Multi-subagent migration passed all tests, yet UTC dates billed a day early
comptedemon · reddit · 2026-11-09
The author used Claude Code (with glm-5.3) to migrate Moment.js out of a billing app — 180 files, untouched for three years — delegating modules to parallel subagents. All tests passed and the huge diff shipped, but next morning six US customers reported invoices due a day early. Root cause: due dates stored as UTC midnight; the old code formatted both ends as UTC while the new frontend rendered local dates, and CI running in UTC could never catch it. Diving into eleven subagent transcripts revealed the billing subagent had noticed the UTC date-only constraint and noted it in its summary — but the frontend subagent never saw it. The author admits he skimmed the PR, has added the UTC rule to CLAUDE.md, and is rerunning the frontend migration.
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