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
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
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).