From 2a849c0fef5d803cec83655b6a2729db15d2cfb3 Mon Sep 17 00:00:00 2001
From: Kit Langton <kit.langton@gmail.com>
Date: Fri, 28 Aug 2026 23:20:20 -0400
Subject: [PATCH] fix(codemode): close symbol cycle gaps

---
 .../codemode/src/interpreter/references.ts    |  2 +-
 packages/codemode/src/stdlib/object.ts        |  1 +
 packages/codemode/test/stdlib.test.ts         | 25 +++++++++++++++++++
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/packages/codemode/src/interpreter/references.ts b/packages/codemode/src/interpreter/references.ts
index 77928e0e638..2682d81585b 100644
--- a/packages/codemode/src/interpreter/references.ts
+++ b/packages/codemode/src/interpreter/references.ts
@@ -109,7 +109,7 @@ export const rejectCircularInsertion = (container: object, value: unknown, label
       throw new InterpreterRuntimeError(`${label} contains a circular value.`, node, "InvalidDataValue")
     if (current === null || typeof current !== "object" || isRuntimeReference(current) || seen.has(current)) continue
     seen.add(current)
-    pending.push(Array.isArray(current) ? current[Symbol.iterator]() : childValues(current))
+    pending.push(childValues(current))
   }
 }
 
diff --git a/packages/codemode/src/stdlib/object.ts b/packages/codemode/src/stdlib/object.ts
index 6d54b1ca34e..bb7c91f7dc1 100644
--- a/packages/codemode/src/stdlib/object.ts
+++ b/packages/codemode/src/stdlib/object.ts
@@ -70,6 +70,7 @@ export const invokeObjectMethod = (name: string, args: Array<unknown>, node: Ast
             continue
           }
           if (key !== AsyncIteratorSymbol && key !== IteratorSymbol) continue
+          if (!Object.prototype.propertyIsEnumerable.call(source, key)) continue
           const item = Reflect.get(source, key)
           rejectCircularInsertion(out, item, "Object.assign result", node)
           Reflect.set(out, key, item)
diff --git a/packages/codemode/test/stdlib.test.ts b/packages/codemode/test/stdlib.test.ts
index f3b455eba2d..65208511d74 100644
--- a/packages/codemode/test/stdlib.test.ts
+++ b/packages/codemode/test/stdlib.test.ts
@@ -17,6 +17,8 @@
 import { describe, expect, test } from "bun:test"
 import { Effect, Schema } from "effect"
 import { CodeMode, Tool } from "../src/index.js"
+import { IteratorSymbol } from "../src/interpreter/model.js"
+import { invokeObjectMethod } from "../src/stdlib/object.js"
 
 // Standard-library value types: Date, RegExp, Map, Set. Programs use them as ordinary JS;
 // intra-CodeMode checkpoints (Object.* helpers, spread, coercion inputs) preserve the live
@@ -824,6 +826,29 @@ describe("stdlib integration", () => {
     expect(await value(`try { Object.assign(null, { a: 1 }); return false } catch { return true }`)).toBe(true)
   })
 
+  test("Object.assign ignores non-enumerable supported symbols without reading them", () => {
+    const target = {}
+    const reads: Array<boolean> = []
+    const source = Object.defineProperty({}, IteratorSymbol, {
+      get() {
+        reads.push(true)
+        return target
+      },
+    })
+    expect(invokeObjectMethod("assign", [target, source], { type: "CallExpression" })).toBe(target)
+    expect(reads).toEqual([])
+    expect(Object.hasOwn(target, IteratorSymbol)).toBe(false)
+  })
+
+  test("Object.assign rejects cycles through supported symbols on nested arrays", () => {
+    const target = {}
+    const nested = Object.defineProperty([], IteratorSymbol, { enumerable: true, value: target })
+    expect(() => invokeObjectMethod("assign", [target, { nested }], { type: "CallExpression" })).toThrow(
+      "Object.assign result contains a circular value.",
+    )
+    expect(Object.hasOwn(target, "nested")).toBe(false)
+  })
+
   test("Object.assign rejects direct and nested cycles", async () => {
     expect(
       await value(`