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docs: document TypeScript passthrough fetch auth origin scoping #6401

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10 changes: 10 additions & 0 deletions fern/products/sdks/deep-dives/generated-sdk.mdx
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Expand Up @@ -65,6 +65,16 @@ When you define webhooks in your API spec, Fern automatically [generates utiliti

When you define [WebSocket channels](/learn/api-definitions/asyncapi/channels/pubsub) in your API spec, Fern can [generate typed WebSocket clients](./websocket-clients) with connection management, typed messages, and automatic reconnection.

## Unmodeled endpoints

TypeScript SDKs expose a passthrough `client.fetch()` escape hatch for calling paths that aren't in your API definition. It mirrors the standard `fetch` signature and reuses the client's configured base URL, default headers, timeouts, retries, and logging. Relative paths resolve against the base URL:

```typescript
const response = await client.fetch("/plants/plant_123/water", { method: "POST" });
```

The SDK attaches its auth headers only when the resolved URL has the same origin as the configured base URL. Relative paths and same-origin absolute URLs still receive credentials; absolute URLs on another origin (including a different port) don't, so those callers must pass their own `Authorization` or other credential headers in the request. Non-auth headers are sent regardless of origin, and no auth headers are attached when the client has no base URL or environment configured.

## Default parameter values

You can configure [default values](/learn/api-definitions/openapi/extensions/default-values) for path, header, and query parameters, including headers defined under [`x-fern-global-headers`](/learn/api-definitions/openapi/extensions/global-headers). The SDK makes those parameters optional and sends the default when the caller doesn't provide one — useful for things like pinning an API version header or default region.
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