diff --git a/.changeset/fix-jsx-clean-teardown-race.md b/.changeset/fix-jsx-clean-teardown-race.md
new file mode 100644
index 00000000..1479c19c
--- /dev/null
+++ b/.changeset/fix-jsx-clean-teardown-race.md
@@ -0,0 +1,12 @@
+---
+"@tko/utils.jsx": patch
+---
+
+Add `options.jsxCleanBatchSize` (default `1000`) controlling JSX node cleanup
+batching. Setting it to `0` runs cleanup synchronously on detach. Registered
+via `defineOption` — the hardcoded `MAX_CLEAN_AT_ONCE` constant is gone, but
+the new default matches it so production behavior is unchanged.
+
+Fixes a `ReferenceError: Element is not defined` in the `cli-happy-dom` test
+project where the 25ms batch timer could fire after happy-dom had torn down
+DOM globals.
diff --git a/AGENTS.md b/AGENTS.md
index e99b7811..98418830 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -105,6 +105,13 @@ packages/example/
 Inter-package dependencies use `@tko/package-name` and are resolved via
 npm workspaces.
 
+### Configurable runtime options (`ko.options.*`)
+
+Register package-owned options via `defineOption` from `@tko/utils`, not as
+fields on the core `Options` class. See
+[`tko.io/public/agents/options.md`](tko.io/public/agents/options.md) for the
+pattern and canonical example.
+
 ## CI/CD
 
 GitHub Actions workflows (`.github/workflows/`):
diff --git a/builds/knockout/helpers/vitest-setup.js b/builds/knockout/helpers/vitest-setup.js
index 2b2557d4..658e5d5d 100644
--- a/builds/knockout/helpers/vitest-setup.js
+++ b/builds/knockout/helpers/vitest-setup.js
@@ -1,5 +1,6 @@
 import * as chai from 'chai'
 import sinon from 'sinon'
+import { options } from '@tko/utils'
 import { isHappyDom } from '../../../packages/utils/helpers/test-env.ts'
 
