DebugJuniorBackendAuto-graded

Software Engineer: Find out why seats were sold twice

Find and fix the bug that sold 23 theatre seats twice, and prove the fix works.A theatre ticketing service sold 23 seats twice on a sold-out night. Use your coding agent to reproduce the double sales from the night's event log, check the incident note's theory before you trust it, fix the cause without leaving any paying customer empty-handed, make the one call two colleagues disagree on, and prove the fix with tests and the replay's numbers.

30 min

python

Start lab

You're done when

No seat is ever sold twice.
Every paying customer ends with exactly one ticket or one refund, however often the payment is sent.
A customer whose hold is active gets a ticket, a late payer gets the seat if it is free, and the open case follows decision.json, which records your choice and your reason.
bin/test is green on your fix, a test fails on the original code, and broken fixes fail your tests.
verification.json holds Saturday's numbers from your own replays, on the original code and after your last change to platea/.

What it measures

Debugging from evidenceReproducing the double sale before changing code, and checking the incident note's theory against the evidence.

Fixing the root causeFixing why seats were sold twice without turning paying customers away or refunding everyone.

VerificationA test that fails on the original code, tests that catch broken fixes, and numbers that match the final code.

Owning the decisionMaking the call two colleagues disagree on, telling the agent, and writing down why.

Your environment

Terminal + editorA full shell in the browser

Claude CodePre-authenticated, no key setup

Isolated microVMYours alone, wiped afterwards

Start this lab

30 min

Start labYou'll sign in to Anthropos to start. The lab opens in a private cloud workspace with Claude Code ready to use.

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