Purpose

Use this skill when creating diagnostics - the error messages, warnings, and hints shown to users. Covers the Diagnostic trait, advice types, and best practices for clear, actionable messages.

Prerequisites

•Read crates/biome_diagnostics/CONTRIBUTING.md for concepts
•Understand Biome's Technical Principles
•Follow the "show don't tell" philosophy

Diagnostic Principles

•Explain what - State what the error is (diagnostic message)
•Explain why - Explain why it's an error (advice notes)
•Tell how to fix - Provide actionable fixes (code actions, diff advice, command advice)

Follow Technical Principles:

•Informative: Explain, don't just state
•Concise: Short messages, rich context via advices
•Actionable: Always suggest how to fix
•Show don't tell: Prefer code frames over textual explanations

Common Workflows

Create a Diagnostic Type

Use the #[derive(Diagnostic)] macro:

rust

use biome_diagnostics::{Diagnostic, category};

#[derive(Debug, Diagnostic)]
#[diagnostic(
    severity = Error,
    category = "lint/correctness/noVar"
)]
struct NoVarDiagnostic {
    #[location(span)]
    span: TextRange,
    
    #[message]
    #[description]
    message: MessageAndDescription,
    
    #[advice]
    advice: NoVarAdvice,
}

#[derive(Debug)]
struct MessageAndDescription;

impl fmt::Display for MessageAndDescription {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Use 'let' or 'const' instead of 'var'")
    }
}

Implement Advices

Create advice types that implement Advices trait:

rust

use biome_diagnostics::{Advices, Visit};
use biome_console::markup;

struct NoVarAdvice {
    is_const_candidate: bool,
}

impl Advices for NoVarAdvice {
    fn record(&self, visitor: &mut dyn Visit) -> std::io::Result<()> {
        if self.is_const_candidate {
            visitor.record_log(
                LogCategory::Info,
                &markup! {
                    "This variable is never reassigned, use 'const' instead."
                }
            )?;
        } else {
            visitor.record_log(
                LogCategory::Info,
                &markup! {
                    "Variables declared with 'var' are function-scoped, use 'let' for block-scoping."
                }
            )?;
        }
        Ok(())
    }
}

Use Built-in Advice Types

rust

use biome_diagnostics::v2::{
    LogAdvice, CodeFrameAdvice, DiffAdvice, CommandAdvice
};

// Log advice - simple text
LogAdvice {
    category: LogCategory::Info,
    message: markup! { "Consider using arrow functions." }
}

// Code frame advice - highlight code location
CodeFrameAdvice {
    location: node.text_range(),
    source_code: ctx.source_code(),
    annotation: markup! { "This code is problematic" },
}

// Diff advice - show before/after
DiffAdvice {
    old: "var x = 1;",
    new: "const x = 1;",
}

// Command advice - suggest CLI command
CommandAdvice {
    command: "biome check --apply",
    message: markup! { "Run this command to fix automatically" },
}

Add Diagnostic to Rule

rust

use biome_analyze::{Rule, RuleDiagnostic};

impl Rule for NoVar {
    fn diagnostic(ctx: &RuleContext<Self>, state: &Self::State) -> Option<RuleDiagnostic> {
        let node = ctx.query();
        
        Some(
            RuleDiagnostic::new(
                rule_category!(),
                node.range(),
                markup! {
                    "Use "<Emphasis>"let"</Emphasis>" or "<Emphasis>"const"</Emphasis>" instead of "<Emphasis>"var"</Emphasis>"."
                },
            )
            .note(markup! {
                "Variables declared with "<Emphasis>"var"</Emphasis>" are function-scoped, not block-scoped."
            })
            .note(markup! {
                "See the "<Hyperlink href="https://developer.mozilla.org/docs/Web/JavaScript/Reference/Statements/var">"MDN documentation"</Hyperlink>" for more details."
            })
        )
    }
}

Use Markup for Rich Text

Biome supports rich markup in diagnostic messages:

rust

use biome_console::markup;

markup! {
    // Emphasis (bold/colored)
    "Use "<Emphasis>"const"</Emphasis>" instead."
    
    // Code/identifiers
    "The variable "<Emphasis>{variable_name}</Emphasis>" is never used."
    
    // Hyperlinks
    "See the "<Hyperlink href="https://example.com">"documentation"</Hyperlink>"."
    
    // Interpolation
    "Found "{count}" issues."
}

Register Diagnostic Category

Add new categories to crates/biome_diagnostics_categories/src/categories.rs:

rust

define_categories! {
    // Existing categories...
    
