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

# E-MCP-009

> The MCP door is wired but VENDO_BASE_URL is not set.

# E-MCP-009

<Warning>
  **what doctor prints** (broken)

  the MCP door is wired but VENDO\_BASE\_URL is not set — discovery, the issuer
  and the token audience all derive from it, so the door advertises whatever
  origin a request happens to carry and outside agents are pointed at the wrong
  server (it surfaces later as "Claude can't find my server"). Set
  VENDO\_BASE\_URL to this deployment's public origin where you deploy, or pass
  mcp: { baseUrl } in the composition.
</Warning>

The door is open, but it does not know its own public address. Everything
that depends on that address, discovery documents, the issuer, the token
audience, is wrong.

`check mcp/base-url` · `error_code E-MCP-009` · `doctor exits 1`

## What you're seeing

This check runs whenever a composition passes `mcp` to `createVendo`. It is a
static check: no dev server or network call is needed, only the composition
source and the environment.

## Why

The door derives its discovery documents, its OAuth issuer, its resource
identifiers, and the RFC 8707 audience it binds tokens to, all from one
value: the deployment's public base URL. Without it, the door falls back to
guessing the origin from whatever request happens to reach it, which is
unreliable behind a proxy, a load balancer, or any host with more than one
public name.

Nothing looks wrong on your own machine, because your own requests carry a
consistent origin. It shows up later, whenever a client calling in from
somewhere else gets pointed at a URL that is not actually the door.

## The fix

Set `VENDO_BASE_URL` to the deployment's full public origin, path prefix
included, in the environment you deploy to:

```bash theme={null}
VENDO_BASE_URL=https://your-app.example.com
```

Or name it directly in the composition instead, which wins over the
environment variable:

```ts theme={null}
createVendo({
  mcp: { baseUrl: "https://your-app.example.com" }, // [!code focus]
});
```

Re-run doctor once the value is set; the discovery documents and the token
audience follow it automatically.

## Related errors

<CardGroup cols={2}>
  <Card title="E-MCP-001" href="/production/troubleshooting/e-mcp-001">protected-resource metadata</Card>
  <Card title="E-MCP-005" href="/production/troubleshooting/e-mcp-005">server.json remote mismatch</Card>
</CardGroup>