vscode/.github/copilot-instructions.md

VS Code Copilot Instructions

Project Overview

Visual Studio Code is built with a layered architecture using TypeScript, web APIs and Electron, combining web technologies with native app capabilities. The codebase is organized into key architectural layers:

Root Folders

• src/: Main TypeScript source code with unit tests in src/vs/*/test/ folders
• build/: Build scripts and CI/CD tools
• extensions/: Built-in extensions that ship with VS Code
• test/: Integration tests and test infrastructure
• scripts/: Development and build scripts
• resources/: Static resources (icons, themes, etc.)
• out/: Compiled JavaScript output (generated during build)

Core Architecture (src/ folder)

• src/vs/base/ - Foundation utilities and cross-platform abstractions
• src/vs/platform/ - Platform services and dependency injection infrastructure
• src/vs/editor/ - Text editor implementation with language services, syntax highlighting, and editing features
• src/vs/workbench/ - Main application workbench for web and desktop
  • workbench/browser/ - Core workbench UI components (parts, layout, actions)
  • workbench/services/ - Service implementations
  • workbench/contrib/ - Feature contributions (git, debug, search, terminal, etc.)
  • workbench/api/ - Extension host and VS Code API implementation
• src/vs/code/ - Electron main process specific implementation
• src/vs/server/ - Server specific implementation
• src/vs/sessions/ - Agent sessions window, a dedicated workbench layer for agentic workflows (sits alongside vs/workbench, may import from it but not vice versa)

The core architecture follows these principles:

• Layered architecture - from base, platform, editor, to workbench
• Dependency injection - Services are injected through constructor parameters
  • If non-service parameters are needed, they need to come before the service parameters
• Contribution model - Features contribute to registries and extension points
• Cross-platform compatibility - Abstractions separate platform-specific code

Built-in Extensions (extensions/ folder)

The extensions/ directory contains first-party extensions that ship with VS Code:

• Language support - typescript-language-features/, html-language-features/, css-language-features/, etc.
• Core features - git/, debug-auto-launch/, emmet/, markdown-language-features/
• Themes - theme-* folders for default color themes
• Development tools - extension-editing/, vscode-api-tests/

Each extension follows the standard VS Code extension structure with package.json, TypeScript sources, and contribution points to extend the workbench through the Extension API.

Finding Related Code

1. Semantic search first: Use file search for general concepts
2. Grep for exact strings: Use grep for error messages or specific function names
3. Follow imports: Check what files import the problematic module
4. Check test files: Often reveal usage patterns and expected behavior

Validating TypeScript changes

Choose validation based on the scope and risk of the change. Large-scale builds and typechecking can be slow, and consume significant resources, so minimize their use. Prefer existing editor or watch-task diagnostics and the smallest targeted tests that cover the changed behavior. Do not start build or watch tasks, run broad type checks, or make type checking a prerequisite for targeted tests solely as a completion ritual.

When running in a VS Code editor window with a workspace folder, use the VS Code task tools for build and watch workflows: inspect the existing task output first, and run an existing task instead of invoking its equivalent shell command. Do not start a duplicate build or watch process when the workspace task already provides current diagnostics. Agents window chats and isolated worktree sessions may not have access to the editor's workspace tasks; use the repository commands directly in those contexts.

Run a targeted type check or build when you are not fully confident in the change, and the change is broad or cross-cutting, it affects build or type configuration, or another validation step reports a compilation problem. When task tools are unavailable or no suitable task exists, useful commands include:

• npm run typecheck-client for the main sources under src/
• npm run gulp compile-extensions for built-in extensions
• npm run typecheck from the build folder for build tooling

Development compile tasks already type-check their inputs. Do not run npm run typecheck-client immediately before npm run compile or npm run compile-client; choose the command that covers the required validation. When tests only need fresh client output, use npm run build-fast -- --client-only. It supports cold builds and file-granular incremental updates without starting a watcher or building extensions. Use npm run build-fast when extension and Copilot outputs are also needed. The client and extension JavaScript lanes transpile rather than type-check; extension media retains its existing type checks. Run explicit type checks when the change warrants them. Reserve npm run transpile-client for an intentional clean client rebuild.

Reuse successful validation while its relevant inputs remain unchanged, including when moving into review, commit, or PR creation. Batch related test selectors in one invocation. Prefer npm run eslint -- <changed files> for targeted linting; this wrapper caches results by file content. Install dependencies only when dependency inputs change or validation identifies missing dependencies; use npm run install-fast for a needed refresh, which skips up-to-date installations and falls back to npm install when required. Never share or symlink node_modules between worktrees.

Use scripts/test.sh (or scripts\test.bat on Windows) for unit tests and scripts/test-integration.sh (or scripts\test-integration.bat on Windows) for integration tests. Add a targeted selector such as --grep whenever possible. Run npm run valid-layers-check only when a change may affect module layering.

