feat(agent): route chat through selected model

src/app/api/agent/chat/route.ts

@@ -2,6 +2,7 @@ import { apiError, apiJson } from "@/lib/api-handler";
 import { resolveApiAuth } from "@/lib/api-auth";
 import { getAssistantPreferences, resolveAssistantSystemPrompt } from "@/lib/assistant-preference";
 import { isLlmConfigured } from "@/lib/llm";
+import { listLlmModels, resolveAssistantModel } from "@/lib/llm/models";
 import { clientChatInputSchema } from "@/modules/agent/messages";
 import { encodeSseEvent } from "@/modules/agent/server/progress";
 import { runAgentChat } from "@/modules/agent/server/run";
@@ -31,6 +32,18 @@ export async function POST(request: Request) {
     return apiError(parsed.error.issues[0]?.message ?? "Validation error", 400);
   }
 
+  const modelList = await listLlmModels();
+  const modelResolution = resolveAssistantModel({
+    requestedModel: parsed.data.model,
+    savedModel: assistant.model,
+    fallbackModel: modelList.fallbackModel,
+    models: modelList.models,
+  });
+
+  if (!modelResolution.ok) {
+    return apiError(modelResolution.error, 400);
+  }
+
   if (parsed.data.stream) {
     const stream = new ReadableStream<Uint8Array>({
       async start(controller) {
@@ -44,6 +57,7 @@ export async function POST(request: Request) {
             messages: parsed.data.messages,
             request,
             systemPrompt,
+            model: modelResolution.model,
             onProgress: send,
           });
 
@@ -78,6 +92,7 @@ export async function POST(request: Request) {
       messages: parsed.data.messages,
       request,
       systemPrompt,
+      model: modelResolution.model,
     });
 
     return apiJson({