Language Server Protocol (LSP) Support

Qwen Code provides native Language Server Protocol (LSP) support, enabling advanced code intelligence features like go-to-definition, find references, diagnostics, and code actions. This integration allows the AI agent to understand your code more deeply and provide more accurate assistance.

Overview

LSP support in Qwen Code works by connecting to language servers that understand your code. Once you configure servers via .lsp.json (or extensions), Qwen Code can start them and use them to:

• Navigate to symbol definitions
• Find all references to a symbol
• Get hover information (documentation, type info)
• View diagnostic messages (errors, warnings)
• Access code actions (quick fixes, refactorings)
• Analyze call hierarchies

Quick Start

LSP is an experimental feature in Qwen Code. To enable it, use the --experimental-lsp command line flag:

qwen --experimental-lsp

LSP servers are configuration-driven. You must define them in .lsp.json (or via extensions) for Qwen Code to start them.

Prerequisites

You need to have the language server for your programming language installed:

Configuration

.lsp.json File

You can configure language servers using a .lsp.json file in your project root. Each top-level key is a language identifier, and its value is the server configuration object.

Basic format:

{
  "typescript": {
    "command": "typescript-language-server",
    "args": ["--stdio"],
    "extensionToLanguage": {
      ".ts": "typescript",
      ".tsx": "typescriptreact",
      ".js": "javascript",
      ".jsx": "javascriptreact"
    }
  }
}

C/C++ (clangd) configuration

Dependencies:

• clangd (LLVM) must be installed and available in PATH.
• A compile database (compile_commands.json) or compile_flags.txt is required for accurate results.

Example:

{
  "cpp": {
    "command": "clangd",
    "args": [
      "--background-index",
      "--clang-tidy",
      "--header-insertion=iwyu",
      "--completion-style=detailed"
    ]
  }
}

Java (jdtls) configuration

Dependencies:

• JDK installed and available in PATH (java).
• JDTLS installed and available in PATH (jdtls).

Example:

{
  "java": {
    "command": "jdtls",
    "args": ["-configuration", ".jdtls-config", "-data", ".jdtls-workspace"]
  }
}

Configuration Options

Required Fields

Optional Fields

TCP/Socket Transport

For servers that use TCP or Unix socket transport:

{
  "remote-lsp": {
    "transport": "tcp",
    "socket": {
      "host": "127.0.0.1",
      "port": 9999
    },
    "extensionToLanguage": {
      ".custom": "custom"
    }
  }
}

Available LSP Operations

Qwen Code exposes LSP functionality through the unified lsp tool. Here are the available operations:

Location-based operations (goToDefinition, findReferences, hover, goToImplementation, and prepareCallHierarchy) require an exact filePath + line + character position. If you do not know the exact position, use workspaceSymbol or documentSymbol first to locate the symbol.

Code Navigation

Go to Definition

Find where a symbol is defined.

Operation: goToDefinition
Parameters:
  - filePath: Path to the file
  - line: Line number (1-based)
  - character: Column number (1-based)

Find References

Find all references to a symbol.

Operation: findReferences
Parameters:
  - filePath: Path to the file
  - line: Line number (1-based)
  - character: Column number (1-based)
  - includeDeclaration: Include the declaration itself (optional)

Go to Implementation

Find implementations of an interface or abstract method.

Operation: goToImplementation
Parameters:
  - filePath: Path to the file
  - line: Line number (1-based)
  - character: Column number (1-based)

Symbol Information

Hover

Get documentation and type information for a symbol.

Operation: hover
Parameters:
  - filePath: Path to the file
  - line: Line number (1-based)
  - character: Column number (1-based)

Document Symbols

Get all symbols in a document.

Operation: documentSymbol
Parameters:
  - filePath: Path to the file

Workspace Symbol Search

Search for symbols across the workspace.

Operation: workspaceSymbol
Parameters:
  - query: Search query string
  - limit: Maximum results (optional)

Call Hierarchy

Prepare Call Hierarchy

Get the call hierarchy item at a position.

Operation: prepareCallHierarchy
Parameters:
  - filePath: Path to the file
  - line: Line number (1-based)
  - character: Column number (1-based)

Incoming Calls

Find all functions that call the given function.

Operation: incomingCalls
Parameters:
  - callHierarchyItem: Item from prepareCallHierarchy

Outgoing Calls

Find all functions called by the given function.

Operation: outgoingCalls
Parameters:
  - callHierarchyItem: Item from prepareCallHierarchy

Diagnostics

File Diagnostics

Get diagnostic messages (errors, warnings) for a file.

