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---
title: server
sidebarTitle: server
---
# `fastmcp.server.server`
FastMCP - A more ergonomic interface for MCP servers.
## Functions
### `default_lifespan` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L237" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
default_lifespan(server: FastMCP[LifespanResultT]) -> AsyncIterator[Any]
```
Default lifespan context manager that does nothing.
**Args:**
- `server`: The server instance this lifespan is managing
**Returns:**
- An empty dictionary as the lifespan result.
### `create_proxy` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2501" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
create_proxy(target: Client[ClientTransportT] | ClientTransport | FastMCP[Any] | SDKServer | AnyUrl | Path | MCPConfig | dict[str, Any] | str, **settings: Any) -> FastMCPProxy
```
Create a FastMCP proxy server for the given target.
This is the recommended way to create a proxy server. For lower-level control,
use `FastMCPProxy` or `ProxyProvider` directly from `fastmcp.server.providers.proxy`.
**Args:**
- `target`: The backend to proxy to. Can be\:
- A Client instance (connected or disconnected)
- A ClientTransport
- A FastMCP server instance
- A URL string or AnyUrl
- A Path to a server script
- An MCPConfig or dict
- `mode`: Protocol-era negotiation for auto-created proxy clients (a
non-Client target). By default (``None``) the backend MIRRORS the
front connection's negotiated era per request, so the whole chain
speaks one era end-to-end\: a modern front reaches a modern backend
(a guard tool's `InputRequiredResult` (SEP-2322) round-trips) and a
handshake front reaches a handshake backend (server-initiated
sampling / elicitation / roots push-forwarding works). Pass an
explicit mode (e.g. ``"auto"`` or a version string) to pin the
backend era regardless of the front; this overrides mirroring and is
appropriate when the backend only speaks one era. Ignored when
`target` is already a `Client` (which carries its own mode).
- `**settings`: Additional settings passed to FastMCPProxy (name, etc.)
**Returns:**
- A FastMCPProxy server that proxies to the target.
## Classes
### `StateValue` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L272" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
Wrapper for stored context state values.
### `FastMCP` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L278" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
**Methods:**
#### `name` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L507" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
name(self) -> str
```
#### `instructions` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L511" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
instructions(self) -> str | None
```
#### `instructions` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L515" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
instructions(self, value: str | None) -> None
```
#### `version` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L519" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
version(self) -> str | None
```
#### `website_url` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L523" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
website_url(self) -> str | None
```
#### `icons` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L527" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
icons(self) -> list[mcp_types.Icon]
```
#### `local_provider` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L534" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
local_provider(self) -> LocalProvider
```
The server's local provider, which stores directly-registered components.
Use this to remove components:
mcp.local_provider.remove_tool("my_tool")
mcp.local_provider.remove_resource("data://info")
mcp.local_provider.remove_prompt("my_prompt")
#### `add_middleware` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L598" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_middleware(self, middleware: Middleware) -> None
```
#### `add_extension` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L601" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_extension(self, extension: ServerExtension) -> None
```
Register a server extension (SEP-2133).
An extension contributes a negotiated capability, additive request
methods, a `tools/call` interceptor, and an optional lifespan — each
with access to FastMCP-level constructs (the component registry,
`Context`, auth scope). Its capability is advertised only while it is
registered.
The extension is bound to this server (so its handlers and interceptor
can reach it), its method bindings are wired onto the low-level server,
and it is recorded for capability advertisement, interception, and
lifespan entry. Registering two extensions with the same identifier is
an error, as is registering after the server's lifespan has started —
the extension's lifespan could no longer run, leaving it silently
half-active.
Extensions are served by the server they are registered on. A mounted
child's extensions do not propagate to the root: the root serves the
wire, so only root-registered extensions advertise capabilities and
answer methods (matching the lifespan, which also defers to the root).
Register extensions on the server you run.
#### `add_provider` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L673" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_provider(self, provider: Provider) -> None
```
Add a provider for dynamic tools, resources, and prompts.
