fix: batch outbox flush (latest-first) and log backoff errors

flushOutbox previously sent the entire pendingOutbox in one POST.
On --resume with 400+ historical messages, this payload was too large
and failed silently (backoff retried forever with no logging),
permanently blocking all subsequent agent messages.

- Batch flushOutbox into max 50 items per POST, sending from the end
  first so the user sees the latest messages immediately
- Add error logging to global backoff so failures are never silent

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

packages/happy-cli/src/api/apiSession.ts

@@ -308,30 +308,32 @@ export class ApiSessionClient extends EventEmitter {
         }
     }
 
-    private async flushOutbox() {
-        if (this.pendingOutbox.length === 0) {
-            return;
-        }
+    private static readonly MAX_OUTBOX_BATCH_SIZE = 50;
 
-        const batch = this.pendingOutbox.slice();
-        const response = await axios.post<V3PostSessionMessagesResponse>(
-            `${configuration.serverUrl}/v3/sessions/${encodeURIComponent(this.sessionId)}/messages`,
-            {
-                messages: batch
-            },
-            {
-                headers: this.authHeaders(),
-                timeout: 60000
-            }
-        );
+    private async flushOutbox() {
+        // Send latest messages first so the user sees recent activity immediately,
+        // then backfill older messages in subsequent batches.
+        while (this.pendingOutbox.length > 0) {
+            const batchSize = Math.min(this.pendingOutbox.length, ApiSessionClient.MAX_OUTBOX_BATCH_SIZE);
+            const batch = this.pendingOutbox.splice(-batchSize, batchSize);
 
-        this.pendingOutbox.splice(0, batch.length);
+            const response = await axios.post<V3PostSessionMessagesResponse>(
+                `${configuration.serverUrl}/v3/sessions/${encodeURIComponent(this.sessionId)}/messages`,
+                {
+                    messages: batch
+                },
+                {
+                    headers: this.authHeaders(),
+                    timeout: 60000
+                }
+            );
 
-        const messages = Array.isArray(response.data.messages) ? response.data.messages : [];
-        const maxSeq = messages.reduce((acc, message) => (
-            message.seq > acc ? message.seq : acc
-        ), this.lastSeq);
-        this.lastSeq = maxSeq;
+            const messages = Array.isArray(response.data.messages) ? response.data.messages : [];
+            const maxSeq = messages.reduce((acc, message) => (
+                message.seq > acc ? message.seq : acc
+            ), this.lastSeq);
+            this.lastSeq = maxSeq;
+        }
     }
 
     private enqueueMessage(content: unknown, invalidate: boolean = true) {

packages/happy-cli/src/claude/utils/sessionScanner.ts

@@ -48,7 +48,6 @@ export async function createSessionScanner(opts: {
 
     // Main sync function
     const sync = new InvalidateSync(async () => {
-        // logger.debug(`[SESSION_SCANNER] Syncing...`);
 
         // Collect session ids - include ALL sessions that have watchers
         // This ensures we continue processing sessions that Claude Code may still write to

packages/happy-cli/src/utils/sync.ts

@@ -71,4 +71,4 @@ export class InvalidateSync {
             this._notifyPendings();
         }
     }
-}
+}

packages/happy-cli/src/utils/time.ts

@@ -1,3 +1,5 @@
+import { logger } from '@/ui/logger';
+
 export async function delay(ms: number) {
     return new Promise(resolve => setTimeout(resolve, ms));
 }
@@ -38,4 +40,8 @@ export function createBackoff(
     };
 }
 
-export let backoff = createBackoff();
+export let backoff = createBackoff({
+    onError: (e, failuresCount) => {
+        logger.debug(`[BACKOFF] retry ${failuresCount}:`, (e as Error)?.message || e);
+    }
+});