# Gemini CLI companion plugin: Interface specification

> Last Updated: September 15, 2025

This document defines the contract for building a companion plugin to enable
Gemini CLI's IDE mode. For VS Code, these features (native diffing, context
awareness) are provided by the official extension
([marketplace](https://marketplace.visualstudio.com/items?itemName=Google.gemini-cli-vscode-ide-companion)).
This specification is for contributors who wish to bring similar functionality
to other editors like JetBrains IDEs, Sublime Text, etc.

## I. The communication interface

Gemini CLI and the IDE plugin communicate through a local communication channel.

### 1. Transport layer: MCP over HTTP

The plugin **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 (for example, `/mcp`)
  for all MCP communication.
- **Port:** The server **MUST** listen on a dynamically assigned port (that is,
  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 plugin **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 plugin 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`
  - `${PID}`: The process ID of the parent IDE process. Your plugin must
    determine this PID and include it in the filename.
  - `${PORT}`: The port your MCP server is listening on.
- **File content and workspace validation:** The file **MUST** contain a JSON
  object with the following structure:

  ```json
  {
    "port": 12345,
    "workspacePath": "/path/to/project1:/path/to/project2",
    "authToken": "a-very-secret-token",
    "ideInfo": {
      "name": "vscode",
      "displayName": "VS Code"
    }
  }
  ```
  - `port` (number, required): The port of the MCP server.
  - `workspacePath` (string, required): 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
    plugin **MUST** provide the correct, absolute path(s) to the root of the
    open workspace(s).
  - `authToken` (string, required): A secret token for securing the connection.
    The CLI will include this token in an `Authorization: Bearer <token>` header
    on all requests.
  - `ideInfo` (object, required): Information about the IDE.
    - `name` (string, required): A short, lowercase identifier for the IDE (for
      example, `vscode`, `jetbrains`).
    - `displayName` (string, required): A user-friendly name for the IDE (for
      example, `VS Code`, `JetBrains IDE`).

- **Authentication:** To secure the connection, the plugin **MUST** generate a
  unique, secret token and include it in the discovery file. The CLI will then
  include this token in the `Authorization` header for all requests to the MCP
  server (for example, `Authorization: Bearer a-very-secret-token`). Your server
  **MUST** validate this token on every request and reject any that are
  unauthorized.
- **Tie-breaking with environment variables (recommended):** For the most
  reliable experience, your plugin **SHOULD** both create the discovery file and
  set the `GEMINI_CLI_IDE_SERVER_PORT` environment variable in the integrated
  terminal. The file serves as the primary discovery mechanism, but the
  environment variable is 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.

## II. The context interface

To enable context awareness, the plugin **MAY** provide the CLI with real-time
information about the user's activity in the IDE.

### `ide/contextUpdate` notification

The plugin **MAY** send an `ide/contextUpdate`
[notification](https://modelcontextprotocol.io/specification/2025-06-18/basic/index#notifications)
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:

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

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

<!-- prettier-ignore -->
> [!NOTE]
> The `openFiles` list should only include files that exist on disk.
> Virtual files (for example, 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 plugin **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 plugin 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
plugin also limits the amount of context it sends.

## III. The diffing interface

To enable interactive code modifications, the plugin **MAY** expose a diffing
interface. This allows the CLI to request that the IDE open a diff view, showing
proposed changes to a file. The user can then review, edit, and ultimately
accept or reject these changes directly within the IDE.

### `openDiff` tool

The plugin **MUST** register an `openDiff` tool on its MCP server.

- **Description:** This tool instructs the IDE to open a modifiable diff view
  for a specific file.
- **Request (`OpenDiffRequest`):** The tool is invoked via a `tools/call`
  request. The `arguments` field within the request's `params` **MUST** be an
  `OpenDiffRequest` object.

  ```typescript
  interface OpenDiffRequest {
    // The absolute path to the file to be diffed.
    filePath: string;
    // The proposed new content for the file.
    newContent: string;
  }
  ```

- **Response (`CallToolResult`):** The tool **MUST** immediately return a
  `CallToolResult` to acknowledge the request and report whether the diff view
  was successfully opened.
  - On Success: If the diff view was opened successfully, the response **MUST**
    contain empty content (that is, `content: []`).
  - On Failure: If an error prevented the diff view from opening, the response
    **MUST** have `isError: true` and include a `TextContent` block in the
    `content` array describing the error.

  The actual outcome of the diff (acceptance or rejection) is communicated
  asynchronously via notifications.

### `closeDiff` tool

The plugin **MUST** register a `closeDiff` tool on its MCP server.

- **Description:** This tool instructs the IDE to close an open diff view for a
  specific file.
- **Request (`CloseDiffRequest`):** The tool is invoked via a `tools/call`
  request. The `arguments` field within the request's `params` **MUST** be an
  `CloseDiffRequest` object.

  ```typescript
  interface CloseDiffRequest {
    // The absolute path to the file whose diff view should be closed.
    filePath: string;
  }
  ```

- **Response (`CallToolResult`):** The tool **MUST** return a `CallToolResult`.
  - On Success: If the diff view was closed successfully, the response **MUST**
    include a single **TextContent** block in the content array containing the
    file's final content before closing.
  - On Failure: If an error prevented the diff view from closing, the response
    **MUST** have `isError: true` and include a `TextContent` block in the
    `content` array describing the error.

### `ide/diffAccepted` notification

When the user accepts the changes in a diff view (for example, by clicking an
"Apply" or "Save" button), the plugin **MUST** send an `ide/diffAccepted`
notification to the CLI.

- **Payload:** The notification parameters **MUST** include the file path and
  the final content of the file. The content may differ from the original
  `newContent` if the user made manual edits in the diff view.

  ```typescript
  {
    // The absolute path to the file that was diffed.
    filePath: string;
    // The full content of the file after acceptance.
    content: string;
  }
  ```

### `ide/diffRejected` notification

When the user rejects the changes (for example, by closing the diff view without
accepting), the plugin **MUST** send an `ide/diffRejected` notification to the
CLI.

- **Payload:** The notification parameters **MUST** include the file path of the
  rejected diff.

  ```typescript
  {
    // The absolute path to the file that was diffed.
    filePath: string;
  }
  ```

## IV. The lifecycle interface

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

- **On activation (IDE startup/plugin enabled):**
  1.  Start the MCP server.
  2.  Create the discovery file.
- **On deactivation (IDE shutdown/plugin disabled):**
  1.  Stop the MCP server.
  2.  Delete the discovery file.
