Features

For a quick overview of which features are supported per language family, see the Support Matrix in README.md.

Many features need your code to expand without errors. After each edit, the server tries re-expanding your file. Each new edit cancels any running expansion and starts a fresh one. Results from the last successful expansion stay available until the new one finishes, so features keep working during editing. If expansion never succeeds, features that require expansion will never return useful results. The code needs to be correct for at least one moment, and stay that correct state for a few seconds to let expansion finish.

Expansion based features mostly use DrRacket's check-syntax APIs. The server expands the module, collects the binding, documentation, diagnostic, and highlighting information that check-syntax reports, and translates those results into LSP responses.

Several features also use the lexer from syntax-color. The lexer dispatches to a language-specific tokenizer based on the #lang declaration. Recognized languages get accurate tokenization. Unrecognized languages fallback to the Racket lexer, which probably does not understand their syntax and produces unreliable tokens.

Integrations

Resyntax

Resyntax provides automated refactoring suggestions. If you have Resyntax installed, it is used automatically with no configuration. Suggestions appear as diagnostics and code actions in your editor. If Resyntax is not installed, the server works normally without it.

racket-fixw

The Formatting feature uses racket-fixw for recognized sexp language indentation. This is a required dependency and is included when you install the server. Other external formatters can be supported, open an issue if you'd like one added.

Code Action (requires expansion)

The Quick Fix menu offers two kinds of actions:

• Unused variable suggests adding a _ prefix to silence the warning.
• Refactoring suggestions powered by Resyntax, shown when Resyntax is installed. Resyntax works automatically with no configuration needed. If it is not installed, these suggestions are simply not shown.

Uses DrRacket's check-syntax for unused variable detection.

Language behavior: not filtered by language family. Works for any language where expansion succeeds and check-syntax or Resyntax produce useful results.

Completion (identifier completion requires expansion)

The autocomplete popup provides two kinds of results:

• Identifiers defined in your file and imported from required modules. These need expansion.
• Module paths for require forms, based on your installed collections. These work without expansion. It covers both bare paths like racket/base and string paths like "racket/base".

Identifier completion is powered by DrRacket's check-syntax. The server registers ( as a trigger character. In VS Code, extensions control the wordPattern setting, it determines whether completion can trigger for other places except after (.

Language behavior: not filtered by language family for either source. Identifier completion works where expansion succeeds and useful binding data is produced. Module-path completion works without expansion. The ( trigger character is sexp-oriented.

Definition (requires expansion)

Jump to the definition of the identifier under the cursor. Works for both local definitions and identifiers imported from other files. Powered by DrRacket's check-syntax.

Language behavior: not filtered by language family. Works where expansion succeeds and check-syntax produces binding data with reliable source ranges.

Diagnostics (expansion for 3 of 4 sources)

Problems are shown from these sources:

• Reader and expander errors (syntax errors, missing modules, broken .zo files). Shown even if expansion fails. .zo version mismatch errors include a suggestion telling you which raco command to run.
• Check-syntax warnings for unused variables and unused require forms. These only appear after a successful expansion. Powered by DrRacket's check-syntax.
• Typed Racket type errors produced during expansion by the type checker. The server reads these from the type checker's log output.
• Language declaration check warns about missing #lang lines or unrecognized language names. Works without expansion.

Language behavior: mostly not filtered by language family. Reader errors, expander errors, and check-syntax warnings work for any language that reads and expands. The language declaration check only recognizes the predefined language families, so other valid #lang names are reported as unrecognized. Typed Racket type errors are specific to Typed Racket.

Document Highlight (requires expansion)

Placing the cursor on an identifier highlights all of its occurrences in the file. Both the definition site and all usage sites light up. The server uses check-syntax to find which declaration the identifier at the cursor resolves to, then looks up every location in the file that refers to the same declaration.

Language behavior: not filtered by language family. Works where expansion succeeds and check-syntax produces binding data with reliable source ranges.

