add sse accept header

[lloydzhou]

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[olaservo]

I don't think this change should be needed since my understanding is that the Accept: text/event-stream header is already automatically added by the SDK's SSEClientTransport: https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/client/sse.ts#L120

Is there a specific issue you're trying to address with this change?

[lloydzhou]

@olaservo
you can see line 115(https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/client/sse.ts#L115)

in this project, we pass eventSourceInit(without send Accept: text/event-stream header) in to SSEClientTransport

[lloydzhou]

in my case, i using nchan.io as Transport and Gateway.
when i test by using inspector, the nchan can not get Accept: text/event-stream header then switch to long-polling.

sequenceDiagram
  MCP Client->>NCNAN: connect
  activate NCNAN
  MCP Client-->>NCNAN: jsonrpc request
  NCNAN-->>FastAPI: nchan_publisher_upstream_request
  FastAPI-->>MCP Server: call_tool(name, args)
  MCP Server-->>FastAPI: result
  FastAPI-->>NCNAN: jsonrpc response
  NCNAN-->> MCP Client: jsonrpc response
  
  MCP Client-->>NCNAN: jsonrpc request
  NCNAN-->>FastAPI: nchan_publisher_upstream_request
  FastAPI-->>MCP Server: call_tool(name, args) in backend
  MCP Server-->>NCNAN: push notification
  NCNAN-->> MCP Client: notification
  MCP Server-->>NCNAN: push jsonrpc response
  NCNAN-->> MCP Client: jsonrpc response
  NCNAN->> MCP Client: close
  deactivate NCNAN

Loading

[cliffhall]

when i test by using inspector, the nchan can not get Accept: text/event-stream header then switch to long-polling
in this project, we pass eventSourceInit(without send Accept: text/event-stream header) in to SSEClientTransport

@lloydzhou This feels like a workaround for a problem that exists with nchan. Is it possible to make a PR there to fix the issue?

[lloydzhou]

@cliffhall
please see code here? i think the issue exists our project, not in nchan.

https://github.com/modelcontextprotocol/inspector/blob/main/server/src/index.ts#L82C7-L82C22

https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/client/sse.ts#L115

[olaservo]

@lloydzhou thanks for clarifying, I see what you mean now. Since we're using a custom fetch (which is filtering the headers to forward) we're not sending the Accept header by default.

[olaservo]

Hi @lloydzhou I think this is good to merge, however could you clarify a little more about how you tested this?

[olaservo]

@lloydzhou looks like you need to fix some formatting issues, could you run the following and also update your branch:

npx prettier server/src/index.ts --write

[lloydzhou]

1. fix by prettier.
2. i have write test code by using nchan + ngx_lua https://github.com/lloydzhou/nchan-mcp-transport-test