Protocol extensions

clangd supports some features that are not in the official Language Server Protocol specification.

We cautious about adding extensions. The most important considerations are:

• Editor support: How many users will the feature be available to?
• Standardization: Is the feature stable? Is it likely to be adopted by more editors over time?
• Utility: Does the feature provide a lot of value?
• Complexity: Is this hard to implement in clangd, or constrain future work? Is the protocol complicated?

These extensions may evolve or disappear over time. If you use them, try to recover gracefully if the structures aren't what's expected.

Switch between the implementation file and the header

This extension is supported in clangd 6 and newer.

Switching between the implementation file and the header is an important feature for C++. A language server that understands C++ can do a better job than the editor.

New client->server request: textDocument/switchSourceHeader.

Lets editors switch between the main source file (*.cpp) and header (*.h).

Parameter: TextDocumentIdentifier: an open file.

Result: string: the URI of the corresponding header (if a source file was provided) or source file (if a header was provided).

If the corresponding file can't be determined, "" is returned.

File status

This extension is supported in clangd 8 and newer.

It is important to provide feedback to the user when the UI is not responsive.

This extension provides information about activity on clangd's per-file worker thread. This information can be displayed to users to let them know that the language server is busy with something. For example, in clangd, building the AST blocks many other operations.

New server->client notification: textDocument/clangd.fileStatus

Sent when the current activity for a file changes. Replaces previous activity for that file.

Parameter: FileStatus object with properties:

• uri : string: the document whose status is being updated.
• state : string: human-readable information about current activity.

New initialization option: initializationOptions.clangdFileStatus : bool

Enables receiving textDocument/clangd.fileStatus notifications.

Compilation commands

This extension is supported in clangd 8 and newer.

clangd relies on knowing accurate compilation options to correctly interpret a file. Typically they are found in a compile_commands.json file in a directory that contains the file, or an ancestor directory. The following extensions allow editors to supply the commands over LSP instead.

New initialization option: initializationOptions.compilationDatabasePath : string

Specifies the directory containing the compilation database (e.g., compile_commands.json). This path will be used for all files, instead of searching their ancestor directories.

New initialization option: initializationOptions.fallbackFlags : string[]

Controls the flags used when no specific compile command is found. The compile command will be approximately clang $FILE $fallbackFlags in this case.

New configuration setting: settings.compilationDatabaseChanges : {string: CompileCommand}

Provides compile commands for files. This can also be provided on startup as initializationOptions.compilationDatabaseChanges.

Keys are file paths (Not URIs!)

Values are {workingDirectory: string, compilationCommand: string[]}.

Force diagnostics generation

This extension is supported in clangd 7 and newer.

Clangd does not regenerate diagnostics for every version of a file (e.g., after every keystroke), as that would be too slow. Its heuristics ensure:

• diagnostics do not get too stale,
• if you stop editing, diagnostics will catch up.

This extension allows editors to force diagnostics to be generated or not generated at a particular revision.

New property of textDocument/didChange request: wantDiagnostics : bool

• if true, diagnostics will be produced for exactly this version.
• if false, diagnostics will not be produced for this version, even if there are no further edits.
• if unset, diagnostics will be produced for this version or some subsequent one in a bounded amount of time.

Diagnostic categories

This extension is supported in clangd 8 and newer.

Clang compiler groups diagnostics into categories (e.g., "Inline Assembly Issue"). Clangd can emit these categories for interested editors.

New property of Diagnostic object: category : string:

A human-readable name for a group of related diagnostics. Diagnostics with the same code will always have the same category.

New client capability: textDocument.publishDiagnostics.categorySupport:

Requests that clangd send Diagnostic.category.

Inline fixes for diagnostics

This extension is supported in clangd 8 and newer.

LSP specifies that code actions for diagnostics (fixes) are retrieved asynchronously using textDocument/codeAction. clangd always computes fixes eagerly. Providing them alongside diagnostics can improve the UX in editors.

New property of Diagnostic object: codeActions : CodeAction[]:

All the code actions that address this diagnostic.

New client capability: textDocument.publishDiagnostics.codeActionsInline : bool

Requests clangd to send Diagnostic.codeActions.

Symbol info request

This extension is supported in clangd 8 and newer.

New client->server request: textDocument/symbolInfo:

This request attempts to resolve the symbol under the cursor, without retrieving further information (like definition location, which may require consulting an index). This request was added to support integration with indexes outside clangd.

Parameter: TextDocumentPositionParams

Response: SymbolDetails, an object with properties:

• name : string the unqualified name of the symbol
• containerName : string the enclosing namespace, class etc (without trailing ::)
• usr : string: the clang-specific "unified symbol resolution" identifier
• id : string?: the clangd-specific opaque symbol ID