Document Symbols (no expansion)

Shows a document outline. The server uses the lexer to produce results. It scans the text for every symbol (identifier), string, and constant and lists each one as a symbol entry with a kind label. This means every occurrence is listed rather than just top-level definitions, making the outline noisy and currently not very useful for navigation.

Language behavior: not filtered by language family. The server does not actively suppress entries for any language family, but actual entries depend on what the lexer can tokenize.

Formatting (no expansion)

Indents Racket code by calling an external formatter. Currently uses racket-fixw. Works for recognized sexp language families. Does not change anything for other languages.

Three trigger modes are supported:

• Format document - indents the whole file.
• Format selection - indents only the selected lines.
• Format on type - indents when you press ), ], or Enter. Pressing ) or ] re-indents the enclosing form; pressing Enter re-indents the current line.

Language behavior: only recognized sexp language families are supported. Other languages return no edits.

Hover (requires expansion)

Hovering over an identifier shows:

1. Type or contract - from check-syntax's mouse-over annotation, the same info DrRacket shows.
2. Online docs link - from check-syntax's doc annotation, turned into a docs.racket-lang.org URL.
3. Locally installed documentation — looked up via the check-syntax doc tag. Scribble blueboxes are preferred for formatted signatures. If they aren't available, the locally installed HTML docs are parsed instead.

Language behavior: not filtered by language family. Works where expansion succeeds and check-syntax produces hover and documentation data with reliable source ranges.

Inlay Hints

A handler is registered, but it is just a stub, not yet implemented.

References (requires expansion)

Finds all references to the identifier under the cursor. Local references are always found via the syncheck:add-jump-to-definition and syncheck:add-arrow/name-dup check-syntax callbacks. Cross-file references only shows for identifiers that were referenced by a file in the workspace that has been opened and expanded, unopened files are not scanned.

Language behavior: not filtered by language family. Works where expansion succeeds and check-syntax produces binding data with reliable source ranges. Cross-file references are limited to files that have been opened and expanded in the workspace.

Rename (requires expansion)

Renames an identifier and all its uses within the current file. Collects the declaration position and all binding positions from check-syntax, then replaces each with the new name. Only identifiers defined in the current file can be renamed.

Language behavior: not filtered by language family. Works where expansion succeeds and check-syntax produces binding data with reliable source ranges.

Prepare Rename (requires expansion)

Called by the editor before the rename dialog opens to check whether renaming is allowed at the cursor position.

Language behavior: same as Rename.

Semantic Tokens (expansion for 2 of 3 sources)

Provides semantic syntax highlighting with token types like function, variable, string, number, and comment. Combines analysis from these sources:

• DrRacket-style highlighting from check-syntax (needs expansion).
• Traverse the syntax tree (needs expansion).
• Sexp comment detection via the lexer, works without expansion.

Supports highlighting the full document or a specified range. Delta (incremental) highlighting is not yet implemented.

Semantic tokens depend on expansion and lexer data. Each request waits for any pending expansion to finish before responding. If expansion succeeds, fresh expansion-based tokens are used. If expansion fails, the last successful expansion tokens may remain available with adjusted ranges, and current lexer-derived sexp-comment tokens can still appear. If no expansion has ever succeeded, only lexer-derived tokens can appear.

In Lisp, due to dynamic typing and the S-expression syntax, many identifiers share the same token type and look similar, so it is recommended to use a semantic token aware editor plugin that gives each different identifier a unique color.

Language behavior: check-syntax color tokens are not filtered by language family and work where expansion succeeds. Sexp-comment tokens are available when the structural lexer can parse s-expression code.

Signature Help (requires expansion)

Shows signature information when you are inside a form. The server finds the head symbol of the enclosing s-expression form and looks up its signatures using locally installed documentation. If expansion is in progress or has failed, the last successful result is used as a fallback. Powered by DrRacket's check-syntax.

Language behavior: sexp-specific. Callee detection relies on s-expression form-head lookup through the structural forest. Known non-sexp languages return no result.