fastmcp.server.providers.proxy

ProxyProvider for proxying to remote MCP servers.

This module provides the ProxyProvider class that proxies components from a remote MCP server via a client factory. It also provides proxy component classes that forward execution to remote servers.

Functions

default_proxy_roots_handler ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1529" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

default_proxy_roots_handler(context: ServerRequestContext[Any, Any]) -> RootsList

Forward list roots request from remote server to proxy's connected clients.

A handshake-era backend can still issue roots/list, and the proxy is that backend's client, so it relays the request onto its own front session. This reaches the wire through the SDK session rather than a Context method: ctx.list_roots() is not part of FastMCP's server-authoring API, because SEP-2577 removed server-initiated requests from the modern protocol. The relay exists only for handshake-era interop on both legs.

default_proxy_sampling_handler ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1547" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

default_proxy_sampling_handler(messages: list[mcp_types.SamplingMessage], params: mcp_types.CreateMessageRequestParams, context: ServerRequestContext[Any, Any]) -> mcp_types.CreateMessageResult

Forward sampling request from remote server to proxy's connected clients.

Relays through the SDK session for the same reason as default_proxy_roots_handler: server-initiated sampling is not part of FastMCP's server-authoring API, and this path only ever runs when both legs of the proxy speak the handshake era.

default_proxy_elicitation_handler ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1581" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

default_proxy_elicitation_handler(message: str, response_type: type, params: mcp_types.ElicitRequestParams, context: ServerRequestContext[Any, Any]) -> ElicitResult

Forward elicitation request from remote server to proxy's connected clients.

default_proxy_log_handler ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1603" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

default_proxy_log_handler(message: LogMessage) -> None

Forward log notification from remote server to proxy's connected clients.

default_proxy_progress_handler ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1611" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

default_proxy_progress_handler(progress: float, total: float | None, message: str | None) -> None

Forward progress notification from remote server to proxy's connected clients.

Classes

ProxyInitializeMiddleware ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L268" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

Deprecated middleware for forwarding instructions during initialization.

Methods:

on_initialize ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L281" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

on_initialize(self, context: MiddlewareContext[mcp_types.InitializeRequest], call_next: CallNext[mcp_types.InitializeRequest, mcp_types.InitializeResult | None]) -> mcp_types.InitializeResult | None

ProxyTool ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L341" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

A Tool that represents and executes a tool on a remote server.

Methods:

model_copy ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L358" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

model_copy(self, **kwargs: Any) -> ProxyTool

Override to preserve _backend_name when name changes.

from_mcp_tool ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L368" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

from_mcp_tool(cls, client_factory: ClientFactoryT, mcp_tool: mcp_types.Tool) -> ProxyTool

Factory method to create a ProxyTool from a raw MCP tool schema.

run ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L386" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

run(self, arguments: dict[str, Any], context: Context | None = None) -> ToolResult

Executes the tool by making a call through the client.

get_span_attributes ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L472" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

get_span_attributes(self) -> dict[str, Any]

ProxyResource ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L479" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

A Resource that represents and reads a resource from a remote server.

Methods:

model_copy ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L504" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

model_copy(self, **kwargs: Any) -> ProxyResource

Override to preserve _backend_uri when uri changes.

from_mcp_resource ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L514" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

from_mcp_resource(cls, client_factory: ClientFactoryT, mcp_resource: mcp_types.Resource) -> ProxyResource

Factory method to create a ProxyResource from a raw MCP resource schema.

read ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L534" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

read(self) -> ResourceResult

Read the resource content from the remote server.

get_span_attributes ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L583" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

get_span_attributes(self) -> dict[str, Any]

ProxyTemplate ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L590" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

A ResourceTemplate that represents and creates resources from a remote server template.

Methods:

model_copy ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L607" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

model_copy(self, **kwargs: Any) -> ProxyTemplate

Override to preserve _backend_uri_template when uri_template changes.

from_mcp_template ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L617" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

from_mcp_template(cls, client_factory: ClientFactoryT, mcp_template: mcp_types.ResourceTemplate) -> ProxyTemplate

Factory method to create a ProxyTemplate from a raw MCP template schema.

create_resource ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L636" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

create_resource(self, uri: str, params: dict[str, Any], context: Context | None = None) -> ProxyResource

Create a resource from the template by calling the remote server.

get_span_attributes ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L716" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

get_span_attributes(self) -> dict[str, Any]

ProxyPrompt ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L725" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

A Prompt that represents and renders a prompt from a remote server.

