MCP prompt string results are returned as JSON string literals

[autocarl]

Summary

Prompt handlers that return string currently surface as JSON string literals in MCP prompt text.

Reproduction

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

Call:

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

Observed prompt content:

{
  "messages": [
    {
      "role": "user",
      "content": {
        "type": "text",
        "text": "\"Investigate the checkout service for this symptom: \\u0027queue depth rising\\u0027. Start with ops_status, inspect failed checks, then propose the smallest safe next step.\""
      }
    }
  ]
}

The text contains the serialized JSON string, including extra quotes and escapes.

Expected behavior

Prompt text should be plain text, e.g.:

Investigate the checkout service for this symptom: 'queue depth rising'. Start with ops_status, inspect failed checks, then propose the smallest safe next step.

Likely cause

McpToolAdapter forces --output:json for MCP command execution. When a Repl handler returns a string, the CLI output is a JSON string literal. ReplMcpServerPrompt.GetAsync() then forwards that text directly into TextContentBlock.

There is already a similar unwrapping pattern for MCP App UI resources (ReplMcpServerUiResource.UnwrapJsonString). Prompts may need equivalent handling, or prompt execution may need a prompt-specific output path.

Environment observed

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