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hooks/useDeferredHookMessages.ts

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+import { useCallback, useEffect, useRef } from 'react'
+import type { HookResultMessage, Message } from '../types/message.js'
+
+/**
+ * Manages deferred SessionStart hook messages so the REPL can render
+ * immediately instead of blocking on hook execution (~500ms).
+ *
+ * Hook messages are injected asynchronously when the promise resolves.
+ * Returns a callback that onSubmit should call before the first API
+ * request to ensure the model always sees hook context.
+ */
+export function useDeferredHookMessages(
+  pendingHookMessages: Promise<HookResultMessage[]> | undefined,
+  setMessages: (action: React.SetStateAction<Message[]>) => void,
+): () => Promise<void> {
+  const pendingRef = useRef(pendingHookMessages ?? null)
+  const resolvedRef = useRef(!pendingHookMessages)
+
+  useEffect(() => {
+    const promise = pendingRef.current
+    if (!promise) return
+    let cancelled = false
+    promise.then(msgs => {
+      if (cancelled) return
+      resolvedRef.current = true
+      pendingRef.current = null
+      if (msgs.length > 0) {
+        setMessages(prev => [...msgs, ...prev])
+      }
+    })
+    return () => {
+      cancelled = true
+    }
+  }, [setMessages])
+
+  return useCallback(async () => {
+    if (resolvedRef.current || !pendingRef.current) return
+    const msgs = await pendingRef.current
+    if (resolvedRef.current) return
+    resolvedRef.current = true
+    pendingRef.current = null
+    if (msgs.length > 0) {
+      setMessages(prev => [...msgs, ...prev])
+    }
+  }, [setMessages])
+}