fix: apply R1 format conversion to Mistral 2512 models

Extends the message format conversion logic to include Mistral 2512 models
(mistralai/*-2512) which require strict role alternation between user and
assistant messages, similar to DeepSeek R1 models.

This fixes the API error: "After the optional system message, conversation
roles must alternate user and assistant roles except for tool calls and
results."

- Updated openrouter.ts to detect Mistral 2512 models via regex pattern
- Added test coverage for both mistral-large-2512 and devstral-2512
- Reuses existing convertToR1Format function for consistent handling

Fixes #10045

src/api/providers/__tests__/openrouter.spec.ts

@@ -489,6 +489,90 @@ describe("OpenRouterHandler", () => {
 			expect(endChunks).toHaveLength(1)
 			expect(endChunks[0].id).toBe("call_openrouter_test")
 		})
+
+		it("uses R1 format for Mistral 2512 models", async () => {
+			const handler = new OpenRouterHandler({
+				openRouterApiKey: "test-key",
+				openRouterModelId: "mistralai/mistral-large-2512",
+			})
+
+			const mockStream = {
+				async *[Symbol.asyncIterator]() {
+					yield {
+						id: "test-id",
+						choices: [{ delta: { content: "test response" } }],
+					}
+				},
+			}
+
+			const mockCreate = vitest.fn().mockResolvedValue(mockStream)
+			;(OpenAI as any).prototype.chat = {
+				completions: { create: mockCreate },
+			} as any
+
+			const systemPrompt = "test system prompt"
+			const messages: Anthropic.Messages.MessageParam[] = [
+				{ role: "user", content: "message 1" },
+				{ role: "user", content: "message 2" },
+			]
+
+			await handler.createMessage(systemPrompt, messages).next()
+
+			// Verify that convertToR1Format was used by checking messages structure
+			// Consecutive user messages should be merged into a single user message
+			expect(mockCreate).toHaveBeenCalledWith(
+				expect.objectContaining({
+					messages: expect.arrayContaining([
+						expect.objectContaining({
+							role: "user",
+							// The system prompt and two user messages should be merged
+							content: expect.stringContaining("test system prompt"),
+						}),
+					]),
+				}),
+				undefined,
+			)
+
+			// Verify that messages were merged (should have fewer messages than original)
+			const callArgs = mockCreate.mock.calls[0][0]
+			// Original would have 3 messages (system converted to user + 2 user messages)
+			// After R1 format conversion, consecutive user messages should be merged into 1
+			expect(callArgs.messages).toHaveLength(1)
+		})
+
+		it("uses R1 format for Mistral devstral-2512 model", async () => {
+			const handler = new OpenRouterHandler({
+				openRouterApiKey: "test-key",
+				openRouterModelId: "mistralai/devstral-2512",
+			})
+
+			const mockStream = {
+				async *[Symbol.asyncIterator]() {
+					yield {
+						id: "test-id",
+						choices: [{ delta: { content: "test response" } }],
+					}
+				},
+			}
+
+			const mockCreate = vitest.fn().mockResolvedValue(mockStream)
+			;(OpenAI as any).prototype.chat = {
+				completions: { create: mockCreate },
+			} as any
+
+			const systemPrompt = "test system prompt"
+			const messages: Anthropic.Messages.MessageParam[] = [
+				{ role: "user", content: "message 1" },
+				{ role: "user", content: "message 2" },
+			]
+
+			await handler.createMessage(systemPrompt, messages).next()
+
+			// Verify R1 format conversion was applied
+			const callArgs = mockCreate.mock.calls[0][0]
+			expect(callArgs.messages).toHaveLength(1)
+			expect(callArgs.messages[0].role).toBe("user")
+		})
 	})
 
 	describe("completePrompt", () => {

src/api/providers/openrouter.ts

@@ -140,7 +140,13 @@ export class OpenRouterHandler extends BaseProvider implements SingleCompletionH
 		]
 
 		// DeepSeek highly recommends using user instead of system role.
-		if (modelId.startsWith("deepseek/deepseek-r1") || modelId === "perplexity/sonar-reasoning") {
+		// Mistral 2512 models also require strict role alternation.
+		const requiresR1Format =
+			modelId.startsWith("deepseek/deepseek-r1") ||
+			modelId === "perplexity/sonar-reasoning" ||
+			/mistralai\/.*-2512/.test(modelId)
+
+		if (requiresR1Format) {
 			openAiMessages = convertToR1Format([{ role: "user", content: systemPrompt }, ...messages])
 		}