Gemini CLI Companion Extension: Interface Specification

This document defines the contract for building a companion extension to enable Gemini CLI's IDE mode. For VS Code, these features (native diffing, context awareness) are provided by the official extension (marketplace). This specification is for contributors who wish to bring similar functionality to other editors like JetBrains IDEs, Sublime Text, etc.

I. The Communication Interface

The foundation of the IDE integration is a local communication channel between Gemini CLI and the IDE extension.

1. Transport Layer: MCP over HTTP

The extension MUST run a local HTTP server that implements the Model Context Protocol (MCP).

Protocol: The server must be a valid MCP server. We recommend using an existing MCP SDK for your language of choice if available.
Endpoint: The server should expose a single endpoint (e.g., /mcp) for all MCP communication.
Port: The server MUST listen on a dynamically assigned port (i.e., listen on port 0).

2. Discovery Mechanism: The Port File

For Gemini CLI to connect, it needs to discover which IDE instance it's running in and what port your server is using. The extension MUST facilitate this by creating a "discovery file."

How the CLI Finds the File: The CLI determines the Process ID (PID) of the IDE it's running in by traversing the process tree. It then looks for a discovery file that contains this PID in its name.
File Location: The file must be created in a specific directory: os.tmpdir()/gemini/ide/. Your extension must create this directory if it doesn't exist.
File Naming Convention: The filename is critical and MUST follow the pattern: gemini-ide-server-${PID}-${PORT}.json
- ${PID}: The process ID of the parent IDE process. Your extension must determine this PID and include it in the filename.
- ${PORT}: The port your MCP server is listening on.
File Content & Workspace Validation: The file MUST contain a JSON object with the following structure:
```
{
  "port": 12345,
  "workspacePath": "/path/to/project1:/path/to/project2"
}
```
- port (number): The port of the MCP server.
- workspacePath (string): A list of all open workspace root paths, delimited by the OS-specific path separator (: for Linux/macOS, ; for Windows). The CLI uses this path to ensure it's running in the same project folder that's open in the IDE. If the CLI's current working directory is not a sub-directory of workspacePath, the connection will be rejected. Your extension MUST provide the correct, absolute path(s) to the root of the open workspace(s).
Tie-Breaking with Environment Variables (Recommended): For the most reliable experience, your extension SHOULD both create the discovery file and set the GEMINI_CLI_IDE_SERVER_PORT and GEMINI_CLI_IDE_WORKSPACE_PATH environment variables in the integrated terminal. The file serves as the primary discovery mechanism, but the environment variables are crucial for tie-breaking. If a user has multiple IDE windows open for the same workspace, the CLI uses the GEMINI_CLI_IDE_SERVER_PORT variable to identify and connect to the correct window's server.
- For prototyping, you may opt to only set the environment variables. However, this is not a robust solution for a production extension, as environment variables may not be reliably set in all terminal sessions (e.g., restored terminals), which can lead to connection failures.
Authentication: (TBD)

II. The Context Interface

A powerful capability of the extension is to provide the CLI with real-time information about the user's activity in the IDE.

`ide/contextUpdate` Notification

The extension MAY send an ide/contextUpdate notification to the CLI whenever the user's context changes.

Triggering Events: This notification should be sent (with a recommended debounce of 50ms) when:
- A file is opened, closed, or focused.
- The user's cursor position or text selection changes in the active file.

Payload (IdeContext): The notification parameters MUST be an IdeContext object (@packages/core/src/ide/types.ts):

interface IdeContext {
  workspaceState?: {
    openFiles?: File[];
    isTrusted?: boolean;
  };
}

interface File {
  path: string; // Absolute path to the file
  timestamp: number; // Last focused Unix timestamp (for ordering)
  isActive?: boolean; // True if this is the currently focused file
  cursor?: {
    line: number; // 1-based line number
    character: number; // 1-based character number
  };
  selectedText?: string; // The text currently selected by the user
}

Note: The openFiles list should only include files that exist on disk. Virtual files (e.g., unsaved files without a path, editor settings pages) MUST be excluded.

How the CLI Uses This Context

After receiving the IdeContext object, the CLI performs several normalization and truncation steps before sending the information to the model.

File Ordering: The CLI uses the timestamp field to determine the most recently used files. It sorts the openFiles list based on this value. Therefore, your extension MUST provide an accurate Unix timestamp for when a file was last focused.
Active File: The CLI considers only the most recent file (after sorting) to be the "active" file. It will ignore the isActive flag on all other files and clear their cursor and selectedText fields. Your extension should focus on setting isActive: true and providing cursor/selection details only for the currently focused file.
Truncation: To manage token limits, the CLI truncates both the file list (to 10 files) and the selectedText (to 16KB).

While the CLI handles the final truncation, it is highly recommended that your extension also limits the amount of context it sends.

III. Supporting Additional IDEs

To add support for a new IDE, two main components in the Gemini CLI codebase need to be updated: the detection logic and the installer logic.

1. IDE Detection (`@packages/core/src/ide/detect-ide.ts`)

// TODO(skeshive): Determine whether we should discover the IDE via the port file

The CLI must be able to identify when it is running inside a specific IDE's integrated terminal. This is primarily done by checking for unique environment variables. As a fallback, it can also inspect process information (like the command name) to help distinguish between IDEs if a unique environment variable is not available.

Add to DetectedIde Enum: First, add your new IDE to the DetectedIde enum.
Update detectIdeFromEnv: Add a check in this function for an environment variable specific to your IDE (e.g., if (process.env['MY_IDE_VAR']) { return DetectedIde.MyIde; }).
Update detectIde (Optional): If your IDE lacks a unique environment variable, you can add logic to the detectIde function to inspect ideProcessInfo (e.g., ideProcessInfo.command) as a secondary detection mechanism.

2. Extension Installation (`@packages/core/src/ide/ide-installer.ts`)

The CLI provides a command (/ide install) to help users automatically install the companion extension. While optional, implementing an IdeInstaller for your IDE is highly recommended to provide a seamless setup experience.

Create an Installer Class: Create a new class that implements the IdeInstaller interface.
Implement install(): The install method should:
1. Locate the IDE's command-line executable. The VsCodeInstaller provides a good example of searching common installation paths for different operating systems.
2. Execute the command to install the extension by its marketplace ID (e.g., "path/to/my-ide-cli" --install-extension my-publisher.my-extension-id).
3. Return a result object indicating success or failure.
Update getIdeInstaller: Add a case to the switch statement in this factory function to return an instance of your new installer class when your DetectedIde enum is matched.

IV. The Lifecycle Interface

The extension MUST manage its resources and the discovery file correctly based on the IDE's lifecycle.

On Activation (IDE startup/extension enabled):
1. Start the MCP server.
2. Create the discovery file.
On Deactivation (IDE shutdown/extension disabled):
1. Stop the MCP server.
2. Delete the discovery file.

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