Cut a long control name or value in a page snapshot between characters, not through an emoji

CHANGELOG.md

@@ -8,6 +8,13 @@ Newest first. `Unreleased` is what is on `main` and not yet tagged.
 
 ## Unreleased
 
+### A long control name or value in a page snapshot is cut between characters
+
+The computer's page snapshot keeps the first 200 UTF-16 code units of each control's accessible
+name and value. When that limit fell between the two halves of an emoji, the Bot was handed text
+ending on half a character, which reads as U+FFFD: a broken character that is not on the page, often
+at the end of a message the Bot had just typed into a text box. The cut now stops one code unit
+short in that case, the same rule tool results and relayed answers already follow.
 ### A Bot's shell can no longer read the deployment's keys from a neighbouring process
 
 In the all-in-one image the API and the browser ran under one account, and a Bot's shell — a child

agent-computer/src/aria-snapshot.ts

@@ -156,6 +156,20 @@ export function parseDescriptor(text: string): Descriptor | null {
   return { role, name, flags };
 }
 
+/**
+ * The first `limit` UTF-16 code units of `text`, one fewer when the cut would split a character.
+ *
+ * `slice` counts code units and an emoji is two, so a limit landing between the halves leaves a lone
+ * high surrogate last: JSON carries it as a bare `\ud83d` and UTF-8 as U+FFFD, and the Bot reads a
+ * broken character that is not on the page. The server's `cutAtCodeUnits` is the same rule; this
+ * process shares no code with the server, so it is repeated here rather than imported.
+ */
+function cutAtCodeUnits(text: string, limit: number): string {
+  const sliced = text.slice(0, limit);
+  const last = sliced.charCodeAt(sliced.length - 1);
+  return last >= 0xd800 && last <= 0xdbff ? sliced.slice(0, -1) : sliced;
+}
+
 /** Build an element from a descriptor and whatever YAML gave as its value, or null if not actionable. */
 function toElement(
   descriptor: Descriptor,
@@ -170,13 +184,13 @@ function toElement(
   const element: SnapshotElement = {
     ref,
     role: descriptor.role,
-    name: descriptor.name.slice(0, 200),
+    name: cutAtCodeUnits(descriptor.name, 200),
   };
 
   // Values arrive as text, with quoting and escapes already resolved.
   if (typeof value === "string") {
     const text = value.trim();
-    if (text) element.value = text.slice(0, 200);
+    if (text) element.value = cutAtCodeUnits(text, 200);
   }
 
   if (descriptor.flags.has("disabled")) element.disabled = true;

agent-computer/tests/aria-snapshot.test.ts

@@ -167,6 +167,39 @@ describe("parseAriaSnapshot, against captured output", () => {
   });
 });
 
+/**
+ * A name or value longer than the 200 kept is cut, and the cut counts UTF-16 code units.
+ *
+ * An emoji is two of them. With its first half as the 200th unit, a plain `slice` leaves a lone high
+ * surrogate as the last character of what the Bot is handed: JSON carries it as a bare `\ud83d` and
+ * UTF-8 as U+FFFD, so the Bot reads back a broken character that is not on the page, in text it may
+ * well have typed itself.
+ */
+describe("a name or value too long to keep whole", () => {
+  const CUT_THROUGH = `${"a".repeat(199)}😀 and the rest`;
+  const ENDS_INSIDE = `${"a".repeat(198)}😀 and the rest`;
+
+  test("a name cut inside a character loses the whole character, not half of it", () => {
+    const { elements } = parseAriaSnapshot(`- link "${CUT_THROUGH}" [ref=e1]`);
+    expect(elements[0]?.name).toBe("a".repeat(199));
+  });
+
+  test("a value cut inside a character loses the whole character, not half of it", () => {
+    const { elements } = parseAriaSnapshot(
+      `- textbox "Message" [ref=e1]: ${CUT_THROUGH}`,
+    );
+    expect(elements[0]?.value).toBe("a".repeat(199));
+  });
+
+  test("a character that ends at the cut is kept", () => {
+    const { elements } = parseAriaSnapshot(
+      `- textbox "${ENDS_INSIDE}" [ref=e1]: ${ENDS_INSIDE}`,
+    );
+    expect(elements[0]?.name).toBe(`${"a".repeat(198)}😀`);
+    expect(elements[0]?.value).toBe(`${"a".repeat(198)}😀`);
+  });
+});
+
 describe("values a real parser handles and a pattern got wrong", () => {
   test("a quoted numeric value is not left with its quotes", () => {
     // Numeric-looking text remains a string, so one-time codes are not coerced.