diff --git a/src/agents/system-prompt.test.ts b/src/agents/system-prompt.test.ts
index e63900ef9..cfed9fb45 100644
--- a/src/agents/system-prompt.test.ts
+++ b/src/agents/system-prompt.test.ts
@@ -216,6 +216,7 @@ describe("buildAgentSystemPrompt", () => {
 
     expect(prompt).toContain("message: Send messages and channel actions");
     expect(prompt).toContain("### message tool");
+    expect(prompt).toContain("respond with ONLY: NO_REPLY");
   });
 
   it("includes runtime provider capabilities when present", () => {
diff --git a/src/agents/system-prompt.ts b/src/agents/system-prompt.ts
index 7c53e501d..dc5c289d5 100644
--- a/src/agents/system-prompt.ts
+++ b/src/agents/system-prompt.ts
@@ -95,6 +95,7 @@ function buildMessagingSection(params: {
           "- Use `message` for proactive sends + channel actions (polls, reactions, etc.).",
           "- For `action=send`, include `to` and `message`.",
           `- If multiple channels are configured, pass \`channel\` (${params.messageChannelOptions}).`,
+          `- If you use \`message\` (\`action=send\`) to deliver your user-visible reply, respond with ONLY: ${SILENT_REPLY_TOKEN} (avoid duplicate replies).`,
           params.inlineButtonsEnabled
             ? "- Inline buttons supported. Use `action=send` with `buttons=[[{text,callback_data}]]` (callback_data routes back as a user message)."
             : params.runtimeChannel