microsoft/semantic-kernel · error · ArgumentException

No handler found for the prompt '{promptName}'.

Error message

No handler found for the prompt '{promptName}'.

What it means

After validating the prompt name, the handler looks up a registered PromptDefinition whose Prompt.Name matches; if none is found it throws ArgumentException naming the missing prompt. So the request was well-formed but asked for a prompt the server did not register.

Source

Thrown at dotnet/samples/Demos/ModelContextProtocolClientServer/MCPServer/Extensions/McpServerBuilderExtensions.cs:212

        });
    }

    private static async ValueTask<GetPromptResult> HandleGetPromptRequestsAsync(RequestContext<GetPromptRequestParams> context, CancellationToken cancellationToken)
    {
        // Make sure the prompt name is provided
        if (context.Params?.Name is not string { } promptName || string.IsNullOrEmpty(promptName))
        {
            throw new ArgumentException("Prompt name is required.");
        }

        // Get all prompt definitions registered in the DI container
        IEnumerable<PromptDefinition> promptDefinitions = context.Server.Services!.GetServices<PromptDefinition>();

        // Look up the prompt definition
        PromptDefinition? definition = promptDefinitions.FirstOrDefault(d => d.Prompt.Name == promptName);
        if (definition is null)
        {
            throw new ArgumentException($"No handler found for the prompt '{promptName}'.");
        }

        // Invoke the handler
        return await definition.Handler(context, cancellationToken);
    }

    private static ValueTask<ReadResourceResult> HandleReadResourceRequestAsync(RequestContext<ReadResourceRequestParams> context, CancellationToken cancellationToken)
    {
        // Make sure the uri of the resource or resource template is provided
        if (context.Params?.Uri is not string { } resourceUri || string.IsNullOrEmpty(resourceUri))
        {
            throw new ArgumentException("Resource uri is required.");
        }

        // Look up in registered resource first
        IEnumerable<ResourceDefinition> resourceDefinitions = context.Server.Services!.GetServices<ResourceDefinition>();

        ResourceDefinition? resourceDefinition = resourceDefinitions.FirstOrDefault(d => d.Resource.Uri == resourceUri);

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Call prompts/list first to discover actually-registered prompt names and use one of those.
2. Verify the server registers the desired PromptDefinition at startup.
3. Check for case/exact-match issues — lookup is an exact Name equality.
4. Return an MCP-level not-found error rather than throwing, if you control the server.

Example fix

// before
PromptDefinition? definition = promptDefinitions.FirstOrDefault(d => d.Prompt.Name == promptName);
if (definition is null)
{
    throw new ArgumentException($"No handler found for the prompt '{promptName}'.");
}

// after (case-insensitive + structured response)
var definition = promptDefinitions.FirstOrDefault(d =>
    string.Equals(d.Prompt.Name, promptName, StringComparison.OrdinalIgnoreCase));
if (definition is null)
{
    return new ValueTask<GetPromptResult>(
        Task.FromResult(new GetPromptResult { /* IsError, message lists available names */ }));
}

var known = promptDefinitions.Select(d => d.Prompt.Name).ToList();
if (!known.Contains(promptName, StringComparer.Ordinal))
    return NotFoundError($"Unknown prompt '{promptName}'. Known: {string.Join(", ", known)}");

static bool PromptExists(IEnumerable<PromptDefinition> defs, string name) =>
    defs.Any(d => string.Equals(d.Prompt.Name, name, StringComparison.Ordinal));

try { return await definition.Handler(context, ct); }
catch (ArgumentException ex) when (ex.Message.Contains("No handler found for the prompt"))
{ return McpError(-32602, ex.Message); }

When it happens

Trigger: Client requests a prompt name that has no matching PromptDefinition registered in the DI container (FirstOrDefault returns null).

Common situations: Client asks for a prompt that exists on another server, a typo in the prompt name, or the server failed to register the prompt at startup.

Related errors

• Prompt name is required.
• No handler found for the resource uri '{resourceUri}'.
• Resource uri is required.
• Plugin creation failed for {pluginName}
• The client does not support sampling.

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/f0a15619637abf38. Report an issue: GitHub.