The Retry That Charged Twice Debugging Lab

The Retry That Charged TwiceReliabilityHardPreparing workspaceAutosaved

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Incident Dossier

The Retry That Charged Twice

A payment client retries on transient transport failures. The first attempt actually succeeded server-side but the response was dropped, so the client retries. The retry uses a fresh idempotency key, the server treats it as a brand-new charge, and the customer is charged twice.

Initial failing signalretry double-records the chargeExpected ledger.length == 1 · received ledger.length == 2

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Look at where the idempotency key is built for each attempt.
Notice the key includes the attempt index, so retries look like new charges.
Compute the key once, outside the retry loop, so retries share it.

Focus Mode

Start

src/payment.js

Likely area

src/payment.js

Progressive hints

Hints reveal broad direction first, then narrow toward the suspect path without showing hidden tests or exact solution code.

TopicReliability

DifficultyHard

Est. time25 min

Workspacecreating

Explorer

Preparing workspace files…

Evidence Map

Failing signal loaded from challenge API
Workspace files loaded from workspace API
Results rendered only after API run/submit

Autosaved

workspace

Outputread-onlyready$ workspace boot

Workspace booting...