> For the complete documentation index, see [llms.txt](https://docs.baas.sh/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.baas.sh/baas-sdk/blockchain/networks.md).

# Networks

The `baas.chains` namespace lets your users pick which network (EVM chain) is used for passkey sign-in. The list comes from your project's configuration, so it's always in sync across BaaS.

{% hint style="info" %}
This is **not** a wallet connector layer (use wagmi or RainbowKit for that). It's a small, opinionated wrapper around the chains your project exposes.
{% endhint %}

## Usage

```ts
const chains = await baas.chains.list();   // async — fetches /networks once, then memoized
// → [{ value: 'base', label: 'Base', chainId: 8453, rpcUrl: '…', isMainnet: true }, …]

baas.chains.active();   // → null until a chain is chosen, then the selected chain id

await baas.chains.switch(10);   // switch to Optimism

const unsubscribe = baas.chains.onChainChange((chainId) => {
  // chainId is `number | null`; fires immediately, then on every switch
});
```

| Method                                  | Description                                                                                |
| --------------------------------------- | ------------------------------------------------------------------------------------------ |
| `list(): Promise<readonly BaasChain[]>` | The chains your project supports, fetched once and then memoized.                          |
| `active(): number \| null`              | The selected chain id, or `null` if the user hasn't chosen one yet.                        |
| `switch(chainId): Promise<void>`        | Switch the active chain. Throws `UNKNOWN_CHAIN` if it isn't in `list()`.                   |
| `onChainChange(cb): () => void`         | Subscribe to changes. Fires immediately (with `number \| null`), then on every `switch()`. |

## The default chain

`active()` returns the selected chain id, or `null` if the user hasn't chosen one. While it's `null`, sign-in uses the **first network** in your project's list by default. The choice is saved in `localStorage` (scoped to your `projectId`) and survives page reloads.

## External wallets are unaffected

`baas.chains` only governs **passkey** sign-in. An external wallet carries its own chain selection, so `signInWithWallet()` ignores `baas.chains.switch()`.

{% hint style="warning" %}
**`baas.chains` is a preference, not a security boundary.** A user can edit `localStorage` (or bypass the SDK entirely) to sign on a different chain. This is intrinsic to any browser-side storage.

If your app must enforce a specific chain, call `await baas.chains.switch(<chainId>)` right after `createBaasClient()` to overwrite any local value at boot. This is a soft defense; real enforcement belongs on your BaaS API.
{% endhint %}

## Next

* [API reference](/baas-sdk/resources/api-reference.md) — the `BaasChain` type and full method signatures.
* [Error handling](/baas-sdk/resources/error-handling.md) — `UNKNOWN_CHAIN` and network errors from `list()` / `switch()`.


---

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