From bd05883c0bcca06d15565cb4efb612c83472b0e3 Mon Sep 17 00:00:00 2001
From: coyaSONG <66289470+coyaSONG@users.noreply.github.com>
Date: Fri, 17 Jul 2026 16:50:28 +0900
Subject: [PATCH] Avoid copying FiberRefs for unchanged updates

---
 .changeset/fuzzy-spiders-smile.md         |  5 ++++
 packages/effect/src/internal/fiberRefs.ts | 36 +++++++++++++++--------
 packages/effect/test/FiberRefs.test.ts    | 26 ++++++++++++++++
 3 files changed, 55 insertions(+), 12 deletions(-)
 create mode 100644 .changeset/fuzzy-spiders-smile.md

diff --git a/.changeset/fuzzy-spiders-smile.md b/.changeset/fuzzy-spiders-smile.md
new file mode 100644
index 00000000000..dcc2681fa7f
--- /dev/null
+++ b/.changeset/fuzzy-spiders-smile.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Avoid copying all FiberRefs when an update keeps the current fiber's value unchanged.
diff --git a/packages/effect/src/internal/fiberRefs.ts b/packages/effect/src/internal/fiberRefs.ts
index 8d9ece313bb..ddaafc84f21 100644
--- a/packages/effect/src/internal/fiberRefs.ts
+++ b/packages/effect/src/internal/fiberRefs.ts
@@ -210,45 +210,57 @@ export const updateAs = dual<
   readonly fiberRef: FiberRef.FiberRef<A>
   readonly value: A
 }) => {
+  const newStack = updatedStack(self.locals, fiberId, fiberRef, value)
+  if (newStack === undefined) {
+    return self
+  }
   if (self.locals.size === 0) {
-    return new FiberRefsImpl(new Map([[fiberRef, [[fiberId, value] as const]]]))
+    return new FiberRefsImpl(new Map([[fiberRef, newStack]]))
   }
   const locals = new Map(self.locals)
-  unsafeUpdateAs(locals, fiberId, fiberRef, value)
+  locals.set(fiberRef, newStack)
   return new FiberRefsImpl(locals)
 })
 
-const unsafeUpdateAs = (
+const updatedStack = (
   locals: Map<FiberRef.FiberRef<any>, Arr.NonEmptyReadonlyArray<readonly [FiberId.Single, any]>>,
   fiberId: FiberId.Single,
   fiberRef: FiberRef.FiberRef<any>,
   value: any
-) => {
+): Arr.NonEmptyReadonlyArray<readonly [FiberId.Single, any]> | undefined => {
   const oldStack: ReadonlyArray<readonly [FiberId.Single, any]> = locals.get(fiberRef) ?? []
-  let newStack: Arr.NonEmptyReadonlyArray<readonly [FiberId.Single, any]> | undefined
-
   if (Arr.isNonEmptyReadonlyArray(oldStack)) {
     const [currentId, currentValue] = Arr.headNonEmpty(oldStack)
     if (currentId[Equal.symbol](fiberId)) {
       if (Equal.equals(currentValue, value)) {
-        return
+        return undefined
       } else {
-        newStack = [
+        return [
           [fiberId, value] as const,
           ...oldStack.slice(1)
         ]
       }
     } else {
-      newStack = [
+      return [
         [fiberId, value] as const,
         ...oldStack
       ]
     }
-  } else {
-    newStack = [[fiberId, value] as const]
   }
 
-  locals.set(fiberRef, newStack)
+  return [[fiberId, value] as const]
+}
+
+const unsafeUpdateAs = (
+  locals: Map<FiberRef.FiberRef<any>, Arr.NonEmptyReadonlyArray<readonly [FiberId.Single, any]>>,
+  fiberId: FiberId.Single,
+  fiberRef: FiberRef.FiberRef<any>,
+  value: any
+) => {
+  const newStack = updatedStack(locals, fiberId, fiberRef, value)
+  if (newStack !== undefined) {
+    locals.set(fiberRef, newStack)
+  }
 }
 
 /** @internal */
diff --git a/packages/effect/test/FiberRefs.test.ts b/packages/effect/test/FiberRefs.test.ts
index e6a77032e7b..c0685ef8468 100644
--- a/packages/effect/test/FiberRefs.test.ts
+++ b/packages/effect/test/FiberRefs.test.ts
@@ -33,6 +33,32 @@ describe("FiberRefs", () => {
     deepStrictEqual(FiberRefs.get(newParentFiberRefs, FiberRef.interruptedCause), Option.some(Cause.empty))
   })
 
+  it("updateAs reuses unchanged FiberRefs for the same fiber", () => {
+    const fiberId = FiberId.make(1, 0) as FiberId.Runtime
+    const childId = FiberId.make(2, 1) as FiberId.Runtime
+    const fiberRef = FiberRef.unsafeMake(0)
+    const fiberRefs = FiberRefs.updateAs(FiberRefs.empty(), {
+      fiberId,
+      fiberRef,
+      value: 1
+    })
+
+    const unchanged = FiberRefs.updateAs(fiberRefs, {
+      fiberId,
+      fiberRef,
+      value: 1
+    })
+    strictEqual(unchanged, fiberRefs)
+
+    const childFiberRefs = FiberRefs.updateAs(fiberRefs, {
+      fiberId: childId,
+      fiberRef,
+      value: 1
+    })
+    assertTrue(childFiberRefs !== fiberRefs)
+    strictEqual(childFiberRefs.locals.get(fiberRef)?.length, 2)
+  })
+
   describe("currentLogAnnotations", () => {
     it("doesnt leak", () => {
       Effect.void.pipe(Effect.annotateLogs("test", "abc"), Effect.runSync)