Practical MCP with FastMCP & LangChain

Structured Data with the Extra Parameter

Every logging method accepts an optional extra parameter, which lets you attach a dictionary of structured data alongside the human-readable message. This is particularly useful for clients that do more than just display log lines. A monitoring dashboard might parse the extra data to build charts. An analytics client might index it for later querying. Even a simple development client benefits from having structured metadata it can format nicely.

The extra dictionary can contain any JSON-serializable data: strings, numbers, booleans, nested dictionaries, and lists. You might include a transaction ID, a count of processed records, request timing information, or any other contextual data that would be useful for understanding the log message in detail. The human-readable message tells the story; the extra data provides the evidence.

Here is an example of using the extra parameter in our code:

from fastmcp import FastMCP
from fastmcp import Context

@mcp.tool
async def dog_to_human_age(ctx: Context, age: int, breed: str) -> int:
    multiplier = get_breed_multiplier(breed