fix(web): big terminal selections no longer break sending

apps/web/src/lib/composerContextRecords.test.ts

@@ -255,7 +255,7 @@ describe("composerContextRecords", () => {
     expect(removeInlineContextReference(prompt, reference.contextId).prompt).toBe("prose tail");
   });
 
-  it("clamps an oversized review selection so the record still encodes", () => {
+  it("clamps oversized review and terminal selections so the records still encode", () => {
     const build = (diffLength: number) =>
       reviewCommentContextRecord({
         id: "rc-big",
@@ -276,6 +276,21 @@ describe("composerContextRecords", () => {
     expect(overLimit.diff.endsWith("… truncated …")).toBe(true);
     expect(() => decodeMessageContext({ version: 1, records: [atLimit] })).not.toThrow();
     expect(() => decodeMessageContext({ version: 1, records: [overLimit] })).not.toThrow();
+
+    const terminal = terminalContextRecord({
+      id: "term-big",
+      threadId: ThreadId.make("t"),
+      createdAt: "2026-01-01T00:00:00.000Z",
+      terminalId: "default",
+      terminalLabel: "Terminal 1",
+      lineStart: 1,
+      lineEnd: 2_000,
+      text: `${"t".repeat(63_985)}${"😀".repeat(8)}`,
+    });
+    expect(terminal.text).toBe(`${"t".repeat(63_985)}\n… truncated …`);
+    expect(decodeMessageContext({ version: 1, records: [terminal] }).records).toMatchObject([
+      { kind: "terminal", text: terminal.text },
+    ]);
   });
 
   it("finds the destination collision for different legacy terminal messages", () => {

apps/web/src/lib/composerContextRecords.ts

@@ -5,6 +5,7 @@ import {
 import {
   COMPOSER_CONTEXT_REVIEW_DIFF_MAX_CHARS,
   COMPOSER_CONTEXT_REVIEW_TEXT_MAX_CHARS,
+  COMPOSER_CONTEXT_TERMINAL_TEXT_MAX_CHARS,
 } from "@t3tools/contracts";
 import type {
   ComposerContextId,
@@ -52,7 +53,11 @@ const TRUNCATION_MARKER = "\n… truncated …";
 
 function clampContextText(value: string, max: number): string {
   if (value.length <= max) return value;
-  return `${value.slice(0, Math.max(0, max - TRUNCATION_MARKER.length))}${TRUNCATION_MARKER}`;
+  let end = Math.max(0, max - TRUNCATION_MARKER.length);
+  // Never end on a lone high surrogate.
+  const last = value.charCodeAt(end - 1);
+  if (last >= 0xd800 && last <= 0xdbff) end -= 1;
+  return `${value.slice(0, end)}${TRUNCATION_MARKER}`;
 }
 
 type ReviewCommentPresentation = ReviewCommentContext | ReviewCommentContextRecord;
@@ -187,7 +192,10 @@ export function terminalContextRecord(context: TerminalContextDraft): TerminalCo
     terminalLabel: context.terminalLabel,
     lineStart: context.lineStart,
     lineEnd: context.lineEnd,
-    text: normalizeTerminalContextText(context.text),
+    text: clampContextText(
+      normalizeTerminalContextText(context.text),
+      COMPOSER_CONTEXT_TERMINAL_TEXT_MAX_CHARS,
+    ),
   };
 }