[Self-hosted] MCP upload using tool retool_start_prepared_import returns internal link

💬 App Building
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  1. My goal: Get external link to upload a file using MCP tool retool_start_prepared_import

  2. Issue: I get internal link `http://retool-agent-sandbox-proxy.prod-retool.svc.cluster.local:3019/sandbox//_/upload`

  3. Steps I've taken to troubleshoot: Configured Retool with RR and MCP.

  4. Additional info: Self-hosted 4.34.6-stable
    Config with domain configuration:

    env:
  BASE_DOMAIN: "https://domain.com"
mcp:
  enabled: true
  config:
    mcpServiceExternalUrl: "https://domain.com"
2

Summary

On self-hosted Retool 4.34.6-stable (new app builder/MCP enabled), calling the MCP tool retool_start_prepared_import to upload a file returns an internal Kubernetes cluster URL (http://retool-agent-sandbox-proxy.prod-retool.svc.cluster.local:3019/sandbox//_/upload) instead of an externally reachable link, despite setting BASE_DOMAIN and mcp.config.mcpServiceExternalUrl.

AI Response

Retool's MCP server (which exposes retool_start_prepared_import) is supported on self-hosted 4.0.0+ and shares the exact same prerequisites as the new app builder, so confirm the full set of new-app-builder/agent sandbox services are enabled in your Helm values rather than MCP alone. The returned host (retool-agent-sandbox-proxy...svc.cluster.local) and the double slash in /sandbox//_/upload point to the external base URL not being applied to the generated link, so verify BASE_DOMAIN and mcpServiceExternalUrl exactly match your externally served origin (scheme + host, no trailing slash) and are picked up by the agent/sandbox proxy service. This mirrors the known self-hosted pattern where generated links fall back to an internal address when the configured external domain isn't propagated to the service producing the URL. No related platform incident accounts for this behavior, so treat it as a configuration/propagation issue.

Sources

:bookmark: Use MCP to build apps | Retool Docs
Confirms Retool's MCP server (including the import tooling) is only available on self-hosted 4.0.0+ and requires the same enabled services/configuration as the new app builder.
:bookmark: Odd url when sending user invites or password reset - self hosted
Solved report of a self-hosted instance generating internal URLs instead of the configured external domain, corroborating the same base-URL misconfiguration symptom class.

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I double checked that BASE_DOMAIN and mcpServiceExternalUrl have the same domain like https://domain.com and without trailing slashes.