Providers are queried in registration order. The first provider to return
a non-None result wins. Static components (registered via decorators)
always take precedence over providers.
**Args:**
- `provider`: A Provider instance that will provide components dynamically.
- `namespace`: Optional namespace prefix. When set\:
- Tools become "namespace_toolname"
- Resources become "protocol\://namespace/path"
- Prompts become "namespace_promptname"
#### `get_tasks` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L785" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
get_tasks(self) -> Sequence[FastMCPComponent]
```
Get task-eligible components with all transforms applied.
Overrides AggregateProvider.get_tasks() to apply server-level transforms
after aggregation. AggregateProvider handles provider-level namespacing.
#### `add_transform` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L814" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_transform(self, transform: Transform) -> None
```
Add a server-level transform.
Server-level transforms are applied after all providers are aggregated.
They transform tools, resources, and prompts from ALL providers.
**Args:**
- `transform`: The transform to add.
#### `list_tools` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L834" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
list_tools(self) -> Sequence[Tool]
```
List all enabled tools from providers.
Overrides Provider.list_tools() to add enabled filtering, auth filtering,
and middleware execution. Returns all versions (no deduplication).
Protocol handlers deduplicate for MCP wire format.
#### `get_tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L917" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
get_tool(self, name: str, version: VersionSpec | None = None) -> Tool | None
```
Get a tool by name, filtering disabled tools.
Overrides Provider.get_tool() to filter disabled tools after all
transforms (including session-level) have been applied. This ensures
session transforms can override provider-level disables.
When the highest version is disabled and no explicit version was
requested, falls back to the next-highest enabled version.
**Args:**
- `name`: The tool name.
- `version`: Version filter (None returns highest version).
**Returns:**
- The tool if found and enabled, None otherwise.
#### `list_resources` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L971" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
list_resources(self) -> Sequence[Resource]
```
List all enabled resources from providers.
Overrides Provider.list_resources() to add visibility filtering, auth filtering,
and middleware execution. Returns all versions (no deduplication).
Protocol handlers deduplicate for MCP wire format.
#### `get_resource` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1056" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
get_resource(self, uri: str, version: VersionSpec | None = None) -> Resource | None
```
Get a resource by URI, filtering disabled resources.
Overrides Provider.get_resource() to add visibility filtering after all
transforms (including session-level) have been applied.
When the highest version is disabled and no explicit version was
requested, falls back to the next-highest enabled version.
**Args:**
- `uri`: The resource URI.
- `version`: Version filter (None returns highest version).
**Returns:**
- The resource if found and enabled, None otherwise.
#### `list_resource_templates` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1106" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
list_resource_templates(self) -> Sequence[ResourceTemplate]
```
List all enabled resource templates from providers.
Overrides Provider.list_resource_templates() to add visibility filtering,
auth filtering, and middleware execution. Returns all versions (no deduplication).
Protocol handlers deduplicate for MCP wire format.
#### `get_resource_template` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1188" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
get_resource_template(self, uri: str, version: VersionSpec | None = None) -> ResourceTemplate | None
```
Get a resource template by URI, filtering disabled templates.
Overrides Provider.get_resource_template() to add visibility filtering after
all transforms (including session-level) have been applied.
When the highest version is disabled and no explicit version was
requested, falls back to the next-highest enabled version.
**Args:**
- `uri`: The template URI.
- `version`: Version filter (None returns highest version).
**Returns:**
- The template if found and enabled, None otherwise.
#### `list_prompts` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1242" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
list_prompts(self) -> Sequence[Prompt]
```
List all enabled prompts from providers.
Overrides Provider.list_prompts() to add visibility filtering, auth filtering,
and middleware execution. Returns all versions (no deduplication).
Protocol handlers deduplicate for MCP wire format.
#### `get_prompt` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1314" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
get_prompt(self, name: str, version: VersionSpec | None = None) -> Prompt | None
```
Get a prompt by name, filtering disabled prompts.
