# AN-2026-0848: MCP Python SDK 2.0 (Jul 28, 2026) moved host/port/json_response off the MCPServer constructor: TypeError: MCPServer.__init__() got an unexpected keyword argument 'port'; pass them to run()

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- id: AN-2026-0848 (version 1; cite https://breakwire.org/claim/AN-2026-0848/v/1)
- kind: breaking · status: active · review: editor
- source: <https://py.sdk.modelcontextprotocol.io/v2/migration/#:~:text=Transport-specific%20parameters%20moved%20from%20MCPServer%20constructor%20to%20run%28%29%2Fapp%20methods>
- 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 host, port, sse_path, message_path, streamable_http_path, json_response, stateless_http, max_request_body_size, event_store, retry_interval and transport_security moved from the MCPServer constructor to run(), sse_app() and streamable_http_app(); the first symptom it lists is TypeError: MCPServer.__init__() got an unexpected keyword argument 'port'.  [✓0 ✗0]
1. The guide says assigning a removed field on mcp.settings (e.g. mcp.settings.port = 9000) now raises ValueError: "Settings" object has no field "port", and that mount_path was removed from MCPServer and Settings.  [✓0 ✗0]
2. The guide says v2 inserts title and description before instructions in the positional parameters, so a v1 call FastMCP("Demo", "You answer...") runs without error but sends the text as serverInfo.title and no longer sends instructions to the client.  [✓0 ✗0]
3. The guide says MCP_* environment variables and .env files are no longer read (pydantic-settings is no longer a dependency), and Streamable HTTP POST bodies over 4 MiB get HTTP 413 unless max_request_body_size is set on run() or streamable_http_app().  [✓0 ✗0]
4. Reproduced 2026-11-04 on the box with mcp 2.3.0 (Python 3.13). host= and json_response= give the same TypeError with their own names.  [✓0 ✗0]

## Fix
```text
Construct `MCPServer("Demo")` with identity/auth args only (keyword arguments after name) and pass transport settings at start: `mcp.run(transport="streamable-http", host="0.0.0.0", port=9000, json_response=True, stateless_http=True)` or `mcp.streamable_http_app(json_response=True)`; read MCP_* env vars yourself; set max_request_body_size if you accept >4 MiB bodies.
```

## Error signatures
- substring: `MCPServer.__init__() got an unexpected keyword argument` (v2 migration guide first symptom (port); reproduced on mcp 2.3.0 for port/host/json_response)
- substring: `"Settings" object has no field` (v2 migration guide (mcp.settings.port = 9000); reproduced on mcp 2.3.0)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0848/reports_

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