Video: Defining hooks
1. Video: Defining hooks
To get a better idea of how hooks work, we're going to take a look at a new sample project. Attached to this lecture is a file called Queries.zip. I'd encourage you to download this project and open your code editor inside of it. Once you've got your editor open, at your terminal, run npm run setup. This is going to install a couple of dependencies and get a couple of hooks ready for use. To better understand hooks, we're going to make our own inside of this project. So here's what I want our hook to do. Inside the root directory of our project is a file called .env. This file contains some sensitive information, and out of an abundance of caution, I want to completely prevent Claude from ever reading this file directly. Let me show you a couple of diagrams to help you understand how we're going to put this hook together. Step one is to decide on whether we need a pre-tool use or a post-tool use hook. In this scenario, we want to prevent Claude from ever reading a particular file. If we make a post-tool use block, then we will have executed our hook or ran our command after Claude already read the file. So in this case, we definitely need a pre-tool use hook to make sure that we can prevent the read operation from occurring. The next thing we need to do is decide exactly which kind of tool calls we want to watch for. I've got a list of all the different current tool names on the right hand side of this diagram. Now, memorizing all the different tool names that are included inside of Claude code can especially since you can add your own custom tools through the use of MCP servers. So let me show you a little trick you can use here. If I go back over and open up Claude code, I can directly ask Claude for a bullet point list of all the different tool names that it has access to right now. Out of all these different tools, there are two that can be used to very easily read the contents of a file. First, there's the read tool, and then it's easy to miss, but this one can actually read the contents of a file as well, the grep tool. That can search the contents of a file. So we really want to watch for tool calls for the read tool and the grep tool. Next up, we need to write out a command that is going to receive some information about the tool call that Claude wants to make. Here's how that part works. We're going to write out a command Claude is going to automatically execute it. And then on standard in to that process, Claude is going to feed in some tool call data as JSON. I've got an example of some tool call data on the top right hand side. So it's going to be a big JSON object that has some information about the tool name and the input to that tool. In this case, the tool name is read. So Claude is trying to call the read tool and it might be trying to read specifically a file path pointing to that .env file. And again, that's the file that we want to prevent a read operation for. So then inside of our program or our command, we need to receive this information through standard in, parse that JSON, and then read the tool name, the tool input arguments and so on and decide what we want to do with this tool call. Then on to step four, in step four, after our command receives that proposed tool call data, we're then going to exit and our exit code is going to provide a signal back to Claude code. An exit code of zero means everything is okay. And we want to allow this tool call to occur. An exit code of two, however, is a sign to Claude code that we want to block this tool call. And that's specifically only applies for the pre tool use hooks. Because remember only in a pre tool use hook, can we actually block a tool call. If we exit with a code of two, then any standard error logs that we generated inside of our command during that time will also be sent as feedback to Claude. So we can both deny the tool call and give Claude a reason at the same time as well. So that's the entire process. And I know once again, there's a lot of stuff going on here. So let's go through this entire process of wiring everything up needed for this hook inside of our project to understand how all these steps come together.2. Let's practice!
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