Overrides Provider.get_prompt() to add visibility filtering after all
transforms (including session-level) have been applied.
When the highest version is disabled and no explicit version was
requested, falls back to the next-highest enabled version.
**Args:**
- `name`: The prompt name.
- `version`: Version filter (None returns highest version).
**Returns:**
- The prompt if found and enabled, None otherwise.
#### `call_tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1364" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
call_tool(self, name: str, arguments: dict[str, Any] | None = None) -> ToolResult
```
Call a tool by name.
This is the public API for executing tools. By default, middleware is applied.
**Args:**
- `name`: The tool name
- `arguments`: Tool arguments (optional)
- `version`: Specific version to call. If None, calls highest version.
- `run_middleware`: If True (default), apply the middleware chain.
Set to False when called from middleware to avoid re-applying.
**Returns:**
- ToolResult.
A guard tool that requests client input (SEP-2322 multi-round-trip)
returns an ``InputRequiredToolResult`` (a ``ToolResult`` subclass); it
flows back through the middleware chain as an ordinary result and the
wire handler unwraps it into an ``InputRequiredResult`` on the response.
**Raises:**
- `NotFoundError`: If tool not found or disabled
- `ToolError`: If tool execution fails
- `ValidationError`: If arguments fail validation
#### `read_resource` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1557" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
read_resource(self, uri: str) -> ResourceResult
```
Read a resource by URI.
This is the public API for reading resources. By default, middleware is applied.
Checks concrete resources first, then templates.
**Args:**
- `uri`: The resource URI
- `version`: Specific version to read. If None, reads highest version.
- `run_middleware`: If True (default), apply the middleware chain.
Set to False when called from middleware to avoid re-applying.
**Returns:**
- ResourceResult.
**Raises:**
- `NotFoundError`: If resource not found or disabled
- `ResourceError`: If resource read fails
#### `render_prompt` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1715" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
render_prompt(self, name: str, arguments: dict[str, Any] | None = None) -> PromptResult
```
Render a prompt by name.
This is the public API for rendering prompts. By default, middleware is applied.
Use get_prompt() to retrieve the prompt definition without rendering.
**Args:**
- `name`: The prompt name
- `arguments`: Prompt arguments (optional)
- `version`: Specific version to render. If None, renders highest version.
- `run_middleware`: If True (default), apply the middleware chain.
Set to False when called from middleware to avoid re-applying.
**Returns:**
- PromptResult.
**Raises:**
- `NotFoundError`: If prompt not found or disabled
- `PromptError`: If prompt rendering fails
#### `add_tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1795" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_tool(self, tool: Tool | Callable[..., Any]) -> Tool
```
Add a tool to the server.
The tool function can optionally request a Context object by adding a parameter
with the Context type annotation. See the @tool decorator for examples.
**Args:**
- `tool`: The Tool instance or @tool-decorated function to register
**Returns:**
- The tool instance that was added to the server.
#### `tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1810" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
tool(self, name_or_fn: F) -> F
```
#### `tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1831" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
tool(self, name_or_fn: str | None = None) -> Callable[[F], F]
```
#### `tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1851" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
tool(self, name_or_fn: str | AnyFunction | None = None) -> Callable[[AnyFunction], FunctionTool] | FunctionTool | partial[Callable[[AnyFunction], FunctionTool] | FunctionTool]
```
Decorator to register a tool.
Tools can optionally request a Context object by adding a parameter with the
Context type annotation. The context provides access to MCP capabilities like
logging, progress reporting, and resource access.
