> ## Documentation Index
> Fetch the complete documentation index at: https://wundergraphinc-ahmet-router-628-demo-environment-to-demonst.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Configuration

> Every configuration key and environment variable for MCP schema discovery.

Schema discovery is configured under `mcp.schema_discovery`.

```yaml router.config.yaml theme={"system"}
mcp:
  enabled: true
  expose_schema: false
  schema_discovery:
    enabled: true
    url: 'https://discovery.example.com'
    token: 'your-token'
    request_timeout: 90s
    index_poll_interval: 2s
    index_timeout: 10m
```

## Keys

| Key                   | Type     | Default | Environment variable                       |
| --------------------- | -------- | ------- | ------------------------------------------ |
| `enabled`             | boolean  | `false` | `MCP_SCHEMA_DISCOVERY_ENABLED`             |
| `url`                 | string   |         | `MCP_SCHEMA_DISCOVERY_URL`                 |
| `token`               | string   |         | `MCP_SCHEMA_DISCOVERY_TOKEN`               |
| `request_timeout`     | duration | `90s`   | `MCP_SCHEMA_DISCOVERY_REQUEST_TIMEOUT`     |
| `index_poll_interval` | duration | `2s`    | `MCP_SCHEMA_DISCOVERY_INDEX_POLL_INTERVAL` |
| `index_timeout`       | duration | `10m`   | `MCP_SCHEMA_DISCOVERY_INDEX_TIMEOUT`       |

### enabled

Turns schema discovery on. The router then indexes the client schema and registers the `search_schema`, `get_symbols` and `generate_query` tools.

### url

The base URL of the schema discovery service. Include the scheme.

The service speaks Connect over HTTP/1.1.

<Warning>
  The router does not start when `enabled` is `true` and `url` is empty. This stops a server whose tools always fail.
</Warning>

### token

The bearer token for the service. The router sends it as `Authorization: Bearer <token>`.

An empty token sends no `Authorization` header. Use an empty token when your service runs with authentication off.

The router never writes the token to a log or to an error message.

### request\_timeout

The timeout for one call to the service.

Query generation takes 10 to 30 seconds. A value below `60s` is too low.

This value also raises the write timeout of the MCP HTTP server, so a slow generation still reaches the caller.

### index\_poll\_interval

The wait between two index status reads while the router waits for a build.

### index\_timeout

The router stops waiting for an index that does not become ready within this time.

A schema of 16,000 lines indexes in about 24 seconds. The default of `10m` only trips on a real fault.

## Interaction with other keys

### expose\_schema

Set `expose_schema` to `false`.

`get_schema` returns the full schema. That is the context cost that schema discovery removes. The two settings do not fail together, but they work against each other.

### enable\_arbitrary\_operations

Set this to `true` when you want the agent to run the operation that `generate_query` returns.

Set it to `false` in production. Use the curated path instead: generate the operation in a development router, review it, publish it as a persisted operation, then deploy that. See [Guides](/router/mcp/schema-discovery/guides).

## Startup behaviour

The router does not wait for the index. It serves GraphQL from the first moment.

Read the log to follow the build.

```
INFO  MCP schema discovery enabled   url=https://discovery.example.com authenticated=true
INFO  schema index is building       index_id=sha256:6926769e...
INFO  schema index is ready          index_id=sha256:6926769e... symbol_count=318
```

The router reindexes after every schema change. An unchanged schema costs nothing, because the router compares the hash locally and makes no network call.

During a rebuild the previous index keeps serving. The router adopts the new index only when it is ready.

## Failure behaviour

| Condition                             | Result                                                                    |
| ------------------------------------- | ------------------------------------------------------------------------- |
| The service is unreachable at startup | The router starts. The tools report that the index is not ready.          |
| The build fails                       | The router logs the reason. The tools report that the index is not ready. |
| The build exceeds `index_timeout`     | The router stops waiting and logs the reason.                             |
| The service drops an unused index     | The router rebuilds it at the same address on the next tool call.         |

A fault in the discovery service never stops the router from serving GraphQL.