[Idea] Delayed Registration of Instance Methods

[strawgate]

Related to: #174

Add delay_registration to the tool decorator (as an example) and a method on the MCP Server to perform delayed registration.

See both objects get their own tools, which are callable, and return data from the data in the corresponding instance.

The prefix implementation is a bit sloppy, ideally prefix would be supported natively on the decorators but anyway

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

src/fastmcp/server/server.py:359

• The parameter name 'object' shadows the built-in type. Consider renaming it (e.g., 'instance' or 'obj') for clarity.

def perform_delayed_registration(self, prefix: str, object: object) -> None:

[jlowin]

This feels to me like it has a lot of overloaded responsibility and extremely tight coupling -- the decorators are tied to a specific mcp instance, which needs to be invoked again later to call mcp.perform_delayed_registration. In the given sample, it seems preferable to simply call mcp.tool()(first_sample.first) explicitly which leaves no mystery about what's happening. It feels a bit outside the scope of the actual FastMCP responsibility.

My suggestion instead is to create a class (below, MCPRegisterable) that has its own decorator (mcp_tool) which collects functions to be used as tools, and then have that class have a register_tools() method that applies those tools to any FastMCP instance. This will avoid the tight coupling and require no changes to FastMCP, while achieving the same goal with considerably greater flexibility

The pseudocode would be:

from src.fastmcp import FastMCP
from some_module import MCPRegisterable, mcp_tool

mcp = FastMCP()

class Sample(MCPRegisterable):
    def __init__(self, name):
        self.name = name
        super().__init__()

    @mcp_tool()
    def first_tool(self):
        """First tool description."""
        return f"Executed first tool {self.name}."

    @mcp_tool()
    def second_tool(self):
        """Second tool description."""
        return f"Executed second tool {self.name}."

first_sample = Sample("First")
second_sample = Sample("Second")

first_sample.register_tools(mcp)
second_sample.register_tools(mcp)

def main():
    mcp.run("sse")

if __name__ == "__main__":
    main()

[strawgate]

Thanks for taking a look and for the feedback. I'll take a look at making something like McpRegisterable and will follow-up unless you've already got an implementation in mind