From 25fc82d255851c028bdb3831673bcf00aea62dc2 Mon Sep 17 00:00:00 2001
From: kevin9327 <5299031+kevin9327@users.noreply.github.com>
Date: Sat, 31 Oct 2026 12:24:39 +0900
Subject: [PATCH] Cut a tool's link fields and a vendor's refusal between
 characters, not inside an emoji

---
 CHANGELOG.md                    |  7 +++++++
 server/src/plugins/mcp.ts       |  4 ++--
 server/tests/mcp-result.test.ts | 10 ++++++++++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2f9d3f16f4..113faadb88 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -78,6 +78,13 @@ Newest first. `Unreleased` is what is on `main` and not yet tagged.
   `COMPUTER_MEMORY_HIGH_PERCENT=0` and `COMPUTER_TAB_IDLE_MS=0` (Helm: `computers.memoryHighPercent`
   and `computers.tabIdleMs`) keep the old behaviour.
 
+### A tool's link and a vendor's refusal are cut between characters
+
+When a tool answers with a link, its title, name and description are each cut to 400 characters
+before the Bot reads them, and the reason a vendor gives for refusing a request is cut to 400 too.
+When the cut landed inside an emoji, the Bot was handed a broken character before the "…". The cut
+now drops the half character instead.
+
 ### A computer closes browsers nobody is using when its memory runs high
 
 A computer runs one browser for each Bot that is browsing, and the only limits were a count and half
diff --git a/server/src/plugins/mcp.ts b/server/src/plugins/mcp.ts
index 49d9db9959..6175093bdd 100644
--- a/server/src/plugins/mcp.ts
+++ b/server/src/plugins/mcp.ts
@@ -74,7 +74,7 @@ function resourceLinkText(item: {
     typeof value === "string" && value.trim() !== "" ? value : null;
   const bounded = (value: string) =>
     value.length > LINK_FIELD_CHARS
-      ? `${value.slice(0, LINK_FIELD_CHARS)}…`
+      ? `${cutAtCodeUnits(value, LINK_FIELD_CHARS)}…`
       : value;
   const lines: string[] = [];
   const uri = field(item.uri);
@@ -274,7 +274,7 @@ function reasonFrom(error: unknown): string | null {
 
 /** Long enough for a sentence and a URL, short enough not to be the wall of JSON this replaced. */
 const trimmed = (value: string) =>
-  value.length > 400 ? `${value.slice(0, 400)}…` : value;
+  value.length > 400 ? `${cutAtCodeUnits(value, 400)}…` : value;
 
 /**
  * A vendor's failure, as one sentence an operator can act on.
diff --git a/server/tests/mcp-result.test.ts b/server/tests/mcp-result.test.ts
index a93dbc39c8..0e8232e481 100644
--- a/server/tests/mcp-result.test.ts
+++ b/server/tests/mcp-result.test.ts
@@ -195,6 +195,16 @@ describe("a result with something in it", () => {
     expect(text).toBe(`uri: file:///a.md\ndescription: ${"d".repeat(400)}…`);
   });
 
+  test("cuts a resource_link's long field between characters, not inside an emoji", () => {
+    // 399 units of text and then an emoji: the 400-unit cut lands between its two halves.
+    const description = `${"d".repeat(399)}${"🙂".repeat(10)}`;
+    const { text } = resultText([
+      { type: "resource_link", uri: "file:///a.md", description },
+    ]);
+    expect(text).toBe(`uri: file:///a.md\ndescription: ${"d".repeat(399)}…`);
+    expect(text.isWellFormed()).toBe(true);
+  });
+
   test("a resource_link that names nothing is still named, rather than dropped", () => {
     expect(resultText([{ type: "resource_link" }]).text).toBe(
       "[resource_link]",