MCP prompt arguments are listed optional but required at invocation

[autocarl]

Summary

A prompt route parameter is listed as optional in prompts/list, but prompts/get without the argument fails validation.

Reproduction

Minimal prerelease sample:

app.Map("ops troubleshoot {symptom}", static (string symptom) =>
        $"Investigate this symptom: '{symptom}'.")
    .WithDescription("Generate troubleshooting instructions for an agent.")
    .AsPrompt();

Using Repl.Mcp 0.11.0-dev.56 with MCP Inspector CLI:

npx --yes @modelcontextprotocol/inspector --cli ./run-mcp.sh --method prompts/list

Observed:

{
  "name": "ops_troubleshoot",
  "arguments": [
    {
      "name": "symptom",
      "description": "Observed symptom",
      "required": false
    }
  ]
}

Then:

npx --yes @modelcontextprotocol/inspector --cli ./run-mcp.sh \
  --method prompts/get \
  --prompt-name ops_troubleshoot

fails with:

Missing value for parameter 'symptom' (expected: string).

Providing the argument works:

npx --yes @modelcontextprotocol/inspector --cli ./run-mcp.sh \
  --method prompts/get \
  --prompt-name ops_troubleshoot \
  --prompt-args symptom="queue depth rising"

Expected behavior

prompts/list metadata and invocation validation should agree.

Possible directions:

• mark route arguments as required if the MCP spec/client behavior allows it;
• make prompt route parameters optional at execution time;
• reject/diagnose .AsPrompt() routes that require positional route parameters unless defaults/optional segments are used.

Notes from source inspection

ReplMcpServerPrompt currently sets prompt argument metadata with:

Required = false // MCP spec: all prompt arguments must be optional.

but invocation still dispatches through the normal Repl route, where {symptom} is required.

Environment observed

• Repl.Mcp: 0.11.0-dev.56
• .NET SDK: 10.0.301
• OS: Ubuntu 24.04 linux-x64