Methods:

model_copy ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L742" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

model_copy(self, **kwargs: Any) -> ProxyPrompt

Override to preserve _backend_name when name changes.

from_mcp_prompt ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L752" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

from_mcp_prompt(cls, client_factory: ClientFactoryT, mcp_prompt: mcp_types.Prompt) -> ProxyPrompt

Factory method to create a ProxyPrompt from a raw MCP prompt schema.

render ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L776" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

render(self, arguments: dict[str, Any]) -> PromptResult

Render the prompt by making a call through the client.

get_span_attributes ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L821" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

get_span_attributes(self) -> dict[str, Any]

ProxyProvider ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L852" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

Provider that proxies to a remote MCP server via a client factory.

This provider fetches components from a remote server and returns Proxy* component instances that forward execution to the remote server.

All components returned by this provider have task_config.mode="forbidden" because tasks cannot be executed through a proxy.

Component lists (tools, resources, templates, prompts) are cached so that individual lookups (e.g. during call_tool) can resolve from the cache instead of opening a new backend connection. The cache stores the backend's raw component metadata and is shared across all sessions; per-session visibility and auth filtering are applied after cache lookup by the server layer. The cache is refreshed whenever a list_* call is made, and entries expire after cache_ttl seconds (default 300). Set cache_ttl=0 to disable caching. Disabling is recommended for backends whose component lists change dynamically.

Methods:

get_tool_by_hash ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L955" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

get_tool_by_hash(self, tool_hash: str, tool_name: str) -> Tool | None

Resolve an identity against the remote listing.

The base implementation looks the tool up by its registered name, which assumes the name survived to here. Across a proxy it need not: a backend that mounts its app under a namespace advertises crm_save, and nothing named save was ever listed. Matching on the identity carried in meta is what the identity is for.

A remote that mounts one app twice sends back two tools claiming one identity, exactly as a local composition would. That is refused here on the same terms AggregateProvider refuses it, so a duplicated app is caught wherever it is composed rather than only nearby.

get_tasks ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1124" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

get_tasks(self) -> Sequence[FastMCPComponent]

Return empty list since proxy components don't support tasks.

Override the base implementation to avoid calling list_tools() during server lifespan initialization, which would open the client before any context is set. All Proxy* components have task_config.mode="forbidden".

ProxyMetadataMiddleware ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1179" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

Forward optional server metadata from a ProxyProvider backend.

Instructions and namespaced metadata are forwarded with frontend values taking precedence. Protocol versions, capabilities, cache policy, and result type are never copied from the backend. identity controls whether server identity remains the gateway's or uses the backend's when available.

Methods:

on_initialize ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1257" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

on_initialize(self, context: MiddlewareContext[mcp_types.InitializeRequest], call_next: CallNext[mcp_types.InitializeRequest, mcp_types.InitializeResult | None]) -> mcp_types.InitializeResult | None

on_discover ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1274" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

on_discover(self, context: MiddlewareContext[mcp_types.DiscoverRequest], call_next: CallNext[mcp_types.DiscoverRequest, mcp_types.DiscoverResult | dict[str, Any]]) -> mcp_types.DiscoverResult | dict[str, Any]

FastMCPProxy ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1451" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

A FastMCP server that acts as a proxy to a remote MCP-compliant server.

This is a convenience wrapper that creates a FastMCP server with a ProxyProvider. For more control, use FastMCP with add_provider(ProxyProvider(...)).

ProxyClient ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1682" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

A proxy client that forwards advanced interactions between a remote MCP server and the proxy's connected clients.

Supports forwarding roots, sampling, elicitation, logging, and progress.

The default forwarding handlers must resolve the proxy's request context so they relay server-initiated requests (roots/sampling/elicitation) back to the proxy's own connected client, not to the upstream server they are talking to. Under SDK v2 an in-memory backend runs in the same event loop as this client, so a naive get_context() inside a handler can resolve to the backend's context and forward the request straight back to the backend — an infinite loop. To avoid that, ProxyTool.run (and the other proxy components) stash the proxy-side RequestContext in _proxy_rc_ref before each backend call, and the handlers are wrapped to restore it before forwarding.

Methods:

new ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1788" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

new(self) -> ProxyClient[ClientTransportT]

StatefulProxyClient ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1798" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

A proxy client that provides a stateful client factory for the proxy server.

The stateful proxy client bound its copy to the server session. And it will be disconnected when the session is exited.

This is useful to proxy a stateful mcp server such as the Playwright MCP server. Note that it is essential to ensure that the proxy server itself is also stateful.

The base ProxyClient already installs the context-restoring handlers (see its docstring); this subclass additionally caches one client per stable Connection and forces disconnect when the connection is torn down.

Methods:

new ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1819" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

new(self) -> StatefulProxyClient[ClientTransportT]

clear ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1830" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

clear(self)

Clear all cached clients and force disconnect them.

new_stateful ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/providers/proxy.py#L1836" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

new_stateful(self) -> Client[ClientTransportT]

Create a new stateful proxy client instance with the same configuration.

Use this method as the client factory for stateful proxy server.