attempts to auto-convert Mistral prompts for its more strict rules

src/proxy/middleware/request/preprocessors/transform-outbound-payload.ts

@@ -106,9 +106,7 @@ export const OpenAIV1ChatCompletionSchema = z
   // Tool usage must be enabled via config because we currently have no way to
   // track quota usage for them or enforce limits.
   .omit(
-    Boolean(config.allowOpenAIToolUsage)
-      ? {}
-      : { tools: true, functions: true }
+    Boolean(config.allowOpenAIToolUsage) ? {} : { tools: true, functions: true }
   )
   .strip();
 
@@ -233,6 +231,15 @@ export const transformOutboundPayload: RequestPreprocessor = async (req) => {
 
   if (alreadyTransformed || notTransformable) return;
 
+  if (req.inboundApi === "mistral-ai") {
+    const messages = req.body.messages;
+    req.body.messages = fixMistralPrompt(messages);
+    req.log.info(
+      { old: messages.length, new: req.body.messages.length },
+      "Fixed Mistral prompt"
+    );
+  }
+
   if (sameService) {
     const result = VALIDATORS[req.inboundApi].safeParse(req.body);
     if (!result.success) {
@@ -540,3 +547,39 @@ function flattenOpenAIMessageContent(
         .join("\n")
     : content;
 }
+
+function fixMistralPrompt(
+  messages: MistralAIChatMessage[]
+): MistralAIChatMessage[] {
+  // Mistral uses OpenAI format but has some additional requirements:
+  // - Only one system message per request, and it must be the first message if
+  //   present.
+  // - Final message must be a user message.
+  // - Cannot have multiple messages from the same role in a row.
+  // While frontends should be able to handle this, we can fix it here in the
+  // meantime.
+
+  const result = messages.reduce<MistralAIChatMessage[]>((acc, msg) => {
+    if (acc.length === 0) {
+      acc.push(msg);
+      return acc;
+    }
+    
+    const copy = { ...msg };
+    // Reattribute subsequent system messages to the user
+    if (msg.role === "system") {
+      copy.role = "user";
+    }
+
+    // Consolidate multiple messages from the same role
+    const last = acc[acc.length - 1];
+    if (last.role === copy.role) {
+      last.content += "\n\n" + copy.content;
+    } else {
+      acc.push(copy);
+    }
+    return acc;
+  }, []);
+
+  return result;
+}