Sample

Set#to_json does not produce a JSON array: it inherits the Object fallback from json gem, which calls to_s.to_json and returns an inspect string, silently corrupting any round-trip

sha256:22a9d32f68b3b4b16f4061794878c5f7c13b8287e92bacc2a67e87c730f3d6b5

PUBLISHED L3_CONTRACT_PASS MIT-0

Case

Goal: Set#to_json does not produce a JSON array: it inherits the Object fallback from json gem, which calls to_s.to_json and returns an inspect string, silently corrupting any round-trip HOW
Packages: set 1.1.1 json 2.9.1
Environment: ruby
Created: 2026-08-17T02:47:04Z

Set[1,2,3].to_json returns the string "[1,2,3]" because Set is an Enumerable collection and the json gem is expected to serialize it like an Array.

The sample's author recorded this as what a developer or model would expect here. The contract below is what actually ran.

Set[1,2,3].to_json returns a JSON string literal (begins with a double-quote character) rather than the JSON array "[1,2,3]", because Set inherits to_json from JSON::Ext::Generator::GeneratorMethods::Object which delegates to to_s.to_json
JSON.parse(Set[1,2,3].to_json) returns a String instance, not a Set or Array
The round-tripped string contains the inspect marker '#<Set:' so none of the original elements are recoverable from the JSON output
Array[1,2,3].to_json round-trips correctly as a JSON array (control: the behaviour is specific to Set, not a general json-gem limitation)
A Set embedded as a Hash value also serialises its value as a JSON string after JSON.parse

ruby 3 · CONTAINER_RUN · compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS · rubygems@1 · 2026-08-17 · ed25519:d91480838ac982c9