 // Set globals that builds/knockout specs and mocha-test-helpers.js expect
@@ -20,3 +21,11 @@ import '../dist/browser.min.js'
 // Now import the helper — it needs chai, expect, ko, and beforeEach/afterEach as globals.
 // beforeEach/afterEach come from vitest globals (globals: true in config).
 import './mocha-test-helpers.js'
+
+// Run JSX cleanup synchronously in tests. In happy-dom the 25ms batch
+// timer can fire after the DOM globals are torn down, throwing from
+// `cleanNode`. browser.min.js bundles its own Options instance, so mirror.
+beforeAll(() => {
+  options.jsxCleanBatchSize = 0
+  globalThis.ko.options.jsxCleanBatchSize = 0
+})
diff --git a/packages/utils.jsx/spec/jsxCleanBehaviors.ts b/packages/utils.jsx/spec/jsxCleanBehaviors.ts
new file mode 100644
index 00000000..78474abf
--- /dev/null
+++ b/packages/utils.jsx/spec/jsxCleanBehaviors.ts
@@ -0,0 +1,125 @@
+import { assert } from 'chai'
+import sinon from 'sinon'
+
+import { addDisposeCallback, options } from '@tko/utils'
+
+import { queueCleanNode } from '../src/jsxClean'
+
+describe('jsxClean queue', function () {
+  let cleaned: Node[]
+  let clock: sinon.SinonFakeTimers
+  let originalBatchSize: number
+
+  function makeNode(): HTMLElement {
+    const node = document.createElement('div')
+    addDisposeCallback(node, () => cleaned.push(node))
+    return node
+  }
+
+  beforeEach(function () {
+    cleaned = []
+    originalBatchSize = options.jsxCleanBatchSize
+    clock = sinon.useFakeTimers()
+  })
+
+  afterEach(function () {
+    clock.restore()
+    options.jsxCleanBatchSize = originalBatchSize
+  })
+
+  describe('jsxCleanBatchSize = 0 (synchronous)', function () {
+    beforeEach(function () {
+      options.jsxCleanBatchSize = 0
+    })
+
+    it('runs cleanup synchronously on queueCleanNode', function () {
+      const node = makeNode()
+      queueCleanNode(node)
+      assert.deepEqual(cleaned, [node])
+    })
+
+    it('drains all queued nodes synchronously in a single call', function () {
+      const nodes = [makeNode(), makeNode(), makeNode()]
+      for (const n of nodes) queueCleanNode(n)
+      assert.deepEqual(cleaned, nodes)
+    })
+
+    it('does not schedule a timer', function () {
+      queueCleanNode(makeNode())
+      assert.equal(clock.countTimers(), 0)
+    })
+  })
+
+  describe('jsxCleanBatchSize > 0 (batched, default 1000)', function () {
+    beforeEach(function () {
+      options.jsxCleanBatchSize = 1000
+    })
+
+    it('defers cleanup until the 25ms timer fires', function () {
+      const node = makeNode()
+      queueCleanNode(node)
+      assert.lengthOf(cleaned, 0, 'not cleaned before timer fires')
+      clock.tick(25)
+      assert.deepEqual(cleaned, [node])
+    })
+
+    it('does not re-trigger the timer while one is already pending', function () {
+      const a = makeNode()
+      const b = makeNode()
+      queueCleanNode(a)
+      queueCleanNode(b)
+      clock.tick(24)
+      assert.lengthOf(cleaned, 0)
+      clock.tick(1)
+      assert.deepEqual(cleaned, [a, b])
+    })
+
+    it('processes at most batchSize nodes per 25ms tick', function () {
+      options.jsxCleanBatchSize = 3
+      const nodes = [makeNode(), makeNode(), makeNode(), makeNode(), makeNode()]
+      for (const n of nodes) queueCleanNode(n)
+
+      clock.tick(25)
+      assert.lengthOf(cleaned, 3, 'first tick processes one batch')
+
+      clock.tick(25)
+      assert.lengthOf(cleaned, 5, 'second tick drains the remainder')
+    })
+
+    it('re-schedules itself when the queue is non-empty after a batch', function () {
+      options.jsxCleanBatchSize = 2
+      const nodes = Array.from({ length: 5 }, makeNode)
+      for (const n of nodes) queueCleanNode(n)
+
+      clock.tick(25)
+      assert.lengthOf(cleaned, 2)
+      clock.tick(25)
+      assert.lengthOf(cleaned, 4)
+      clock.tick(25)
+      assert.lengthOf(cleaned, 5)
+    })
+
+    it('drains synchronously if batchSize is flipped to 0 while a timer is pending', function () {
+      const nodes = [makeNode(), makeNode(), makeNode()]
+      for (const n of nodes) queueCleanNode(n)
+      assert.equal(clock.countTimers(), 1)
+
+      options.jsxCleanBatchSize = 0
+      clock.tick(25)
+
+      assert.deepEqual(cleaned, nodes)
+      assert.equal(clock.countTimers(), 0, 'no re-armed timer')
+    })
+
+    it('drains synchronously if batchSize becomes non-finite while a timer is pending', function () {
+      const nodes = [makeNode(), makeNode()]
+      for (const n of nodes) queueCleanNode(n)
+
+      options.jsxCleanBatchSize = Number.NaN as unknown as number
+      clock.tick(25)
+
+      assert.deepEqual(cleaned, nodes)
+      assert.equal(clock.countTimers(), 0)
+    })
+  })
+})
diff --git a/packages/utils.jsx/src/jsxClean.ts b/packages/utils.jsx/src/jsxClean.ts
index ca61253d..7e06b18f 100644
--- a/packages/utils.jsx/src/jsxClean.ts
+++ b/packages/utils.jsx/src/jsxClean.ts
@@ -1,26 +1,62 @@
-import { cleanNode } from '@tko/utils'
+import { cleanNode, defineOption, options } from '@tko/utils'
 
 const DELAY_MS = 25
-const MAX_CLEAN_AT_ONCE = 1000
 const cleanNodeQueue = new Array()
 let cleanNodeTimeoutID: ReturnType<typeof setTimeout> | null = null
 
+// Extend the Options type so ko.options.jsxCleanBatchSize is strongly typed.
+declare module '@tko/utils' {
+  interface Options {
+    jsxCleanBatchSize: number
+  }
+}
+
+// `0` runs cleanup synchronously on detach. Test environments that tear
+// down DOM globals between files use that to avoid a pending 25ms timer
+// firing against a dead global.
+defineOption('jsxCleanBatchSize', { default: 1000 })
+
 export function queueCleanNode(node) {
   cleanNodeQueue.push(node)
-  triggerCleanTimeout()
+  if (options.jsxCleanBatchSize === 0) {
+    flushAll()
+  } else {
+    scheduleBatch()
+  }
 }
 
-function triggerCleanTimeout() {
+function scheduleBatch() {
   if (!cleanNodeTimeoutID && cleanNodeQueue.length) {
-    cleanNodeTimeoutID = setTimeout(flushCleanQueue, DELAY_MS)
+    cleanNodeTimeoutID = setTimeout(flushBatch, DELAY_MS)
   }
 }
 
