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

[kevin9327]

What this changes

cap in server/src/plugins/composio.ts is the cap every string the Composio transport puts in front of a model goes through: an action's answer (resultOf) and a failure sentence (failure). It cut at MAX_RESULT_CHARS with slice, which counts UTF-16 code units. When the cut landed between the two halves of an emoji, or any other character outside the Basic Multilingual Plane, the text ended on a lone high surrogate. JSON carries that as a bare \ud83d and UTF-8 as U+FFFD, so the model read a broken character that Composio never sent. Mail bodies, chat messages and documents are exactly the answers long enough to reach the cap and likely to hold emoji.

#525 fixed this cut in the MCP transport and both built-in transports with cutAtCodeUnits in channels/text.ts. The Composio transport landed afterwards (#481) with its own slice, so this is the fourth transport's copy of the same cut. It now calls the same helper: one import and one call.

Text that fits, a cut that lands between characters, the [truncated] note and truncated: true are all unchanged. The result is at most one code unit shorter, never longer than the cap.

Where it runs

• New state that outlives a request? None. A pure string function on the call path.
• What happens on the second replica? Same as the first; nothing is held.
• Anything serialised? No.
• Anything fanned out to a browser? No.
• New listener, port, or schedule? No.

Boundary and audit

• Every acting call still goes through the gateway: resolve, decide, audit, then act. The call path is untouched; only the text of an answer that was already being cut changes.
• New refusals and new failures each write a row. None are added.
• Nothing new is trusted from the client that the server can resolve itself.

Changelog

• A line in CHANGELOG.md under Unreleased.

Unreleased was empty, so this entry sits at its top. If another PR lands there first, I'm happy to rebase.

Proof

New test in server/tests/composio-transport.test.ts: "a result or a failure over the cap is cut between characters, not through an emoji". It drives both answers through callTool with an emoji whose high surrogate lands on the cap's last unit. The result goes through JSON.stringify(data, null, 2), and the test accounts for its 12-unit opening.

On main without the fix, both come back cut through the emoji:

error: expect(received).toEqual(expected)

  [
-   "result: truncated true, marked true, kept 19999, ends on a lone surrogate false",
-   "failure: truncated true, marked true, kept 19999, ends on a lone surrogate false",
+   "result: truncated true, marked true, kept 20000, ends on a lone surrogate true",
+   "failure: truncated true, marked true, kept 20000, ends on a lone surrogate true",
  ]
(fail) calling one action > a result or a failure over the cap is cut between characters, not through an emoji

With the fix:

• bun test tests/composio-transport.test.ts tests/mcp-result.test.ts: 122 pass, 0 fail. This includes the existing boundary test ("a result the exact size of the cap is not cut, and one character more is") and "a result is capped visibly rather than silently", both unchanged.
• bun run typecheck (app, server, worker): exit 0.
• bunx biome check on the changed files: clean.

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[davidmckayv]

Code-verified clean; CI green on this sha.