# AN-2026-0856: MCP Python SDK 2.0 (Jul 28, 2026): ctx.elicit()/create_message()/list_roots() raise NoBackChannelError on 2026-07-28 connections, incl. in-process Client(server)

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- id: AN-2026-0856 (version 1; cite https://breakwire.org/claim/AN-2026-0856/v/1)
- kind: breaking · status: active · review: editor
- source: <https://py.sdk.modelcontextprotocol.io/v2/migration/#:~:text=Server-initiated%20sampling%2C%20elicitation%2C%20and%20roots%20raise%20NoBackChannelError>
- 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 2026-07-28 protocol has no server-initiated requests, so ctx.elicit(), ctx.elicit_url(), ctx.session.create_message(), ctx.session.list_roots() and other ServerSession request helpers raise NoBackChannelError on such a connection instead of sending.  [✓0 ✗0]
1. The guide says NoBackChannelError (mcp.shared.exceptions) subclasses MCPError with code -32600 and message Cannot send '<method>': this transport context has no back-channel for server-initiated requests., reaches the client as a top-level JSON-RPC error, and is also raised on legacy sessions against stateless_http=True servers and on the request channel of json_response=True servers.  [✓0 ✗0]
2. The guide says v2's Client defaults to mode='auto' (probes server/discover, falls back to initialize), and an in-process Client(server) negotiates 2026-07-28 by default, so the first smoke test of an unchanged v1 sampling or elicitation tool fails even with sampling_callback= / elicitation_callback= set.  [✓0 ✗0]

## Fix
```text
Short term: connect with `Client(server, mode="legacy", sampling_callback=..., elicitation_callback=...)`. Long term: return the question instead of pushing it with a `Resolve(...)` parameter whose resolver returns `Elicit`, `Sample` or `ListRoots` (mcp.server.mcpserver).
```

## Error signatures
- substring: `this transport context has no back-channel for server-initiated requests` (v2 migration guide NoBackChannelError message (method name varies))

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0856/reports_

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