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Auto-populate completion handlers from AllowedValuesAttribute on prompt/resource parameters (#1380)

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Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>
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36
docs/concepts/completions/completions.md
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@ -81,6 +81,42 @@ builder.Services.AddMcpServer()
});
```
### Automatic completions with AllowedValuesAttribute
For parameters with a known set of valid values, you can use `System.ComponentModel.DataAnnotations.AllowedValuesAttribute` on `string` parameters of prompts or resource templates. The server will automatically surface those values as completions without needing a custom completion handler.
#### Prompt parameters
```csharp
[McpServerPromptType]
public class MyPrompts
{
[McpServerPrompt, Description("Generates a code review prompt")]
public static ChatMessage CodeReview(
[Description("The programming language")]
[AllowedValues("csharp", "python", "javascript", "typescript", "go", "rust")]
string language,
[Description("The code to review")] string code)
=> new(ChatRole.User, $"Please review the following {language} code:\n\n```{language}\n{code}\n```");
}
```
#### Resource template parameters
```csharp
[McpServerResourceType]
public class MyResources
{
[McpServerResource("config://settings/{section}"), Description("Reads a configuration section")]
public static string ReadConfig(
[AllowedValues("general", "network", "security", "logging")]
string section)
=> GetConfig(section);
}
```
With these attributes in place, when a client sends a `completion/complete` request for the `language` or `section` argument, the server will automatically filter and return matching values based on what the user has typed so far. This approach can be combined with a custom completion handler registered via `WithCompleteHandler`; the handler's results are returned first, followed by any matching `AllowedValues`.
### Requesting completions on the client
Clients request completions using <xref:ModelContextProtocol.Client.McpClient.CompleteAsync*>. Provide a reference to the prompt or resource template, the argument name, and the current partial value: