Cut a long Composio result or failure between characters, not through an emoji

CHANGELOG.md

@@ -8,6 +8,15 @@ Newest first. `Unreleased` is what is on `main` and not yet tagged.
 
 ## Unreleased
 
+### A long Composio result or failure is cut between characters, not through an emoji
+
+A Composio action's answer over 20,000 characters, and a failure sentence as long, were cut by UTF-16
+code unit. When the cut landed inside an emoji or any other character outside the Basic Multilingual
+Plane, the text handed to the model ended on half of it: a lone surrogate that JSON carries as a bare
+`\ud83d` and UTF-8 turns into a replacement character. The cut now stops one unit short in that case,
+the way the MCP and built-in transports' cuts already do. Anything that fits is untouched, and the
+note saying the answer was cut reads as before.
+
 ## 0.0.12
 
 ### A deployment can broker its Bots into a few hundred apps through Composio

server/src/plugins/composio.ts

@@ -1,3 +1,4 @@
+import { cutAtCodeUnits } from "../channels/text";
 import { brokerSentence, flagOf } from "./broker";
 import { type ListedTool, MAX_RESULT_CHARS, type McpCallResult } from "./mcp";
 
@@ -1320,7 +1321,7 @@ function listingSentence(toolkit: string, error: unknown): string {
 function cap(text: string): { text: string; truncated: boolean } {
   if (text.length <= MAX_RESULT_CHARS) return { text, truncated: false };
   return {
-    text: `${text.slice(0, MAX_RESULT_CHARS)}\n\n[truncated]`,
+    text: `${cutAtCodeUnits(text, MAX_RESULT_CHARS)}\n\n[truncated]`,
     truncated: true,
   };
 }

server/tests/composio-transport.test.ts

@@ -2377,6 +2377,59 @@ describe("calling one action", () => {
     expect(result.text.length).toBe(CAPPED_LENGTH);
   });
 
+  test("a result or a failure over the cap is cut between characters, not through an emoji", async () => {
+    /*
+     * `slice` counts UTF-16 code units and an emoji is two of them. When the cap lands between the
+     * halves, the last unit a model reads is a lone high surrogate: JSON carries it as a bare
+     * `\ud83d` and UTF-8 as U+FFFD, a broken character that was never in what Composio sent. A
+     * result and a failure go through the same cap, so both are driven to the same boundary.
+     */
+    const straddling = (lead: number) =>
+      `${"x".repeat(lead)}😀${"x".repeat(100)}`;
+    // `JSON.stringify(data, null, 2)` writes `{\n  "body": "` before the string, so this puts the
+    // emoji's high surrogate on the cap's last unit.
+    const opening = '{\n  "body": "'.length;
+    const answers = [
+      {
+        name: "result",
+        execute: async () =>
+          answered({ body: straddling(RESULT_CAP - opening - 1) }),
+      },
+      {
+        name: "failure",
+        execute: async () => {
+          throw new Error(straddling(RESULT_CAP - 1));
+        },
+      },
+    ];
+
+    const seen: string[] = [];
+    for (const { name, execute } of answers) {
+      useComposioClient(recording({ execute }).client);
+
+      const result = await callTool(
+        { url: "composio://gmail", actorId: "user_asker" },
+        "GMAIL_FETCH_EMAILS",
+        { __version: "20260903_00" },
+      );
+
+      const kept = result.text.endsWith(TRUNCATION_MARKER)
+        ? result.text.slice(0, -TRUNCATION_MARKER.length)
+        : result.text;
+      const last = kept.charCodeAt(kept.length - 1);
+      const lone = last >= 0xd800 && last <= 0xdbff;
+      seen.push(
+        `${name}: truncated ${result.truncated}, marked ${kept !== result.text}, kept ${kept.length}, ends on a lone surrogate ${lone}`,
+      );
+    }
+
+    // The orphan is dropped rather than completed, so the cap is never exceeded.
+    expect(seen).toEqual([
+      `result: truncated true, marked true, kept ${RESULT_CAP - 1}, ends on a lone surrogate false`,
+      `failure: truncated true, marked true, kept ${RESULT_CAP - 1}, ends on a lone surrogate false`,
+    ]);
+  });
+
   test("an empty answer says so in words rather than being empty", async () => {
     useComposioClient(recording({ execute: async () => answered({}) }).client);