Question about MCP server tools: How does client infer what the tool's parameter values should be?

[bsloan666]

Pre-submission Checklist

• I have checked that this question would not be more appropriate as an issue in a specific repository
• I have searched existing discussions and documentation for answers

Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
• Documentation
• Other

Your Question

In the MCP server python example, weather.py (https://modelcontextprotocol.io/docs/develop/build-server), tools are defined using the @mcp.tool() decorator on global methods get_alerts(state) and get_forecast(lat, long). Does the caller of the tool (the client?) have any more information than the function signature of the tool's method and perhaps the parameter names? Is there some magic by which the mcp library can introspect the tool method's docstrings or the method's code?

Otherwise, I would expect that get_alerts(state: str) would not provide the client enough of a clue as to what a valid input param should be -- in this case a 2-letter USA state code. Are detailed docstrings a requirement. Are there other recommended ways to provide parameter hints to the client?

Also, this screen-shot from the tutorial is confusing (apologies if this is the wrong forum):

"View result from get-weather from weather (local)" I assume get_forecast() may have been named get_weather() at some point.

Thanks in advance!