diff --git a/server/supabase/migrations/20260927000036_builtin_instruction_prompt_sync.sql b/server/supabase/migrations/20260927000036_builtin_instruction_prompt_sync.sql
new file mode 100644
index 0000000..20c6740
--- /dev/null
+++ b/server/supabase/migrations/20260927000036_builtin_instruction_prompt_sync.sql
@@ -0,0 +1,101 @@
+-- Preset command prompts edited on the desktop reach the phone.
+--
+-- Built-in custom_instructions rows are not writable by clients (RLS allows
+-- only builtin_key IS NULL), and the phone executes every command with the
+-- row's prompt. These RPCs let the owner change only the prompt of their own
+-- preset row, or restore its default; name, key and order stay server-owned.
+-- Defaults live in one function that bootstrap_custom_instructions' values
+-- mirror, so "is this the default?" has a single answer.
+
+BEGIN;
+
+CREATE OR REPLACE FUNCTION public.builtin_instruction_default_prompt_v1(p_builtin_key text)
+RETURNS text
+LANGUAGE sql
+IMMUTABLE
+SET search_path = ''
+AS $$
+  SELECT CASE p_builtin_key
+    WHEN 'translate_en' THEN 'Translate the following text into natural English. Return only the translation.'
+    WHEN 'summarize' THEN 'Summarize the following text in no more than three concise lines.'
+    WHEN 'formal' THEN 'Rewrite the following text in a formal business style while preserving its meaning. Return only the rewritten text.'
+    WHEN 'explain_code' THEN 'Explain the following code in Korean, including its responsibilities and important caveats.'
+  END
+$$;
+
+REVOKE ALL ON FUNCTION public.builtin_instruction_default_prompt_v1(text) FROM PUBLIC, anon;
+GRANT EXECUTE ON FUNCTION public.builtin_instruction_default_prompt_v1(text) TO authenticated;
+
+-- p_prompt NULL (or blank) restores the default prompt.
+CREATE OR REPLACE FUNCTION public.sync_set_builtin_instruction_prompt_v1(
+  p_builtin_key text,
+  p_prompt text DEFAULT NULL
+)
+RETURNS public.custom_instructions
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path = pg_catalog, public, auth
+AS $$
+DECLARE
+  actor_id uuid := auth.uid();
+  default_prompt text := public.builtin_instruction_default_prompt_v1(p_builtin_key);
+  next_prompt text;
+  saved public.custom_instructions;
+BEGIN
+  IF actor_id IS NULL THEN
+    RAISE EXCEPTION 'authentication_required' USING ERRCODE = '42501';
+  END IF;
+  IF default_prompt IS NULL THEN
+    RAISE EXCEPTION 'invalid_builtin_key' USING ERRCODE = '22023';
+  END IF;
+
+  next_prompt := coalesce(nullif(btrim(p_prompt), ''), default_prompt);
+  IF char_length(next_prompt) > 4000 THEN
+    RAISE EXCEPTION 'invalid_instruction_prompt' USING ERRCODE = '22023';
+  END IF;
+
+  -- The preset rows are created lazily; make sure this user has them.
+  PERFORM public.bootstrap_custom_instructions();
+
+  UPDATE public.custom_instructions
+  SET prompt = next_prompt
+  WHERE user_id = actor_id
+    AND builtin_key = p_builtin_key
+    AND prompt IS DISTINCT FROM next_prompt
+  RETURNING * INTO saved;
+
+  IF saved.id IS NULL THEN
+    SELECT * INTO saved
+    FROM public.custom_instructions
+    WHERE user_id = actor_id AND builtin_key = p_builtin_key;
+  END IF;
+  RETURN saved;
+END;
+$$;
+
+REVOKE ALL ON FUNCTION public.sync_set_builtin_instruction_prompt_v1(text, text) FROM PUBLIC, anon;
+GRANT EXECUTE ON FUNCTION public.sync_set_builtin_instruction_prompt_v1(text, text) TO authenticated;
+
+-- Preset rows with a flag telling whether the prompt is still the default.
+CREATE OR REPLACE FUNCTION public.sync_list_builtin_instructions_v1()
+RETURNS TABLE (builtin_key text, prompt text, is_default boolean, updated_at timestamptz)
+LANGUAGE sql
+STABLE
+SECURITY INVOKER
+SET search_path = ''
+AS $$
+  SELECT
+    instruction.builtin_key,
+    instruction.prompt,
+    instruction.prompt = public.builtin_instruction_default_prompt_v1(instruction.builtin_key),
+    instruction.updated_at
+  FROM public.custom_instructions AS instruction
+  WHERE instruction.user_id = auth.uid()
+    AND instruction.builtin_key IS NOT NULL
+  ORDER BY instruction.sort_order
+$$;
+
+REVOKE ALL ON FUNCTION public.sync_list_builtin_instructions_v1() FROM PUBLIC, anon;
+GRANT EXECUTE ON FUNCTION public.sync_list_builtin_instructions_v1() TO authenticated;
+
+COMMIT;