core/json-invariant — assert a structured file's shape

Hold a JSON file to a shape rule — like “package.json scripts must stay empty”. Parses the file properly, so formatting doesn’t matter.

What it catches

A structured config file drifting from its intended shape. The starter example: keeping package.json scripts empty so the justfile stays the single command source.

Config

rules:
  justfile:
    - id: no-package-scripts
      use: core/json-invariant
      on: "package.json"
      assert: { path: scripts, keysPrefixedWith: "//" }
      message: "package.json scripts must stay empty — commands live in the justfile."

Field	What it does
`on`	Glob(s) for the JSON file(s) to check.
`assert.path`	A dotted path into the JSON to inspect (e.g. `scripts`, `compilerOptions.paths`).
`assert.keysPrefixedWith`	Every key at that path must start with this prefix (here, only `//` comment keys are allowed). Anything else is a violation.
`message`	The reason, shown on a block.

Legal vs illegal

{ "scripts": { "//": "see justfile" } }            ✓ passes
{ "scripts": { "//": "x", "dev": "vite" } }        ✗ blocked: "scripts.dev"

Could this be ast-grep? Possibly — there are JSON/YAML grammars, so some structured-file checks could fold into core/ast-grep. Today core/json-invariant is the simple path for JSON; a bespoke structured-file rule (YAML, the justfile) is a small own-script until it earns a place in core.