Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

[zach-blumenfeld]

Describe the bug
Error Connecting. Results in persistent issues requiring server reboot. This is inconsistent. It works fine sometimes other times when my colleagues try and connect it creates this error then no one can connect and I need to reboot the inspector server.

To Reproduce
I use the below setup on Google Cloud VM with Google MCP toolbox.

#install git
sudo apt install git-all

#sudo su 'cause I'm lazy.... then make directories
sudo su
mkdir /ai
cd /ai

# get the things
export VERSION=0.3.0
curl -O https://storage.googleapis.com/genai-toolbox/v$VERSION/linux/amd64/toolbox
chmod +x toolbox
git clone https://github.com/neo4j-examples/neo4j-gcp-vertex-ai-langchain.git

# create a new screen so it isn't tied to your ssh session
screen -S mcp

#start mcp server
./toolbox --tools-file neo4j-gcp-vertex-ai-langchain/toolbox-companies/tools.yaml -a 0.0.0.0

detach from screen using Ctrl+a d

Now you will have the MCP server you can connect to on http://xxxxxxxxxx:5000/mcp/sse
. For MCP inspector:

#install npm
sudo apt install npm

#I like my screens....
screen -S inspector

#run
npx @modelcontextprotocol/inspector

Expected behavior
For the connection to work fine.

Logs

Received message for sessionId 3e2e5cf9-695e-4a8e-8d5c-842425b35e7b
Received message for sessionId 3e2e5cf9-695e-4a8e-8d5c-842425b35e7b
New SSE connection
Query parameters: { transportType: 'sse', url: 'http://xxxxxxx:5000/mcp/sse' }
SSE transport: url=http://xxxxxxxxx:5000/mcp/sse, headers=Accept
Connected to SSE transport
Connected MCP client to backing server transport
Created web app transport
Created web app transport
Set up MCP proxy
Received message for sessionId 3e2e5cf9-695e-4a8e-8d5c-842425b35e7b
Error in /message route: Error: SSE connection not established
    at SSEServerTransport.handlePostMessage (file:///root/.npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:53:19)
    at file:///root/.npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/inspector/server/build/index.js:130:25
    at Layer.handle [as handle_request] (/root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/router/layer.js:95:5)
    at next (/root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/router/route.js:149:13)
    at Route.dispatch (/root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/router/route.js:119:3)
    at Layer.handle [as handle_request] (/root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/router/layer.js:95:5)
    at /root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/router/index.js:284:15
    at Function.process_params (/root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/router/index.js:346:12)
    at next (/root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/router/index.js:280:10)
    at cors (/root/.npm/_npx/5a9d879542beca3a/node_modules/cors/lib/index.js:188:7)
node:internal/errors:496
    ErrorCaptureStackTrace(err);
    ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at new NodeError (node:internal/errors:405:5)
    at ServerResponse.setHeader (node:_http_outgoing:648:11)
    at ServerResponse.header (/root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/response.js:794:10)
    at ServerResponse.json (/root/.npm/_npx/5a9d879542beca3a/node_modules/express/lib/response.js:275:10)
    at file:///root/.npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/inspector/server/build/index.js:134:25
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  code: 'ERR_HTTP_HEADERS_SENT'
}

[pulkitsharma07]

#290

[cliffhall]

@zach-blumenfeld Thanks for this. Going to try and get a 0.8.3 out today or tomorrow that has #290 in it, which fixes this issue.

[cliffhall]

Duplicate of #273