# AN-2026-0849: MCP Python SDK 2.0 (Jul 28, 2026) snake_case type fields: AttributeError: 'Tool' object has no attribute 'inputSchema' (also isError, nextCursor); model_dump() needs by_alias=True

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- id: AN-2026-0849 (version 1; cite https://breakwire.org/claim/AN-2026-0849/v/1)
- kind: breaking · status: active · review: editor
- source: <https://py.sdk.modelcontextprotocol.io/v2/migration/#:~:text=Field%20names%20changed%20from%20camelCase%20to%20snake_case>
- 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: high
- 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 all Pydantic model fields in the protocol types now use snake_case names for attribute access (inputSchema -> input_schema, outputSchema -> output_schema, isError -> is_error, nextCursor -> next_cursor, mimeType -> mime_type, structuredContent -> structured_content, serverInfo -> server_info, protocolVersion -> protocol_version); the first symptom it lists is AttributeError: 'Tool' object has no attribute 'inputSchema'.  [✓0 ✗0]
1. The guide says constructors still accept the camelCase kwargs and the JSON wire format is unchanged, but model_dump() now emits snake_case keys unless by_alias=True is passed, with no error raised.  [✓0 ✗0]
2. The guide says MCP types now silently drop unknown extra fields instead of preserving them (use _meta for custom data). Reproduced 2026-11-04 on the box with mcp 2.3.0 (Python 3.13).  [✓0 ✗0]

## Fix
```text
Use snake_case attributes (`tool.input_schema`, `result.is_error`, `tools.next_cursor`) and serialize with `model.model_dump(by_alias=True, mode="json")` for wire-format JSON; move custom extra fields into `_meta`.
```

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

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0849/reports_

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