Fix incorrect MapMcp default-path guidance in mcp-csharp-create

[leslierichardson95]

Summary

Fixes factually incorrect HTTP-endpoint path guidance in mcp-csharp-create that could cause frontier models to emit /mcp client URLs against a root-mapped server (404s). Part of the dotnet-ai skills-eval follow-up (#889); this is the "protect + surgical fix" action for mcp-csharp-create, which is otherwise an efficient win on Sonnet/Haiku.

What was wrong

The HTTP transport example and Key HTTP details claimed plain app.MapMcp() "defaults to /mcp". Verified against the C# MCP SDK source (McpEndpointRouteBuilderExtensions.MapMcp):

public static IEndpointConventionBuilder MapMcp(
    this IEndpointRouteBuilder endpoints,
    [StringSyntax("Route")] string pattern = "")

The default pattern = "" maps the server at the root route (/), not /mcp. A client told to connect to /mcp against a default-mapped server gets a 404.

Change

• app.MapMcp(); → app.MapMcp("/mcp"); in the HTTP Program.cs example (matches the inline comment's stated intent).
• Rewrote Key HTTP details to describe the pattern argument accurately and tell clients to use a matching URL.

Consistent with references/transport-config.md, which already documents MapMcp("/custom-mcp-path") as a custom path prefix. Surgical, 2-line change — no expansion of an already-winning skill.

Refs #889.

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Moved from fork branch to the primary dotnet/skills repo; supersedes #937.

[github-actions]

Skill Coverage Report

	Plugin	Skill	Covered	Coverage
❌	dotnet-ai	mcp-csharp-create	0/25	0%

Uncovered: dotnet-ai/mcp-csharp-create

• [Validation] Project builds with no errors (dotnet build) (line 232)
• [Validation] All tool classes have [McpServerToolType] attribute (line 233)
• [Validation] All tool methods have [McpServerTool] and [Description] attributes (line 234)
• [Validation] All parameters have [Description] attributes (line 235)
• [Validation] stdio: logging directed to stderr, not stdout (line 236)
• [Validation] HTTP: app.MapMcp() is called in Program.cs (line 237)
• [Validation] Server starts successfully with dotnet run (line 238)
• [Pitfall] stdio server outputs garbage or hangs (line 244)
• [Pitfall] Tool not discovered by LLM clients (line 245)
• [Pitfall] LLM doesn't understand when to use a tool (line 246)
• [Pitfall] WithToolsFromAssembly() fails in AOT (line 247)
• [Pitfall] Parameters not appearing in tool schema (line 248)
• [Pitfall] HTTP server returns 404 (line 249)
• [WorkflowStep] Step 1: Verify prerequisites (line 47)
• [WorkflowStep] Step 2: Choose transport (line 57)
• [WorkflowStep] Step 3: Scaffold the project (line 68)
• [WorkflowStep] Step 4: Implement tools (line 86)
• [WorkflowStep] Step 5: Add prompts and resources (optional) (line 151)
• [WorkflowStep] Step 6: Configure Program.cs (line 176)
• [WorkflowStep] Step 7: Verify the server starts (line 219)
• [CodePattern] [McpServerToolType] (line 90)
• [CodePattern] [McpServerResourceType] (line 166)
• [CodePattern] [McpServerPromptType] (line 154)
• [CodePattern] CancellationToken (line 90)
• [CodePattern] [Description] (line 90)

[Copilot]

Pull request overview

This PR corrects the mcp-csharp-create skill’s HTTP transport guidance so the documented MapMcp route pattern matches the client URL, avoiding 404s caused by incorrectly assuming MapMcp() defaults to /mcp.

Changes:

• Updates the HTTP Program.cs example to explicitly map the MCP endpoint at /mcp via app.MapMcp("/mcp").
• Rewrites the “Key HTTP details” section to accurately describe MapMcp(pattern) behavior and instruct clients to use a matching URL.

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[leslierichardson95]

Superseded by #943, which retires this skill (and the rest of the mcp-csharp-* family) per the #889 eval + telemetry review. Closing as obsolete.