Using with Open AI Agent Builder

[lolz0r]

Summary:
When using the MCP server with HTTP transport behind an Nginx reverse proxy, the Open AI agent builder can detect the server and list available tools. However, tool calls fail most of the time (>90%), returning:

Tool: mcp_mongo.connect
Response: { "code": 32600, "message": "Session terminated" }

Expected behavior:
Tool calls should consistently succeed once the MCP server is reachable.

Observed behavior:
Most requests to /mcp end with a Session terminated error, even though initial handshake and occasional tool calls succeed.

Condensed Server Log:

nginx | "POST /mcp" 200 / 202 responses alternating
mongo-mcp | [INFO] Server started (StreamableHTTPServerTransport)
mongo-mcp | [INFO] Attempting MongoDB connection...
mongo-mcp | [DEBUG] Telemetry: start → stop sequence repeats within seconds
mongo-mcp | [INFO] Server shutdown / "Session terminated"

• Repeated server restarts and session closures observed within 1–2 seconds.
• Same behavior across multiple session IDs.
• Nginx reverse proxy appears to receive valid POST/DELETE cycles but connections terminate immediately after.

I am running the MCP server via docker:
mongo-mcp:
image: mongodb/mongodb-mcp-server:latest
restart: always
env_file:
- .env
environment:
MDB_MCP_CONNECTION_STRING: ${MDB_MCP_CONNECTION_STRING}
MDB_MCP_READ_ONLY: ${MDB_MCP_READ_ONLY:-true}
MDB_MCP_TRANSPORT: ${MDB_MCP_TRANSPORT:-http}
MDB_MCP_HTTP_HOST: ${MDB_MCP_HTTP_HOST:-0.0.0.0}
MDB_MCP_HTTP_PORT: ${MDB_MCP_HTTP_PORT:-8080}
MDB_MCP_API_CLIENT_ID: ${MDB_MCP_API_CLIENT_ID}
MDB_MCP_API_CLIENT_SECRET: ${MDB_MCP_API_CLIENT_SECRET}
MDB_MCP_LOGGERS: "disk,stderr"

[github-actions]

Thanks for opening this issue. The ticket MCP-267 was created for internal tracking.

[jeroenvervaeke]

Thank you for reporting this issue! The detailed logs and Docker configuration are very helpful.
To help us reproduce this problem, could you please share your Nginx configuration? Specifically the server block that proxies to your MCP server, including any timeout or connection handling settings you have configured.
This will help us set up a similar environment and better understand what might be causing the session terminations.

[lolz0r]

Hi @jeroenvervaeke
Below is the nginx config:

gzip on;
gzip_min_length  1000; # recommended minimum length for compression
gzip_proxied     any; # ensures gzip compression works even for proxied requests
gzip_types       text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
gzip_vary on; # tells proxies to cache both gzipped and non-gzipped versions

upstream app_server {
    server app:8000;
}

upstream mcp_upstream {
    server mongo-mcp:8080;
}

#map $http_x_use_mcp $route_to_mcp {
#    default 0;
#    "secret-header" 1;
#}

server {
    listen 80;
    listen [::]:80;
    server_name localhost;

    location /mcp {
        proxy_pass http://mcp_upstream;
        proxy_http_version 1.1;
        proxy_read_timeout 1h;
        proxy_send_timeout 1h;
        proxy_buffering off;
        proxy_request_buffering off;
        proxy_set_header   Connection "keep-alive";
        proxy_set_header   Proxy-Connection "keep-alive";
        proxy_set_header   X-Accel-Buffering no;
        proxy_set_header   Keep-Alive "timeout=3600";
        proxy_set_header   Upgrade $http_upgrade;

        # Standard proxy headers
        proxy_set_header   Host $host;
        proxy_set_header   X-Real-IP $remote_addr;
        proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header   X-Forwarded-Proto $scheme;
    }

    location / {
        #if ($route_to_mcp = 1) {
        #    rewrite ^ /mcp break;
        #}

        proxy_pass http://app_server;
        proxy_http_version 1.1;

        proxy_connect_timeout 180s;
        proxy_read_timeout 180s;

        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_set_header Host $host;
    }

    error_page 500 502 503 504 /50x.html;
}

server { 
    listen                  443 ssl;
    listen                  [::]:443 ssl;
    server_name             localhost;
    ssl_certificate         /root/ssl/cert.pem;
    ssl_certificate_key     /root/ssl/key.pem;

    location /mcp {
        proxy_pass http://mcp_upstream;
        proxy_http_version 1.1;

        proxy_buffering off;
        proxy_request_buffering off;
        proxy_set_header Connection "keep-alive";
        proxy_set_header Proxy-Connection "keep-alive";
        proxy_set_header X-Accel-Buffering no;
        proxy_read_timeout 1h;        # long-running calls
        proxy_send_timeout 1h;
        proxy_set_header Keep-Alive "timeout=3600";
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;

    }

    location / {
        #if ($route_to_mcp = 1) {
        #    rewrite ^ /mcp break;
        #}

        proxy_pass http://app_server;
        proxy_http_version 1.1;

        proxy_connect_timeout 180s;
        proxy_read_timeout 180s;

        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_set_header Host $host;
    }

    error_page   500 502 503 504  /50x.html;
}