fix: keep ACP stdout protocol-clean

Route AIAgent print output to stderr via _print_fn for ACP stdio sessions.
Gate quiet-mode spinner startup on _should_start_quiet_spinner() so JSON-RPC
on stdout isn't corrupted. Child agents inherit the redirect.

Co-authored-by: Git-on-my-level <Git-on-my-level@users.noreply.github.com>

run_agent.py

@@ -1491,6 +1491,25 @@ class AIAgent:
             return
         self._safe_print(*args, **kwargs)
 
+    def _should_start_quiet_spinner(self) -> bool:
+        """Return True when quiet-mode spinner output has a safe sink.
+
+        In headless/stdio-protocol environments, a raw spinner with no custom
+        ``_print_fn`` falls back to ``sys.stdout`` and can corrupt protocol
+        streams such as ACP JSON-RPC. Allow quiet spinners only when either:
+        - output is explicitly rerouted via ``_print_fn``; or
+        - stdout is a real TTY.
+        """
+        if self._print_fn is not None:
+            return True
+        stream = getattr(sys, "stdout", None)
+        if stream is None:
+            return False
+        try:
+            return bool(stream.isatty())
+        except (AttributeError, ValueError, OSError):
+            return False
+
     def _emit_status(self, message: str) -> None:
         """Emit a lifecycle status message to both CLI and gateway channels.
 
@@ -6066,7 +6085,7 @@ class AIAgent:
 
         # Start spinner for CLI mode (skip when TUI handles tool progress)
         spinner = None
-        if self.quiet_mode and not self.tool_progress_callback:
+        if self.quiet_mode and not self.tool_progress_callback and self._should_start_quiet_spinner():
             face = random.choice(KawaiiSpinner.KAWAII_WAITING)
             spinner = KawaiiSpinner(f"{face} ⚡ running {num_tools} tools concurrently", spinner_type='dots', print_fn=self._print_fn)
             spinner.start()
@@ -6294,7 +6313,7 @@ class AIAgent:
                     goal_preview = (function_args.get("goal") or "")[:30]
                     spinner_label = f"🔀 {goal_preview}" if goal_preview else "🔀 delegating"
                 spinner = None
-                if self.quiet_mode and not self.tool_progress_callback:
+                if self.quiet_mode and not self.tool_progress_callback and self._should_start_quiet_spinner():
                     face = random.choice(KawaiiSpinner.KAWAII_WAITING)
                     spinner = KawaiiSpinner(f"{face} {spinner_label}", spinner_type='dots', print_fn=self._print_fn)
                     spinner.start()
@@ -7120,9 +7139,9 @@ class AIAgent:
                     # CLI TUI mode: use prompt_toolkit widget instead of raw spinner
                     # (works in both streaming and non-streaming modes)
                     self.thinking_callback(f"{face} {verb}...")
-                elif not self._has_stream_consumers():
-                    # Raw KawaiiSpinner only when no streaming consumers
-                    # (would conflict with streamed token output)
+                elif not self._has_stream_consumers() and self._should_start_quiet_spinner():
+                    # Raw KawaiiSpinner only when no streaming consumers and the
+                    # spinner output has a safe sink.
                     spinner_type = random.choice(['brain', 'sparkle', 'pulse', 'moon', 'star'])
                     thinking_spinner = KawaiiSpinner(f"{face} {verb}...", spinner_type=spinner_type, print_fn=self._print_fn)
                     thinking_spinner.start()