider.complete error mapping', () => {

src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.ts

@@ -61,6 +61,38 @@ export class MistralAIProvider implements IChatProvider {
         return ids;
     }
 
+    /**
+     * Mistral's API expects `image_url` content parts to carry a plain
+     * string URL, not the OpenAI-style `{ url: string }` object.
+     * This method normalises any `{ type: 'image_url', image_url: { url } }`
+     * parts to `{ type: 'image_url', image_url: url }` before the request
+     * is sent. Messages whose `content` is a plain string are left untouched.
+     */
+    #coerceImageUrls(
+        messages: { role: string; content: unknown }[],
+    ): { role: string; content: unknown }[] {
+        return messages.map((message) => {
+            if (!Array.isArray(message.content)) return message;
+            const content = message.content.map(
+                (part: { type?: string; image_url?: unknown }) => {
+                    if (
+                        part.type === 'image_url' &&
+                        part.image_url !== null &&
+                        typeof part.image_url === 'object' &&
+                        'url' in (part.image_url as object)
+                    ) {
+                        return {
+                            ...part,
+                            image_url: (part.image_url as { url: string }).url,
+                        };
+                    }
+                    return part;
+                },
+            );
+            return { ...message, content };
+        });
+    }
+
     async complete({
         messages,
         stream,
@@ -70,6 +102,7 @@ export class MistralAIProvider implements IChatProvider {
         temperature,
     }: ICompleteArguments): Promise<IChatCompleteResult> {
         messages = await OpenAIUtil.process_input_messages(messages);
+        messages = this.#coerceImageUrls(messages);
         for (const message of messages) {
             if (message.tool_calls) {
                 message.toolCalls = message.tool_calls;