> ## Documentation Index
> Fetch the complete documentation index at: https://docs.0mcp.io/llms.txt
> Use this file to discover all available pages before exploring further.

# MCP connection issues

> Troubleshoot clients that can't connect to your MCP server.

If your client can't establish a connection, work through the checks below in order.

## 1. Verify your server is published

Only [**published versions**](/guides/versioning) can be accessed by MCP clients.

If your server only has draft changes, publish a version before attempting to connect.

<Note>
  Creating or editing a draft does not update the live MCP endpoint.
</Note>

## 2. Check that serving is enabled

If your server is paused, it won't accept incoming MCP requests.

Resume serving and try connecting again.

## 3. Verify the endpoint

Open the [**Setup** page](/guides/setup) and copy the endpoint again.

Small differences in the URL, such as an incorrect hostname or path, can prevent clients from connecting.

## 4. Verify the transport

0mcp uses **Streamable HTTP**.

Make sure your MCP client is configured to use the hosted HTTP endpoint instead of a local command or an unsupported transport.

<Warning>
  Do not place a hosted 0mcp URL in a client field that expects a local command.
</Warning>

## 5. Review authentication

If your server requires authentication, confirm that:

* Required environment variables are configured.
* Authentication headers are present.
* Credentials are valid.

See [**Authentication errors**](/troubleshooting/authentication-errors) if requests are being rejected with `401` or `403`.

## 6. Restart your client

Many MCP clients load their configuration only when they start.

After updating your configuration or credentials, restart or reconnect the client before testing again.

## 7. Check the logs

Open [**Logs**](/guides/logs) and look for activity at the time you attempted to connect.

* **A matching log entry exists:** The request reached 0mcp. Review the error details to continue troubleshooting.
* **No log entry exists:** The request never reached 0mcp. The problem is likely in the client configuration, network, DNS, or endpoint URL.

<Info>
  No matching log entry is a useful signal. Check the client and network path before changing your tools or upstream API.
</Info>

## Common HTTP responses

| Status                      | What it usually means                                                   |
| --------------------------- | ----------------------------------------------------------------------- |
| **404 Not Found**           | The endpoint or hostname is incorrect.                                  |
| **401 Unauthorized**        | Authentication credentials are missing or invalid.                      |
| **403 Forbidden**           | The credentials don't have permission to access the requested resource. |
| **503 Service Unavailable** | The server is paused or temporarily unavailable.                        |

## Using a custom domain

If you're connecting through a [custom domain](/guides/custom-domain):

* Wait until the domain status is **Active**.
* Update your client to use the custom endpoint.
* Verify that DNS propagation and TLS provisioning have completed before reconnecting.