# Functions limits
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This page lists the limits that apply to Functions. These limits are enforced at authoring time (when you create or update a function) and at runtime (when the function runs).

## Hook execution limits
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| Hook | Limit | Value | 
| --- | --- | --- | 
| PRE\_SESSION\_INITIALIZATION | Total timeout | 2,000 ms | 
| PRE\_SESSION\_INITIALIZATION | Total player\_params size | 1,000 characters | 
| PRE\_ADS\_REQUEST | Total timeout | 2,000 ms | 

These timeouts cover the entire hook execution, including all function steps and HTTP calls. If the hook exceeds its timeout, MediaTailor discards all output and proceeds as if no function were attached.

Individual function timeouts (such as `RequestTimeoutMilliseconds` for HTTP\_REQUEST functions) must fit within the hook's total timeout. For example, if the hook timeout is 2,000 ms and an HTTP\_REQUEST function sets `RequestTimeoutMilliseconds` to 2,000 ms, the function may time out before completing if any processing occurs before or after the HTTP call.

The `player_params` total size includes the combined character count of all output key names and values. For example, outputting `player_params.deviceType = "mobile"` and `player_params.region = "us-east-1"` counts `deviceType`, `mobile`, `region`, and `us-east-1` toward the 1,000-character total.

## Expression limits
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| Limit | Value | Enforced at | 
| --- | --- | --- | 
| CPU time per expression | 100 ms | Runtime | 
| Stack depth | 100 | Runtime | 
| Expression length | 1,000 characters | Authoring | 

When an expression exceeds a limit at runtime, MediaTailor stops the expression and records the error. For details on specific error types, see [Troubleshooting and monitoring](monetization-functions-troubleshooting.md).

## Function composition limits
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| Limit | Value | 
| --- | --- | 
| Steps per sequential executor | 1–10 | 
| Maximum nesting depth | 2 | 
| Total function executions per hook | 20 | 
| Output entries per function | 20 | 
| Circular references | Not allowed | 

These limits are enforced at authoring time.

## HTTP request limits
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| Limit | Value | 
| --- | --- | 
| URL length | 2,048 characters | 
| Request body size | 64 KB | 
| Header count | 50 | 
| Header name length | 256 characters | 
| Header value length | 8,192 characters | 
| Request timeout | 100–2,000 ms (customer-configurable via RequestTimeoutMilliseconds) | 
| Allowed URL schemes | https, http | 
| Restricted headers | Host, Transfer-Encoding, Content-Length, Connection | 

URL length, body size, header count, and header size limits are enforced at authoring time. Request timeout and restricted headers are enforced at runtime.

If a function sets a restricted header, MediaTailor accepts the function configuration at authoring time but drops the header when the HTTP request is sent during execution.

## Response limits
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| Limit | Value | 
| --- | --- | 
| response.body maximum size | 20,000 characters | 
| response.text maximum length | 20,000 characters | 

These limits are enforced at runtime.

## Output limits
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| Limit | Value | 
| --- | --- | 
| Output value max length (per key) | 1,000 characters | 

These limits are enforced at runtime.

## Allowed JSONata functions
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See [JSONata expression reference](monetization-functions-jsonata.md) for the full list.

For help diagnosing errors related to these limits, see [Troubleshooting and monitoring](monetization-functions-troubleshooting.md).