This decorator supports multiple calling patterns:
- @server.tool (without parentheses)
- @server.tool (with empty parentheses)
- @server.tool("custom_name") (with name as first argument)
- @server.tool(name="custom_name") (with name as keyword argument)
- server.tool(function, name="custom_name") (direct function call)
**Args:**
- `name_or_fn`: Either a function (when used as @tool), a string name, or None
- `name`: Optional name for the tool (keyword-only, alternative to name_or_fn)
- `description`: Optional description of what the tool does
- `tags`: Optional set of tags for categorizing the tool
- `output_schema`: Optional JSON schema for the tool's output
- `annotations`: Optional annotations about the tool's behavior
- `meta`: Optional meta information about the tool
**Examples:**
Register a tool with a custom name:
```python
@server.tool
def my_tool(x: int) -> str:
return str(x)
# Register a tool with a custom name
@server.tool
def my_tool(x: int) -> str:
return str(x)
@server.tool("custom_name")
def my_tool(x: int) -> str:
return str(x)
@server.tool(name="custom_name")
def my_tool(x: int) -> str:
return str(x)
# Direct function call
server.tool(my_function, name="custom_name")
```
#### `add_resource` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1948" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_resource(self, resource: Resource | Callable[..., Any]) -> Resource | ResourceTemplate
```
Add a resource to the server.
**Args:**
- `resource`: A Resource instance or @resource-decorated function to add
**Returns:**
- The resource instance that was added to the server.
#### `add_template` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1961" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_template(self, template: ResourceTemplate) -> ResourceTemplate
```
Add a resource template to the server.
**Args:**
- `template`: A ResourceTemplate instance to add
**Returns:**
- The template instance that was added to the server.
#### `resource` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L1972" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
resource(self, uri: str) -> Callable[[F], F]
```
Decorator to register a function as a resource.
The function will be called when the resource is read to generate its content.
The function can return:
- str for text content
- bytes for binary content
- other types will be converted to JSON
Resources can optionally request a Context object by adding a parameter with the
Context type annotation. The context provides access to MCP capabilities like
logging, progress reporting, and session information.
If the URI contains parameters (e.g. "resource://{param}") or the function
has parameters, it will be registered as a template resource.
**Args:**
- `uri`: URI for the resource (e.g. "resource\://my-resource" or "resource\://{param}")
- `name`: Optional name for the resource
- `description`: Optional description of the resource
- `mime_type`: Optional MIME type for the resource
- `tags`: Optional set of tags for categorizing the resource
- `annotations`: Optional annotations about the resource's behavior
- `meta`: Optional meta information about the resource
**Examples:**
Register a resource with a custom name:
```python
@server.resource("resource://my-resource")
def get_data() -> str:
return "Hello, world!"
@server.resource("resource://my-resource")
async get_data() -> str:
data = await fetch_data()
return f"Hello, world! {data}"
@server.resource("resource://{city}/weather")
def get_weather(city: str) -> str:
return f"Weather for {city}"
@server.resource("resource://{city}/weather")
async def get_weather_with_context(city: str, ctx: Context) -> str:
await ctx.info(f"Fetching weather for {city}")
return f"Weather for {city}"
@server.resource("resource://{city}/weather")
async def get_weather(city: str) -> str:
data = await fetch_weather(city)
return f"Weather for {city}: {data}"
```
#### `add_prompt` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2091" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_prompt(self, prompt: Prompt | Callable[..., Any]) -> Prompt
```
Add a prompt to the server.
**Args:**
- `prompt`: A Prompt instance or @prompt-decorated function to add
**Returns:**
- The prompt instance that was added to the server.
#### `prompt` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2103" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
prompt(self, name_or_fn: F) -> F
```
#### `prompt` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2118" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
prompt(self, name_or_fn: str | None = None) -> Callable[[F], F]
```
#### `prompt` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2132" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
prompt(self, name_or_fn: str | AnyFunction | None = None) -> Callable[[AnyFunction], FunctionPrompt] | FunctionPrompt | partial[Callable[[AnyFunction], FunctionPrompt] | FunctionPrompt]
```
Decorator to register a prompt.
Prompts can optionally request a Context object by adding a parameter with the
Context type annotation. The context provides access to MCP capabilities like
logging, progress reporting, and session information.
