When I use two pages to access the same MCP server and run the tool, I encounter "Error POSTing to endpoint (HTTP 500): SSE connection not established". Even if I start two inspectors on the same computer using different ports, this problem still exists

[gemo12123]

Describe the bug
When I started the inspector, I opened two pages to access the same MCP server and run the tool at the same time, but it prompted me with "Error POSTing to endpoint (HTTP 500): SSE connection not established". Even though I started two inspectors on the same computer using different ports, this problem still exists

To Reproduce
Steps to reproduce the behavior:

1. Launch the Inspector and start the mcp server.
   2.Open two pages and visit ‘http://127.0.0.1:6274/’
   3.Connect to the same MCP server and run the tool

Expected behavior
Both pages can return the tool execution results normally

Logs
Received message for sessionId c265c82d-4810-4d59-9cd5-da2ca7c39baa
Error in /message route: Error: SSE connection not established
at SSEServerTransport.handlePostMessage (file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:61:19)
at file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/inspector/server/build/index.js:262:25
at Layer.handleRequest (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\layer.js:152:17)
at next (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\route.js:157:13)
at Route.dispatch (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\route.js:117:3)
at handle (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:435:11)
at Layer.handleRequest (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\layer.js:152:17)
at D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:295:15
at processParams (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:582:12)
at next (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:291:5)
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at ServerResponse.setHeader (node:_http_outgoing:699:11)
at ServerResponse.header (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\express\lib\response.js:684:10)
at ServerResponse.json (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\express\lib\response.js:247:10)
at file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/inspector/server/build/index.js:266:25
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
Received message for sessionId c265c82d-4810-4d59-9cd5-da2ca7c39baa
Error in /message route: Error: SSE connection not established
at SSEServerTransport.handlePostMessage (file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:61:19)
at file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/inspector/server/build/index.js:262:25
at Layer.handleRequest (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\layer.js:152:17)
at next (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\route.js:157:13)
at Route.dispatch (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\route.js:117:3)
at handle (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:435:11)
at Layer.handleRequest (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\layer.js:152:17)
at D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:295:15
at processParams (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:582:12)
at next (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:291:5)
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at ServerResponse.setHeader (node:_http_outgoing:699:11)
at ServerResponse.header (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\express\lib\response.js:684:10)
at ServerResponse.json (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\express\lib\response.js:247:10)
at file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/inspector/server/build/index.js:266:25
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
Received message for sessionId c265c82d-4810-4d59-9cd5-da2ca7c39baa
Error in /message route: Error: SSE connection not established
at SSEServerTransport.handlePostMessage (file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:61:19)
at file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/inspector/server/build/index.js:262:25
at Layer.handleRequest (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\layer.js:152:17)
at next (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\route.js:157:13)
at Route.dispatch (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\route.js:117:3)
at handle (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:435:11)
at Layer.handleRequest (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\layer.js:152:17)
at D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:295:15
at processParams (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:582:12)
at next (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:291:5)
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at ServerResponse.setHeader (node:_http_outgoing:699:11)
at ServerResponse.header (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\express\lib\response.js:684:10)
at ServerResponse.json (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\express\lib\response.js:247:10)
at file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/inspector/server/build/index.js:266:25
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
Received message for sessionId c265c82d-4810-4d59-9cd5-da2ca7c39baa
Error in /message route: Error: SSE connection not established
at SSEServerTransport.handlePostMessage (file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:61:19)
at file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/inspector/server/build/index.js:262:25
at Layer.handleRequest (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\layer.js:152:17)
at next (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\route.js:157:13)
at Route.dispatch (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\route.js:117:3)
at handle (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:435:11)
at Layer.handleRequest (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\lib\layer.js:152:17)
at D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:295:15
at processParams (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:582:12)
at next (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\router\index.js:291:5)
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at ServerResponse.setHeader (node:_http_outgoing:699:11)
at ServerResponse.header (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\express\lib\response.js:684:10)
at ServerResponse.json (D:\DevEnv\Node\node_cache_npx\4482475f1ce046f8\node_modules\express\lib\response.js:247:10)
at file:///D:/DevEnv/Node/node_cache/_npx/4482475f1ce046f8/node_modules/@modelcontextprotocol/inspector/server/build/index.js:266:25
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)

[cherry0226]

i went through the same issues

[olaservo]

I think this PR might resolve this for you once its released: #428

[cliffhall]

Note that your server needs to be implemented properly to handle multiple client connections. While #428 added support for multiple client connections in the inspector, in order to test, we also had to refactor server-everything to support multiple inbound client connections properly.

[cliffhall]

Closing, since the support exists and is demonstrated. Feel free to reopen if you're unable to get your server supporting the multiple connections.