After giving a GitHub Copilot training last week where I introduced the concept of hooks, one of the attendants asked me what would be a good example for a hook. Great question!
A first use case I could think of is that we use a hook to format the AI generated code to match the style preferences and static analysis recommendations specified in an .editorconfig file.
Tip: If you are looking for some inspiration, check out the hooks section in Awesome Copilot: awesome-copilot/docs/README.hooks.md at main · github/awesome-copilot
What event should we use?
There are multiple hook events that you can use: sessionStart, sessionEnd, userPromptSubmitted, preToolUse, postToolUse, and errorOccurred. As the formatting should be done after every code change, postToolUse seems the logical choice.
Why not at SessionEnd?
postToolUse formats the file immediately after each edit. This means the agent sees clean, correctly structured usings before it reads the file again for its next step. If the agent inspects the file later in the same session, it's working with already-formatted code, which leads to better decisions.
sessionEnd has a key problem: by the time it runs, the agent has already finished. Any subsequent reads or edits it made during the session were based on the unformatted state. You'd also get a bulk formatting pass at the end, which makes the diff noisier and harder to review — it mixes the agent's logical changes with formatting changes.
Think of it like this: postToolUse is like a formatter running on save, while sessionEnd is like running it only when you close your IDE. The former is almost always more useful.
The one case where sessionEnd could make sense is if dotnet format is slow on your project and you're okay with the trade-offs above, batching it once at the end avoids running it after every single file touch. But dotnet format is fast enough that this isn't usually a concern.
So postToolUse wins here for correctness and agent quality during the session.
Writing our script
Now it’s time to write our script. The approach is simple. The script will be fired after any file edit, it checks if the changed file is a .cs file, and then runs dotnet-format to apply our formatting rules.
Add the script to the following folder: .github/hooks/scripts/format-csharp.ps1
Creating the hook
Almost there! Create a json file with the hook configuration and store it inside the .github/hooks folder. Make sure that the hook correctly points to our script above:
Testing the hook
After you save the hook file, VS Code automatically loads it — no restart needed. Put your agent to work:
You can verify it ran by checking the GitHub Copilot Chat Hooks output channel:
Happy coding!
More information
GitHub Copilot CLI Tips & Tricks — Part 4: Automating and enforcing policies with hooks