fix(workflow): prevent fixed speech continuation

backend/services/fixed_speech.py

@@ -12,6 +12,20 @@ from services.brains.base import BrainRuntime
 from services.runtime_variables import DynamicVariableStore
 
 
+FIXED_SPEECH_CONTEXT_MARKER = "[会话事实：以下固定消息已向用户播报]"
+
+
+def fixed_speech_context_message(content: str) -> dict[str, str] | None:
+    """Keep deterministic speech in context without inviting assistant continuation."""
+    text = content.strip()
+    if not text:
+        return None
+    return {
+        "role": "system",
+        "content": f"{FIXED_SPEECH_CONTEXT_MARKER}\n{text}",
+    }
+
+
 class FixedSpeechOutput:
     """Display and synthesize fixed speech without waiting for playback."""