From 8cb4e49f1b0c9c0fe7b7c4a76525254e233f855e Mon Sep 17 00:00:00 2001
From: Sebastian Lorenz <fubhy@fubhy.com>
Date: Sat, 1 Aug 2026 21:12:11 +0000
Subject: [PATCH 1/2] Add reproduction for Array issue

---
 packages/effect/test/ArrayNaNIndex.test.ts | 30 ++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 packages/effect/test/ArrayNaNIndex.test.ts

diff --git a/packages/effect/test/ArrayNaNIndex.test.ts b/packages/effect/test/ArrayNaNIndex.test.ts
new file mode 100644
index 00000000000..a7abedcffc0
--- /dev/null
+++ b/packages/effect/test/ArrayNaNIndex.test.ts
@@ -0,0 +1,30 @@
+import { assert, describe, it } from "@effect/vitest"
+import { Array as Arr, Option } from "effect"
+
+describe("Array NaN indexes", () => {
+  const input = [1, 2, 3]
+
+  it("rejects NaN in get", () => {
+    assert.deepStrictEqual(Arr.get(input, Number.NaN), Option.none())
+  })
+
+  it("rejects NaN in getUnsafe", () => {
+    assert.throws(() => Arr.getUnsafe(input, Number.NaN), /Index out of bounds/)
+  })
+
+  it("rejects NaN in insertAt", () => {
+    assert.deepStrictEqual(Arr.insertAt(input, Number.NaN, 4), Option.none())
+  })
+
+  it("rejects NaN in replace", () => {
+    assert.deepStrictEqual(Arr.replace(input, Number.NaN, 4), Option.none())
+  })
+
+  it("rejects NaN in modify", () => {
+    assert.deepStrictEqual(Arr.modify(input, Number.NaN, (n) => n * 2), Option.none())
+  })
+
+  it("treats removing NaN as an out-of-bounds no-op", () => {
+    assert.deepStrictEqual(Arr.remove(input, Number.NaN), input)
+  })
+})

From 878bc0e7a17a8e9b125c1f40c62c78f9b7b65d55 Mon Sep 17 00:00:00 2001
From: Giulio Canti <giulio.canti@gmail.com>
Date: Sun, 2 Aug 2026 09:40:03 +0200
Subject: [PATCH 2/2] Fix Array index handling

---
 .changeset/fix-array-non-finite-indexes.md |  5 +++++
 packages/effect/src/Array.ts               | 19 +++++++++++--------
 packages/effect/test/ArrayNaNIndex.test.ts | 14 ++++++++++++--
 3 files changed, 28 insertions(+), 10 deletions(-)
 create mode 100644 .changeset/fix-array-non-finite-indexes.md

diff --git a/.changeset/fix-array-non-finite-indexes.md b/.changeset/fix-array-non-finite-indexes.md
new file mode 100644
index 00000000000..dcfd9db0fd6
--- /dev/null
+++ b/.changeset/fix-array-non-finite-indexes.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fix Array index operations handling `NaN` and fractional indexes.
diff --git a/packages/effect/src/Array.ts b/packages/effect/src/Array.ts
index 35eb7002231..c35cf612a90 100644
--- a/packages/effect/src/Array.ts
+++ b/packages/effect/src/Array.ts
@@ -909,7 +909,7 @@ export const length = <A>(self: ReadonlyArray<A>): number => self.length
 
 /** @internal */
 export function isOutOfBounds<A>(i: number, as: ReadonlyArray<A>): boolean {
-  return i < 0 || i >= as.length
+  return !Number.isFinite(i) || i < 0 || i >= as.length
 }
 
 const clamp = <A>(i: number, as: ReadonlyArray<A>): number => Math.floor(Math.min(Math.max(0, i), as.length))
@@ -1875,10 +1875,11 @@ export const insertAt: {
   <A, B>(self: Iterable<A>, i: number, b: B): Option.Option<NonEmptyArray<A | B>>
 } = dual(3, <A, B>(self: Iterable<A>, i: number, b: B): Option.Option<NonEmptyArray<A | B>> => {
   const out: Array<A | B> = Array.from(self) // copy because `splice` mutates the array
-  if (i < 0 || i > out.length) {
+  const index = Math.floor(i)
+  if (index !== out.length && isOutOfBounds(index, out)) {
     return Option.none()
   }
-  out.splice(i, 0, b)
+  out.splice(index, 0, b)
   return Option.some(out as any)
 })
 
@@ -1966,12 +1967,13 @@ export const modify: {
   ): Option.Option<ReadonlyArray.With<S, ReadonlyArray.Infer<S> | B>>
 } = dual(3, <A, B>(self: Iterable<A>, i: number, f: (a: A) => B): Option.Option<Array<A | B>> => {
   const arr = Array.from(self)
-  if (isOutOfBounds(i, arr)) {
+  const index = Math.floor(i)
+  if (isOutOfBounds(index, arr)) {
     return Option.none()
   }
   const out: Array<A | B> = arr
-  const b = f(arr[i])
-  out[i] = b
+  const b = f(arr[index])
+  out[index] = b
   return Option.some(out)
 })
 
@@ -2004,10 +2006,11 @@ export const remove: {
   <A>(self: Iterable<A>, i: number): Array<A>
 } = dual(2, <A>(self: Iterable<A>, i: number): Array<A> => {
   const out = Array.from(self)
-  if (isOutOfBounds(i, out)) {
+  const index = Math.floor(i)
+  if (isOutOfBounds(index, out)) {
     return out
   }
-  out.splice(i, 1)
+  out.splice(index, 1)
   return out
 })
 
diff --git a/packages/effect/test/ArrayNaNIndex.test.ts b/packages/effect/test/ArrayNaNIndex.test.ts
index a7abedcffc0..9c5f258c338 100644
--- a/packages/effect/test/ArrayNaNIndex.test.ts
+++ b/packages/effect/test/ArrayNaNIndex.test.ts
@@ -1,9 +1,9 @@
 import { assert, describe, it } from "@effect/vitest"
 import { Array as Arr, Option } from "effect"
 
-describe("Array NaN indexes", () => {
-  const input = [1, 2, 3]
+const input = [1, 2, 3]
 
+describe("Array NaN indexes", () => {
   it("rejects NaN in get", () => {
     assert.deepStrictEqual(Arr.get(input, Number.NaN), Option.none())
   })
@@ -28,3 +28,13 @@ describe("Array NaN indexes", () => {
     assert.deepStrictEqual(Arr.remove(input, Number.NaN), input)
   })
 })
+
+describe("Array fractional indexes", () => {
+  it("floors the index in replace", () => {
+    assert.deepStrictEqual(Arr.replace(input, 1.5, 4), Option.some([1, 4, 3]))
+  })
+
+  it("floors the index in modify", () => {
+    assert.deepStrictEqual(Arr.modify(input, 1.5, (n) => n * 2), Option.some([1, 4, 3]))
+  })
+})