Biome GritQL Custom Lint Rules

Create AST-based custom lint rules using Biome's GritQL plugin system.

Quick Start

Create a rule file (rules/no-use-effect-fetch.grit):

`useEffect($callback, $deps)` where {
    $callback <: contains `fetch`,
    register_diagnostic(
        span = $callback,
        message = "Don't fetch inside useEffect. Use TanStack Query instead.",
        severity = "error"
    )
}

Add to biome.json:

{
  "plugins": ["./rules/no-use-effect-fetch.grit"]
}

Run: bunx biome check or any equivalent command to run linter in the codebase

GritQL Syntax

Basic Pattern Matching

`pattern_to_match` where {
    register_diagnostic(
        span = $matched_var,
        message = "Your message here",
        severity = "error"  // hint | info | warn | error
    )
}

make sure to read documentation

• BiomeJS Documentation
• Blog examples
• BiomeJS GitHub Repository
• GritQL GitHub Repository

Variables

• $name - captures any single node
• $args - captures arguments
• $_ - wildcard (match but don't capture)

Operators

Operator	Meaning
<:	matches / contains
<: contains	deep contains
<: r"regex"	regex match
not	negation
and	both conditions
or	either condition

Common Rule Patterns

Ban a Function Call

`console.log($args)` where {
    register_diagnostic(
        span = $args,
        message = "Remove console.log before committing",
        severity = "warn"
    )
}

Ban a Hook (React)

`useMemo($args)` where {
    register_diagnostic(
        span = $args,
        message = "useMemo unnecessary with React Compiler. Remove it.",
        severity = "warn"
    )
}

Ban Dynamic Imports

`await import($path)` where {
    register_diagnostic(
        span = $path,
        message = "Use static imports at the top of the file",
        severity = "error"
    )
}

Require Pattern in Context

`useState($initial)` where {
    not $initial <: contains `atom`,
    register_diagnostic(
        span = $initial,
        message = "Prefer Jotai atoms over useState for shared state",
        severity = "info"
    )
}

CSS Rules

language css;

`$selector { $props }` where {
    $props <: contains `color: $color` as $rule,
    not $selector <: r"\.color-.*",
    register_diagnostic(
        span = $rule,
        message = "Use .color-* utility classes instead of explicit colors"
    )
}

Project Structure

project/
├── biome.json
└── rules/
    ├── no-console-log.grit
    ├── no-use-memo.grit
    ├── no-use-callback.grit
    ├── no-dynamic-import.grit
    └── no-fetch-in-effect.grit

biome.json Configuration

{
  "$schema": "https://biomejs.dev/schemas/2.0.0/schema.json",
  "plugins": [
    "./rules/no-console-log.grit",
    "./rules/no-use-memo.grit",
    "./rules/no-use-callback.grit"
  ],
  "linter": {
    "enabled": true
  }
}

Creating Rules Workflow

1. Identify the pattern to ban/enforce
2. Create .grit file in rules/ directory
3. Write the GritQL pattern with clear error message
4. Add path to biome.json plugins array
5. Test with bunx biome check or any equivalent command to run linter in the codebase

Guidelines

• Keep error messages actionable - tell the user what to do instead
• Use severity = "error" for hard requirements, "warn" for preferences
• Put all .grit files in a rules/ directory
• Name files descriptively: no-X.grit or prefer-Y.grit
• Test rules against real code before committing

Why This Matters

Instructions in CLAUDE.md degrade as context fills up. Linter rules provide immediate, persistent feedback. When Claude violates a pattern, it sees the error and self-corrects - no context drift.

Resources

• BiomeJS Documentation
• Blog examples
• BiomeJS GitHub Repository
• GritQL GitHub Repository