Force VS Code to Trigger Autocomplete For Keywords Starting with Non-Alphanumeric Symbols

I am trying to make a simple autocomplete for the LSP using:

import * as vscode from 'vscode';

export function activate(context: vscode.ExtensionContext) {

    const provider = vscode.languages.registerCompletionItemProvider('exa', {

        provideCompletionItems(document: vscode.TextDocument, position: vscode.Position, token: vscode.CancellationToken, context: vscode.CompletionContext) {
            const completions = [];

            const linePrefix = document.lineAt(position).text.slice(0, position.character);
            if (linePrefix.includes('@')) {
                completions.push(new vscode.CompletionItem("@END"));
                return completions;
            }
        }
    });

    context.subscriptions.push(provider);
}

But when I type @, no autocomplete is triggered:

It isn't until I start typing alphanumeric characters that it is triggered. Notice that it seems to actively ignore the @ sign as part of the autocomplete trigger:

This is confirmed if the alphanumeric trigger is then selected and an extra @ sign is added as if the first hadn't even been typed:

How could I correct this code to accept @ as part of the autocomplete trigger and completion?

I am not sure why your code doesn't work. It may be related to @ not being a word character in the exa language and thus needing to be specifically listed as a trigger character.

Making 2 CompletionProviders does work. Try this code:

// const provider = vscode.languages.registerCompletionItemProvider('exa', {
const provider = vscode.languages.registerCompletionItemProvider('plaintext', {

        provideCompletionItems(document, position, token, context) {
            const completions = [];

            const linePrefix = document.lineAt(position).text.slice(0, position.character);
            
            let item;
            
            if (linePrefix.startsWith('@')) item = new vscode.CompletionItem("@END");
            if (linePrefix.startsWith('!')) item = new vscode.CompletionItem("!START");
            
            if (item) {
                item.range = new vscode.Range(position.translate(0, -1), position);
                completions.push(item);
                return completions;
            }
        }
    },
   '@', '!'    // list any special characters here
);

context.subscriptions.push(provider);

const provider2 = vscode.languages.registerCompletionItemProvider('plaintext', {

    provideCompletionItems(document, position, token, context) {
        const completions = [];

        const linePrefix = document.lineAt(position).text.slice(0, position.character);
        if (linePrefix.includes('H')) {
                completions.push(new vscode.CompletionItem("HOWDY"));
                return completions;
        }
    }
});

context.subscriptions.push(provider2);

Note: I used 'plaintext' for my testing so change that. provider1 gets called for its list of trigger characters. provider2 gets called for regular word characters.

Possibly related to @ not being a word character is the double @@END you were seeing. I fixed that with

        const item = new vscode.CompletionItem("@END");
        item.range = new vscode.Range(position.translate(0, -1), position);

so that the range would include that first @ character.