Coding Guidelines

Indentation

We use tabs, not spaces.

Naming Conventions

• Use PascalCase for type names
• Use PascalCase for enum values
• Use camelCase for function and method names
• Use camelCase for property names and local variables
• Use whole words in names when possible

Types

• Do not export types or functions unless you need to share it across multiple components
• Do not introduce new types or values to the global namespace

Comments

• Use JSDoc style comments for functions, interfaces, enums, and classes

Strings

• Use "double quotes" for strings shown to the user that need to be externalized (localized)
• Use 'single quotes' otherwise
• All strings visible to the user need to be externalized using the vs/nls module
• Externalized strings must not use string concatenation. Use placeholders instead ({0}).

UI labels

• Use title-style capitalization for command labels, buttons and menu items (each word is capitalized).
• Don't capitalize prepositions of four or fewer letters unless it's the first or last word (e.g. "in", "with", "for").

Designing UI

• When creating, editing, or reviewing any visual surface, reason in design terms, not pixels: name the feeling (Calm, Focused, Consistent, Delightful), find the principle it breaks, then reach for the move (token/tier/ramp) that restores it. Describe a bug by its role/tier/ramp (e.g. "this overlay is rounded at the control tier"), not its number.
• See the design-philosophy skill for the full Values→Principles→Moves vocabulary, worked examples, and feedback guidance, and design-tokens.instructions.md for the token reference.

Style

• Use arrow functions => over anonymous function expressions
• Only surround arrow function parameters when necessary. For example, (x) => x + x is wrong but the following are correct:

x => x + x
(x, y) => x + y
<T>(x: T, y: T) => x === y

• Always surround loop and conditional bodies with curly braces
• Open curly braces always go on the same line as whatever necessitates them
• Parenthesized constructs should have no surrounding whitespace. A single space follows commas, colons, and semicolons in those constructs. For example:

for (let i = 0, n = str.length; i < 10; i++) {
    if (x < 10) {
        foo();
    }
}
function f(x: number, y: string): void { }

• Whenever possible, in top-level scopes, use export function x(…) {…} instead of export const x = (…) => {…}. One advantage of using the function keyword is that the stack trace shows a good name when debugging.

Code Quality

• All files must include Microsoft copyright header
• Prefer async and await over Promise and then calls
• All user facing messages must be localized using the applicable localization framework (for example nls.localize() method)
• Don't add tests to the wrong test suite (e.g., adding to end of file instead of inside relevant suite)
• Look for existing test patterns before creating new structures
• Use describe and test consistently with existing patterns
• Prefer regex capture groups with names over numbered capture groups.
• If you create any temporary new files, scripts, or helper files for iteration, clean up these files by removing them at the end of the task
• Never duplicate imports. Always reuse existing imports if they are present.
• When removing an import, do not leave behind blank lines where the import was. Ensure the surrounding code remains compact.
• Do not use any or unknown as the type for variables, parameters, or return values unless absolutely necessary. If they need type annotations, they should have proper types or interfaces defined.
• When adding file watching, prefer correlated file watchers (via fileService.createWatcher) to shared ones.
• When adding tooltips to UI elements, prefer the use of IHoverService service.
• Do not duplicate code. Always look for existing utility functions, helpers, or patterns in the codebase before implementing new functionality. Reuse and extend existing code whenever possible.
• You MUST deal with disposables by registering them immediately after creation for later disposal. Use helpers such as DisposableStore, MutableDisposable or DisposableMap. Do NOT register a disposable to the containing class if the object is created within a method that is called repeatedly to avoid leaks. Instead, return an IDisposable from such method and let the caller register it.
• You MUST NOT use storage keys of another component only to make changes to that component. You MUST come up with proper API to change another component.
• Use IEditorService to open editors instead of IEditorGroupsService.activeGroup.openEditor to ensure that the editor opening logic is properly followed and to avoid bypassing important features such as revealIfOpened or preserveFocus.
• Avoid using bind(), call() and apply() solely to control this or partially apply arguments; prefer arrow functions or closures to capture the necessary context, and use these methods only when required by an API or interoperability.
• Avoid using events to drive control flow between components. Instead, prefer direct method calls or service interactions to ensure clearer dependencies and easier traceability of logic. Events should be reserved for broadcasting state changes or notifications rather than orchestrating behavior across components.
• Service dependencies MUST be declared in constructors and MUST NOT be accessed through the IInstantiationService at any other point in time.

Learnings

• Minimize the amount of assertions in tests. Prefer one snapshot-style assert.deepStrictEqual over multiple precise assertions, as they are much more difficult to understand and to update.
• Do not stub a global object (e.g. (mainWindow as any).ResizeObserver = ...) or use any casts to install fakes in tests. Instead, make the dependency injectable: add an optional constructor parameter on the production class that defaults to the real implementation (e.g. targetWindow.ResizeObserver), and have the test pass a fake that implements the real interface.