fastmcp.tools.base

Functions

default_serializer ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L72" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

default_serializer(data: Any) -> str

Classes

ToolResult ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L95" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

Methods:

from_mcp_result ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L160" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

from_mcp_result(cls, result: CallToolResult) -> ToolResult

Wrap a protocol result while preserving its exact wire representation.

to_mcp_result ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L171" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

to_mcp_result(self) -> list[ContentBlock] | tuple[list[ContentBlock], dict[str, Any]] | CallToolResult

InputRequiredToolResult ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L193" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

The full result of a single multi-round-trip leg (SEP-2322).

The protocol is stateless: each MRTR leg is a complete request→response cycle. When a guard tool returns an InputRequiredResult from its body to ask the client for input, that ask is the legitimate result of this tool call — not a pause, not an error, not a third control-flow outcome. FastMCP wraps it in this ToolResult subclass so it flows through the middleware chain as an ordinary return value: call_next(...) returns it, default middleware completes normally on the leg, and middleware authors can identify an ask with a simple isinstance(result, InputRequiredToolResult) check.

Invariant: the wrapped InputRequiredResult is never serialized as tool content. content is always empty; the wire handler (_on_call_tool) reads .input_required and returns it to the runner as the input_required result. Do not read .content / .structured_content on this subclass — they carry nothing.

Tool ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L231" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

Internal tool registration info.

Methods:

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

to_mcp_tool(self, **overrides: Any) -> MCPTool

Convert the FastMCP tool to an MCP tool.

from_function ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L311" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

from_function(cls, fn: Callable[..., Any]) -> FunctionTool

Create a Tool from a function.

run ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L349" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

run(self, arguments: dict[str, Any]) -> ToolResult

Run the tool with arguments.

This method is not implemented in the base Tool class and must be implemented by subclasses.

run() can EITHER return a list of ContentBlocks, or a tuple of (list of ContentBlocks, dict of structured output).

A tool that requests client input (SEP-2322 multi-round-trip) does so by returning an InputRequiredResult from its body; the run machinery wraps that in an InputRequiredToolResult — a ToolResult subclass — so it stays inside the declared ToolResult result type and flows through the middleware chain as an ordinary result (see FunctionTool.run).

convert_result ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L367" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

convert_result(self, raw_value: Any) -> ToolResult

Convert a raw result to ToolResult.

Handles ToolResult passthrough and converts raw values using the tool's attributes (output_schema) for proper conversion.

from_tool ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L443" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

from_tool(cls, tool: Tool | Callable[..., Any]) -> TransformedTool

get_span_attributes ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/base.py#L489" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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