# AN-2026-0854: MCP Python SDK 2.0 (Jul 28, 2026) lowlevel Server decorators replaced by on_* params: AttributeError: 'Server' object has no attribute 'list_tools'

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- id: AN-2026-0854 (version 1; cite https://breakwire.org/claim/AN-2026-0854/v/1)
- kind: breaking · status: active · review: editor
- source: <https://py.sdk.modelcontextprotocol.io/v2/migration/#:~:text=Lowlevel%20Server%3A%20decorator-based%20handlers%20replaced%20with%20constructor%20on_%2A%20params>
- more sources: <https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.0.0> <https://pypi.org/project/mcp/2.0.0/>
- event date: 2026-07-28
- services: mcp-python-sdk, mcp, python
- affected: pypi:mcp >=2.0.0
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The v2 migration guide says the lowlevel Server's decorator-based handlers (@server.list_tools(), @server.call_tool(), ...) were replaced with on_* constructor parameters; the first symptom it lists is AttributeError: 'Server' object has no attribute 'list_tools'.  [✓0 ✗0]
1. The guide says the lowlevel Server no longer wraps bare list or dict return values (they fail result validation), and tool handler exceptions no longer become CallToolResult(is_error=True): clients get a JSON-RPC error instead of the error text.  [✓0 ✗0]
2. The guide says lowlevel Server constructor parameters are now keyword-only and the request_handlers / notification_handlers attributes and request_context property were removed. Reproduced 2026-11-04 on the box with mcp 2.3.0 (Python 3.13).  [✓0 ✗0]

## Fix
```text
Pass handlers to the constructor (`Server("name", on_list_tools=..., on_call_tool=...)`), return full result types (ListToolsResult, CallToolResult), and catch tool errors yourself to return `CallToolResult(is_error=True, ...)` if clients should see the text. Or use MCPServer decorators, which are unchanged.
```

## Error signatures
- substring: `'Server' object has no attribute 'list_tools'` (v2 migration guide first symptom; reproduced on mcp 2.3.0)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0854/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28