[Bug] Subagent Task tool spawns terminate during setup phase with spurious "user interrupt" on Windows

Bug Description
Subagent spawns (Task tool) consistently die during their setup phase (~20-30 tool calls in, typically during heavy initial file reading / repo copying), reported as "user interrupt" even when the user confirms no interaction (verified 4+ times, hands off keyboard, Alt-Tab away). Retries in foreground sometimes work; background spawns also stalled via watchdog (600s no progress). Reproducible across multiple sessions and days on Windows. Full reproducible writeup with timestamps and patterns is in our repo: docs/harness-bug-spawn-setup.md (beslagic/ai-embedded-starter). Impact: independent verification agents must run inline instead (weaker guarantees), and per-box token measurements come back empty (12 unmeasured boxes in one pass). Environment: Windows, native terminal (MINGW64 paths visible), Claude Code with Opus 5 / Sonnet 5, large repo context.

Environment Info

• Platform: win32
• Terminal: cursor
• Version: 2.1.222
• Feedback ID: 56959448-1c83-456d-9478-855021732d5a

Errors

[{"error":"TelemetrySafeError: VirtualMessageList: itemKeys/messages length desync (keys=283 messages=282 range=[180,219))\n    at h8S (B:/~BUN/root/src/entrypoints/cli.js:22821:8181)\n    at VJf (B:/~BUN/root/src/entrypoints/cli.js:22821:1103)\n    at En (B:/~BUN/root/src/entrypoints/cli.js:2543:21369)\n    at oT (B:/~BUN/root/src/entrypoints/cli.js:2543:40466)\n    at PF (B:/~BUN/root/src/entrypoints/cli.js:2543:51390)\n    at K$e (B:/~BUN/root/src/entrypoints/cli.js:2543:89052)\n    at Vke (B:/~BUN/root/src/entrypoints/cli.js:2543:87996)\n    at q$e (B:/~BUN/root/src/entrypoints/cli.js:2543:87815)\n    at sP (B:/~BUN/root/src/entrypoints/cli.js:2543:84072)\n    at Je (B:/~BUN/root/src/entrypoints/cli.js:2543:6665)","timestamp":"2026-08-06T01:12:10.251Z"}]