fix(slack): truncate oversized tool input in approval blocks

The HITL approval block serialized the tool input without a bound into a Slack section text field, which is capped near 3000 characters. A large input made chat.postMessage fail with invalid_blocks so the approve and deny controls never appeared. The serialized input is now capped to keep the block within the limit.

Author: ericallam

packages/slack/src/index.test.ts

@@ -50,6 +50,25 @@ describe("slack channel", () => {
     expect(values).toContain("call-1::deny");
   });
 
+  it("renderInteraction truncates an oversized tool input to stay under the Slack block limit", () => {
+    const c = slack({ id: "s-hitl-big", token: "t" });
+    const msg = c.renderInteraction?.(
+      [
+        {
+          toolCallId: "call-big",
+          toolName: "requestApproval",
+          input: { blob: "x".repeat(10_000) },
+        },
+      ],
+      { event: messageEvent(), deliveryId: "d1" }
+    );
+    expect(msg).not.toBeNull();
+    const section = (msg!.blocks as any[]).find((b) => b.type === "section");
+    const text = section.text.text as string;
+    expect(text).toContain("... (truncated)");
+    expect(text.length).toBeLessThan(3000);
+  });
+
   it("onInteraction resolves a block_actions click to a tool output; ignores messages", () => {
     const c = slack({ id: "s-hitl2", token: "t" });
     const approve = c.onInteraction?.({

packages/slack/src/index.ts

@@ -188,6 +188,13 @@ export function slack<TEvent = SlackMessageEvent>(
   });
 }
 
+/**
+ * Cap for the serialized tool input in an approval block. A Slack section `text` field accepts about
+ * 3000 characters; staying well under keeps a large input from failing chat.postMessage with
+ * `invalid_blocks` and dropping the approval controls.
+ */
+const MAX_INTERACTION_INPUT_CHARS = 2500;
+
 /**
  * Default HITL controls: render each pending human-decision tool as a Block Kit approve/deny pair. The
  * button `value` carries `${toolCallId}::${decision}` so `onInteraction` can resolve the exact tool.
@@ -198,7 +205,15 @@ function defaultSlackRenderInteraction(
 ): ChannelMessage | null {
   const call = pending[0];
   if (!call) return null;
-  const detail = call.input !== undefined ? "\n```" + safeStringify(call.input) + "```" : "";
+  let detail = "";
+  if (call.input !== undefined) {
+    const serialized = safeStringify(call.input);
+    const shown =
+      serialized.length > MAX_INTERACTION_INPUT_CHARS
+        ? serialized.slice(0, MAX_INTERACTION_INPUT_CHARS) + "\n... (truncated)"
+        : serialized;
+    detail = "\n```" + shown + "```";
+  }
   return {
     text: `Approval needed: ${call.toolName}`,
     blocks: [