## Cause
The client never registered an elicitation handler, so the SDK cannot prompt the user; paths that do not pre-check capabilities surface Elicitation not supported instead of a silent decline.
## Steps
1. Implement async def handle_elicitation(context, params) -> ElicitResult per docs (accept/decline/cancel).
2. Construct Client with elicitation_callback=handle_elicitation inside async with.
3. Retry the tool call; confirm the callback receives params.message and requested_schema.
4. If the server uses ctx.elicit on 2026-07-28 connections, migrate to resolver-based elicitation instead—mid-call elicitation is refused on that protocol.
## Expected result
Tool calls that require user input either invoke the callback or fail upfront with the capability error (-32021) before work starts when using resolvers that pre-check.
## Misleading approaches
- Expecting the tool to receive a synthetic 'decline' when no callback is registered.
- Adding elicitation_callback only on the server side.
## Limitations
- On 2026-07-28 protocol, ctx.elicit in a tool cannot work regardless of callback; use resolvers or multi-round-trip responses.
- Legacy stateless_http or json_response servers may lack a back-channel for server-initiated elicitation even with a callback.
## Unknowns
- Which protocol version and server transport a given deployment negotiates without inspecting initialize traffic.
## Evidence claims
- (documented_platform_behavior) MCPError: Elicitation not supported means the client registered no elicitation_callback when the server needed an elicitation answered. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/docs/troubleshooting.md
- (documented_platform_behavior) Registering elicitation_callback on Client declares elicitation capability; there is no separate switch. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/docs/troubleshooting.md
- (documented_platform_behavior) A client that cannot be asked causes the tool call to fail rather than returning an implicit decline. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/docs/troubleshooting.md
Proposed approach
- Problem id
- 7b8f6553-8ed8-47f4-a3e3-d54105ff90e6
- Proposed action
- Pass elicitation_callback= to Client(...) (registering the callback declares elicitation capability); alternatively redesign server tools to avoid mid-call elicitation on clients that cannot answer (resolvers / InputRequiredResult for modern protocol).
- Applicability
- State
- partial
- Text
- MCP Python SDK 2.x client calling tools that elicit user input; error MCPError: Elicitation not supported; client constructed without elicitation_callback.; affected: mcp Python SDK 2.x clients without elicitation_callback; fix_status: CONFIRMED_SOLUTION
- Limitations
- State
- partial
- Text
- On 2026-07-28 protocol, ctx.elicit in a tool cannot work regardless of callback; use resolvers or multi-round-trip responses. Legacy stateless_http or json_response servers may lack a back-channel for server-initiated elicitation even with a callback.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- MCP Python SDK 2.x client calling tools that elicit user input; error MCPError: Elicitation not
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