diff --git a/packages/@sanity/types/src/portableText/asserters.ts b/packages/@sanity/types/src/portableText/asserters.ts
index 0bce91b7fce..3a18ef77186 100644
--- a/packages/@sanity/types/src/portableText/asserters.ts
+++ b/packages/@sanity/types/src/portableText/asserters.ts
@@ -1,26 +1,70 @@
-import type {Block, Span} from './types'
+import type {PortableTextTextBlock, PortableTextSpan, PortableTextObject} from './types'
 
 function isRecord(value: unknown): value is Record<string, unknown> {
   return !!value && (typeof value == 'object' || typeof value == 'function')
 }
 
-/** @internal */
-export function isBlock<T = Span>(value: unknown): value is Block<T> {
+/**
+ * Assert that a given object is a portable-text text-block type object
+ *
+ * @remarks
+ * * The `markDefs` and `style` property of a block is optional.
+ * * Block types can be named, so expect anything of the _type property.
+ *
+ * @alpha
+ */
+export function isPortableTextTextBlock<T = PortableTextSpan | PortableTextObject>(
+  value: unknown
+): value is PortableTextTextBlock<T> {
   return (
     isRecord(value) &&
     typeof value._type === 'string' && // block types can be named, so expect anything here.
-    typeof value.style === 'string' &&
     Array.isArray(value.children) &&
-    Array.isArray(value.markDefs)
+    value.children.every((child) => isRecord(child)) &&
+    ('markDefs' in value // optional property
+      ? Array.isArray(value.markDefs) && value.markDefs.every((def) => isRecord(def))
+      : false) &&
+    ('style' in value ? typeof value.style === 'string' : true) // optional property
   )
 }
 
-/** @internal */
-export function isSpan(value: unknown): value is Span {
+/**
+ * Assert that a given object is a portable-text span-type object
+ *
+ * @remarks
+ * The `marks` property of a block is optional.
+ *
+ * @alpha
+ */
+export function isPortableTextSpan(value: unknown): value is PortableTextSpan {
   return (
     isRecord(value) &&
     value._type === 'span' &&
     typeof value.text === 'string' &&
-    Array.isArray(value.marks)
+    ('marks' in value // optional property
+      ? Array.isArray(value.marks) && value.marks.every((mark) => typeof mark === 'string')
+      : true)
+  )
+}
+
+/**
+ * Assert that a given object is a portable-text list-text-block-type object
+ *
+ * @remarks
+ * Uses `isPortableTextTextBlock` and checks for `listItem` and `level`
+ *
+ * @see isPortableTextTextBlock
+ *
+ * @alpha
+ */
+export function isPortableTextListBlock<T = PortableTextSpan | PortableTextObject>(
+  value: unknown
+): value is PortableTextTextBlock<T> {
+  return (
+    isPortableTextTextBlock(value) &&
+    'listItem' in value &&
+    typeof value.listItem === 'string' &&
+    'level' in value &&
+    Number.isInteger(value.level)
   )
 }
diff --git a/packages/@sanity/types/src/portableText/types.ts b/packages/@sanity/types/src/portableText/types.ts
index 07dcd28316c..0fb4a01ef08 100644
--- a/packages/@sanity/types/src/portableText/types.ts
+++ b/packages/@sanity/types/src/portableText/types.ts
@@ -1,23 +1,38 @@
-/** @public */
-export interface Block<TChild = Span> {
+/** @alpha */
+
+export type PortableTextBlock = PortableTextTextBlock | PortableTextObject
+
+/** @alpha */
+export interface PortableTextTextBlock<TChild = PortableTextSpan | PortableTextObject> {
   _type: string
   _key: string
-  style: string
   children: TChild[]
-  markDefs: MarkDefinition[]
+  markDefs?: PortableTextObject[]
+  listItem?: string
+  style?: string
+  level?: number
 }
 
-/** @public */
-export interface Span {
-  _type: 'span'
+/** @alpha */
+export interface PortableTextObject {
+  _type: string
   _key: string
-  marks: string[]
-  text: string
+  [other: string]: unknown
 }
 
-/** @public */
-export interface MarkDefinition {
-  [key: string]: unknown
-  _type: string
+/** @alpha */
+export interface PortableTextSpan {
   _key: string
+  _type: 'span'
+  text: string
+  marks?: string[]
+}
+
+/** @alpha */
+export type PortableTextChild = PortableTextObject | PortableTextSpan
+
+/** @alpha */
+export interface PortableTextListBlock extends PortableTextTextBlock {
+  listItem: string
+  level: number
 }