fixes token counting for streaming Mistral Text prompts

src/proxy/middleware/response/streaming/transformers/mistral-ai-to-openai.ts

@@ -7,19 +7,25 @@ const log = logger.child({
   transformer: "mistral-ai-to-openai",
 });
 
-type MistralAIStreamEvent = {
+type MistralChatCompletionEvent = {
   choices: {
     index: number;
     message: { role: string; content: string };
     stop_reason: string | null;
   }[];
+};
+type MistralTextCompletionEvent = {
+  outputs: { text: string; stop_reason: string | null }[];
+};
+
+type MistralAIStreamEvent = {
   "amazon-bedrock-invocationMetrics"?: {
     inputTokenCount: number;
     outputTokenCount: number;
     invocationLatency: number;
     firstByteLatency: number;
   };
-};
+} & (MistralChatCompletionEvent | MistralTextCompletionEvent);
 
 export const mistralAIToOpenAI = (params: SSEResponseTransformArgs) => {
   const { data } = params;
@@ -34,29 +40,49 @@ export const mistralAIToOpenAI = (params: SSEResponseTransformArgs) => {
     return { position: -1 };
   }
 
-  const newEvent = {
-    id: params.fallbackId,
-    object: "chat.completion.chunk" as const,
-    created: Date.now(),
-    model: params.fallbackModel,
-    choices: [
-      {
-        index: completionEvent.choices[0].index,
-        delta: { content: completionEvent.choices[0].message.content },
-        finish_reason: completionEvent.choices[0].stop_reason,
-      },
-    ],
-  };
+  if ("choices" in completionEvent) {
+    const newChatEvent = {
+      id: params.fallbackId,
+      object: "chat.completion.chunk" as const,
+      created: Date.now(),
+      model: params.fallbackModel,
+      choices: [
+        {
+          index: completionEvent.choices[0].index,
+          delta: { content: completionEvent.choices[0].message.content },
+          finish_reason: completionEvent.choices[0].stop_reason,
+        },
+      ],
+    };
+    return { position: -1, event: newChatEvent };
+  } else if ("outputs" in completionEvent) {
+    const newTextEvent = {
+      id: params.fallbackId,
+      object: "chat.completion.chunk" as const,
+      created: Date.now(),
+      model: params.fallbackModel,
+      choices: [
+        {
+          index: 0,
+          delta: { content: completionEvent.outputs[0].text },
+          finish_reason: completionEvent.outputs[0].stop_reason,
+        },
+      ],
+    };
+    return { position: -1, event: newTextEvent };
+  }
 
-  return { position: -1, event: newEvent };
+  // should never happen
+  return { position: -1 };
 };
 
 function asCompletion(event: ServerSentEvent): MistralAIStreamEvent | null {
   try {
     const parsed = JSON.parse(event.data);
     if (
-      Array.isArray(parsed.choices) &&
-      parsed.choices[0].message !== undefined
+      (Array.isArray(parsed.choices) &&
+        parsed.choices[0].message !== undefined) ||
+      (Array.isArray(parsed.outputs) && parsed.outputs[0].text !== undefined)
     ) {
       return parsed;
     } else {