When sanitize/encode while implementing tags system like on SO

Question

In my development I have a tag system that closely matches the one SO has. And it also allows non-Latin characters.

• User can enter new tag and it is saved to the DB.
• Existing tags are shown to the user when they type tag prefix. Fetch API is used for this.

I'm using Razor pages. At what point and how should I sanitize/encode strings in this flow?

Here is an example of my fetch request:

try {
    const response = await fetch("api/tags?" + new URLSearchParams({ prefix: curPrefix, count: 12 }));
    if (!response.ok) throw new Error("Network response was not OK");
    const jsonData = await response.json();
    if (jsonData.prefix === getPrefix()) {
        var newTags = jsonData.tags.filter(tag => !selectedTags.find(x => x.name == tag.name));
        setSuggestedTags(newTags);
    }
} catch (error) {
    console.error("There has been a problem with your fetch operation:", error);
}