What Problem This Solves

truncateLine in src/shared/subagents-format.ts truncates a subagent display
label with a raw value.slice(0, maxLength). UTF-16 .slice cuts at a code
unit boundary, so when an emoji (a surrogate pair) straddles the limit the
slice keeps only the high surrogate half. trimEnd() is a no-op on a lone
surrogate, and "..." is appended after it — producing a label that ends in an
unpaired surrogate, which renders as a broken � glyph (or worse, corrupts
downstream encoding).

Repro — truncateLine("y".repeat(47) + "\u{1F600}" + "tail", 48)
(47 ys, then 😀 occupying UTF-16 units 47-48, then tail; string length 53,
max 48):

• Before (wrong): "yyy…y" + "\uD83D" + "..." — the slice stops at unit 48,
  keeping the lone high surrogate \uD83D (the 😀 cut in half) right before the
  ellipsis.
• After (fixed): "yyy…y..." — the cut backs off to the code-point boundary
  before the emoji (unit 47), so the emoji is dropped whole and the label never
  contains a lone surrogate.

The fix reuses the already-shared truncateUtf16Safe helper from src/utils.ts
(the same helper used by tasks/task-status.ts, the embedded-agent tools, etc.).
For any input without a surrogate pair on the boundary, truncateUtf16Safe
returns exactly value.slice(0, maxLength), so non-emoji behavior — including
the trailing-whitespace trim and the short-string passthrough — is unchanged.

Evidence

Node red-green proof. The script replicates the canonical sliceUtf16Safe /
truncateUtf16Safe helpers and both the buggy and fixed truncateLine bodies,
then asserts the repro input maps to the expected output and that unaffected
inputs are byte-for-byte unchanged:

reproInput length: 53 (expected 53)
BEFORE codes: 2e,2e,2e
AFTER  : "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy..."
PASS: RED: buggy output contains a lone surrogate
PASS: RED: buggy output != expected
PASS: GREEN: fixed output === expectedOutput got="yyy…yyy..."
PASS: GREEN: fixed output has no lone surrogate
PASS: unchanged: short string passthrough
PASS: unchanged: trailing-whitespace trim case
PASS: unchanged: ascii truncation matches buggy (no surrogates) fixed="abcd..."
PASS: preserved: emoji below limit kept whole got="yyy…yy😀..."

ALL CHECKS PASSED

(In the RED case, walking the buggy output's code units finds an unpaired high
surrogate \uD83D immediately before the 2e,2e,2e = ... tail.)

A matching unit test is added in src/shared/subagents-format.test.ts
("never cuts a surrogate pair in half when truncating"): it asserts the repro
maps to "y".repeat(47) + "...", scans the result for any lone surrogate code
unit, and confirms an emoji that fully fits under the limit is preserved intact.

🤖 Generated with Claude Code