docs/core/index.md

# Gemini CLI core

Gemini CLI's core package (`packages/core`) is the backend portion of Gemini
CLI, handling communication with the Gemini API, managing tools, and processing
requests sent from `packages/cli`. For a general overview of Gemini CLI, see the
[main documentation page](../index.md).

## Navigating this section

- **[Sub-agents](./subagents.md):** Learn how to create and use specialized
  sub-agents for complex tasks.
- **[Core tools reference](../reference/tools.md):** Information on how tools
  are defined, registered, and used by the core.
- **[Memory Import Processor](../reference/memport.md):** Documentation for the
  modular GEMINI.md import feature using @file.md syntax.
- **[Policy Engine](../reference/policy-engine.md):** Use the Policy Engine for
  fine-grained control over tool execution.
- **[Local Model Routing (experimental)](./gemma-setup.md):** Learn how to
  enable use of a local Gemma model for model routing decisions using the
  automated setup command.

## Role of the core

While the `packages/cli` portion of Gemini CLI provides the user interface,
`packages/core` is responsible for:

- **Gemini API interaction:** Securely communicating with the Google Gemini API,
  sending user prompts, and receiving model responses.
- **Prompt engineering:** Constructing effective prompts for the Gemini model,
  potentially incorporating conversation history, tool definitions, and
  instructional context from `GEMINI.md` files.
- **Tool management & orchestration:**
  - Registering available tools (for example, file system tools, shell command
    execution).
  - Interpreting tool use requests from the Gemini model.
  - Executing the requested tools with the provided arguments.
  - Returning tool execution results to the Gemini model for further processing.
- **Session and state management:** Keeping track of the conversation state,
  including history and any relevant context required for coherent interactions.
- **Configuration:** Managing core-specific configurations, such as API key
  access, model selection, and tool settings.

## Security considerations

The core plays a vital role in security:

- **API key management:** It handles the `GEMINI_API_KEY` and ensures it's used
  securely when communicating with the Gemini API.
- **Tool execution:** When tools interact with the local system (for example,
  `run_shell_command`), the core (and its underlying tool implementations) must
  do so with appropriate caution, often involving sandboxing mechanisms to
  prevent unintended modifications.

## Chat history compression

To ensure that long conversations don't exceed the token limits of the Gemini
model, the core includes a chat history compression feature.

When a conversation approaches the token limit for the configured model, the
core automatically compresses the conversation history before sending it to the
model. This compression is designed to be lossless in terms of the information
conveyed, but it reduces the overall number of tokens used.

You can find the token limits for each model in the
[Google AI documentation](https://ai.google.dev/gemini-api/docs/models).

## Model fallback

Gemini CLI includes a model fallback mechanism to ensure that you can continue
to use the CLI even if the default "pro" model is rate-limited.

If you are using the default "pro" model and the CLI detects that you are being
rate-limited, it automatically switches to the "flash" model for the current
session. This lets you continue working without interruption.

Internal utility calls that use `gemini-2.5-flash-lite` (for example, prompt
completion and classification) silently fall back to `gemini-2.5-flash` and
`gemini-2.5-pro` when quota is exhausted, without changing the configured model.

## File discovery service

The file discovery service is responsible for finding files in the project that
are relevant to the current context. It is used by the `@` command and other
tools that need to access files.

## Memory discovery service

The memory discovery service is responsible for finding and loading the
`GEMINI.md` files that provide context to the model. It searches for these files
in a hierarchical manner, starting from the current working directory and moving
up to the project root and the user's home directory. It also searches in
subdirectories.

This lets you have global, project-level, and component-level context files,
which are all combined to provide the model with the most relevant information.

You can use the [`/memory` command](../reference/commands.md) to `show`, `add`,
and `refresh` the content of loaded `GEMINI.md` files.

## Citations

When Gemini finds it is reciting text from a source it appends the citation to
the output. It is enabled by default but can be disabled with the
ui.showCitations setting.

- When proposing an edit the citations display before giving the user the option
  to accept.
- Citations are always shown at the end of the model’s turn.
- We deduplicate citations and display them in alphabetical order.