-function flushCleanQueue() {
+function flushBatch() {
   cleanNodeTimeoutID = null
-  const nodes = cleanNodeQueue.splice(0, MAX_CLEAN_AT_ONCE)
+  // If the option was flipped to a non-positive / non-finite value while the
+  // timer was pending, fall through to synchronous drain — otherwise
+  // splice(0, <=0|NaN) removes nothing and scheduleBatch re-arms forever.
+  const batchSize = Math.trunc(options.jsxCleanBatchSize)
+  if (!Number.isFinite(batchSize) || batchSize <= 0) {
+    flushAll()
+    return
+  }
+  const nodes = cleanNodeQueue.splice(0, batchSize)
   for (const node of nodes) {
     cleanNode(node)
   }
-  triggerCleanTimeout()
+  scheduleBatch()
+}
+
+function flushAll() {
+  if (cleanNodeTimeoutID !== null) {
+    clearTimeout(cleanNodeTimeoutID)
+    cleanNodeTimeoutID = null
+  }
+  // Outer `while` is for re-enqueues triggered by cleanNode side effects.
+  while (cleanNodeQueue.length) {
+    for (const node of cleanNodeQueue.splice(0)) {
+      cleanNode(node)
+    }
+  }
 }
diff --git a/tko.io/public/agents/guide.md b/tko.io/public/agents/guide.md
index d5f363d9..3346dcbb 100644
--- a/tko.io/public/agents/guide.md
+++ b/tko.io/public/agents/guide.md
@@ -4,6 +4,8 @@ Test-backed behavior summaries live under `/agents/verified-behaviors/`. Treat t
 
 For preferred state/binding/DOM architecture in examples and prototypes, read `/agents/contract.md`.
 
+For how to add or configure runtime options (`ko.options.*`), read `/agents/options.md`.
+
 ## Setup
 
 ```html
diff --git a/tko.io/public/agents/options.md b/tko.io/public/agents/options.md
new file mode 100644
index 00000000..88d21d03
--- /dev/null
+++ b/tko.io/public/agents/options.md
@@ -0,0 +1,66 @@
+# `ko.options.*` — Configurable Runtime Options
+
+`ko.options` is TKO's runtime configuration object. It is a singleton `Options`
+instance defined in `packages/utils/src/options.ts`. This page documents how
+to add new options and which of the two available mechanisms to reach for.
+
+## Two mechanisms — when to use which
+
+### `defineOption` (default — almost always use this)
+
+For options that belong to a specific package (batch sizes, feature toggles,
+plugin behavior), register via `defineOption` inside the owning package.
+
+```ts
+// packages/utils.jsx/src/jsxClean.ts
+import { defineOption, options } from '@tko/utils'
+
+// Extend the Options type so ko.options.<name> is strongly typed.
+declare module '@tko/utils' {
+  interface Options {
+    jsxCleanBatchSize: number
+  }
+}
+
+// Register at module load, with an optional side-effect setter.
+defineOption('jsxCleanBatchSize', { default: 1000 })
+
+// Read wherever the option applies.
+if (options.jsxCleanBatchSize === 0) { /* sync path */ }
+```
+
+Rules:
+
+- The `declare module '@tko/utils' { interface Options { ... } }` augmentation
+  lives in the same file (or at least the same package) that defines the option.
+- `defineOption` registers at module-side-effect time; the option is available
+  as soon as the owning package is imported.
+- An optional `set(value)` runs side effects at configuration time (not
+  retroactively on already-parsed bindings) — use for options that swap
+  implementations or rebuild internal state.
+
+**Canonical example with a side-effect setter:**
+`packages/utils.parser/src/operators.ts` → `strictEquality` swaps the `==`
+and `!=` operator implementations when the option flips.
+
+### Core `Options` class field (rare — only for `@tko/utils` intrinsics)
+
+Add a field directly to the `Options` class in `packages/utils/src/options.ts`
+**only** when the option is intrinsic to `@tko/utils` itself — something that
+`@tko/utils` uses or enforces without knowing about any downstream package.
+
+Current core fields include `templateSizeLimit`, `deferUpdates`,
+`useOnlyNativeEvents`, `onError`, `sanitizeHtmlTemplate`.
+
+Never add a core-class field for an option that belongs to another package.
+Doing so forces `@tko/utils` to carry concepts from downstream packages and
+creates a wrong-way dependency.
+
+## Quick test
+
+> Is the option intrinsic to `@tko/utils`?
+
+- Yes → core class field.
+- No → `defineOption` in the owning package.
+
+In practice, new options are almost always the second case.
diff --git a/tko.io/public/llms.txt b/tko.io/public/llms.txt
index 49ffe591..b6d1ce88 100644
--- a/tko.io/public/llms.txt
+++ b/tko.io/public/llms.txt
@@ -17,6 +17,7 @@
 - /agents/glossary.md — domain terms, concepts, packages
 - /agents/testing.md — how to run and verify TKO code
 - /agents/contract.md — state/binding/DOM architecture
+- /agents/options.md — ko.options.* — when to use defineOption vs core Options fields
 - /agents/verified-behaviors/index.md — test-backed behavior contracts
 - /agents/sample-tsx.html — minimal browser TSX scaffold
 - /examples/ — interactive self-contained HTML examples