validation

Shared validation functions for server requests.

This module provides validation logic for sampling and elicitation requests that is shared across normal and task-augmented code paths.

check_sampling_tools_capability

check_sampling_tools_capability(
    client_caps: ClientCapabilities | None,
) -> bool

Check if the client supports sampling tools capability.

Parameters:

Name	Type	Description	Default
client_caps	ClientCapabilities | None	The client's declared capabilities	required

Returns:

Type	Description
bool	True if client supports sampling.tools, False otherwise

Source code in src/mcp/server/validation.py

def check_sampling_tools_capability(client_caps: ClientCapabilities | None) -> bool:
    """Check if the client supports sampling tools capability.

    Args:
        client_caps: The client's declared capabilities

    Returns:
        True if client supports sampling.tools, False otherwise
    """
    if client_caps is None:
        return False
    if client_caps.sampling is None:
        return False
    if client_caps.sampling.tools is None:
        return False
    return True

validate_sampling_tools

validate_sampling_tools(
    client_caps: ClientCapabilities | None,
    tools: list[Tool] | None,
    tool_choice: ToolChoice | None,
) -> None

Validate that the client supports sampling tools if tools are being used.

Parameters:

Name	Type	Description	Default
client_caps	ClientCapabilities | None	The client's declared capabilities	required
tools	list[Tool] | None	The tools list, if provided	required
tool_choice	ToolChoice | None	The tool choice setting, if provided	required

Raises:

Type	Description
MCPError	If tools/tool_choice are provided but client doesn't support them

Source code in src/mcp/server/validation.py

def validate_sampling_tools(
    client_caps: ClientCapabilities | None,
    tools: list[Tool] | None,
    tool_choice: ToolChoice | None,
) -> None:
    """Validate that the client supports sampling tools if tools are being used.

    Args:
        client_caps: The client's declared capabilities
        tools: The tools list, if provided
        tool_choice: The tool choice setting, if provided

    Raises:
        MCPError: If tools/tool_choice are provided but client doesn't support them
    """
    if tools is not None or tool_choice is not None:
        if not check_sampling_tools_capability(client_caps):
            raise MCPError(code=INVALID_PARAMS, message="Client does not support sampling tools capability")

validate_tool_use_result_messages

validate_tool_use_result_messages(
    messages: list[SamplingMessage],
) -> None

Validate tool_use/tool_result message structure per SEP-1577.

This validation ensures: 1. Messages with tool_result content contain ONLY tool_result content 2. tool_result messages are preceded by a message with tool_use 3. tool_result IDs match the tool_use IDs from the previous message

See: https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1577

Parameters:

Name	Type	Description	Default
messages	list[SamplingMessage]	The list of sampling messages to validate	required

Raises:

Type	Description
ValueError	If the message structure is invalid

Source code in src/mcp/server/validation.py

def validate_tool_use_result_messages(messages: list[SamplingMessage]) -> None:
    """Validate tool_use/tool_result message structure per SEP-1577.

    This validation ensures:
    1. Messages with tool_result content contain ONLY tool_result content
    2. tool_result messages are preceded by a message with tool_use
    3. tool_result IDs match the tool_use IDs from the previous message

    See: https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1577

    Args:
        messages: The list of sampling messages to validate

    Raises:
        ValueError: If the message structure is invalid
    """
    if not messages:
        return

    last_content = messages[-1].content_as_list
    has_tool_results = any(c.type == "tool_result" for c in last_content)

    previous_content = messages[-2].content_as_list if len(messages) >= 2 else None
    has_previous_tool_use = previous_content and any(c.type == "tool_use" for c in previous_content)

    if has_tool_results:
        # Per spec: "SamplingMessage with tool result content blocks
        # MUST NOT contain other content types."
        if any(c.type != "tool_result" for c in last_content):
            raise ValueError("The last message must contain only tool_result content if any is present")
        if previous_content is None:
            raise ValueError("tool_result requires a previous message containing tool_use")
        if not has_previous_tool_use:
            raise ValueError("tool_result blocks do not match any tool_use in the previous message")

    if has_previous_tool_use and previous_content:
        tool_use_ids = {c.id for c in previous_content if c.type == "tool_use"}
        tool_result_ids = {c.tool_use_id for c in last_content if c.type == "tool_result"}
        if tool_use_ids != tool_result_ids:
            raise ValueError("ids of tool_result blocks and tool_use blocks from previous message do not match")