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If your client can’t establish a connection, work through the checks below in order. The official MCP debugging guide can help you inspect client, server, and transport behavior.

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1. Verify your server is published

Only published versions can be accessed by MCP clients. If your server only has draft changes, publish a version before attempting to connect.
Creating or editing a draft does not update the live MCP endpoint.

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2. Check that serving is enabled

If your server is paused, it won’t accept incoming MCP requests. Resume serving and try connecting again.

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3. Verify the endpoint

Open the Setup page and copy the endpoint again. Small differences in the URL, such as an incorrect hostname or path, can prevent clients from connecting.

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

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5. Check protocol compatibility

Modern MCP clients may use MCP 2026-07-28 discovery and request headers. If a browser-based client fails before any log appears in 0mcp, check whether the browser request completed its CORS preflight and whether the client supports remote Streamable HTTP. Use the generated configuration from Setup whenever possible. See MCP 2026-07-28 compatibility for more detail.

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6. Review authentication

If your server requires authentication, confirm that:
  • Required environment variables are configured.
  • Authentication headers are present.
  • Credentials are valid.
  • OAuth issuer settings are configured when Setup reports they are required.
See Authentication errors if requests are being rejected with 401 or 403.

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7. 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.

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8. Check the logs

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

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Common HTTP responses

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Using a custom domain

If you’re connecting through a 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.