Fix: Remove charset=utf-8 from Content-Type: application/json in HTTP transport

[jayaraman-venkatesan]

Fixes #1527

Summary

The HttpClientTransport was sending Content-Type: application/json; charset=utf-8 on all POST requests. Per RFC 8259 §11 and the IANA registration for application/json, charset is not a defined parameter for application/json — JSON is always Unicode. Strict servers such as GitHub Copilot's MCP endpoint (https://api.githubcopilot.com/mcp) reject the extra parameter with HTTP 415 Unsupported Media Type, making it impossible to connect without a custom DelegatingHandler workaround.

Changes

src/ModelContextProtocol.Core/Client/McpHttpClient.cs

• Removed CharSet = "utf-8" from s_applicationJsonContentType — fixes the non-NET path and ClientOAuthProvider which shares the constant.

// Before
internal static readonly MediaTypeHeaderValue s_applicationJsonContentType = new("application/json") { CharSet = "utf-8" };
// ...
return JsonContent.Create(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);

// After
internal static readonly MediaTypeHeaderValue s_applicationJsonContentType = new("application/json");
// ...
return JsonContent.Create(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage, s_applicationJsonContentType);

Tests

Two new tests added to HttpClientTransportTests.cs using a shared StrictJsonContentTypeHandler mock that mirrors GitHub Copilot's behavior — returns 200 only for bare application/json, otherwise 415:

References

• Issue: #1527
• RFC 8259 §11: https://datatracker.ietf.org/doc/html/rfc8259#section-11
• IANA application/json registration: https://www.iana.org/assignments/media-types/application/json
• Python SDK sends bare application/json: src/mcp/client/streamable_http.py
• TypeScript SDK sends bare application/json: src/client/streamableHttp.ts