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

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

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"
}

deps: upgrade modelcontextprotocol/go-sdk to v1.5.0

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.

deps: upgrade modelcontextprotocol/go-sdk to v1.5.0

Reopening as the SDK upgrade didn't resolve the issue.

El vie., 17 de abril de 2026 12:25 p. m., Sam Morrow < ***@***.***> escribió:

…

*SamMorrowDrums* left a comment (github/github-mcp-server#2333) <#2333 (comment)> Reopening as the SDK upgrade didn't resolve the issue. — Reply to this email directly, view it on GitHub <#2333?email_source=notifications&email_token=B7EZGWLYRIFJTCAOORH7AGL4WKAK5A5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTIMRXGA3TAMRYGAY2M4TFMFZW63VKON2WE43DOJUWEZLEUVSXMZLOOSWGM33PORSXEX3DNRUWG2Y#issuecomment-4270702801>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/B7EZGWMPHAY2G3KAOC3277L4WKAK5AVCNFSM6AAAAACX2F7XFGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DENZQG4YDEOBQGE> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

Hi! Checking in as this is a blocker for us. Since it was recently reopened, do you recommend we wait for a fix or implement a temporary workaround on our side? Thanks!

I'm seeing the same behavior when integrating the GitHub MCP Server with a Co-Pilot Studio Agent. Both from a custom connector and the offical GitHub MCP Server tool.

{"reasonCode":"RequestFailure","errorMessage":"Connector request failed","HttpStatusCode":"unsupportedMediaType","errorResponse":"[{\"jsonrpc\":\"2.0\",\"error\":{\"code\":415,\"message\":\"The response is not in a JSON format.\"}}]

{"reasonCode":"RequestFailure","errorMessage":"Connector request failed","HttpStatusCode":"unsupportedMediaType","errorResponse":"Content-Type must be 'application/json'\n"

Fix Content-Type rejection for application/json; charset=utf-8

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'Content-Type must be 'application/json' - when setting charset #2333

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'Content-Type must be 'application/json' - when setting charset #2333

Description

Describe the bug

Affected version

Steps to reproduce the behavior

Expected vs actual behavior

Activity

weltonrodrigo commented on Apr 15, 2026

SamMorrowDrums commented on Apr 15, 2026

SamMorrowDrums commented on Apr 17, 2026

pachecocordovamoiseseduardo-byte commented on Apr 19, 2026

maor-co-MA commented on Apr 20, 2026

dharris-bt commented on Apr 21, 2026

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