McpClientPrompt having a way to specify metdata

[nschuessler]

Is your feature request related to a problem? Please describe.
MCP protocol can send prompts to clients, however, prompts are often tuned for certain models or types of models.
E.g. reasoning models, coding models, etc.

I didn't see a way to send metadata along with the prompt from server to client.

Describe the solution you'd like
If there was a way to attach metadata to the prompts using attributes using
name/value pairs that would be returned with the prompt to the client that would be helpful.
e.g. [Meta(Name="Model", Value="gpt-4o")]

Or some other.

Describe alternatives you've considered
There may be better ways to do it.

Additional context
We spend a lot of time tuning prompts against different models, so having alternative versions of the prompt and being able to tag or sort them out based on the client-side model availability would be helpful.

[stephentoub]

Up until the 06-18-2025 spec, there was nothing in the MCP protocol to enable this.

The 06-18-2025 spec does add https://github.com/modelcontextprotocol/modelcontextprotocol/blob/169021c2e1332eb9197a6f47fb6ec30e0a70b1d0/docs/specification/2025-06-18/basic/index.mdx#_meta, which shows up in the C# SDK here:

csharp-sdk/src/ModelContextProtocol.Core/Protocol/Result.cs

Lines 16 to 23 in 9d835a0

	/// <summary>
	/// Gets or sets metadata reserved by MCP for protocol-level metadata.
	/// </summary>
	/// <remarks>
	/// Implementations must not make assumptions about its contents.
	/// </remarks>
	[JsonPropertyName("_meta")]
	public JsonObject? Meta { get; init; }

You can populate that however you like if you implement a handler manually, but currently there's no way to do it if you purely rely on attributed prompts.

We could consider adding some kind of [McpMeta] attribute as you suggest.

You could also add this with your own DelegatingMcpServerPrompt that would wrap one created with McpServerPrompt.Create and augment the returned GetPromptResult with the additional state.