Add support for statically-defined parametric prompts

[asklar]

Is your feature request related to a problem? Please describe.
In MCPB, it is expected that when statically defining a prompt, the developer can say "Order a ${sandwichKind} sandwich" as the text of the prompt.
It would be beneficial for tools operating on the assembly to be able to extract the string template for the prompt. But today, all we have is whatever the code decided to return at runtime.

Describe the solution you'd like

    [McpServerPrompt(Name = "complex_prompt", 
    Template = "Set the temperature to ${arguments.temperature} and reply in ${arguments.style} style"), 
    Description("A prompt with arguments")]
    public partial static IEnumerable<ChatMessage> ComplexPrompt(
        [Description("Temperature setting")] int temperature,
        [Description("Output style")] string? style = null);

Then the source generator will use the new Template attribute parameter to generate the code to return $"Set the temperature to {temperature} and reply in {style} style".

Describe alternatives you've considered
n/a

Additional context
n/a

[stephentoub]

Thanks for the suggestion. I'm nervous at the idea of building a custom prompt template language. If this is an important scenario, would it not be preferable to instead be able to import from existing templating languages like liquid, or from prompty, or the like? Those also wouldn't need to be part of the MCP C# SDK; they can just be separate libraries that create McpServerPrompt instances that are added to the prompt collection. Thoughts?

[asklar]

agree on reluctance to create a template language; I was hoping we could reuse C#'s own interpolated strings syntax (at least for prompts that are text-only).

The scenario is that we have servers which we want to use static analysis tooling on to produce MCPB manifests. Those manifests have descriptions of tools and prompts. Without a way to discover these prompts' structure (e.g. via attributes), we would rely on users manually specifying their prompts and worst of all they'd have to do it twice: once in code, and once in the manifest.

[stephentoub]

I see https://github.com/anthropics/mcpb/blob/main/MANIFEST.md#prompt-structure.

Do any of the other official MCP SDKs have built-in support for MCPB?

[asklar]

AFAIK, creating the MCPB manifest is mostly manual (the mcpb CLI helps with getting started, validating, etc.).

I am in the process of forking/porting the mcpb CLI to .net in coordination with our friends at Anthropic 🚀 so .net would get 1st class MCPB support.

[stephentoub]

we want to use static analysis tooling on to produce MCPB manifests

Are these separate tools that run over binaries? Roslyn source generators?

[asklar]

I'm using asklar/McpExtract - it loads the assembly and inspects it for McpServerToolAttribute and related attributes from the csharp-sdk.