diff --git a/packages/builder/spec/builderBehaviors.ts b/packages/builder/spec/builderBehaviors.ts
index 8d6fc6759..52b1f9a52 100644
--- a/packages/builder/spec/builderBehaviors.ts
+++ b/packages/builder/spec/builderBehaviors.ts
@@ -1,3 +1,4 @@
+import { assert } from 'chai'
 import { VirtualProvider } from '@tko/provider.virtual'
 import { bindings as ifBindings } from '@tko/binding.if'
 import { options } from '@tko/utils'
@@ -22,3 +23,73 @@ describe('Builder', () => {
     const builder = new Builder({ filters: {}, provider: new VirtualProvider(), bindings: [ifBindings], options: {} })
   })
 })
+
+describe('Builder.create()', () => {
+  let builder: Builder
+
+  beforeEach(() => {
+    // @ts-ignore — global helper from mocha-test-helpers.js
+    restoreAfter(options, 'filters')
+    // @ts-ignore — global helper from mocha-test-helpers.js
+    restoreAfter(options, 'bindingProviderInstance')
+    builder = new Builder({ filters: {}, provider: new VirtualProvider(), bindings: [ifBindings], options: {} })
+  })
+
+  it('merges additional properties onto the knockout instance', function () {
+    const extensions = { version: '1.0.0', myProp: 42 }
+    const ko = builder.create(extensions)
+    assert.equal(ko.version, '1.0.0')
+    assert.equal(ko.myProp, 42)
+  })
+
+  it('preserves core KnockoutInstance properties', function () {
+    const ko = builder.create({})
+    assert.isFunction(ko.observable)
+    assert.isFunction(ko.computed)
+    assert.isFunction(ko.applyBindings)
+    assert.isFunction(ko.isObservable)
+  })
+
+  it('sets options.knockoutInstance to the returned instance', function () {
+    const ko = builder.create({})
+    assert.strictEqual(ko.options.knockoutInstance, ko)
+  })
+
+  it('handles an empty object as additional properties', function () {
+    const ko = builder.create({})
+    assert.isFunction(ko.observable)
+  })
+
+  it('handles nested object additional properties', function () {
+    const jsx = { createElement: () => {}, Fragment: Symbol('Fragment') }
+    const ko = builder.create({ jsx })
+    assert.deepEqual(ko.jsx, jsx)
+  })
+
+  it('additional properties do not override core knockout properties', function () {
+    // observable is a core KO property — if overridden, it should come from additionalProperties
+    // but the standard behavior is: knockout core < providedProperties < additionalProperties
+    const customObservable = () => 'custom'
+    const ko = builder.create({ customField: customObservable })
+    // Existing core props still present
+    assert.isFunction(ko.observable)
+    // Custom field also present
+    assert.isFunction(ko.customField)
+  })
+
+  it('returns an instance with the getBindingHandler getter', function () {
+    const ko = builder.create({})
+    // getBindingHandler should be accessible as a property (delegating to options)
+    assert.isDefined(ko.getBindingHandler)
+  })
+
+  it('additionalProperties object is treated as single merge (not spread)', function () {
+    // Previously create() used rest params, now it takes a single typed object.
+    // Verify a plain object with multiple keys all appear on the result.
+    const extensions = { alpha: 1, beta: 'two', gamma: true }
+    const ko = builder.create(extensions)
+    assert.equal(ko.alpha, 1)
+    assert.equal(ko.beta, 'two')
+    assert.equal(ko.gamma, true)
+  })
+})
diff --git a/packages/utils.jsx/spec/jsxRenderBehaviors.ts b/packages/utils.jsx/spec/jsxRenderBehaviors.ts
new file mode 100644
index 000000000..aee472a88
--- /dev/null
+++ b/packages/utils.jsx/spec/jsxRenderBehaviors.ts
@@ -0,0 +1,86 @@
+import { assert } from 'chai'
+
+import { render, createElement, Fragment } from '../src'
+
+describe('render()', function () {
+  it('returns an object with node and dispose properties', function () {
+    const jsx = createElement('div', null)
+    const result = render(jsx)
+    assert.property(result, 'node')
+    assert.property(result, 'dispose')
+    assert.isFunction(result.dispose)
+    result.dispose()
+  })
+
+  it('returns the first child node when JSX produces exactly one node', function () {
+    const jsx = createElement('span', { id: 'test-single' })
+    const result = render(jsx)
+    assert.isNotNull(result.node)
+    // A single element renders as a plain child node, not a fragment
+    assert.notInstanceOf(result.node, DocumentFragment)
+    assert.equal((result.node as HTMLElement).tagName.toLowerCase(), 'span')
+    result.dispose()
+  })
+
+  it('returns a DocumentFragment when JSX produces multiple nodes', function () {
+    // Fragment children expand to multiple nodes
+    const jsx = createElement(Fragment, null, createElement('div', null), createElement('span', null))
+    const result = render(jsx)
+    assert.instanceOf(result.node, DocumentFragment)
+    result.dispose()
+  })
+
+  it('dispose() can be called without throwing', function () {
+    const jsx = createElement('p', null, 'Hello')
+    const result = render(jsx)
+    assert.doesNotThrow(() => result.dispose())
+  })
+
+  it('returns node=null-or-fragment when jsx is null/undefined (empty output)', function () {
+    // An empty JSX Fragment produces no child nodes → node is the DocumentFragment itself
+    const jsx = createElement(Fragment, null)
+    const result = render(jsx)
+    // Empty fragment: childNodes.length === 0, so node should be the fragment (not firstChild)
+    assert.instanceOf(result.node, DocumentFragment)
+    result.dispose()
+  })
+
+  it('renders a text node for a string child', function () {
+    const jsx = createElement('div', null, 'hello world')
+    const result = render(jsx)
+    // The outer div is a single node
+    assert.notInstanceOf(result.node, DocumentFragment)
+    const el = result.node as HTMLElement
+    assert.equal(el.textContent, 'hello world')
+    result.dispose()
+  })
+
+  it('renders nested elements correctly', function () {
+    const jsx = createElement('section', null, createElement('h1', null, 'Title'))
+    const result = render(jsx)
+    assert.notInstanceOf(result.node, DocumentFragment)
+    const el = result.node as HTMLElement
+    assert.equal(el.tagName.toLowerCase(), 'section')
+    assert.equal(el.querySelector('h1')?.textContent, 'Title')
+    result.dispose()
+  })
+
+  it('multiple render() calls each return independent instances', function () {
+    const jsx1 = createElement('div', { class: 'first' })
+    const jsx2 = createElement('div', { class: 'second' })
+    const result1 = render(jsx1)
+    const result2 = render(jsx2)
+    assert.notStrictEqual(result1.node, result2.node)
+    result1.dispose()
+    result2.dispose()
+  })
+
+  it('dispose() can be called multiple times without throwing', function () {
+    const jsx = createElement('div', null)
+    const result = render(jsx)
+    assert.doesNotThrow(() => {
+      result.dispose()
+      result.dispose()
+    })
+  })
+})