Error Handling in FastMCP Servers
47%
Exception Types
FastMCP offers specific exception types for different operations, for example:
ToolError: For tool execution failuresResourceError: For resource read failuresPromptError: For prompt retrieval failuresMcpError: For general MCP protocol errors (imported frommcp, notfastmcp.exceptions)AuthorizationError: For authentication/authorization failures
These exceptions can be raised with custom messages that are intended to be seen by the parts of the system that consume them.
Basic Pattern: Raising ToolError
Our tool code looks like this:
@mcp.tool
def dog_to_human_age(
age: Annotated[int, Field(ge=0, le=30, description="The dog's age in years")],
multiplier: int = Depends(get_age_multiplier),
) -> int:
"""Calculate the real age of a dog in human years."""
return age * multiplier
We are going to evolve this tool to do more complex things. We will base our age calculator on the dog breed, which we will fetch from an external function:
def get_breed_multiplier(breed: str) -> int | None:
"""Fetch the age multiplier for a given dog breed."""
breed_multipliers = {
"labrador": 7,
"chihuahua": 5,
"german shepherd": 8,
"bulldog": 6,
# We have a lot more breeds
# but for brevity we only list a few here
}
return breed_multipliers.get(breed.lower())
@mcp.tool
def dog_to_human_age(
age: Annotated[int, Field(ge=0, le=30, description="The dog's age in years")],
breed: str,
) -> int:
"""Calculate the real age of a dog in human years based on its breed."""
multiplier = get_breed_multiplier(breed)
return age *Practical MCP with FastMCP & LangChain
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