BuildMidBackendAuto-graded

Fix a JSON Pointer that mangles escaped tokens

Fiddly escaping rules and clear error reporting decide this one, rather than anything conceptually hard.Paths into JSON documents appear all over the codebase — webhook routing rules, config overrides, event selectors — written as /payload/items/0/sku. That syntax is RFC 6901, JSON Pointer. The resolve() here is close but violates the spec in three subtle ways, and a missing path escapes as a raw KeyError or IndexError straight to the client. Make it match the RFC, and fail cleanly with a ResolveError.

30 min

python

Start lab

You're done when

The RFC's own reference cases all resolve to the values the specification says.
Numeric tokens index into arrays rather than being treated as object keys.
Escaped tokens decode correctly, and in the order the spec mandates — the tilde and slash escapes are not interchangeable.
A path that does not exist raises ResolveError instead of leaking a raw lookup error.
Behaviour matches a reference implementation across a broad set of generated inputs.

What it measures

Data structures and algorithmsWalking a document by path across both object and array nodes, decoding tokens correctly at each step.

Test case designTesting against the specification's own cases and a differential oracle rather than a handful of hand-picked paths.

Specification fidelityHonouring details like escape decoding order, where an implementation that looks right is wrong on exactly the inputs an attacker picks.

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.

AI Labs is part of Anthropos Workforce. Not on your plan yet? Book a demo