From e465cac713a4d335a6f7ab1ac14f7224844d237f Mon Sep 17 00:00:00 2001
From: Abhishek Govindarasu <abhishekgovindarasu@gmail.com>
Date: Wed, 3 Dec 2025 00:51:20 -0800
Subject: [PATCH 1/2] fix: add seen weakset during mergeDeep

---
 src/utils.ts | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/utils.ts b/src/utils.ts
index 4a8ec09a..88ec6cbc 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -1,4 +1,3 @@
-import { isStringTextContainingNode } from 'typescript'
 import type { Sucrose } from './sucrose'
 import type { TraceHandler } from './trace'
 
@@ -59,14 +58,19 @@ export const mergeDeep = <
 		skipKeys?: string[]
 		override?: boolean
 		mergeArray?: boolean
+		seen?: WeakSet<object>
 	}
 ): A & B => {
 	const skipKeys = options?.skipKeys
 	const override = options?.override ?? true
 	const mergeArray = options?.mergeArray ?? false
+	const seen = options?.seen ?? new WeakSet<object>()
 
 	if (!isObject(target) || !isObject(source)) return target as A & B
 
+	if (seen.has(source)) return target as A & B
+	seen.add(source)
+
 	for (const [key, value] of Object.entries(source)) {
 		if (
 			skipKeys?.includes(key) ||
@@ -98,7 +102,7 @@ export const mergeDeep = <
 				target[key as keyof typeof target] = mergeDeep(
 					(target as any)[key] as any,
 					value,
-					{ skipKeys, override, mergeArray }
+					{ skipKeys, override, mergeArray, seen }
 				)
 			} catch {}
 	}

From 1819ba315afa5a08a89636cd6c33e8c58af8e0ae Mon Sep 17 00:00:00 2001
From: Abhishek Govindarasu <abhishekgovindarasu@gmail.com>
Date: Wed, 3 Dec 2025 01:02:36 -0800
Subject: [PATCH 2/2] add tests

---
 src/utils.ts                  |  2 +
 test/units/merge-deep.test.ts | 74 +++++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)

diff --git a/src/utils.ts b/src/utils.ts
index 88ec6cbc..c564990e 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -107,6 +107,8 @@ export const mergeDeep = <
 			} catch {}
 	}
 
+	seen.delete(source)
+
 	return target as A & B
 }
 export const mergeCookie = <const A extends Object, const B extends Object>(
diff --git a/test/units/merge-deep.test.ts b/test/units/merge-deep.test.ts
index f2767269..74e040a5 100644
--- a/test/units/merge-deep.test.ts
+++ b/test/units/merge-deep.test.ts
@@ -86,4 +86,78 @@ describe('mergeDeep', () => {
 			.decorate('db', Object.freeze({ hello: 'world' }))
 			.guard({}, (app) => app)
 	})
+
+	it('handle circular references', () => {
+		const a: {
+			x: number
+			toB?: typeof b
+		} = { x: 1 }
+		const b: {
+			y: number
+			toA?: typeof a
+		} = { y: 2 }
+
+		a.toB = b
+		b.toA = a
+
+		const target = {}
+		const source = { prop: a }
+
+		const result = mergeDeep(target, source)
+
+		expect(result.prop.x).toBe(1)
+		expect(result.prop.toB?.y).toBe(2)
+	})
+
+	it('handle shared references in different branches', () => {
+		const shared = { value: 123 }
+		const target = { x: {}, y: {} }
+		const source = { x: shared, y: shared }
+
+		const result = mergeDeep(target, source)
+
+		expect(result.x.value).toBe(123)
+		expect(result.y.value).toBe(123)
+	})
+
+	it('deduplicate plugin with circular decorators', async () => {
+		const a: {
+			x: number
+			toB?: typeof b
+		} = { x: 1 }
+		const b: {
+			y: number
+			toA?: typeof a
+		} = { y: 2 }
+		a.toB = b
+		b.toA = a
+
+		const complex = { a }
+
+		const Plugin = new Elysia({ name: 'Plugin', seed: 'seed' })
+			.decorate('dep', complex)
+			.as('scoped')
+
+		const ModuleA = new Elysia({ name: 'ModuleA' })
+			.use(Plugin)
+			.get('/moda/a', ({ dep }) => dep.a.x)
+			.get('/moda/b', ({ dep }) => dep.a.toB?.y)
+
+		const ModuleB = new Elysia({ name: 'ModuleB' })
+			.use(Plugin)
+			.get('/modb/a', ({ dep }) => dep.a.x)
+			.get('/modb/b', ({ dep }) => dep.a.toB?.y)
+
+		const app = new Elysia().use(ModuleA).use(ModuleB)
+
+		const resA = await app.handle(req('/moda/a')).then((x) => x.text())
+		const resB = await app.handle(req('/modb/a')).then((x) => x.text())
+		const resC = await app.handle(req('/moda/b')).then((x) => x.text())
+		const resD = await app.handle(req('/modb/b')).then((x) => x.text())
+
+		expect(resA).toBe('1')
+		expect(resB).toBe('1')
+		expect(resC).toBe('2')
+		expect(resD).toBe('2')
+	})
 })