fastmcp.prompts.base

Base classes for FastMCP prompts.

Classes

Message ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L38" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

Wrapper for prompt message with auto-serialization.

Accepts any content - strings pass through, other types (dict, list, BaseModel) are JSON-serialized to text.

Methods:

to_mcp_prompt_message ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L93" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

to_mcp_prompt_message(self) -> PromptMessage

Convert to MCP PromptMessage.

PromptArgument ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L98" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

An argument that can be passed to a prompt.

PromptResult ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L110" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

Canonical result type for prompt rendering.

Provides explicit control over prompt responses: multiple messages, roles, and metadata at both the message and result level.

Methods:

to_mcp_prompt_result ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L182" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

to_mcp_prompt_result(self) -> GetPromptResult

Convert to MCP GetPromptResult.

InputRequiredPromptResult ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L192" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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

InputRequiredResult is a result type, not a tools/call feature: any request may resolve to one. When a prompt returns an InputRequiredResult from its body to ask the client for input, that ask is the legitimate result of this prompts/get — so FastMCP wraps it in this PromptResult subclass, mirroring InputRequiredToolResult, and it flows through the middleware chain as an ordinary return value.

Invariant: the wrapped InputRequiredResult is never rendered as prompt messages. messages is always empty; the wire handler (_on_get_prompt) reads .input_required and returns it to the runner.

Prompt ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L224" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

A prompt template that can be rendered with parameters.

Methods:

to_mcp_prompt ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L236" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

to_mcp_prompt(self, **overrides: Any) -> SDKPrompt

Convert the prompt to an MCP prompt.

from_function ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L262" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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

Create a Prompt from a function.

The function can return:

• str: wrapped as single user Message
• list[Message | str]: converted to list[Message]
• PromptResult: used directly

render ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L296" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

render(self, arguments: dict[str, Any] | None = None) -> str | list[Message | str] | PromptResult

Render the prompt with arguments.

Subclasses must implement this method. Return one of:

• str: Wrapped as single user Message
• list[Message | str]: Converted to list[Message]
• PromptResult: Used directly

convert_result ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L309" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

convert_result(self, raw_value: Any) -> PromptResult

Convert a raw return value to PromptResult.

Raises:

• TypeError: for unsupported types

get_span_attributes ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/prompts/base.py#L364" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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