microsoft/semantic-kernel · error · ArgumentException

Resource uri is required.

Error message

Resource uri is required.

What it means

The ReadResource handler throws ArgumentException when GetResourceRequestParams.Uri is null or empty. The `is not string { } resourceUri` pattern requires a non-null string, then string.IsNullOrEmpty guards empties. This validates the client request before resource/template lookup.

Source

Thrown at dotnet/samples/Demos/ModelContextProtocolClientServer/MCPServer/Extensions/McpServerBuilderExtensions.cs:224

        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);
        if (resourceDefinition is not null)
        {
            return resourceDefinition.InvokeHandlerAsync(context, cancellationToken);
        }

        // Look up in registered resource templates
        IEnumerable<ResourceTemplateDefinition> resourceTemplateDefinitions = context.Server.Services!.GetServices<ResourceTemplateDefinition>();

        foreach (var resourceTemplateDefinition in resourceTemplateDefinitions)
        {
            if (resourceTemplateDefinition.IsMatch(resourceUri))
            {

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Ensure the client always sends a non-empty `uri` in resources/read params.
2. Fix or upgrade the client SDK.
3. Pre-validate uris client-side before issuing the read.
4. On the server, translate this into a structured MCP error response.

Example fix

// before
if (context.Params?.Uri is not string { } resourceUri || string.IsNullOrEmpty(resourceUri))
{
    throw new ArgumentException("Resource uri is required.");
}

// after (return MCP error response)
if (string.IsNullOrWhiteSpace(context.Params?.Uri))
{
    return new ValueTask<ReadResourceResult>(
        Task.FromResult(new ReadResourceResult { /* IsError = true */ }));
}

if (string.IsNullOrWhiteSpace(context.Params?.Uri))
    return BadRequestError("resources/read requires a non-empty 'uri'.");

static bool HasResourceUri(ReadResourceRequestParams? p) =>
    !string.IsNullOrEmpty(p?.Uri);

try { return await HandleReadResourceRequestAsync(context, ct); }
catch (ArgumentException ex) when (ex.Message.Contains("Resource uri is required"))
{ return McpError(-32602, ex.Message); }

When it happens

Trigger: context.Params is null, or context.Params.Uri is null or empty string — a malformed resources/read request.

Common situations: Custom/buggy MCP client omits the uri field, an SDK serialization issue, or a hand-crafted JSON-RPC call missing uri.

Related errors

• Prompt name is required.
• No handler found for the resource uri '{resourceUri}'.
• No handler found for the prompt '{promptName}'.
• Resource constraint is not set. Do not try to call this meth
• Invalid choice

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/cd8f568c97094369. Report an issue: GitHub.