> ## Documentation Index
> Fetch the complete documentation index at: https://docs.seasonlabs.co/llms.txt
> Use this file to discover all available pages before exploring further.
# Choose an SDK
> Pick the Era SDKs that match where your application runs.
Most applications use two SDKs:
1. A client SDK creates a stable `visitorId`.
2. The Node SDK sends the conversation to Era and returns a context block.
## SDK comparison
| Your code runs in | Package | Use it for | Key type |
| ----------------- | ---------------------- | --------------------------------------------- | ----------- |
| Browser | `@seasonlabs/era-web` | Create a visitor ID and verify email or phone | Publishable |
| Flutter app | `era_flutter` | Create a visitor ID and verify email or phone | Publishable |
| Trusted backend | `@seasonlabs/era-node` | Get the context block for a conversation | Secret |
Never put a Node SDK API key in browser or mobile code.
## Browser application
Use the Web SDK in the browser and the Node SDK on your backend.
```typescript Browser theme={null}
const visitorId = await eraWeb.init();
await fetch("/api/chat", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify({ messages, visitorId }),
});
```
```typescript Trusted backend theme={null}
const block = await era.turnContext(messages, {
visitorId,
sessionId,
});
```
## Flutter application
Use `era_flutter` in the app and the Node SDK on your backend.
```dart Flutter theme={null}
final visitorId = await era.init();
```
Send `visitorId` with the chat request. Your backend uses it in the same
`turnContext()` call shown above.
## Backend-only application
The Node SDK still needs a stable `visitorId`. Create and store one for each
user or visitor, then pass it on every turn.
## Signed-in users
When your backend knows the signed-in user, add your own stable user ID:
```typescript theme={null}
const block = await era.turnContext(messages, {
visitorId,
partnerEndUserId: authenticatedUser.id,
sessionId,
});
```
`partnerEndUserId` adds trusted account identity, but it does not replace
`visitorId`. Only send email or phone assertions that your backend has already
verified.
Request your first context block.
Create a browser visitor.
Create a mobile visitor.