diff --git a/.changeset/fix-single-row-optional-ref-proxy.md b/.changeset/fix-single-row-optional-ref-proxy.md
new file mode 100644
index 0000000000..e8911f6375
--- /dev/null
+++ b/.changeset/fix-single-row-optional-ref-proxy.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/db': patch
+---
+
+Allow collection index and change-filter callbacks to traverse optional or nullable nested plain objects with optional chaining while preserving built-in values and functions as query leaves.
diff --git a/packages/db/src/query/builder/ref-proxy.ts b/packages/db/src/query/builder/ref-proxy.ts
index 121c0da40e..3b19330871 100644
--- a/packages/db/src/query/builder/ref-proxy.ts
+++ b/packages/db/src/query/builder/ref-proxy.ts
@@ -1,6 +1,6 @@
 import { PropRef, Value } from '../ir.js'
 import type { BasicExpression } from '../ir.js'
-import type { RefLeaf } from './types.js'
+import type { IsPlainObject, RefLeaf } from './types.js'
 import type { VirtualRowProps } from '../../virtual-props.js'
 
 export interface RefProxy<T = any> {
@@ -22,6 +22,18 @@ export type VirtualPropsRefProxy<
   readonly [K in keyof VirtualRowProps<TKey>]: RefLeaf<VirtualRowProps<TKey>[K]>
 }
 
+// Strip nullish members before deciding whether a schema field is traversable,
+// then restore them outside the proxy so the schema still requires a guard.
+// The tuple guard keeps exact null/undefined fields as leaves: `never` would
+// otherwise satisfy the plain-object check.
+type SingleRowField<V, TKey extends string | number> = [
+  NonNullable<V>,
+] extends [never]
+  ? RefLeaf<V>
+  : IsPlainObject<NonNullable<V>> extends true
+    ? SingleRowRefProxy<NonNullable<V>, TKey> | Extract<V, null | undefined>
+    : RefLeaf<V>
+
 /**
  * Type for creating a RefProxy for a single row/type without namespacing
  * Used in collection indexes and where clauses
@@ -35,9 +47,7 @@ export type SingleRowRefProxy<
 > =
   T extends Record<string, any>
     ? {
-        [K in keyof T]: T[K] extends Record<string, any>
-          ? SingleRowRefProxy<T[K], TKey> & RefProxy<T[K]>
-          : RefLeaf<T[K]>
+        [K in keyof T]: SingleRowField<T[K], TKey>
       } & RefProxy<T> &
         VirtualPropsRefProxy<TKey>
     : RefProxy<T> & VirtualPropsRefProxy<TKey>
diff --git a/packages/db/src/query/builder/types.ts b/packages/db/src/query/builder/types.ts
index 2651f1fe3f..00af2e97e3 100644
--- a/packages/db/src/query/builder/types.ts
+++ b/packages/db/src/query/builder/types.ts
@@ -1320,7 +1320,7 @@ export type Prettify<T> = {
  * - Objects with `Symbol.toStringTag` (class instances like Temporal types,
  *   TypedArrays not already in JsBuiltIns, etc.) — these are not plain data objects
  */
-type IsPlainObject<T> = T extends unknown
+export type IsPlainObject<T> = T extends unknown
   ? T extends object
     ? T extends ReadonlyArray<any>
       ? false
diff --git a/packages/db/tests/query/builder/ref-proxy.test.ts b/packages/db/tests/query/builder/ref-proxy.test.ts
index ec41351daf..e003d34d07 100644
--- a/packages/db/tests/query/builder/ref-proxy.test.ts
+++ b/packages/db/tests/query/builder/ref-proxy.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it } from 'vitest'
 import {
   createRefProxy,
+  createSingleRowRefProxy,
   isRefProxy,
   toExpression,
   val,
@@ -8,6 +9,28 @@ import {
 import { PropRef, Value } from '../../../src/query/ir.js'
 
 describe(`ref-proxy`, () => {
+  describe(`createSingleRowRefProxy`, () => {
+    it(`records a nested path through an optional schema field`, () => {
+      type Row = { timestamp?: { seconds: number } }
+      const proxy = createSingleRowRefProxy<Row>()
+
+      const expression = toExpression(proxy.timestamp?.seconds)
+
+      expect(expression).toBeInstanceOf(PropRef)
