# PRE\_ADS\_REQUEST
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## When it fires
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MediaTailor runs the function mapped to `PRE_ADS_REQUEST` once per ad break, immediately before sending the request to the ADS. The function runs each time an ad opportunity is encountered during manifest processing.

## Input
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All fields from `PRE_SESSION_INITIALIZATION`, plus `avail.*`, `scte.*`, and `adsRequest.*` (url, method, headers, body). For all available fields, see [Input field reference](monetization-functions-hooks.md#monetization-functions-hooks-input-ref).

## Output namespace allowed
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| Namespace | Accepted types | How the output is used | 
| --- | --- | --- | 
| player\_params.\* | Strings, numbers, booleans | Overrides session player parameters for this ad break. Available to the ADS request URL through [MediaTailor dynamic ad variables for ADS requests](variables.md). | 
| session.\* | Strings, numbers, booleans | Overrides session variables for this ad break. Available to the ADS request URL through dynamic variable substitution. | 
| avail.\* | Strings, numbers, booleans | Overrides avail variables for this ad break. Available to the ADS request URL through dynamic variable substitution. | 
| scte.\* | Strings, numbers, booleans | Overrides SCTE variables for this ad break. Available to the ADS request URL through dynamic variable substitution. | 
| adsRequest.\* | String | Overrides the ADS request for this ad break only. Supported fields: url, method, headers.<name>, body. The url value is treated as a template and supports [MediaTailor dynamic ad variables for ADS requests](variables.md) after the function runs. Not persisted. | 

**Note**  
All output from the `PRE_ADS_REQUEST` hook are transient overrides — they apply only to the current ad break's ADS request and are not persisted to the session.

**Example — rewriting the ADS request:**

```
{
    "Output": {
        "adsRequest.url": "{%'https://ads.example.com/v1/vast?sid=' & session.id & '&genre=' & player_params.genre%}",
        "adsRequest.headers.X-Custom-Token": "{%player_params.auth_token%}"
    }
}
```

This replaces the ADS URL and adds a custom header for the current ad break.

## Typical use cases
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+ Rewrite the ADS request URL to route traffic between different ad servers for A/B testing.
+ Append enrichment data (audience segments, identity tokens) to the ADS request URL or headers.
+ Conditionally modify ADS request parameters based on SCTE-35 signal data or avail index.

## Failure behavior
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If a function attached to `PRE_ADS_REQUEST` fails for any reason, MediaTailor discards the function's output and proceeds as if no function were attached. The ADS request is sent using the original session and request parameters without modification.