# Configuring per-message SMS or MMS fallback
<a name="rcs-fallback-per-message"></a>

When you send an RCS message, the recipient's device might not support RCS, or the message might expire before delivery. With per-message fallback, you can specify alternative SMS or MMS content that AWS End User Messaging delivers automatically when the RCS message cannot reach the recipient. You configure fallback on each `SendRcsMessage` request by including the `FallbackConfiguration` parameter.

## When fallback triggers
<a name="rcs-fallback-per-message-triggers"></a>

AWS End User Messaging triggers the fallback message in two situations:
+ **Immediate RCS failure**: The recipient's device or carrier does not support RCS, or the RCS channel rejects the message.
+ **TTL expiry**: You set a `TimeToLive` value and the message is not delivered before the timer expires. For more information about message expiration, see [Configuring RCS message expiration](rcs-message-expiration.md).

If you do not include `FallbackConfiguration` in the request and the RCS message fails or expires, no fallback message is sent.

## Per-message fallback compared to pool-based fallback
<a name="rcs-fallback-per-message-vs-pool"></a>

Per-message fallback on `SendRcsMessage` differs from the automatic pool-based fallback available with `SendTextMessage`. For details on pool-based fallback, see [RCS to SMS fallback using phone pools](rcs-sms-fallback.md).


**Comparison of fallback mechanisms**  

| Feature | Pool-based fallback (`SendTextMessage`) | Per-message fallback (`SendRcsMessage`) | 
| --- | --- | --- | 
| Configuration | Automatic when a pool contains both an RCS agent and SMS identities | Explicit: you include FallbackConfiguration in each request | 
| Fallback channel | SMS (selected by the pool) | SMS or MMS (you choose with Channel) | 
| Fallback content | Same message body | You provide separate content tailored for SMS or MMS | 
| Identity selection | Pool selects the origination identity | You specify a phone number or sender ID | 
| Trigger | RCS not supported (timeout-based) | Immediate RCS failure or TTL expiry | 

## FallbackConfiguration structure
<a name="rcs-fallback-per-message-structure"></a>

The `FallbackConfiguration` parameter is a single object on the `SendRcsMessage` request. You set the `Channel` field to indicate whether the fallback is SMS or MMS, then provide the corresponding content fields.


**FallbackConfiguration fields**  

| Field | Type | Required | Description | 
| --- | --- | --- | --- | 
| Channel | String | Yes | The fallback channel. Valid values: SMS or MMS. | 
| MessageBody | String | Conditional | The text body of the fallback message. Maximum 1,600 characters. Required when Channel is SMS. For MMS, at least one of MessageBody or MediaUrls must be provided. | 
| MediaUrls | List of strings | Conditional | One or more media URLs for an MMS fallback. For MMS, at least one of MessageBody or MediaUrls must be provided. | 
| OriginationIdentity | String | Conditional | The origination identity for the fallback message. Accepts a phone number or sender ID (by value, ID, or ARN); you cannot specify a pool or an RCS agent. If you omit this field and the original message was sent through a pool, the service selects a suitable number from that pool. | 

**Important**  
The fallback `MessageBody` limit is 1,600 characters. This is shorter than the 3,072-character limit for the RCS `TextMessage` body. Craft your fallback content to fit within the SMS or MMS character constraints.

The following JSON shows the structure of a `FallbackConfiguration` object for an SMS fallback:

```
{
    "Channel": "SMS",
    "MessageBody": "Your order has shipped. Track at https://example.com/track/12345",
    "OriginationIdentity": "+18005550199"
}
```

The following JSON shows the structure for an MMS fallback:

```
{
    "Channel": "MMS",
    "MessageBody": "Your order has shipped.",
    "MediaUrls": ["https://cdn.example.com/shipping-confirmation.jpg"],
    "OriginationIdentity": "+18005550199"
}
```

## Configuring SMS fallback
<a name="rcs-fallback-per-message-sms"></a>

To fall back to SMS, set `Channel` to `SMS` and provide a `MessageBody`. Because SMS does not support suggestion chips or rich cards, include any important URLs as plain text in the message body.

## Configuring MMS fallback
<a name="rcs-fallback-per-message-mms"></a>

To fall back to MMS, set `Channel` to `MMS` and provide one or more media URLs in `MediaUrls`. Use MMS fallback when your RCS message contains visual content (such as a rich card with an image) and you want to preserve the visual experience for recipients who do not support RCS.

## Combining fallback with message expiration
<a name="rcs-fallback-per-message-ttl"></a>

When you set both `TimeToLive` and `FallbackConfiguration` on a request, AWS End User Messaging attempts RCS delivery for the duration of the TTL. If the message is not delivered before the timer expires, the service sends the fallback message. This pattern is useful for time-sensitive content such as one-time passwords or appointment reminders.

For more information about configuring message expiration, see [Configuring RCS message expiration](rcs-message-expiration.md).

The following example sets a 60-second TTL with SMS fallback:

```
{
    "DestinationPhoneNumber": "+12065550100",
    "OriginationIdentity": "rcs-agent-1234567890",
    "RcsMessageContent": "{\"Content\":{\"TextMessage\":{\"Body\":\"Your verification code is 483291.\"}}}",
    "TimeToLive": 60,
    "FallbackConfiguration": {
        "Channel": "SMS",
        "MessageBody": "Your verification code is 483291.",
        "OriginationIdentity": "+18005550199"
    }
}
```

## Best practices
<a name="rcs-fallback-per-message-best-practices"></a>
+ **Write fallback content separately.** Rich RCS content (suggestion chips, rich cards, carousels) does not translate directly to SMS or MMS. Write a concise, self-contained fallback message rather than reusing the RCS body verbatim.
+ **Include URLs as plain text in SMS fallback.** Replace interactive suggestion chips (such as `OpenUrl` actions) with inline URLs in the `MessageBody`.
+ **Use MMS fallback for visual content.** If your RCS message features a rich card or carousel with images, MMS fallback preserves the visual context for recipients without RCS support.
+ **Set a reasonable TTL.** When you combine fallback with `TimeToLive`, choose a TTL that balances delivery attempts with timeliness. For time-sensitive messages, a TTL of 30 to 60 seconds gives a brief RCS delivery window before falling back. For general messages, a longer window (minutes to hours) gives the RCS channel more opportunity to succeed.
+ **Verify your origination identity.** The `OriginationIdentity` in the fallback configuration must be a phone number or sender ID that is registered in your AWS End User Messaging account and capable of sending SMS or MMS to the destination country.

## Related resources
<a name="rcs-fallback-per-message-related"></a>
+ [RCS to SMS fallback using phone pools](rcs-sms-fallback.md) for information about automatic pool-based fallback with `SendTextMessage`.
+ [Configuring RCS message expiration](rcs-message-expiration.md) for information about configuring TTL and expiration behavior.
+ [Sending rich RCS messages](rcs-rich-messaging.md) for an overview of RCS rich message types and the `SendRcsMessage` API.