Show upgrade-required error to old freebuff clients

When the waiting-room gate rejects a free-mode request and no
freebuff_instance_id was sent, return 426 with a "please restart to
upgrade" message. Old CLI versions render the message verbatim in
their error banner; new clients still get the normal gate responses.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jahooma

web/src/app/api/v1/chat/completions/_post.ts

@@ -410,11 +410,32 @@ export async function postChatCompletions(params: {
     // Runs before the rate limiter so rejected requests don't burn a queued
     // user's free-mode counters.
     if (isFreeModeRequest) {
+      const claimedInstanceId =
+        typedBody.codebuff_metadata?.freebuff_instance_id
       const gate = await checkSession({
         userId,
-        claimedInstanceId: typedBody.codebuff_metadata?.freebuff_instance_id,
+        claimedInstanceId,
       })
       if (!gate.ok) {
+        // Old freebuff clients (pre-waiting-room) never send an instance_id.
+        // Return a 426 with a clear "please restart to upgrade" message that
+        // their existing error banner will render verbatim.
+        if (!claimedInstanceId) {
+          trackEvent({
+            event: AnalyticsEvent.CHAT_COMPLETIONS_VALIDATION_ERROR,
+            userId,
+            properties: { error: 'freebuff_update_required' },
+            logger,
+          })
+          return NextResponse.json(
+            {
+              error: 'freebuff_update_required',
+              message:
+                'This version of freebuff is out of date. Please restart freebuff to upgrade and continue using free mode.',
+            },
+            { status: 426 },
+          )
+        }
         trackEvent({
           event: AnalyticsEvent.CHAT_COMPLETIONS_VALIDATION_ERROR,
           userId,