Why does McpError("message") raise AttributeError: 'str' object has no attribute 'message'?

Question

In the Python MCP SDK from the modelcontextprotocol/servers repo (Python 3.11.9 on Linux), raising McpError("Error") fails with AttributeError: 'str' object has no attribute 'message' at mcp/shared/exceptions.py line 13. Does the constructor accept a plain string, and what is the correct way to raise McpError?

Originally asked on GitHub: https://github.com/modelcontextprotocol/servers/issues/1847

AgentArmorHQ team · Accepted Answer

The issue is closed (NOT_PLANNED) as superseded by the v2 release: a maintainer explained that the reference servers were rebuilt for the 2026-07-28 MCP specification (#4857) and released as v2 (2.0.0 on npm for the TypeScript servers, 2026.10.19 on PyPI for the Python servers), and that the behavior described "has either been fixed or no longer exists in the same form." The Python servers now use the v2 SDK's MCPError with keyword `code` and `message` arguments, so call sites no longer pass a bare string.

The reporter's traceback shows that `McpError.__init__` in `mcp/shared/exceptions.py` (line 13) accesses `error.message` on its argument, so the constructor expects an `ErrorData` object, not a string. A commenter in the thread reported that wrapping the message in `ErrorData` works:

```python
from mcp.shared.exceptions import McpError
from mcp.types import ErrorData

# Instead of:
# raise McpError("Error message")

raise McpError(ErrorData(code=0, message="Error message"))
```

The maintainer added that if this still happens on the v2 release, you should open a fresh issue with the version you are running and steps to reproduce; the repository accepts issues rather than pull requests.

AgentArmorHQ team · Answer

**What a GitHub user tried on GitHub** (Didn't work)

The reporter called `raise McpError("Error")` in Python 3.11.9 on Linux and got `AttributeError: 'str' object has no attribute 'message'`, traced to `super().__init__(error.message)` in `mcp/shared/exceptions.py`.

Source: https://github.com/modelcontextprotocol/servers/issues/1847

AgentArmorHQ team · Answer

**What @sstklen tried on GitHub** (Worked)

A commenter reported that `raise McpError(ErrorData(code=0, message="Error message"))` — importing ErrorData from `mcp.types` — resolves the error.

Source: https://github.com/modelcontextprotocol/servers/issues/1847#issuecomment-3981217324

Why does McpError("message") raise AttributeError: 'str' object has no attribute 'message'?

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Why does McpError("message") raise AttributeError: 'str' object has no attribute 'message'?

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