+      expect((expression as PropRef).path).toEqual([`timestamp`, `seconds`])
+    })
+
+    it(`records built-in method paths only when the type boundary is bypassed`, () => {
+      type Row = { updatedAt?: Date }
+      const proxy = createSingleRowRefProxy<Row>()
+
+      const expression = toExpression((proxy as any).updatedAt?.getTime)
+
+      expect(expression).toBeInstanceOf(PropRef)
+      expect((expression as PropRef).path).toEqual([`updatedAt`, `getTime`])
+    })
+  })
+
   describe(`createRefProxy`, () => {
     it(`creates a proxy with correct basic properties`, () => {
       const proxy = createRefProxy<{ users: { id: number; name: string } }>([
diff --git a/packages/db/tests/single-row-ref-proxy.test-d.ts b/packages/db/tests/single-row-ref-proxy.test-d.ts
new file mode 100644
index 0000000000..ed89b5af5c
--- /dev/null
+++ b/packages/db/tests/single-row-ref-proxy.test-d.ts
@@ -0,0 +1,256 @@
+/**
+ * Oracle owner: SingleRowRefProxy compile-time projection.
+ *
+ * Runtime callbacks receive one truthy path-recording proxy. Schema-level
+ * nullishness nevertheless owns whether nested object access requires optional
+ * chaining. TypeScript's structural assignability and `@ts-expect-error` are
+ * the independent judges; the paired runtime test owns the recorded path.
+ */
+import { describe, expectTypeOf, test } from 'vitest'
+import { createCollection } from '../src/collection/index.js'
+import { eq, isNull } from '../src/query/builder/functions.js'
+import type { Collection } from '../src/collection/index.js'
+import type { SingleRowRefProxy } from '../src/query/builder/ref-proxy.js'
+import type { RefLeaf } from '../src/query/builder/types.js'
+
+interface Timestamp {
+  seconds: number
+  nanoseconds: number
+}
+
+type Variant =
+  | { kind: `created`; payload: { author: string } }
+  | { kind: `deleted`; payload: { reason: string } }
+
+type Row = {
+  id: string
+  name: string
+  nickname?: string
+  deletedBy: string | null
+  optionalTimestamp?: Timestamp
+  nullableTimestamp: Timestamp | null
+  nullishTimestamp?: Timestamp | null
+  nested: { timestamp?: Timestamp }
+  exactNull: null
+  exactUndefined: undefined
+  variant?: Variant
+  mixed: Timestamp | string | undefined
+  requiredDate: Date
+  optionalDate?: Date
+  nullableDate: Date | null
+  requiredTags: Array<string>
+  optionalTags?: Array<string>
+  nullableTags: Array<string> | null
+  requiredMap: Map<string, number>
+  optionalMap?: Map<string, number>
+  nullableMap: Map<string, number> | null
+  requiredCallback: () => string
+  optionalCallback?: () => string
+  nullableCallback: (() => string) | null
+  mixedObjects: Timestamp | Date | null
+}
+
+const collection = createCollection<Row, string>({
+  getKey: (row) => row.id,
+  sync: { sync: () => {} },
+})
+
+describe(`SingleRowRefProxy type algebra`, () => {
+  test(`optional and nullable objects remain traversable with schema guards`, () => {
+    collection.createIndex((row) => {
+      expectTypeOf(row.optionalTimestamp).toEqualTypeOf<
+        SingleRowRefProxy<Timestamp> | undefined
+      >()
+      expectTypeOf(row.optionalTimestamp?.seconds).toEqualTypeOf<
+        RefLeaf<number> | undefined
+      >()
+      expectTypeOf(
+        row.nullableTimestamp,
+      ).toEqualTypeOf<SingleRowRefProxy<Timestamp> | null>()
+      expectTypeOf(row.nullableTimestamp?.seconds).toEqualTypeOf<
+        RefLeaf<number> | undefined
+      >()
+      expectTypeOf(row.nullishTimestamp).toEqualTypeOf<
+        SingleRowRefProxy<Timestamp> | null | undefined
+      >()
+      expectTypeOf(row.nested.timestamp?.nanoseconds).toEqualTypeOf<
+        RefLeaf<number> | undefined
+      >()
+
+      // @ts-expect-error Optional objects require a guard before traversal.
