Postmortem: Claude Code Hanging in Sandbox

Date: 2026-01-19
Severity: P0
Duration: ~4 hours

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SUMMARY

Production agent runs were failing silently. Claude Code started but never produced output, timing out after 15+ minutes.

Root Cause: stdin was configured as "pipe" but never closed, causing Claude Code to hang waiting for EOF.

Why CI missed it: CI uses mock-claude which doesn't check stdin state. Real Claude Code does.

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THE BUG

Before (hangs - stdin pipe never closed):
spawn(cmd, args, { stdio: ["pipe", "pipe", "pipe"] })

After (works - stdin is /dev/null, immediate EOF):
spawn(cmd, args, { stdio: ["ignore", "pipe", "pipe"] })

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HOW WE FOUND IT

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WHY NO ROLLBACK

Release included database migration. Forward-fix was safer.

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INVESTIGATION NOISE

Runner npm publish failure (@vm0/core not built) was unrelated but consumed investigation time.

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ACTION ITEMS

✅ Fix stdin → "ignore" (#1316)
✅ Add spawn unit tests (#1319)
✅ Fix CI publish jobs (#1306, #1318)
🔲 Add real Claude test in CI (TODO)

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KEY LESSON

Mock ≠ Real: CI must include at least one test with real Claude Code to catch behavior differences like stdin handling.

VM0 - undefined undefined