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Docker MCP Gateway does not have one universal TCP port. Running docker mcp gateway run uses the stdio transport by default, so it listens to the client process rather than opening a network socket. Port 8811 is used in Docker’s official SSE-based agentic-AI Compose example, where clients connect to http://mcp-gateway:8811/sse. If your command or Compose file specifies another port, that configured value is the one you must use.
The short answer
There are two different answers, depending on how the gateway is started:
- Default CLI invocation: no TCP port.
docker mcp gateway run --profile <profile-id>usesstdio. - Network deployment: the port is whatever you pass with
--port N. Docker documents--portas the “TCP port to listen on” and does not give it a numeric default. - Docker’s official SSE Compose example: port 8811, with the endpoint
http://mcp-gateway:8811/sse.
Therefore, seeing 8811 in a tutorial does not mean every Docker MCP Gateway listens there. Read the actual gateway command and the client endpoint in your deployment.
Why the default has no port
The docker mcp gateway run command documents stdio as its default transport. In this mode, Docker starts the gateway as a process and exchanges MCP messages through standard input and standard output. There is no TCP listener for another process to reach, so there is no port to expose, publish, or put in a URL.
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A typical direct launch looks like this:
docker mcp gateway run --profile <profile-id>A client configured to launch that command directly communicates over stdio. Its configuration needs the command and arguments, not an address such as localhost:8811. If you are debugging a stdio setup, checking for an open port will give a misleading result: the gateway can be working normally without one.
When Docker MCP Gateway does use a TCP port
Docker supports three transport values for the gateway:
| Transport | How it is selected | Port behavior | Typical client address |
|---|---|---|---|
stdio |
Default, or --transport=stdio |
No TCP port | Client launches the Docker command |
sse |
--transport=sse |
Uses the number supplied with --port |
Configured HTTP URL, commonly ending in /sse |
streaming |
--transport=streaming |
Uses the number supplied with --port |
Route depends on the deployment |
Selecting sse or streaming changes the gateway from a local process connection to a network service. At that point, --port determines the TCP listening port. Because Docker does not publish a universal numeric default for this option, always set it explicitly when a network client must connect.
Why 8811 appears so often
Docker’s official “Build and run agentic AI applications with Docker” Compose example sets the application environment variable to:
MCPGATEWAY_ENDPOINT=http://mcp-gateway:8811/sseThe same Compose service starts the gateway with the SSE transport. In that particular deployment, the service name is mcp-gateway, the TCP port is 8811, and /sse is the client route. The value is a Compose configuration choice, not a protocol-wide requirement.
If you copy the example and retain its service name and port mapping, use that exact endpoint from the application container. If you change the published or internal port, update the endpoint as well. A client pointing at 8811 cannot reach a gateway that is listening on another configured number.
How to determine the port in your deployment
- Find the transport. Inspect the gateway command for
--transport=stdio,--transport=sse, or--transport=streaming. If the option is absent, the documented default is stdio. - Find the port option. For a network transport, locate
--port N. That number is the gateway’s TCP listening port. - Check the client URL. Look for an environment variable such as
MCPGATEWAY_ENDPOINTor an MCP client setting. Confirm that its host, port, and route match the gateway deployment. - Check container networking. In Compose, distinguish the port used between containers from a host-published port. The application container may use the service name and internal port, while a browser or host process may need the published host port.
- Restart both sides after changes. Changing
--portwithout changing the client endpoint leaves the client attempting the old address.
Connecting over SSE with port 8811
For the official Compose arrangement, configure the consuming application with:
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Keep the path as well as the port. http://mcp-gateway:8811 and http://mcp-gateway:8811/sse are not interchangeable when the client expects the SSE route.
Changing the port deliberately
Choose an unused TCP port and pass it explicitly with a network transport. Conceptually, the gateway command has this form:
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docker mcp gateway run --profile <profile-id> --transport=sse --port=<PORT>Then change the client endpoint to the same configured value and the route required by your deployment. For example, if you select 9000 for an SSE service, the client must use the corresponding host and :9000 URL rather than 8811. The exact route should come from the gateway configuration you are running; the official example uses /sse.
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Do not add --port to a stdio-only setup just because a tool asks for a port. Either configure the tool to launch the Docker command, or intentionally switch the gateway and client to a network transport.
Common connection failures and fixes
The client says connection refused
Usually the gateway is not listening at the address being used. Verify that a network transport is selected, that --port matches the endpoint, and that the gateway process is running. If the gateway is still using stdio, remove the URL-based configuration and launch the command directly from the client.
The client connects to 8811 but receives no response
8811 only has meaning when your deployment configured it. Check the actual Compose command and port mapping. Also verify the route: Docker’s example uses /sse, so omitting that path can target the wrong endpoint.
A container cannot resolve mcp-gateway
The name is the Compose service name used in Docker’s example. A client outside that Compose network may not resolve it. Use the reachable host name for that client and preserve the configured port and route.
Changing the Compose port had no effect
Update every related value: the gateway’s --port, any container or host port mapping, and the client’s endpoint variable. Restart the affected services so they do not retain the old configuration.
You cannot find any listening socket
That is expected when the gateway is running with the default stdio transport. Confirm the client is launching docker mcp gateway run --profile <profile-id> and communicating through the process streams instead of expecting HTTP.
Port, transport and deployment checklist
- Use stdio when one MCP client can launch and supervise the gateway; it requires no TCP port.
- Use SSE when clients need a network endpoint, and set
--portexplicitly. - Use streaming when that transport is what your client and deployment require; its port is also configured rather than universally fixed.
- Document the hostname, port and route together. A port number by itself is insufficient for an HTTP-based client.
- Treat 8811 as the value from Docker’s official SSE Compose example, not as an installation-wide default.
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For a URL that is reachable from the machine making the request, use the API documented at https://screenshotneo.com/docs/:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpimport requests
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open("shot.webp", "wb").write(r.content)const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
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FAQ
Is port 8811 reserved for Docker MCP Gateway?
No. It is the port selected in Docker’s official SSE Compose example. Another deployment can select a different number with --port.
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Can an MCP client use Docker MCP Gateway without networking?
Yes. With the documented stdio default, the client launches the gateway command and exchanges messages through standard input and output, so no TCP port is required.
What should I copy when reporting a gateway endpoint?
Copy the complete configured host, port and route, such as http://mcp-gateway:8811/sse, and state whether the client runs inside the same Compose network.
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Frequently Asked Questions
Does Docker MCP Gateway always listen on 8811?
No. 8811 belongs to Docker’s official SSE Compose example; the default CLI transport is stdio, and network deployments use the number supplied with –port.
Why does my stdio gateway have no URL?
Stdio connects through the launched process’s input and output streams. It is not a TCP service, so a URL and port are unnecessary.
Which value must change when I choose a different port?
Keep the gateway’s –port setting, the container or host mapping, and the client endpoint consistent, including the required route such as /sse.
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