mcp-elicitation

name	mcp-elicitation
description	How to safely use MCP Elicitation (ElicitAsync) in .NET MCP servers
domain	mcp, error-handling
confidence	high
source	earned — diagnosed and fixed broken elicitation flow

Context

MCP Elicitation lets a server request structured user input from the client (host). The client must advertise the capability during initialize. If the client doesn't support it, the SDK throws InvalidOperationException — which is easy to miss in catch filters.

Patterns

1. Always pre-check client capabilities before calling ElicitAsync:

   if (mcpServer.ClientCapabilities?.Elicitation?.Form is null)
       throw new InvalidOperationException("Client does not support elicitation.");
   

2. ElicitRequestParams.Mode defaults to "form" in SDK 1.1.0 — no need to set it explicitly for form-mode elicitation.

3. Elicitation is client-advertised, not server-declared. ServerCapabilities has no elicitation property. The client sends { capabilities: { elicitation: { form: {} } } } during initialize.

4. SDK throws InvalidOperationException, not McpException for capability checks — handle accordingly.

Examples

// Good: pre-flight check + defensive catch
private bool IsElicitationSupported() =>
    mcpServer.ClientCapabilities?.Elicitation?.Form is not null;

public async Task<T> PromptAsync(CancellationToken ct)
{
    if (!IsElicitationSupported())
        throw new InvalidOperationException("Client doesn't support elicitation. Workaround: ...");

    try { return await mcpServer.ElicitAsync(params, ct); }
    catch (InvalidOperationException) when (!IsElicitationSupported()) { /* re-wrap */ }
    catch (Exception ex) when (ex is not InvalidOperationException) { /* other failures */ }
}

Anti-Patterns

1. Don't use catch (Exception ex) when (ex is not InvalidOperationException) as a catch-all — SDK capability errors ARE InvalidOperationException and will slip through.

2. Don't call ElicitAsync without checking capabilities first — the SDK throws internally, but the error message is generic and not actionable for users.

3. Don't assume all MCP hosts support elicitation — it's an optional client capability. Always provide a fallback or workaround path.