MCP client `resources/read` calls corrupt certain template URIs

[eiriktsarpalis]

Type: Bug

Hi, I maintain the official C# SDK and I am currently investigating customer reports that certain resources/read calls from the VS Code MCP client to C# MCP servers are failing. I have been able to create the following minimal reproduction:

using ModelContextProtocol.Server;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddMcpServer()
    .WithHttpTransport()
    .WithResources<SimpleResourceType>();

var app = builder.Build();
app.MapMcp();
app.Run();

[McpServerResourceType]
public class SimpleResourceType
{
    [McpServerResource(UriTemplate = "resource:///DirectTextResource", MimeType = "text/plain")]
    public static string DirectTextResource() => "This is a direct resource";
}

The issue appears to be caused by the fact that the VS Code client is normalizing the reported resource:///DirectTextResource URI to resource://directtextresource, i.e:

1. It trims the third / character, making its name part of the host segment of the uri when in fact it was part of its path.
2. It converts the name to all-lowercase, even though URI paths are case sensitive.

This effectively changes the URI, meaning that the server responds with an error because it cannot find the requested resource.

VS Code version: Code 1.101.1 (18e3a1e, 2025-06-18T13:35:12.605Z)
OS version: Windows_NT arm64 10.0.26100
Modes:

System Info

Item	Value
CPUs	Apple Silicon (8 x 3200)
GPU Status	2d_canvas: enabled canvas_oop_rasterization: enabled_on direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: unavailable_off raw_draw: disabled_off_ok skia_graphite: disabled_off video_decode: enabled video_encode: enabled vulkan: disabled_off webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	19.99GB (3.47GB free)
Process Argv	--disable-extensions --crash-reporter-id bcbd6c16-914e-4ec8-9d99-80056caf9469
Screen Reader	no
VM	100%

Extensions disabled

A/B Experiments

vsliv368:30146709
vspor879:30202332
vspor708:30202333
vspor363:30204092
vswsl492:30256859
vscod805cf:30301675
binariesv615:30325510
c4g48928:30535728
azure-dev_surveyone:30548225
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pythontbext0:30879054
cppperfnew:31000557
dwnewjupyter:31046869
pythonrstrctxt:31112756
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dwcopilot:31170013
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pythoneinst12:31285622
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c7cif404:31314491
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pythonrdcb7cf:31303019
usemplatestapi:31297334
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aj953862:31281341
generatesymbolt:31295002
convertfstringf:31295003
pylancequickfixt:31329274
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usemarketplace:31331644
nesew2to15:31330639
1292j425:31329156
agentclaude:31331946
048e0538:31329543

cc @stephentoub @halter73