biome-anti-slop

Opinionated Biome lint rules that reject low-evidence TypeScript and JavaScript patterns: unknown smuggled through contracts, type assertions that fabricate evidence, dictionary types that erase their values, module mocks standing in for real seams.

A port of dmmulroy/anti-slop from Oxlint's JavaScript plugin API to Biome's GritQL plugins. The rules and opinions are upstream's; the implementations are new.

Biome plugins have no scope analysis, type information, or dataflow, so some rules catch less than the Oxlint originals. Every divergence is measured in docs/PARITY.md.

Install

npm install --save-dev biome-anti-slop
npx biome-anti-slop init

init adds one line to biome.json:

{
	"extends": ["biome-anti-slop"]
}

Then verify the rules load:

npx biome-anti-slop doctor

Requires Biome 2.5.0+ and a node_modules directory.

Start smaller

recommended drops the five noisiest rules:

{
	"extends": ["biome-anti-slop/recommended"]
}

Or list rules individually:

{
	"plugins": [
		"./node_modules/biome-anti-slop/plugins/no-unknown-returns.grit",
		"./node_modules/biome-anti-slop/plugins/no-chained-type-assertions.grit"
	]
}

Write paths from the project root, ./node_modules/ prefix included. Biome resolves plugin paths against the consuming project, not against the config that declares them.

Vendor instead

npx biome-anti-slop vendor biome/anti-slop

Copies the .grit files into your repo and prints the plugins array. The files are then yours to edit.

With a coding agent

skills/install-biome-anti-slop/ installs, verifies, and reports the new diagnostic count per rule.

Rules

| Rule | Rejects | Parity | | --- | --- | --- | | no-chained-type-assertions | nested assertions that fabricate evidence, input as object as User | full | | no-conditional-empty-object-spread | conditional spreads that use {} to omit fields | approximate | | no-known-value-widening | broad target types that discard known value evidence | partial | | no-module-mocking | Vitest and Jest module mocks in place of real dependency seams | partial | | no-object-parameters | the broad object type on function inputs | partial | | no-reflect-apply | Reflect.apply in place of typed function calls | partial | | no-reflect-get | Reflect.get in place of typed property access | partial | | no-runtime-typeof | ad hoc typeof narrowing instead of boundary parsing | partial | | no-shape-in-symbol-names | shape in symbol names, interface UserShape | partial | | no-unknown-parameters | unknown inputs, except the explicit cause convention | partial | | no-unknown-returns | contracts returning unknown or Promise<unknown> | partial | | no-unknown-type-aliases | aliases that merely conceal unknown | partial | | no-unsafe-dictionary-type | dictionaries valued by unknown, any, object, {} | partial | | no-widen-then-assert | flows that widen known values and later assert them back | partial | | require-safety-comment-for-type-assertion | assertions that do not document their checked invariant | partial |

recommended excludes no-known-value-widening, no-runtime-typeof, no-shape-in-symbol-names, no-widen-then-assert, and require-safety-comment-for-type-assertion.

Suppressing a diagnostic

// biome-ignore lint: parsed by the boundary schema directly above
const config = raw as Config;

Biome puts every plugin diagnostic in one plugin category, so a suppression comment silences all anti-slop rules on that line. There is no per-rule suppression, and rules cannot be toggled through linter.rules. Granularity comes from which plugins you enable.

Troubleshooting

Run npx biome-anti-slop doctor first.

Error(s) during loading of plugins: Cannot read file means the package is not resolvable from the project root.

• pnpm: works for a direct dependency. If it is only transitive, add it directly.
• Monorepos: install it in the package whose biome.json you are editing.
• Yarn PnP: no node_modules to point at, so use npx biome-anti-slop vendor.

A plugin that fails to load aborts the entire lint run: Biome lints nothing and --reporter=json emits no JSON. Do not read a quiet run as a clean one.

No diagnostics at all: check extends contains biome-anti-slop, and that files.includes covers the code you expect to lint.

Development

npm install
npm test

• npm run test:rules runs every rule against its fixtures. Fixtures mark expected violations with a trailing //~ error; the harness fails on false negatives and false positives, and treats a plugin that fails to compile as a hard failure. 924 violation cases, ~1,900 lines of valid code.
• npm run test:robustness runs every rule against hostile constructs (destructuring, braceless switch cases, deep parens, JSX, generated-size files) under a time budget. Both failure modes it catches are silent: a rule that aborts reports nothing and looks like a clean codebase, and an accidentally quadratic rule fails as a hang rather than a wrong answer. Neither shows up in a rule's own fixtures, and both shipped once.
• npm run test:cli covers the installer, including that it never clobbers an existing config.
• npm run test:install builds a throwaway consumer project and proves the install resolves.
• npm run check verifies plugins, presets, fixtures, and docs have not drifted apart.

Read docs/GRITQL-NOTES.md before changing a rule. It records what Biome's GritQL engine can and cannot express, verified empirically.

License

MIT. Ported from dmmulroy/anti-slop (MIT, © Dillon Mulroy).