'Content-Type must be 'application/json' - when setting charset

[pedrobrasilborges]

Describe the bug

I'm using the https://github.com/modelcontextprotocol/csharp-sdk for MCP integration and when sending the initial POST it adds the Content-Type as "application/json; charset=utf-8" and the server replies with 'Content-Type must be 'application/json' , while reduntant due to RFC 8259, it should just be stripped instead of enforced. This was working previously working.

Affected version

Remote version of Github MCP Server.

Steps to reproduce the behavior

Working:

Non-Working:

Expected vs actual behavior

Expected -> ability to initialize the McpClient
Actual -> throws exceptions

[weltonrodrigo]

this problem is affecting usage of Github MCP on azure foundry agents.

An agent with a definition like this is being affected:

object: agent.version
id: <my id>
name: <my agent name>
version: "62"
description: ""
created_at: 1776277369
definition:
  kind: prompt
  model: gpt-5.2
  instructions: "<here the agent prompt>"
  tools:
    - type: mcp
      server_label: github
      server_url: https://api.githubcopilot.com/mcp
      require_approval: never
      project_connection_id: github-mcp

Here is the raw JSON of the github-mcp connection retrieved from the Azure Foundry Connections API. Note that Azure masks the credentials object in the response for security reasons.

{
  "id": "/subscriptions/<MYSUB>/resourceGroups/agent-foundry-ab/providers/Microsoft.CognitiveServices/accounts/<MY-ACCOUNT>/projects/<MY-PROJECT>/connections/github-mcp",
  "location": null,
  "name": "github-mcp",
  "properties": {
    "authType": "CustomKeys",
    "category": "CustomKeys",
    "credentials": {
        "keys": {
            "Authorization": "Bearer <github_token>",
            "Content-Type": "application/json",
            "Accept": "application/json, text/event-stream"
        }
    },
    "error": null,
    "expiryTime": null,
    "group": "AzureAI",
    "isDefault": true,
    "isSharedToAll": false,
    "metadata": {},
    "peRequirement": "NotRequired",
    "peStatus": "NotApplicable",
    "sharedUserList": [],
    "target": "https://api.githubcopilot.com/mcp",
    "useWorkspaceManagedIdentity": false
  },
  "systemData": {
    "createdAt": "2026-04-15T18:45:50.653997Z",
    "createdBy": "4fe1a9c6-492a-4973-8816-47141b188eb6",
    "createdByType": "Application",
    "lastModifiedAt": "2026-04-15T18:45:50.653997Z",
    "lastModifiedBy": "4fe1a9c6-492a-4973-8816-47141b188eb6",
    "lastModifiedByType": "Application"
  },
  "tags": null,
  "type": "Microsoft.CognitiveServices/accounts/projects/connections"
}

[SamMorrowDrums]

Closing — this is resolved by upgrading the MCP Go SDK to v1.5.0 in #2336. The SDK (since v1.4.1) already uses mime.ParseMediaType in StreamableHTTPHandler to correctly handle Content-Type: application/json; charset=utf-8, so no additional middleware is needed.