From f8f716d186299fb5db1fe446715569310011fb6f Mon Sep 17 00:00:00 2001
From: Sebastian Lorenz <fubhy@fubhy.com>
Date: Sat, 1 Aug 2026 20:48:51 +0000
Subject: [PATCH 1/2] Add reproduction for Sink issue

---
 packages/effect/test/Sink.test.ts | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/packages/effect/test/Sink.test.ts b/packages/effect/test/Sink.test.ts
index 6b2fbaba705..680eb0c8374 100644
--- a/packages/effect/test/Sink.test.ts
+++ b/packages/effect/test/Sink.test.ts
@@ -106,6 +106,18 @@ describe("Sink", () => {
         strictEqual(result, 45)
       }))
   })
+
+  describe("reduceWhileArray", () => {
+    it.effect("applies the reducer once per non-empty input array", () =>
+      Effect.gen(function*() {
+        const result = yield* Stream.fromArrays([1, 2, 3]).pipe(
+          Stream.run(Sink.reduceWhileArray(() => 0, constTrue, (count) => count + 1))
+        )
+
+        strictEqual(result, 1, "the reducer must run once for each input array")
+      }))
+  })
+
   describe("reduceWhileEffect", () => {
     it.effect("short circuits", () =>
       Effect.gen(function*() {

From 33a6afa22217c6a36b8d19d7848a7b2988037c15 Mon Sep 17 00:00:00 2001
From: Tim Smart <hello@timsmart.co>
Date: Mon, 3 Aug 2026 10:49:53 +1200
Subject: [PATCH 2/2] Fix Sink reduceWhileArray reducer calls

---
 .changeset/calm-wolves-reduce.md | 5 +++++
 packages/effect/src/Sink.ts      | 8 +++-----
 2 files changed, 8 insertions(+), 5 deletions(-)
 create mode 100644 .changeset/calm-wolves-reduce.md

diff --git a/.changeset/calm-wolves-reduce.md b/.changeset/calm-wolves-reduce.md
new file mode 100644
index 00000000000..1b255645666
--- /dev/null
+++ b/.changeset/calm-wolves-reduce.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fix `Sink.reduceWhileArray` applying its reducer more than once per input array.
diff --git a/packages/effect/src/Sink.ts b/packages/effect/src/Sink.ts
index 9bb15d78140..47865c92d9f 100644
--- a/packages/effect/src/Sink.ts
+++ b/packages/effect/src/Sink.ts
@@ -1336,11 +1336,9 @@ export const reduceWhileArray = <S, In>(
     }
     return upstream.pipe(
       Effect.flatMap((arr) => {
-        for (let i = 0; i < arr.length; i++) {
-          state = f(state, arr)
-          if (!contFn(state)) {
-            return Cause.done()
-          }
+        state = f(state, arr)
+        if (!contFn(state)) {
+          return Cause.done()
         }
         return Effect.void
       }),