> ## Documentation Index
> Fetch the complete documentation index at: https://gofastmcp.com/llms.txt
> Use this file to discover all available pages before exploring further.
# extension_hooks
# `fastmcp.client.extension_hooks`
Registry for FastMCP-internal client extensions (SEP-2133).
Core ships the client wiring for opt-in extensions but no extension of its own.
A companion package (`fastmcp-tasks`) provides an extension the `Client`
folds in automatically once the package is imported — so a caller that uses
tasks (importing `fastmcp_tasks` for `call_tool_task`, or to register the
server extension) gets transparent client task support without passing anything
per `Client`. The package cannot reach into core's `Client` constructor, so
core exposes this hook instead: the package registers a factory on import, and
`Client` folds the factory's extension in alongside the user's own.
This mirrors the server-side `set_background_context_factory` hook: core
declares the extension point, the tasks package fills it. Task support is
opt-in — with `fastmcp_tasks` unimported the registry is empty and `Client`
behaves exactly as core alone, so a plain `from fastmcp import Client` never
advertises the tasks capability and the server never runs its calls as tasks.
A factory receives the client's elicitation callback (so a task resolver can
answer in-task input prompts) and returns a `ClientExtension` to register, or
`None` to contribute nothing for this client.
## Functions
### `register_internal_client_extension_factory`
```python theme={"theme":{"light":"snazzy-light","dark":"dark-plus"}}
register_internal_client_extension_factory(factory: InternalClientExtensionFactory) -> None
```
Register a factory whose extension every `Client` folds in automatically.
Idempotent: registering the same factory object twice is a no-op, so a
package importing more than once does not double-register.
### `build_internal_client_extensions`
```python theme={"theme":{"light":"snazzy-light","dark":"dark-plus"}}
build_internal_client_extensions(elicitation_callback: ElicitationFnT | None) -> list[ClientExtension]
```
Build the internal extensions to fold into a `Client` under construction.
Each registered factory is invoked with the client's elicitation callback;
factories that return `None` contribute nothing. Empty when no companion
package has registered a factory (plain core, or `fastmcp_tasks` unimported).