MCPIntegration bypasses ignore_errors when FastMCP wraps exceptions in ToolError

[flostadler]

Problem

The MCPIntegration (auto-enabled since sentry-sdk 2.45) captures exceptions that should be filtered by ignore_errors because it sees the FastMCP-wrapped ToolError instead of the original exception type.

When using ignore_errors to suppress specific exception types raised by MCP tool handlers, the filtering doesn't work because of the order of exception wrapping:

1. Tool handler raises a custom exception (e.g. ExpectedToolError)
2. FastMCP's tool_manager.call_tool() catches it and wraps it in ToolError: raise ToolError(...) from e
3. ToolError propagates up through FastMCP's _call_tool_mcp
4. The MCPIntegration's _async_handler_wrapper catches ToolError and calls sentry_sdk.capture_exception(e) (link)
5. _is_ignored_error checks issubclass(ToolError, ExpectedToolError) → False → event is sent to Sentry

The ignore_errors configuration never sees the original exception type because FastMCP has already wrapped it.

Expected Behavior

Exceptions listed in ignore_errors (or their subclasses) should not be sent to Sentry, even when FastMCP wraps them in ToolError. The integration should either walk the exception chain (__cause__) or skip capturing exceptions that are already handled by the framework.

Reproduction

import sentry_sdk
from sentry_sdk.integrations.mcp import MCPIntegration
from fastmcp import FastMCP

class ExpectedToolError(Exception):
    """Should not be reported to Sentry."""
    pass

sentry_sdk.init(
    dsn="...",
    ignore_errors=[ExpectedToolError],
)

mcp = FastMCP("test")

@mcp.tool()
def my_tool() -> str:
    raise ExpectedToolError("This is expected and should not go to Sentry")

When my_tool is called, the ExpectedToolError ends up in Sentry because the MCPIntegration captures the wrapping ToolError.

Workaround

Disable the MCPIntegration if you handle MCP error reporting yourself:

sentry_sdk.init(
    dsn="...",
    disabled_integrations=[MCPIntegration()],
    ignore_errors=[ExpectedToolError],
)

Environment

• sentry-sdk: 2.51.0
• fastmcp: 2.14.4
• mcp: 1.26.0
• Python: 3.13

[linear]

PY-2086

[alexander-alderman-webb]

Nice report @flostadler!

We'll need to decide whether to report the exceptions as handled=True or remove them completely.
But they certainly must not be reported as unhandled when the library handled them.