+      row.optionalTimestamp.seconds
+      // @ts-expect-error Nullable objects require a guard before traversal.
+      row.nullableTimestamp.seconds
+      // @ts-expect-error Unknown nested keys must not be fabricated.
+      row.optionalTimestamp?.milliseconds
+
+      return row.optionalTimestamp?.seconds
+    })
+  })
+
+  test(`scalar and exact-nullish leaves retain their declared domains`, () => {
+    collection.createIndex((row) => {
+      expectTypeOf(row.name).toEqualTypeOf<RefLeaf<string>>()
+      expectTypeOf(row.nickname).toEqualTypeOf<
+        RefLeaf<string | undefined> | undefined
+      >()
+      expectTypeOf(row.deletedBy).toEqualTypeOf<RefLeaf<string | null>>()
+      expectTypeOf(row.exactNull).toEqualTypeOf<RefLeaf<null>>()
+      expectTypeOf(row.exactUndefined).toEqualTypeOf<RefLeaf<undefined>>()
+
+      // @ts-expect-error Scalar leaves are not traversable objects.
+      row.nickname?.length
+      // @ts-expect-error Exact null does not acquire object fields.
+      row.exactNull.value
+
+      return row.name
+    })
+  })
+
+  test(`JavaScript built-ins and functions remain scalar leaves`, () => {
+    collection.createIndex((row) => {
+      expectTypeOf(row.requiredDate).toEqualTypeOf<RefLeaf<Date>>()
+      expectTypeOf(row.optionalDate).toEqualTypeOf<
+        RefLeaf<Date | undefined> | undefined
+      >()
+      expectTypeOf(row.nullableDate).toEqualTypeOf<RefLeaf<Date | null>>()
+
+      expectTypeOf(row.requiredTags).toEqualTypeOf<RefLeaf<Array<string>>>()
+      expectTypeOf(row.optionalTags).toEqualTypeOf<
+        RefLeaf<Array<string> | undefined> | undefined
+      >()
+      expectTypeOf(row.nullableTags).toEqualTypeOf<
+        RefLeaf<Array<string> | null>
+      >()
+
+      expectTypeOf(row.requiredMap).toEqualTypeOf<
+        RefLeaf<Map<string, number>>
+      >()
+      expectTypeOf(row.optionalMap).toEqualTypeOf<
+        RefLeaf<Map<string, number> | undefined> | undefined
+      >()
+      expectTypeOf(row.nullableMap).toEqualTypeOf<
+        RefLeaf<Map<string, number> | null>
+      >()
+
+      expectTypeOf(row.requiredCallback).toEqualTypeOf<RefLeaf<() => string>>()
+      expectTypeOf(row.optionalCallback).toEqualTypeOf<
+        RefLeaf<(() => string) | undefined> | undefined
+      >()
+      expectTypeOf(row.nullableCallback).toEqualTypeOf<
+        RefLeaf<(() => string) | null>
+      >()
+
+      // @ts-expect-error Date methods are value behavior, not query paths.
+      row.requiredDate.getTime
+      // @ts-expect-error Optional Date methods are not traversable query paths.
+      row.optionalDate?.toISOString
+      // @ts-expect-error Array members are not traversable query paths.
