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genai-toolbox/docs/en/samples/pre_post_processing/js.md
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---
title: "Javascript"
type: docs
weight: 2
description: >
How to add pre- and post- processing to your Agents using JS.
---
## Prerequisites
This tutorial assumes that you have set up Toolbox with a basic agent as described in the [local quickstart](../../getting-started/local_quickstart_js.md).
This guide demonstrates how to implement these patterns in your Toolbox applications.
## Implementation
{{< tabpane persist=header >}}
{{% tab header="ADK" text=true %}}
Coming soon.
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{{% tab header="Langchain" text=true %}}
The following example demonstrates how to use `ToolboxClient` with LangChain's middleware to implement pre- and post- processing for tool calls.
```js
{{< include "js/langchain/agent.js" >}}
```
You can also use the `wrapModelCall` hook to intercept messages before and after model calls.
You can also use [node-style hooks](https://docs.langchain.com/oss/javascript/langchain/middleware/custom#node-style-hooks) to intercept messages at the agent and model level.
See the [LangChain Middleware documentation](https://docs.langchain.com/oss/javascript/langchain/middleware/custom#tool-call-monitoring) for details on these additional hook types.
{{% /tab %}}
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## Results
The output should look similar to the following. Note that exact responses may vary due to the non-deterministic nature of LLMs and differences between orchestration frameworks.
```
AI: Booking Confirmed! You earned 500 Loyalty Points with this stay.
AI: Error: Maximum stay duration is 14 days.
```