fix: title generation + ai sdk upgrade (#1392)

lib/ai/prompts.ts

@@ -123,10 +123,17 @@ export const updateDocumentPrompt = (
 ${currentContent}`;
 };
 
-export const titlePrompt = `Generate a very short chat title (2-5 words max) based on the user's message.
-Rules:
-- Maximum 30 characters
-- No quotes, colons, hashtags, or markdown
-- Just the topic/intent, not a full sentence
-- If the message is a greeting like "hi" or "hello", respond with just "New conversation"
-- Be concise: "Weather in NYC" not "User asking about the weather in New York City"`;
+export const titlePrompt = `Generate a short chat title (2-5 words) summarizing the user's message.
+
+Output ONLY the title text. No prefixes, no formatting.
+
+Examples:
+- "what's the weather in nyc" → Weather in NYC
+- "help me write an essay about space" → Space Essay Help
+- "hi" → New Conversation
+- "debug my python code" → Python Debugging
+
+Bad outputs (never do this):
+- "# Space Essay" (no hashtags)
+- "Title: Weather" (no prefixes)
+- ""NYC Weather"" (no quotes)`;

lib/ai/providers.ts

@@ -51,7 +51,7 @@ export function getTitleModel() {
   if (isTestEnvironment && myProvider) {
     return myProvider.languageModel("title-model");
   }
-  return gateway.languageModel("anthropic/claude-haiku-4.5");
+  return gateway.languageModel("google/gemini-2.5-flash-lite");
 }
 
 export function getArtifactModel() {