Add MCP service exposed from workflow failed, 503 Service Unavailable

[sanpatricky]

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Dify version

1.13.0

Cloud or Self Hosted

Self Hosted (Docker)

Steps to reproduce

1. Enter a workflow editor view
2. Switch on MCP on and copy its endpoint url
3. Open MCP menu and add a new MCP service
4. Fill in the url copied in step2.
5. Click add and authentication.
6. Click authentication.
   Then the page says: Failed to connect to MCP server: code=503 message="Server error '503 Service Unavailable' for url 'http://localhost/mcp/server/cl2cAH7KRScwaUek/mcp'

log:
ssrf_proxy

1773360616.329 0 172.18.0.8 TCP_MISS_ABORTED/503 3477 POST http://localhost/mcp/server/cl2cAH7KRScwaUek/mcp - HIER_DIRECT/::1 text/html
api

2026-03-13 00:10:16.329 INFO [ThreadPoolExecutor-37_0] [_client.py:1038] - HTTP Request: POST http://localhost/mcp/server/cl2cAH7KRScwaUek/mcp "HTTP/1.1 503 Service Unavailable"

Other MCP services not exposed from workflow for example written by python can be added without this issue.

✔️ Expected Behavior

The MCP service exposed from the workflow can be added.

❌ Actual Behavior

Failed to connect to MCP server: code=503 message="Server error '503 Service Unavailable' for url 'http://localhost/mcp/server/cl2cAH7KRScwaUek/mcp'

[dosubot]

This is a known Docker networking issue. The 503 error occurs because localhost inside a Docker container refers to the container itself, not the host machine or other containers.

When the SSRF proxy tries to connect to http://localhost/mcp/server/cl2cAH7KRScwaUek/mcp, it's trying to reach its own container's localhost, which fails.

Solution: Replace localhost in the MCP endpoint URL with a network-accessible address:

1. Your host machine's IP address:

   http://192.168.x.x/mcp/server/cl2cAH7KRScwaUek/mcp
   

2. Docker service name (if the workflow is on the same Dify instance):

   http://api/mcp/server/cl2cAH7KRScwaUek/mcp
   

3. host.docker.internal (Docker Desktop on Mac/Windows):

   http://host.docker.internal/mcp/server/cl2cAH7KRScwaUek/mcp
   

   On Linux, you'd need to add host.docker.internal via extra_hosts in your docker-compose.yml.

This is why your other Python MCP services work—they likely use proper network-accessible addresses instead of localhost.

See this discussion for more details on configuring MCP endpoints in Docker environments.

To reply, just mention @dosu.

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[sanpatricky]

you'd need to add host.docker.internal via extra_hosts in your docker-compose.yml - for what services of dify?

[dosubot]

For MCP connectivity, you'd primarily need to add extra_hosts to the ssrf_proxy service, since MCP requests are routed through the SSRF proxy for security:

ssrf_proxy:
  extra_hosts:
    - "host.docker.internal:host-gateway"

If you want to be thorough, you can also add it to api, worker, plugin_daemon, and sandbox services—these handle external requests and plugin execution. However, since external HTTP calls typically go through the SSRF proxy, starting with ssrf_proxy should resolve your 503 error.

Alternatively, using your host machine's actual IP address in the MCP endpoint URL avoids needing any extra_hosts configuration.

To reply, just mention @dosu.

---

^{How did I do? Good | Irrelevant | Incorrect | Verbose | Hallucination | Report 🐛 | Other}

   

[sanpatricky]

how to find host machine's actual IP address which can be used in this case on macbook

[sanpatricky]

new error in log:1773369220.137 0 172.18.0.9 TCP_MISS_ABORTED/503 3519 POST http://host.docker.internal/mcp/server/cl2cAH7KRScwaUek/mcp - HIER_DIRECT/fdc4:f303:9324::254 text/html