# 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:

```bash
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:

| Language              | Language Server            | Install Command                                                                |
| --------------------- | -------------------------- | ------------------------------------------------------------------------------ |
| TypeScript/JavaScript | typescript-language-server | `npm install -g typescript-language-server typescript`                         |
| Python                | pylsp                      | `pip install python-lsp-server`                                                |
| Go                    | gopls                      | `go install golang.org/x/tools/gopls@latest`                                   |
| Rust                  | rust-analyzer              | [Installation guide](https://rust-analyzer.github.io/manual.html#installation) |
| C/C++                 | clangd                     | Install LLVM/clangd via your package manager                                   |
| Java                  | jdtls                      | Install JDTLS and a JDK                                                        |

## 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:**

```json
{
  "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:

```json
{
  "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:

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

### Configuration Options

#### Required Fields

| Option    | Type   | Description                                                                                                                                       |
| --------- | ------ | ------------------------------------------------------------------------------------------------------------------------------------------------- |
| `command` | string | Command to start the LSP server. Supports bare command names resolved via `PATH` (e.g. `clangd`) and absolute paths (e.g. `/opt/llvm/bin/clangd`) |

#### Optional Fields

| Option                  | Type     | Default   | Description                                             |
| ----------------------- | -------- | --------- | ------------------------------------------------------- |
| `args`                  | string[] | `[]`      | Command line arguments                                  |
| `transport`             | string   | `"stdio"` | Transport type: `stdio`, `tcp`, or `socket`             |
| `env`                   | object   | -         | Environment variables                                   |
| `initializationOptions` | object   | -         | LSP initialization options                              |
| `settings`              | object   | -         | Server settings via `workspace/didChangeConfiguration`  |
| `extensionToLanguage`   | object   | -         | Maps file extensions to language identifiers            |
| `workspaceFolder`       | string   | -         | Override workspace folder (must be within project root) |
| `startupTimeout`        | number   | `10000`   | Startup timeout in milliseconds                         |
| `shutdownTimeout`       | number   | `5000`    | Shutdown timeout in milliseconds                        |
| `restartOnCrash`        | boolean  | `false`   | Auto-restart on crash                                   |
| `maxRestarts`           | number   | `3`       | Maximum restart attempts                                |
| `trustRequired`         | boolean  | `true`    | Require trusted workspace                               |

### TCP/Socket Transport

For servers that use TCP or Unix socket transport:

```json
{
  "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:

```json
{
  "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:

```bash
# 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:

```bash
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:

```bash
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.
