microsoft/autogen · error · ValueError

Tool '{tool_name}' not found, available tools: {', '.join([t

Error message

Tool '{tool_name}' not found, available tools: {', '.join([t.name for t in tools_response.tools])}

What it means

McpToolAdapter.from_server_params opens a session, calls session.list_tools(), and searches for the requested tool_name; if no advertised tool matches exactly, ValueError is raised listing all available tool names. This is a discovery mismatch: the name you asked for is not what the server exposes.

Source

Thrown at python/packages/autogen-ext/src/autogen_ext/tools/mcp/_base.py:153

        Args:
            server_params (TServerParams): Parameters for the MCP server connection.
            tool_name (str): The name of the tool to wrap.

        Returns:
            McpToolAdapter[TServerParams]: An instance of McpToolAdapter.

        Raises:
            ValueError: If the tool with the specified name is not found.
        """
        async with create_mcp_server_session(server_params) as session:
            await session.initialize()

            tools_response = await session.list_tools()
            matching_tool = next((t for t in tools_response.tools if t.name == tool_name), None)

            if matching_tool is None:
                raise ValueError(
                    f"Tool '{tool_name}' not found, available tools: {', '.join([t.name for t in tools_response.tools])}"
                )

        return cls(server_params=server_params, tool=matching_tool)

    def return_value_as_string(self, value: list[Any]) -> str:
        """Return a string representation of the result."""

        def serialize_item(item: Any) -> dict[str, Any]:
            if isinstance(item, (TextContent, ImageContent, AudioContent)):
                dumped = item.model_dump()
                # Remove the 'meta' field if it exists and is None (for backward compatibility)
                if dumped.get("meta") is None:
                    dumped.pop("meta", None)
                return dumped
            elif isinstance(item, EmbeddedResource):
                type = item.type
                resource = {}

View on GitHub (pinned to 027ecf0a37)

Solutions

  1. Log the error message — it enumerates available tools; use the exact name it lists.
  2. Before creating the adapter, call session.list_tools() yourself and match names programmatically (strip/casefold) to find the right one.
  3. Verify server_params (command, args, url) point to the server that actually exposes the tool.
  4. Pin the MCP server version whose tool names your code expects.

Example fix

# before
adapter = await McpToolAdapter.from_server_params(params, "search")

# after
async with create_mcp_server_session(params) as session:
    await session.initialize()
    names = [t.name for t in (await session.list_tools()).tools]
target = next(n for n in names if n.casefold() == "search")
adapter = await McpToolAdapter.from_server_params(params, target)
Defensive patterns

Strategy: validation

Validate before calling

async with create_mcp_server_session(server_params) as session:
    await session.initialize()
    names = [t.name for t in (await session.list_tools()).tools]
if tool_name not in names:
    close = [n for n in names if n.casefold() == tool_name.casefold()]
    raise ValueError(f"{tool_name!r} not in {names}; closest: {close}")

Try / catch

try:
    adapter = await McpToolAdapter.from_server_params(params, name)
except ValueError as e:
    if "not found" in str(e):
        # message lists available tools; pick the right one and retry once
        available = parse_available_tools(str(e))
        name = next(n for n in available if n.casefold() == name.casefold())
        adapter = await McpToolAdapter.from_server_params(params, name)
    else:
        raise

Prevention

When it happens

Trigger: Calling from_server_params(server_params, tool_name='search') when the server registers the tool as 'web_search'; trailing whitespace or different casing in tool_name; the server exposing tools conditionally (disabled feature flags, env-gated tools); connecting to the wrong server entirely.

Common situations: Server upgraded and tool renamed; connecting to a different MCP server (wrong command/URL) than the one hosting the tool; case-sensitivity mistakes ('GitHub' vs 'github'); a fork of the server with renamed tools; the server returning an empty list because initialization failed partially.

Related errors

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/6dae7abc3536ad55. Report an issue: GitHub.