5. cheshire for JSON on the JVM, js/JSON elsewhere

Status: accepted, 2026-10-16

Context

The HTTP transport and decision handoff both need JSON. ClojureScript and nbb have js/JSON, but the JVM has no JSON support in the standard library. The options were to resolve clojure.data.json at runtime if it happened to be there, to make the host pass in encode and decode functions, or to take a real dependency.

Decision

fieldtest depends on cheshire on the JVM. fieldtest.json only requires it inside a #?(:clj ...) branch and uses js/JSON in the :cljs branch.

On every runtime, keywords encode with their namespace (:a/b becomes "a/b"), insts encode as UTC ISO-8601 with milliseconds, and object keys decode to keywords.

Keeping cheshire out of nbb and ClojureScript

nbb and ClojureScript builds must never load cheshire. Two checks make sure of that:

• bb lint:no-infra reads every file under src/ with the :cljs and nbb reader features and fails if any cheshire symbol is still there.
• test-consumers/nbb is a small nbb project that depends on fieldtest through :local/root, which puts the cheshire and promesa jars on its classpath. It checks that cheshire never loads, that nbb uses its own promesa instead of the jar's source, and that decide, branch and handoff all work. It runs as part of bb test.

Consequences

JVM projects pull in cheshire and Jackson. If a project already uses another JSON library, it ends up with both. A project that never touches fieldtest.http or fieldtest.json can exclude cheshire with :exclusions, because nothing else requires it.