Operation: diagnostics
Parameters:
  - filePath: Path to the file

Workspace Diagnostics

Get all diagnostic messages across the workspace.

Operation: workspaceDiagnostics
Parameters:
  - limit: Maximum results (optional)

Code Actions

Get Code Actions

Get available code actions (quick fixes, refactorings) at a location.

Operation: codeActions
Parameters:
  - filePath: Path to the file
  - line: Start line number (1-based)
  - character: Start column number (1-based)
  - endLine: End line number (optional, defaults to line)
  - endCharacter: End column (optional, defaults to character)
  - diagnostics: Diagnostics to get actions for (optional)
  - codeActionKinds: Filter by action kind (optional)

Code action kinds:

• quickfix - Quick fixes for errors/warnings
• refactor - Refactoring operations
• refactor.extract - Extract to function/variable
• refactor.inline - Inline function/variable
• source - Source code actions
• source.organizeImports - Organize imports
• source.fixAll - Fix all auto-fixable issues

Security

LSP servers are only started in trusted workspaces by default. This is because language servers run with your user permissions and can execute code.

Trust Controls

• Trusted Workspace: LSP servers start if configured
• Untrusted Workspace: LSP servers won't start unless trustRequired: false is set in the server configuration

To mark a workspace as trusted, use the /trust command.

Per-Server Trust Override

You can override trust requirements for specific servers in their configuration:

{
  "safe-server": {
    "command": "safe-language-server",
    "args": ["--stdio"],
    "trustRequired": false,
    "extensionToLanguage": {
      ".safe": "safe"
    }
  }
}

Troubleshooting

Server Not Starting

1. Verify --experimental-lsp flag: Make sure you're using the flag when starting Qwen Code
2. Check if the server is installed: Run the command manually (e.g. clangd --version) to verify
3. Check the command: The server binary must be in your system PATH, or specified as an absolute path (e.g. /opt/llvm/bin/clangd). Relative paths that escape the workspace are blocked
4. Check workspace trust: The workspace must be trusted for LSP (use /trust)
5. Check logs: Look for [LSP] entries in the debug log (see Debugging section below)
6. Check the process: Run ps aux | grep <server-name> to verify the server process is running

Slow Performance

1. Large projects: Consider excluding node_modules and other large directories
2. Server timeout: Increase startupTimeout in server configuration for slow servers

No Results

1. Server not ready: The server may still be indexing. For C/C++ projects with clangd, ensure --background-index is in the args and a compile_commands.json (or compile_flags.txt) exists in the project root or a parent directory. Use --compile-commands-dir=<path> if it is in a build subdirectory
2. File not saved: Save your file for the server to pick up changes
3. Wrong language: Check if the correct server is running for your language
4. Check the process: Run ps aux | grep <server-name> to verify the server is actually running

Debugging

LSP debug logs are automatically written to session log files in ~/.qwen/debug/. To check LSP-related entries:

# View the latest session log
grep '\[LSP\]' ~/.qwen/debug/latest

# Common error messages to look for:
#   "command path is unsafe"  → relative path escapes workspace, use absolute path or add to PATH
#   "command not found"       → server binary not installed or not in PATH
#   "requires trusted workspace" → run /trust first

You can also verify the server process is running:

ps aux | grep clangd   # or typescript-language-server, jdtls, etc.

Extension LSP Configuration

Extensions can provide LSP server configurations through the lspServers field in their plugin.json. This can be either an inline object or a path to a .lsp.json file. Qwen Code loads these configs when the extension is enabled. The format is the same language-keyed layout used in project .lsp.json files.

Best Practices

1. Install language servers globally: This ensures they're available in all projects
2. Use project-specific settings: Configure server options per project when needed via .lsp.json
3. Keep servers updated: Update your language servers regularly for best results
4. Trust wisely: Only trust workspaces from trusted sources

FAQ

Q: How do I enable LSP?

Use the --experimental-lsp flag when starting Qwen Code:

qwen --experimental-lsp

Q: How do I know which language servers are running?

Check the debug log for [LSP] entries (grep '\[LSP\]' ~/.qwen/debug/latest), or verify the process directly with ps aux | grep <server-name>.

Q: Can I use multiple language servers for the same file type?

Yes, but only one will be used for each operation. The first server that returns results wins.

Q: Does LSP work in sandbox mode?

LSP servers run outside the sandbox to access your code. They're subject to workspace trust controls.