Biome Gritql Plugins

Biome GritQL Plugins

Overview

Biome 2.x supports custom lint rules via GritQL .grit files. Rules use pattern matching against the AST. Several patterns look like they should work but silently don't — this skill documents what actually does.

File Structure

rules/
  no-barrel-reexport.grit
  no-direct-gaia-icons-import.grit
biome.json   ← plugins registered here at ROOT level

Rules live at the workspace root in rules/. Sub-directory biome.json files shadow root plugins (see Config Discovery below).

.grit File Format

engine biome(1.0)
language js(typescript, jsx)

`import { $_ } from $source` as $node where {
  $source <: `"some-package"`,
  register_diagnostic(
    span = $node,
    message = "Human-readable error message.",
    severity = "error"
  )
}

Severity options: "error", "warn", "info".

biome.json Registration

{
  "plugins": ["./rules/my-rule.grit"],
  "overrides": [
    {
      "includes": ["src/features/**/index.ts"],
      "plugins": ["./rules/no-barrel-reexport.grit"]
    }
  ]
}

Paths are relative to CWD when biome runs, not to the config file.

What Works vs What Doesn't

Import source matching

// ✅ WORKS — exact string literal in backticks
`import { $_ } from $source` where {
  $source <: `"@my-package"`
}

// ✅ WORKS — multiple sources via or
`import { $_ } from $source` where {
  $source <: or {
    `"@my-package"`,
    `"@my-package/variant-a"`,
    `"@my-package/variant-b"`
  }
}

// ❌ DOES NOT WORK — regex on string content
$source <: r"@my-package"

// ❌ DOES NOT WORK — $_ inside string for partial match
`import { $_ } from "@my-package/$_"`

Regex (r"...") and metavariables inside string literals do not match string content. Enumerate every package variant explicitly in an or clause.

Named import specifier matching

// ✅ WORKS — capture all named imports, match exactly
`import { $_ } from $source`

// ✅ WORKS — positional $_ to find a specific name
or {
  `import { ChevronDown } from "@icons"`,
  `import { ChevronDown, $_ } from "@icons"`,
  `import { $_, ChevronDown } from "@icons"`,
  `import { $_, ChevronDown, $_ } from "@icons"`
} as $node

// ❌ DOES NOT WORK — contains on specifier list
`import { $imports } from "@icons"` where {
  $imports <: contains `ChevronDown`
}

// ❌ DOES NOT WORK — $node <: contains on the full import
`import { $_ } from "@icons"` as $node where {
  $node <: contains `ChevronDown`
}

contains does not recurse into import/export specifier lists. Use explicit positional patterns (add variants for each position).

Export patterns

// ✅ WORKS — star exports
`export * from $source` as $node
`export * as $_ from $source` as $node

// ❌ DOES NOT WORK — metavariables in named export specifiers
`export { $_ } from $source`
`export { $name } from $source`

// ✅ WORKS — exact literal specifier
`export { SpecificName } from "./path"`

Named export specifier lists (export { $_ }) do not match with metavariables in Biome's GritQL engine. Only export * is generically matchable. Named re-exports must be caught via exact patterns or other enforcement.

Critical: Config Discovery and Plugin Shadowing

Plugins only fire from the config Biome stops at when walking up from the file.

workspace-root/
  biome.json          ← has plugins: ["./rules/foo.grit"]
  apps/web/
    biome.json        ← extends "//", NO plugins key
    src/
      file.ts         ← Biome finds apps/web/biome.json first → STOPS HERE

Result: root plugins do NOT fire on file.ts. The sub-directory config shadows the root, even with extends: "//". The extends mechanism does not inherit plugins.

Fix: consolidate into root biome.json

Move sub-directory settings to overrides in the root config, then delete the sub-directory biome.json:

{
  "files": {
    "maxSize": 2097152,
    "includes": ["!!apps/web/public", "...rest of root excludes..."]
  },
  "overrides": [
    {
      "includes": ["apps/web/**"],
      "linter": { "domains": { "next": "recommended" } }
    }
  ],
  "plugins": ["./rules/my-rule.grit"]
}

With no apps/web/biome.json, Biome walks up to the root config for all files — plugins fire everywhere.

Debugging Plugins

# Isolate plugin output (no formatter/import noise)
pnpm biome check --formatter-enabled=false --assist-enabled=false path/to/file.ts

# Verify plugin path resolves (if plugins key is read, wrong path → "Cannot read file.")
# Temporarily set a nonexistent path — if you see the error, the key is active

# Verify rule pattern matches (use absolute path in temp config for fast iteration)
cat > /tmp/test.json << 'EOF'
{
  "$schema": "https://biomejs.dev/schemas/2.3.7/schema.json",
  "plugins": ["/absolute/path/to/rule.grit"]
}
EOF
pnpm biome check --config-path=/tmp/test.json --formatter-enabled=false --assist-enabled=false path/to/file.ts

Start with a catch-all pattern to confirm the plugin loads and the rule fires at all:

engine biome(1.0)
language js(typescript, jsx)

`import $_ from $source` as $node where {
  register_diagnostic(span = $node, message = "catch-all: plugin loaded", severity = "error")
}

If the catch-all fires but your real rule doesn't, the issue is the pattern. If neither fires, the plugin isn't being applied (config discovery problem).

Common Mistakes