diff --git a/src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.test.ts b/src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.test.ts
index 3c38ba1f8..54da23411 100644
--- a/src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.test.ts
+++ b/src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.test.ts
@@ -210,6 +210,51 @@ describe('MistralAIProvider.complete request shape', () => {
         expect(args.temperature).toBe(0.4);
     });
 
+    it('forwards custom.prompt_mode as the SDK promptMode', async () => {
+        const { provider } = makeProvider();
+        completeMock.mockResolvedValueOnce({
+            choices: [
+                {
+                    message: { role: 'assistant', content: 'ok' },
+                    finishReason: 'stop',
+                },
+            ],
+            usage: { promptTokens: 1, completionTokens: 1 },
+        });
+
+        await withTestActor(() =>
+            provider.complete({
+                model: 'magistral-small-latest',
+                messages: [{ role: 'user', content: 'think' }],
+                custom: { prompt_mode: 'reasoning' },
+            }),
+        );
+
+        expect(completeMock.mock.calls[0]![0].promptMode).toBe('reasoning');
+    });
+
+    it('omits promptMode when custom does not carry prompt_mode', async () => {
+        const { provider } = makeProvider();
+        completeMock.mockResolvedValueOnce({
+            choices: [
+                {
+                    message: { role: 'assistant', content: 'ok' },
+                    finishReason: 'stop',
+                },
+            ],
+            usage: { promptTokens: 1, completionTokens: 1 },
+        });
+
+        await withTestActor(() =>
+            provider.complete({
+                model: 'mistral-small-2603',
+                messages: [{ role: 'user', content: 'hi' }],
+            }),
+        );
+
+        expect('promptMode' in completeMock.mock.calls[0]![0]).toBe(false);
+    });
+
     it('omits the `tools` key when no tools are supplied', async () => {
         const { provider } = makeProvider();
         completeMock.mockResolvedValueOnce(baseCompletion);
diff --git a/src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.ts b/src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.ts
index 96de9da5a..1618cb8be 100644
--- a/src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.ts
+++ b/src/backend/drivers/ai-chat/providers/mistral/MistralAiProvider.ts
@@ -152,7 +152,19 @@ export class MistralAIProvider implements IChatProvider {
         temperature,
         normalize,
         response,
+        custom,
     }: ICompleteArguments): Promise<IChatCompleteResult> {
+        // Mistral's reasoning prompt mode: with `prompt_mode: 'reasoning'`,
+        // magistral models return their thinking as structured ThinkChunk
+        // content (which the splitter below separates into `reasoning`)
+        // instead of inlining it as answer prose. Opt-in passthrough rather
+        // than a default because the API rejects it on accounts/models where
+        // the mode is not enabled ('Reasoning prompt mode is not enabled for
+        // this model', code 3051).
+        const customParams =
+            custom && typeof custom === 'object' && !Array.isArray(custom)
+                ? (custom as { prompt_mode?: 'reasoning' | null })
+                : {};
         messages = await OpenAIUtil.process_input_messages(messages);
         messages = this.#coerceImageUrls(messages);
         for (const message of messages) {
@@ -177,6 +189,9 @@ export class MistralAIProvider implements IChatProvider {
         ]({
             model: selectedModel.id,
             ...(tools ? { tools: tools as any[] } : {}),
+            ...(customParams.prompt_mode !== undefined
+                ? { promptMode: customParams.prompt_mode }
+                : {}),
             messages,
             maxTokens: max_tokens,
             temperature,