+      row.optionalTags?.length
+      // @ts-expect-error Map methods are not traversable query paths.
+      row.nullableMap.get
+      // @ts-expect-error Function values remain leaves, not callable proxies.
+      row.requiredCallback()
+
+      return row.requiredDate
+    })
+  })
+
+  test(`leaf-compatible helpers retain required and nullish built-ins`, () => {
+    collection.createIndex((row) => {
+      const requiredDate: RefLeaf<Date> = row.requiredDate
+      const nullableDate: RefLeaf<Date | null> = row.nullableDate
+      const optionalDate: RefLeaf<Date | undefined> | undefined =
+        row.optionalDate
+      const requiredTags: RefLeaf<Array<string>> = row.requiredTags
+      const optionalTags: RefLeaf<Array<string> | undefined> | undefined =
+        row.optionalTags
+      const nullableMap: RefLeaf<Map<string, number> | null> = row.nullableMap
+      const optionalCallback: RefLeaf<(() => string) | undefined> | undefined =
+        row.optionalCallback
+
+      void [
+        requiredDate,
+        nullableDate,
+        optionalDate,
+        requiredTags,
+        optionalTags,
+        nullableMap,
+        optionalCallback,
+      ]
+      return row.id
+    })
+  })
+
+  test(`direct expression consumers continue accepting built-in leaves`, () => {
+    collection.createIndex((row) => {
+      const equality = eq(row.nullableDate, new Date(0))
+      const nullCheck = isNull(row.optionalDate)
+      void [equality, nullCheck]
+      return row.requiredDate
+    })
+
+    collection.subscribeChanges(() => {}, {
+      where: (row) => isNull(row.nullableMap),
+    })
+  })
+
+  test(`object unions expose shared structure without inventing variant keys`, () => {
+    collection.createIndex((row) => {
+      expectTypeOf(row.variant?.kind).toEqualTypeOf<
+        RefLeaf<`created`> | RefLeaf<`deleted`> | undefined
+      >()
+
+      // Ref leaves record expressions; they do not provide value-level
+      // discriminant narrowing for sibling proxy fields.
+      // @ts-expect-error Variant-only nested fields remain unavailable.
+      row.variant?.payload.author
+      // @ts-expect-error Mixed object/scalar unions are opaque leaves.
+      row.mixed.seconds
+      expectTypeOf(row.mixedObjects).toEqualTypeOf<
+        RefLeaf<Timestamp | Date | null>
+      >()
+      // @ts-expect-error Mixed plain/built-in object unions stay opaque.
+      row.mixedObjects.seconds
+
+      return row.variant?.kind
+    })
+  })
+
+  test(`constrained generic optional objects retain guaranteed fields`, () => {
+    function addTimestampIndex<T extends { id: string; timestamp?: Timestamp }>(
+      rows: Collection<T, string>,
+    ) {
+      rows.createIndex((row) => row.timestamp?.seconds)
+      rows.subscribeChanges(() => {}, {
+        where: (row) => eq(row.timestamp?.nanoseconds, 0),
+      })
+    }
+
+    void addTimestampIndex
+
+    function keepBuiltInLeaves<
+      T extends {
+        id: string
+        date?: Date
+        tags: Array<string> | null
+      },
+    >(rows: Collection<T, string>) {
+      rows.createIndex((row) => {
+        const date: RefLeaf<Date | undefined> | undefined = row.date
+        const tags: RefLeaf<Array<string> | null> = row.tags
+        void [date, tags]
+        return row.id
+      })
+    }
+
+    void keepBuiltInLeaves
+  })
+
+  test(`all public SingleRowRefProxy callback paths share the same projection`, () => {
+    collection.createIndex((row) => row.optionalTimestamp?.seconds)
+    collection.subscribeChanges(() => {}, {
+      where: (row) => eq(row.nullishTimestamp?.nanoseconds, 0),
+    })
+  })
+})