This decorator supports multiple calling patterns:
- @server.prompt (without parentheses)
- @server.prompt() (with empty parentheses)
- @server.prompt("custom_name") (with name as first argument)
- @server.prompt(name="custom_name") (with name as keyword argument)
- server.prompt(function, name="custom_name") (direct function call)
Args:
name_or_fn: Either a function (when used as @prompt), a string name, or None
name: Optional name for the prompt (keyword-only, alternative to name_or_fn)
description: Optional description of what the prompt does
tags: Optional set of tags for categorizing the prompt
meta: Optional meta information about the prompt
Examples:
```python
@server.prompt
def analyze_table(table_name: str) -> list[Message]:
schema = read_table_schema(table_name)
return [
{
"role": "user",
"content": f"Analyze this schema:
{schema}"
}
]
@server.prompt()
async def analyze_with_context(table_name: str, ctx: Context) -> list[Message]:
await ctx.info(f"Analyzing table {table_name}")
schema = read_table_schema(table_name)
return [
{
"role": "user",
"content": f"Analyze this schema:
{schema}"
}
]
@server.prompt("custom_name")
async def analyze_file(path: str) -> list[Message]:
content = await read_file(path)
return [
{
"role": "user",
"content": {
"type": "resource",
"resource": {
"uri": f"file://{path}",
"text": content
}
}
}
]
@server.prompt(name="custom_name")
def another_prompt(data: str) -> list[Message]:
return [{"role": "user", "content": data}]
# Direct function call
server.prompt(my_function, name="custom_name")
```
#### `add_completion_handler` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2230" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
add_completion_handler(self, handler: CompletionHandler) -> None
```
Register the server's argument-completion handler.
A server has a single completion handler that answers every
`completion/complete` request, switching on the reference (a prompt or
resource template) and the argument being completed. Registering it also
registers the low-level `completion/complete` handler, which is what
makes the SDK declare the completions capability — so the capability is
advertised exactly when the server can answer. Calling this again
replaces the handler.
**Args:**
- `handler`: A callable taking the reference, the
`CompletionArgument`, and the optional `CompletionContext`, and
returning candidate values (a `Completion`, a list of strings,
or None). May be sync or async.
#### `completion` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2251" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
completion(self, handler: CompletionHandler) -> CompletionHandler
```
#### `completion` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2254" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
completion(self) -> Callable[[CompletionHandler], CompletionHandler]
```
#### `completion` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2258" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
completion(self, handler: CompletionHandler | None = None) -> CompletionHandler | Callable[[CompletionHandler], CompletionHandler]
```
Decorator to register the server's argument-completion handler.
The handler answers `completion/complete` requests for prompt arguments
and resource-template parameters. It receives the reference being
completed, the argument (its name and the partial value typed so far),
and the context of arguments already supplied, and returns candidate
values. Return a list of strings, a `Completion` (to include pagination
hints), or None when the reference/argument is not one it handles — an
unhandled reference yields an empty completion, not an error.
Registering a handler declares the completions capability; a server with
none does not advertise it. This works identically on the handshake and
modern protocol eras.
Supports both `@mcp.completion` and `@mcp.completion()`.
Example:
```python
from fastmcp import FastMCP
from mcp_types import Completion, PromptReference
mcp = FastMCP("Completion Server")
@mcp.prompt
def poem(theme: str) -> str:
return f"Write a poem about {theme}"
@mcp.completion
def complete(ref, argument, context):
if isinstance(ref, PromptReference) and ref.name == "poem":
if argument.name == "theme":
options = ["nature", "love", "adventure"]
return [o for o in options if o.startswith(argument.value)]
return None
```
#### `mount` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2308" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
mount(self, server: FastMCP[LifespanResultT], namespace: str | None = None, tool_names: dict[str, str] | None = None) -> None
```
Mount another FastMCP server on this server with an optional namespace.
Mounting establishes a dynamic connection between servers. When a client
interacts with a mounted server's objects through the parent server, requests
are forwarded to the mounted server in real-time. This means changes to the
mounted server are immediately reflected when accessed through the parent.
