Semantic Token Colors

semanticTokenColors is the newest and least understood theme layer. It provides language-aware styling powered by language servers rather than TextMate grammars. Where tokenColors pattern-matches against syntax, semanticTokenColors styles based on meaning.

Why It Exists

TextMate grammars are powerful but limited. They parse code as text patterns — they don't understand what the code means. Consider this JavaScript:

javascript

const name = "hello"
console.log(name)

A TextMate grammar sees name on both lines as a generic variable. It can't tell you that the first is a declaration and the second is a reference. It can't distinguish a readonly variable from a mutable one, or a local variable from a global one.

A language server can. It performs full semantic analysis — type checking, scope resolution, symbol tracking. Semantic tokens expose this information to the theme.

Structure

semanticTokenColors is an object (not an array like tokenColors). Each key is a semantic token selector, and values can be either a string or an object:

json

{
"semanticTokenColors": {
  "variable.declaration": {
    "foreground": "#e6e6e6",
    "fontStyle": "italic"
  },
  "function.declaration": {
    "foreground": "#56b6c2",
    "fontStyle": "bold"
  },
  "string:escape": "#ffd93d"
}
}

Value Forms

Form	Example	Meaning
String	"#ffd93d"	Shorthand — sets foreground only
Object	{"foreground": "#e6e6e6", "fontStyle": "italic"}	Full control over foreground and/or fontStyle

Both forms are valid. The string form is a convenience for when you only need to set the colour.

Semantic Token Selectors

Selectors follow the pattern: type.modifier:language

Component	Required	Example	Purpose
type	Yes	variable, function, parameter	The kind of token
.modifier	No	.declaration, .readonly, .async	Refines the type
:language	No	:javascript, :python	Targets a specific language

Common Token Types

Type	What It Targets
variable	Variables
variable.declaration	Variable declarations specifically
variable.readonly	Constants and readonly variables
function	Function references
function.declaration	Function declarations
parameter	Function parameters
property	Object properties
property.readonly	Readonly properties
class	Class references
class.declaration	Class declarations
interface	Interface references
type	Type references
enum	Enum types
enumMember	Enum members
namespace	Namespaces/modules
string	String tokens
string:escape	Escape sequences within strings
comment	Comments
keyword	Keywords
number	Numeric literals
operator	Operators
decorator	Decorators/annotations
macro	Macros (Rust, C/C++)

Modifiers

Modifiers refine a token type. Multiple modifiers can be combined:

Modifier	Meaning
.declaration	Where the symbol is declared
.definition	Where the symbol is defined (similar to declaration)
.readonly	Immutable (const, final, readonly)
.static	Static members
.async	Async functions/methods
.deprecated	Deprecated symbols (often paired with strikethrough)
.modification	Where a variable is modified
.documentation	Documentation comments
.defaultLibrary	Standard library symbols

json

{
"semanticTokenColors": {
  "variable.readonly": "#e5c07b",
  "function.async": { "fontStyle": "italic" },
  "class.deprecated": { "fontStyle": "strikethrough" },
  "function.declaration:python": { "foreground": "#61afef" }
}
}

The Colon Separator

The : separator targets specific languages. string:escape is a special case — it's not a language filter but a well-known compound token type for escape sequences inside strings.

How It Overrides tokenColors

When a language server is active and provides semantic tokens, VS Code applies them on top of TextMate scopes. The override is per-token:

1. VS Code first applies the matching tokenColors rule (TextMate)
2. If a semanticTokenColors entry also matches, its properties override the TextMate result
3. Properties not set by semanticTokenColors are kept from the tokenColors match

This means:

json

{
"tokenColors": [
  {
    "scope": "variable",
    "settings": { "foreground": "#abb2bf" }
  }
],
"semanticTokenColors": {
  "variable.readonly": { "fontStyle": "bold" }
}
}

A readonly variable gets foreground: #abb2bf from tokenColors and fontStyle: bold from semanticTokenColors. The semantic layer adds to the TextMate base rather than replacing it entirely.

Gotchas

You Must Opt In

This is the single most common reason semantic token colours don't work: custom themes must explicitly enable semantic highlighting by setting "semanticHighlighting": true at the root of the theme JSON.

json

{
"name": "My Theme",
"type": "dark",
"semanticHighlighting": true,
"colors": {},
"tokenColors": [],
"semanticTokenColors": {}
}

Without this flag, VS Code treats your semanticTokenColors entries as if they don't exist. Only VS Code's built-in themes (like "Dark+" and "Light+") have this enabled by default. Every other theme must opt in.

The user setting editor.semanticHighlighting.enabled also plays a role. When set to configuredByTheme (the default), it defers to the theme's semanticHighlighting property. Users can override this to true or false to force semantic highlighting on or off regardless of the theme.

In Sassy, you enable this via config.custom:

yaml

config:
  name: "My Theme"
  type: dark
  custom:
    semanticHighlighting: true

Not All Languages Have Semantic Tokens

Semantic tokens require a language server that supports the textDocument/semanticTokens capability. Most popular languages have this (TypeScript, Python, Rust, Go, Java, C#, C/C++), but some don't. For those languages, semanticTokenColors has no effect and tokenColors is all you've got.

Object vs String Merge Behaviour

When defining semanticTokenColors across imports, the merge behaviour depends on value type:

• String values are replaced entirely by later definitions
• Object values are deep-merged — you can override just fontStyle in an import while keeping foreground from the base

Discovering Available Tokens

Unlike TextMate scopes (which you can inspect with Developer: Inspect Editor Tokens and Scopes), semantic tokens are harder to discover. The same inspector command shows semantic token information when available — look for the "semantic token type" and "semantic token modifiers" fields in the inspector popup.

The Empty fontStyle Trick

Just like with tokenColors, setting fontStyle to "" (empty string) explicitly clears all font styling. This is useful when you want semantic tokens to strip italic or bold from a TextMate base rule.

In Sassy

Sassy maps semanticTokenColors under theme.semanticTokenColors. Both value forms are supported:

yaml

# Raw VS Code JSON
# "semanticTokenColors": {
#   "variable.declaration": {
#     "foreground": "#e6e6e6",
#     "fontStyle": "italic"
#   },
#   "string:escape": "#ffd93d"
# }

# Sassy equivalent
theme:
  semanticTokenColors:
    variable.declaration:
      foreground: $(std.fg)
      fontStyle: italic
    function.declaration:
      foreground: $(accent)
      fontStyle: bold
    "string:escape": $$yellow

Variable references, palette aliases, and colour functions work in both the string form and inside object properties. Sassy resolves everything to hex and writes the standard VS Code structure.

tip

Use sassy resolve --semanticTokenColor variable.declaration my-theme.yaml to trace how a semantic token value is resolved through your variable chain. This is especially helpful when debugging imported themes with overrides.