    "lint/correctness/noVar": "https://biomejs.dev/linter/rules/no-var",
    "lint/style/useConst": "https://biomejs.dev/linter/rules/use-const",
}

Create Multi-Advice Diagnostics

rust

#[derive(Debug, Diagnostic)]
#[diagnostic(severity = Warning)]
struct ComplexDiagnostic {
    #[location(span)]
    span: TextRange,
    
    #[message]
    message: &'static str,
    
    // Multiple advices
    #[advice]
    first_advice: LogAdvice,
    
    #[advice]
    code_frame: CodeFrameAdvice,
    
    #[verbose_advice]
    verbose_help: LogAdvice,
}

Add Tags to Diagnostics

rust

#[derive(Debug, Diagnostic)]
#[diagnostic(
    severity = Warning,
    tags(FIXABLE, DEPRECATED_CODE)  // Add diagnostic tags
)]
struct MyDiagnostic {
    // ...
}

Available tags:

•FIXABLE - Diagnostic has fix information
•INTERNAL - Internal error in Biome
•UNNECESSARY_CODE - Code is unused
•DEPRECATED_CODE - Code uses deprecated features

Best Practices

Message Guidelines

Good messages:

rust

// ✅ Specific and actionable
"Use 'let' or 'const' instead of 'var'"

// ✅ Explains why
"This variable is never reassigned, consider using 'const'"

// ✅ Shows what to do
"Remove the unused import statement"

Bad messages:

rust

// ❌ Too vague
"Invalid syntax"

// ❌ Just states the obvious
"Variable declared with 'var'"

// ❌ No guidance
"This code has a problem"

Advice Guidelines

Show, don't tell:

rust

// ✅ Good - shows code frame
CodeFrameAdvice {
    location: node.range(),
    source_code: source,
    annotation: markup! { "This expression is always truthy" }
}

// ❌ Less helpful - just text
LogAdvice {
    message: markup! { "The expression at line 5 is always truthy" }
}

Provide actionable fixes:

rust

// ✅ Good - shows exact change
DiffAdvice {
    old: "var x = 1;",
    new: "const x = 1;",
}

// ❌ Less helpful - describes change
LogAdvice {
    message: markup! { "Change 'var' to 'const'" }
}

Severity Levels

Choose appropriate severity:

rust

// Fatal - Biome can't continue
severity = Fatal

// Error - Must be fixed (correctness, security, a11y)
severity = Error

// Warning - Should be fixed (suspicious code)
severity = Warning

// Information - Style suggestions
severity = Information

// Hint - Minor improvements
severity = Hint

Common Patterns

rust

// Pattern 1: Simple diagnostic with note
RuleDiagnostic::new(
    rule_category!(),
    node.range(),
    markup! { "Main message" },
)
.note(markup! { "Additional context" })

// Pattern 2: Diagnostic with code frame
RuleDiagnostic::new(
    rule_category!(),
    node.range(),
    markup! { "Main message" },
)
.detail(
    node.syntax().text_range(),
    markup! { "This part is problematic" }
)

// Pattern 3: Diagnostic with link
RuleDiagnostic::new(
    rule_category!(),
    node.range(),
    markup! { "Main message" },
)
.note(markup! {
    "See "<Hyperlink href="https://biomejs.dev/linter">"documentation"</Hyperlink>"."
})

// Pattern 4: Conditional advice
impl Advices for MyAdvice {
    fn record(&self, visitor: &mut dyn Visit) -> std::io::Result<()> {
        if self.show_hint {
            visitor.record_log(
                LogCategory::Info,
                &markup! { "Hint: ..." }
            )?;
        }
        Ok(())
    }
}

Tips

•Category format: Use area/group/ruleName format (e.g., lint/correctness/noVar)
•Markup formatting: Use markup! macro for all user-facing text
•Hyperlinks: Always link to documentation for more details
•Code frames: Include for spatial context when helpful
•Multiple advices: Chain multiple pieces of information
•Verbose advices: Use for extra details users can opt into
•Description vs Message: Description for plain text contexts (IDE popover), message for rich display
•Register categories: Don't forget to add to categories.rs

References

•Full guide: crates/biome_diagnostics/CONTRIBUTING.md
•Technical principles: https://biomejs.dev/internals/philosophy/#technical
•Diagnostic trait: crates/biome_diagnostics/src/diagnostic.rs
•Advice types: crates/biome_diagnostics/src/v2/
•Examples: Search for #[derive(Diagnostic)] in codebase