When a server is mounted with a namespace:
- Tools from the mounted server are accessible with namespaced names.
Example: If server has a tool named "get_weather", it will be available as "namespace_get_weather".
- Resources are accessible with namespaced URIs.
Example: If server has a resource with URI "weather://forecast", it will be available as
"weather://namespace/forecast".
- Templates are accessible with namespaced URI templates.
Example: If server has a template with URI "weather://location/{id}", it will be available
as "weather://namespace/location/{id}".
- Prompts are accessible with namespaced names.
Example: If server has a prompt named "weather_prompt", it will be available as
"namespace_weather_prompt".
When a server is mounted without a namespace (namespace=None), its tools, resources, templates,
and prompts are accessible with their original names. Multiple servers can be mounted
without namespaces, and they will be tried in order until a match is found.
The mounted server's lifespan is executed when the parent server starts, and its
middleware chain is invoked for all operations (tool calls, resource reads, prompts).
**Args:**
- `server`: The FastMCP server to mount.
- `namespace`: Optional namespace to use for the mounted server's objects. If None,
the server's objects are accessible with their original names.
- `tool_names`: Optional mapping of original tool names to custom names. Use this
to override namespaced names. Keys are the original tool names from the
mounted server.
#### `from_openapi` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2379" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
from_openapi(cls, openapi_spec: dict[str, Any], client: httpx2.AsyncClient | None = None, name: str = 'OpenAPI Server', route_maps: list[RouteMap] | None = None, route_map_fn: OpenAPIRouteMapFn | None = None, mcp_component_fn: OpenAPIComponentFn | None = None, mcp_names: dict[str, str] | None = None, tags: set[str] | None = None, validate_output: bool = True, **settings: Any) -> Self
```
Create a FastMCP server from an OpenAPI specification.
**Args:**
- `openapi_spec`: OpenAPI schema as a dictionary
- `client`: Optional httpx2 AsyncClient for making HTTP requests.
If not provided, a default client is created using the first
server URL from the OpenAPI spec with a 30-second timeout.
Legacy httpx clients are temporarily accepted with a deprecation
warning.
- `name`: Name for the MCP server
- `route_maps`: Optional list of RouteMap objects defining route mappings
- `route_map_fn`: Optional callable for advanced route type mapping
- `mcp_component_fn`: Optional callable for component customization
- `mcp_names`: Optional dictionary mapping operationId to component names
- `tags`: Optional set of tags to add to all components
- `validate_output`: If True (default), tools use the output schema
extracted from the OpenAPI spec for response validation. If
False, a permissive schema is used instead, allowing any
response structure while still returning structured JSON.
- `**settings`: Additional settings passed to FastMCP
**Returns:**
- A FastMCP server with an OpenAPIProvider attached.
#### `from_fastapi` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2432" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
from_fastapi(cls, app: Any, name: str | None = None, route_maps: list[RouteMap] | None = None, route_map_fn: OpenAPIRouteMapFn | None = None, mcp_component_fn: OpenAPIComponentFn | None = None, mcp_names: dict[str, str] | None = None, httpx_client_kwargs: dict[str, Any] | None = None, tags: set[str] | None = None, **settings: Any) -> Self
```
Create a FastMCP server from a FastAPI application.
**Args:**
- `app`: FastAPI application instance
- `name`: Name for the MCP server (defaults to app.title)
- `route_maps`: Optional list of RouteMap objects defining route mappings
- `route_map_fn`: Optional callable for advanced route type mapping
- `mcp_component_fn`: Optional callable for component customization
- `mcp_names`: Optional dictionary mapping operationId to component names
- `httpx_client_kwargs`: Optional kwargs passed to httpx2.AsyncClient.
Use this to configure timeout and other client settings.
- `tags`: Optional set of tags to add to all components
- `**settings`: Additional settings passed to FastMCP
**Returns:**
- A FastMCP server with an OpenAPIProvider attached.
#### `generate_name` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/server.py#L2487" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
generate_name(cls, name: str | None = None) -> str
```