> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hasmcp.com/llms.txt
> Use this file to discover all available pages before exploring further.

# How can I write JavaScript Interceptors to modify API responses before sending them to the LLM?

> Creating programmable mutations inside HasMCP natively dynamically.

# Writing JavaScript Interceptors

To inject a custom Goja interceptor onto any specific Provider API Route, you utilize the **Data Transformation** UI block securely nested within the Provider Tool configuration page.

<img src="https://mintcdn.com/hasmcp/9Embd1XJnDsV9Wi6/images/kb/data-transformation.png?fit=max&auto=format&n=9Embd1XJnDsV9Wi6&q=85&s=d71265e8f4fa6ef1fd439cf1af403935" alt="Data Transformation UI" width="2560" height="1440" data-path="images/kb/data-transformation.png" />

### The Interceptor Interface

All JavaScript logic inside HasMCP must structurally adhere to a strict interface. You must implement a single global function exactly named `intercept(response)` for Response Interceptors, or `intercept(request)` for Request Interceptors.

> \[!NOTE]
> **Interceptor Availability**: Request Interceptors can strictly be attached to `POST`, `PUT`, and `PATCH` methods to modify outgoing bodies or arguments. Response Interceptors are universally available to all HTTP methods uniformly.

```javascript theme={null}
// The HasMCP Proxy will inherently pass the Provider's raw REST Response as the 'input' object.
 
// 1. Execute any arbitrary stateful logic securely on the object
input.hasmcp_injected_timestamp = new Date().toISOString();
 
if (input.status === 404) {
  input.message = "The external API failed to find the document.";
}

// 2. Return the evaluated object directly
return input;
```

### Execution Behavior

1. The originating MCP client asks the HasMCP Server to trigger a tool.
2. HasMCP negotiates the upstream HTTP execution accurately against the external third-party API.
3. The target API returns an HTTP response payload.
4. HasMCP buffers the response and inherently executes your `intercept(response)` function locally physically.
5. The LLM exclusively receives the modified stringified response intelligently, completely unaware of the physical interception step.

> \[!IMPORTANT]
> **Execution Timeouts**: Both GoJA and JMESPath interceptors have a strict **100ms execution timeout** on HasMCP Cloud versions to prevent runaway scripts. Enterprise on-prem deployments can explicitly define custom timeout limits on their own hosted infrastructure.