diff --git a/src/slack/send.blocks.test.ts b/src/slack/send.blocks.test.ts
index 2b70b6c29b2..690f95120f0 100644
--- a/src/slack/send.blocks.test.ts
+++ b/src/slack/send.blocks.test.ts
@@ -4,6 +4,52 @@ import { createSlackSendTestClient, installSlackBlockTestMocks } from "./blocks.
 installSlackBlockTestMocks();
 const { sendMessageSlack } = await import("./send.js");
 
+describe("sendMessageSlack NO_REPLY guard", () => {
+  it("suppresses NO_REPLY text before any Slack API call", async () => {
+    const client = createSlackSendTestClient();
+    const result = await sendMessageSlack("channel:C123", "NO_REPLY", {
+      token: "xoxb-test",
+      client,
+    });
+
+    expect(client.chat.postMessage).not.toHaveBeenCalled();
+    expect(result.messageId).toBe("suppressed");
+  });
+
+  it("suppresses NO_REPLY with surrounding whitespace", async () => {
+    const client = createSlackSendTestClient();
+    const result = await sendMessageSlack("channel:C123", "  NO_REPLY  ", {
+      token: "xoxb-test",
+      client,
+    });
+
+    expect(client.chat.postMessage).not.toHaveBeenCalled();
+    expect(result.messageId).toBe("suppressed");
+  });
+
+  it("does not suppress substantive text containing NO_REPLY", async () => {
+    const client = createSlackSendTestClient();
+    await sendMessageSlack("channel:C123", "This is not a NO_REPLY situation", {
+      token: "xoxb-test",
+      client,
+    });
+
+    expect(client.chat.postMessage).toHaveBeenCalled();
+  });
+
+  it("does not suppress NO_REPLY when blocks are attached", async () => {
+    const client = createSlackSendTestClient();
+    const result = await sendMessageSlack("channel:C123", "NO_REPLY", {
+      token: "xoxb-test",
+      client,
+      blocks: [{ type: "section", text: { type: "mrkdwn", text: "content" } }],
+    });
+
+    expect(client.chat.postMessage).toHaveBeenCalled();
+    expect(result.messageId).toBe("171234.567");
+  });
+});
+
 describe("sendMessageSlack blocks", () => {
   it("posts blocks with fallback text when message is empty", async () => {
     const client = createSlackSendTestClient();
diff --git a/src/slack/send.ts b/src/slack/send.ts
index 5905473970f..ede97bafd71 100644
--- a/src/slack/send.ts
+++ b/src/slack/send.ts
@@ -9,6 +9,7 @@ import {
   resolveChunkMode,
   resolveTextChunkLimit,
 } from "../auto-reply/chunk.js";
+import { isSilentReplyText } from "../auto-reply/tokens.js";
 import { loadConfig } from "../config/config.js";
 import { resolveMarkdownTableMode } from "../config/markdown-tables.js";
 import { logVerbose } from "../globals.js";
@@ -231,6 +232,10 @@ export async function sendMessageSlack(
   opts: SlackSendOpts = {},
 ): Promise<SlackSendResult> {
   const trimmedMessage = message?.trim() ?? "";
+  if (isSilentReplyText(trimmedMessage) && !opts.mediaUrl && !opts.blocks) {
+    logVerbose("slack send: suppressed NO_REPLY token before API call");
+    return { messageId: "suppressed", channelId: "" };
+  }
   const blocks = opts.blocks == null ? undefined : validateSlackBlocksArray(opts.blocks);
   if (!trimmedMessage && !opts.mediaUrl && !blocks) {
     throw new Error("Slack send requires text, blocks, or media");