Code change:
assert.equal(editor.getValue(), '    let name = foo'); }); <ins> test('use additionalTextEdits sync when possible', async function () { disposables.add(CompletionProviderRegistry.register({ scheme: 'test-ctrl' }, { provideCompletionItems(doc, pos) { return { suggestions: [{ kind: CompletionItemKind.Snippet, label: 'let', insertText: 'hello', range: Range.fromPositions(pos), additionalTextEdits: [{ text: 'I came sync', range: { startLineNumber: 1, startColumn: 1, endLineNumber: 1, endColumn: 1 } }] }] }; }, async resolveCompletionItem(item) { return item; } })); editor.setValue('hellonhallo'); editor.setSelection(new Selection(2, 6, 2, 6)); // trigger let p1 = Event.toPromise(controller.model.onDidSuggest); controller.triggerSuggest(); await p1; // let p2 = Event.toPromise(controller.model.onDidCancel); controller.acceptSelectedSuggestion(false, false); await p2; // insertText happens sync! assert.equal(editor.getValue(), 'I came synchellonhallohello'); }); test('resolve additionalTextEdits async when needed', async function () { let resolveCallCount = 0; disposables.add(CompletionProviderRegistry.register({ scheme: 'test-ctrl' }, { provideCompletionItems(doc, pos) { return { suggestions: [{ kind: CompletionItemKind.Snippet, label: 'let', insertText: 'hello', range: Range.fromPositions(pos) }] }; }, async resolveCompletionItem(item) { resolveCallCount += 1; await timeout(10); item.additionalTextEdits = [{ text: 'I came late', range: { startLineNumber: 1, startColumn: 1, endLineNumber: 1, endColumn: 1 } }]; return item; } })); editor.setValue('hellonhallo'); editor.setSelection(new Selection(2, 6, 2, 6)); // trigger let p1 = Event.toPromise(controller.model.onDidSuggest); controller.triggerSuggest(); await p1; // let p2 = Event.toPromise(controller.model.onDidCancel); controller.acceptSelectedSuggestion(false, false); await p2; // insertText happens sync! assert.equal(editor.getValue(), 'hellonhallohello'); assert.equal(resolveCallCount, 1); // additional edits happened after a litte wait await timeout(20); assert.equal(editor.getValue(), 'I came latehellonhallohello'); // single undo stop editor.getModel()?.undo(); assert.equal(editor.getValue(), 'hellonhallo'); }); test('resolve additionalTextEdits async when needed (typing)', async function () { let resolveCallCount = 0; let resolve: Function = () => { }; disposables.add(CompletionProviderRegistry.register({ scheme: 'test-ctrl' }, { provideCompletionItems(doc, pos) { return { suggestions: [{ kind: CompletionItemKind.Snippet, label: 'let', insertText: 'hello', range: Range.fromPositions(pos) }] }; }, async resolveCompletionItem(item) { resolveCallCount += 1; await new Promise(_resolve => resolve = _resolve); item.additionalTextEdits = [{ text: 'I came late', range: { startLineNumber: 1, startColumn: 1, endLineNumber: 1, endColumn: 1 } }]; return item; } })); editor.setValue('hellonhallo'); editor.setSelection(new Selection(2, 6, 2, 6)); // trigger let p1 = Event.toPromise(controller.model.onDidSuggest); controller.triggerSuggest(); await p1; // let p2 = Event.toPromise(controller.model.onDidCancel); controller.acceptSelectedSuggestion(false, false); await p2; // insertText happens sync! assert.equal(editor.getValue(), 'hellonhallohello'); assert.equal(resolveCallCount, 1); // additional edits happened after a litte wait assert.ok(editor.getSelection()?.equalsSelection(new Selection(2, 11, 2, 11))); editor.trigger('test', 'type', { text: 'TYPING' }); assert.equal(editor.getValue(), 'hellonhallohelloTYPING'); resolve(); await timeout(10); assert.equal(editor.getValue(), 'I came latehellonhallohelloTYPING'); assert.ok(editor.getSelection()?.equalsSelection(new Selection(2, 17, 2, 17))); }); // additional edit come late and are AFTER the selection -> cancel test('resolve additionalTextEdits async when needed (simple conflict)', async function () { let resolveCallCount = 0; let resolve: Function = () => { }; disposables.add(CompletionProviderRegistry.register({ scheme: 'test-ctrl' }, { provideCompletionItems(doc, pos) { return { suggestions: [{ kind: CompletionItemKind.Snippet, label: 'let', insertText: 'hello', range: Range.fromPositions(pos) }] }; }, async resolveCompletionItem(item) { resolveCallCount += 1; await new Promise(_resolve => resolve = _resolve); item.additionalTextEdits = [{ text: 'I came late', range: { startLineNumber: 1, startColumn: 6, endLineNumber: 1, endColumn: 6 } }]; return item; } })); editor.setValue(''); editor.setSelection(new Selection(1, 1, 1, 1)); // trigger let p1 = Event.toPromise(controller.model.onDidSuggest); controller.triggerSuggest(); await p1; // let p2 = Event.toPromise(controller.model.onDidCancel); controller.acceptSelectedSuggestion(false, false); await p2; // insertText happens sync! assert.equal(editor.getValue(), 'hello'); assert.equal(resolveCallCount, 1); resolve(); await timeout(10); assert.equal(editor.getValue(), 'hello'); }); // additional edit come late and are AFTER the position at which the user typed -> cancelled test('resolve additionalTextEdits async when needed (conflict)', async function () { let resolveCallCount = 0; let resolve: Function = () => { }; disposables.add(CompletionProviderRegistry.register({ scheme: 'test-ctrl' }, { provideCompletionItems(doc, pos) { return { suggestions: [{ kind: CompletionItemKind.Snippet, label: 'let', insertText: 'hello', range: Range.fromPositions(pos) }] }; }, async resolveCompletionItem(item) { resolveCallCount += 1; await new Promise(_resolve => resolve = _resolve); item.additionalTextEdits = [{ text: 'I came late', range: { startLineNumber: 1, startColumn: 2, endLineNumber: 1, endColumn: 2 } }]; return item; } })); editor.setValue('hellonhallo'); editor.setSelection(new Selection(2, 6, 2, 6)); // trigger let p1 = Event.toPromise(controller.model.onDidSuggest); controller.triggerSuggest(); await p1; // let p2 = Event.toPromise(controller.model.onDidCancel); controller.acceptSelectedSuggestion(false, false); await p2; // insertText happens sync! assert.equal(editor.getValue(), 'hellonhallohello'); assert.equal(resolveCallCount, 1); // additional edits happened after a litte wait editor.setSelection(new Selection(1, 1, 1, 1)); editor.trigger('test', 'type', { text: 'TYPING' }); assert.equal(editor.getValue(), 'TYPINGhellonhallohello'); resolve(); await timeout(10); assert.equal(editor.getValue(), 'TYPINGhellonhallohello'); assert.ok(editor.getSelection()?.equalsSelection(new Selection(1, 7, 1, 7))); }); test('resolve additionalTextEdits async when needed (cancel)', async function () { let resolve: Function[] = []; disposables.add(CompletionProviderRegistry.register({ scheme: 'test-ctrl' }, { provideCompletionItems(doc, pos) { return { suggestions: [{ kind: CompletionItemKind.Snippet, label: 'let', insertText: 'hello', range: Range.fromPositions(pos) }, { kind: CompletionItemKind.Snippet, label: 'let', insertText: 'hallo', range: Range.fromPositions(pos) }] }; }, async resolveCompletionItem(item) { await new Promise(_resolve => resolve.push(_resolve)); item.additionalTextEdits = [{ text: 'additionalTextEdits', range: { startLineNumber: 1, startColumn: 2, endLineNumber: 1, endColumn: 2 } }]; return item; } })); editor.setValue('abc'); editor.setSelection(new Selection(1, 1, 1, 1)); // trigger let p1 = Event.toPromise(controller.model.onDidSuggest); controller.triggerSuggest(); await p1; // let p2 = Event.toPromise(controller.model.onDidCancel); controller.acceptSelectedSuggestion(true, false); await p2; // insertText happens sync! assert.equal(editor.getValue(), 'helloabc'); // next controller.acceptNextSuggestion(); // resolve additional edits (MUST be cancelled) resolve.forEach(fn => fn); resolve.length = 0; await timeout(10); // next suggestion used assert.equal(editor.getValue(), 'halloabc'); }); </ins> });

Related discussion:
text: so how do servers know that clients support lazy-loading of additional textedits? Surely this should be a client capability advertised by vscode (that needs to be specced in the LSP)
is the right person for that and you should probably create a follow up issue in the LSP land
corresponding LSP feature request created here please add more details if I miss anything.
Thanks
To verify: use TypeScript have a suggestion that will import a symbol make sure that suggestion isn't first or focused in the UI use the mouse(!) to select the suggestion make sure the import is there make sure undo, redo works as expected