For a custom language, syntax highlighting and code completion are separate features: start by teaching your editor which files belong to the language and how to tokenize them, then add a language server if completion needs to understand symbols or project context. In VS Code, a TextMate grammar and language configuration provide a useful first step; Tree-sitter and the Language Server Protocol (LSP) address different, deeper needs.
Choose the smallest setup that meets your needs
A language extension can combine several independent features. File recognition tells the editor when a file belongs to your language. A grammar assigns token scopes for highlighting. Language configuration adds editing conveniences, and snippets insert known templates. Programmatic completion—especially completion based on declarations, project files, or semantic analysis—usually requires a language server or another analysis component.
| Approach | Useful for | What it does not provide by itself |
|---|---|---|
| TextMate grammar | Lexical highlighting for comments, strings, keywords, operators, and similar constructs in editors that support the format. | Semantic or project-aware completion. |
| Snippets and language configuration | Reusable text templates and editor behavior such as comments, bracket handling, indentation, and folding. | Understanding declarations, types, or project symbols. |
| Tree-sitter parser and queries | Structural highlighting based on nodes in a syntax tree. | Language-server features such as semantic completion or diagnostics. |
| Language server over LSP | Analysis features such as context-aware completion, diagnostics, and navigation, depending on what the server implements. | Automatic highlighting unless a separate highlighting feature is also configured. |
This is not a universal ranking. A small language may need only recognition, a grammar, and a few snippets. A parser becomes useful when structural syntax matters; a server is warranted when suggestions must reflect more than fixed keywords and templates. VS Code distinguishes declarative extension contributions from programmatic features in its Language Extensions Overview.
Set up basic support in VS Code
The following is a practical sequence for a VS Code extension. The exact extension scaffolding and configuration formats can vary with editor versions; use the current language configuration guide and syntax highlighting guide when creating your extension.
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1. Connect files to a language ID
Choose a stable, unique language ID, such as acme, and associate it with the file extensions or filenames your language uses. Use the same ID in the file association and grammar contribution. The grammar contribution also identifies the grammar file and its root scope. If these identifiers do not line up, VS Code may not activate the grammar for the files you expect.
2. Write a TextMate grammar
Create a JSON grammar with a root scope and rules for the constructs your language supports: comments, strings, numbers, keywords, operators, and punctuation are common starting points. Organize related rules in a repository and include them where appropriate. Prefer established scope names so existing themes can style tokens without requiring users to install a custom theme.
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TextMate rules tokenize text and assign scopes; the theme decides how those scopes appear. That separation is why a token can be recognized correctly yet look similar to surrounding text in a particular theme.
3. Add language configuration
Configure only the editor behaviors that fit the language: line or block comments, bracket pairs, auto-closing or surrounding pairs, indentation rules, and folding markers. These conveniences do not perform semantic analysis. Check how the rules behave inside strings and comments—for example, bracket matching or auto-closing can be disruptive when applied to text where a bracket is only literal content.
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4. Add snippets for fixed templates
Snippets are appropriate for recurring text patterns such as a function skeleton or a standard declaration. They can appear among completion suggestions, but they are templates rather than analysis: they do not discover variables, resolve imports, or infer types from the current project.
5. Verify tokenization on real and broken files
- Open a sample file with the expected extension and check that VS Code recognizes the intended language.
- Run Developer: Inspect Editor Tokens and Scopes from the Command Palette on representative keywords, strings, comments, and punctuation.
- Confirm the grammar has loaded and that its scopes follow conventions a theme can style.
- Test escaped quotes, nested delimiters, incomplete expressions, and malformed source, as well as a file with an unexpected extension.
Testing incomplete and misidentified files matters because users often edit code before it parses cleanly, and incorrect file associations can make a sound grammar appear not to work.
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Add context-aware completion with a language server
When suggestions must account for names, symbols, project files, or language rules, implement analysis and expose it through a language server. In a VS Code language-server extension, a client starts or connects to the server; the server advertises completion support and responds to completion requests. The official Language Server Extension Guide demonstrates this client/server arrangement and also includes diagnostics.
- Build the client and server using the current guide’s extension structure.
- Have the server advertise completion capability and return suggestions for the document position requested by the client.
- Begin with the smallest useful behavior, then add symbol resolution or project-wide analysis only if the language semantics support it.
- Launch an Extension Development Host and debug the client and server there.
- Test suggestions in representative contexts, check that selected items behave as intended, and inspect logs for server errors.
A client starting successfully is not proof that completion is useful. Validate the actual suggestions against sample programs, including contexts where a suggestion should be offered and contexts where it should not. LSP standardizes communication between an editor client and a language-analysis server; as Microsoft explains, this can let an author reuse one analysis program in multiple compatible editors.
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When Tree-sitter or Neovim is a better fit
Tree-sitter for structural highlighting
Tree-sitter highlighting uses queries over parser nodes rather than relying only on lexical patterns. A language needs a parser, and highlighting queries can match nodes and assign captures such as @keyword, @function, @type, and @string. The Tree-sitter syntax-highlighting documentation explains this query-based model. It is a distinct choice from LSP: Tree-sitter captures support structural highlighting, while an LSP server supplies analysis features such as completion.
Neovim setup paths
For lexical highlighting in Neovim, a traditional syntax file can live in a user runtime directory, but files also need to be detected as the intended filetype. Neovim documents syntax files in its syntax help.
For Tree-sitter highlighting, provide or install a parser, add a highlights.scm query, and ensure the filetype maps to the parser language when their names differ. Neovim looks for query files under runtimepath, for example queries/<language>/highlights.scm; the nvim-treesitter documentation describes its integration. For completion based on language analysis, configure a compatible LSP client and server separately.
Quick Recap
Choose based on language complexity and editor coverage
- Language complexity: regex-oriented tokenization may cover straightforward lexical constructs; nesting, structural patterns, or context-sensitive syntax may call for a parser.
- Feature depth: fixed keywords and templates can be handled simply; symbol-aware completion, diagnostics, and navigation require analysis, commonly through LSP.
- Editor coverage: TextMate grammars, Tree-sitter parsers, and LSP servers each depend on compatible editor support and packaging; one configuration should not be assumed to work identically everywhere.
- Maintenance: a grammar is a smaller first deliverable than a parser and analysis server, which also bring ongoing compatibility and implementation work.
- Embedded syntax and incomplete code: test how your chosen approach handles embedded languages and partially written programs before relying on it for users.
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