From 3bfd093cdba945e7ebcfe79d800a4565d5303ba0 Mon Sep 17 00:00:00 2001
From: Vincent Koc <vincentkoc@ieee.org>
Date: Sun, 15 Mar 2026 20:39:43 -0700
Subject: [PATCH] Slack: fail oversized merged block payloads

---
 .../outbound/slack.sendpayload.test.ts        | 23 +++++++++++++++++++
 src/channels/plugins/outbound/slack.ts        |  4 +++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/src/channels/plugins/outbound/slack.sendpayload.test.ts b/src/channels/plugins/outbound/slack.sendpayload.test.ts
index ffa86d8ed95..0bb551d0395 100644
--- a/src/channels/plugins/outbound/slack.sendpayload.test.ts
+++ b/src/channels/plugins/outbound/slack.sendpayload.test.ts
@@ -144,4 +144,27 @@ describe("slackOutbound sendPayload", () => {
     );
     expect(result).toMatchObject({ channel: "slack", messageId: "sl-controls" });
   });
+
+  it("fails when merged Slack blocks exceed the platform limit", async () => {
+    const { run, sendMock } = createHarness({
+      payload: {
+        channelData: {
+          slack: {
+            blocks: Array.from({ length: 50 }, () => ({ type: "divider" })),
+          },
+        },
+        interactive: {
+          blocks: [
+            {
+              type: "buttons",
+              buttons: [{ label: "Allow", value: "pluginbind:approval-123:o" }],
+            },
+          ],
+        },
+      },
+    });
+
+    await expect(run()).rejects.toThrow(/Slack blocks cannot exceed 50 items/i);
+    expect(sendMock).not.toHaveBeenCalled();
+  });
 });
diff --git a/src/channels/plugins/outbound/slack.ts b/src/channels/plugins/outbound/slack.ts
index 9785abf503a..13729acb2ee 100644
--- a/src/channels/plugins/outbound/slack.ts
+++ b/src/channels/plugins/outbound/slack.ts
@@ -139,7 +139,9 @@ function resolveSlackBlocks(payload: {
     return undefined;
   }
   if (mergedBlocks.length > SLACK_MAX_BLOCKS) {
-    return existingBlocks?.length ? existingBlocks : undefined;
+    throw new Error(
+      `Slack blocks cannot exceed ${SLACK_MAX_BLOCKS} items after interactive render`,
+    );
   }
   return mergedBlocks;
 }