diff --git a/src/config/config-paths.test.ts b/src/config/config-paths.test.ts
new file mode 100644
index 000000000000..4947a20b16f5
--- /dev/null
+++ b/src/config/config-paths.test.ts
@@ -0,0 +1,91 @@
+import { describe, expect, it, vi } from "vitest";
+import {
+  getConfigValueAtPath,
+  setConfigValueAtPath,
+  unsetConfigValueAtPath,
+} from "./config-paths.js";
+
+describe("config path own-property traversal", () => {
+  for (const key of ["toString", "valueOf", "hasOwnProperty"]) {
+    it(`does not treat inherited ${key} as config`, () => {
+      const parent: Record<string, unknown> = {};
+      const root: Record<string, unknown> = { parent };
+
+      expect(getConfigValueAtPath(root, ["parent", key])).toBeUndefined();
+      expect(unsetConfigValueAtPath(root, ["parent", key])).toBe(false);
+      expect(root).toEqual({ parent: {} });
+
+      setConfigValueAtPath(root, ["parent", key], "own");
+      expect(Object.hasOwn(parent, key)).toBe(true);
+      expect(getConfigValueAtPath(root, ["parent", key])).toBe("own");
+      expect(unsetConfigValueAtPath(root, ["parent", key])).toBe(true);
+      expect(root).toEqual({});
+    });
+  }
+
+  it("replaces an inherited parent instead of traversing it", () => {
+    const prototypeBranch = { leaf: "prototype" };
+    const root = Object.create({ branch: prototypeBranch }) as Record<string, unknown>;
+
+    expect(getConfigValueAtPath(root, ["branch", "leaf"])).toBeUndefined();
+    expect(unsetConfigValueAtPath(root, ["branch", "leaf"])).toBe(false);
+    expect(prototypeBranch).toEqual({ leaf: "prototype" });
+
+    setConfigValueAtPath(root, ["branch", "leaf"], "own");
+    expect(Object.hasOwn(root, "branch")).toBe(true);
+    expect(root.branch).toEqual({ leaf: "own" });
+    expect(prototypeBranch).toEqual({ leaf: "prototype" });
+
+    expect(unsetConfigValueAtPath(root, ["branch", "leaf"])).toBe(true);
+    expect(Object.hasOwn(root, "branch")).toBe(false);
+    expect(getConfigValueAtPath(root, ["branch", "leaf"])).toBeUndefined();
+    expect(prototypeBranch).toEqual({ leaf: "prototype" });
+  });
+
+  for (const inheritedKind of ["setter", "non-writable"] as const) {
+    it(`creates an own parent over an inherited ${inheritedKind} property`, () => {
+      const setter = vi.fn();
+      const prototype = {};
+      Object.defineProperty(
+        prototype,
+        "branch",
+        inheritedKind === "setter"
+          ? { configurable: true, set: setter }
+          : { configurable: true, value: { leaf: "prototype" }, writable: false },
+      );
+      const root = Object.create(prototype) as Record<string, unknown>;
+
+      expect(() => setConfigValueAtPath(root, ["branch", "leaf"], "own")).not.toThrow();
+      expect(setter).not.toHaveBeenCalled();
+      expect(Object.getOwnPropertyDescriptor(root, "branch")).toMatchObject({
+        configurable: true,
+        enumerable: true,
+        value: { leaf: "own" },
+        writable: true,
+      });
+    });
+
+    it(`creates an own leaf over an inherited ${inheritedKind} property`, () => {
+      const setter = vi.fn();
+      const prototype = {};
+      Object.defineProperty(
+        prototype,
+        "leaf",
+        inheritedKind === "setter"
+          ? { configurable: true, set: setter }
+          : { configurable: true, value: "prototype", writable: false },
+      );
+      const parent = Object.create(prototype) as Record<string, unknown>;
+      const root = { parent };
+
+      expect(() => setConfigValueAtPath(root, ["parent", "leaf"], "own")).not.toThrow();
+      expect(setter).not.toHaveBeenCalled();
+      expect(Object.getOwnPropertyDescriptor(parent, "leaf")).toMatchObject({
+        configurable: true,
+        enumerable: true,
+        value: "own",
+        writable: true,
+      });
+    });
+  }
+});
diff --git a/src/config/config-paths.ts b/src/config/config-paths.ts
index dee4feda26e0..6037dde34bd8 100644
--- a/src/config/config-paths.ts
+++ b/src/config/config-paths.ts
@@ -1,9 +1,22 @@
+import { isBlockedObjectKey } from "../infra/prototype-keys.js";
 // Resolves and classifies config paths for reads, writes, and metadata.
 import { isPlainObject } from "../utils.js";
-import { isBlockedObjectKey } from "../infra/prototype-keys.js";
 
 type PathNode = Record<string, unknown>;
 
+function setOwnConfigProperty(node: PathNode, key: string, value: unknown): void {
+  if (Object.hasOwn(node, key)) {
+    node[key] = value;
+    return;
+  }
+  Object.defineProperty(node, key, {
+    configurable: true,
+    enumerable: true,
+    value,
+    writable: true,
+  });
+}
+
 /** Parses CLI/config dot-notation paths and rejects unsafe object-key segments. */
 export function parseConfigPath(raw: string): {
   ok: boolean;
@@ -37,13 +50,14 @@ export function setConfigValueAtPath(root: PathNode, path: string[], value: unkn
   let cursor: PathNode = root;
   for (let idx = 0; idx < path.length - 1; idx += 1) {
     const key = path[idx];
-    const next = cursor[key];
-    if (!isPlainObject(next)) {
-      cursor[key] = {};
+    const existing = Object.hasOwn(cursor, key) ? cursor[key] : undefined;
+    const next: PathNode = isPlainObject(existing) ? existing : {};
+    if (next !== existing) {
+      setOwnConfigProperty(cursor, key, next);
     }
-    cursor = cursor[key] as PathNode;
+    cursor = next;
   }
-  cursor[path[path.length - 1]] = value;
+  setOwnConfigProperty(cursor, path[path.length - 1], value);
 }
 
 /** Removes a value at a config path and prunes empty parent objects created by setters. */
@@ -52,6 +66,9 @@ export function unsetConfigValueAtPath(root: PathNode, path: string[]): boolean
   let cursor: PathNode = root;
   for (let idx = 0; idx < path.length - 1; idx += 1) {
     const key = path[idx];
+    if (!Object.hasOwn(cursor, key)) {
+      return false;
+    }
     const next = cursor[key];
     if (!isPlainObject(next)) {
       return false;
@@ -60,7 +77,7 @@ export function unsetConfigValueAtPath(root: PathNode, path: string[]): boolean
     cursor = next;
   }
   const leafKey = path[path.length - 1];
-  if (!(leafKey in cursor)) {
+  if (!Object.hasOwn(cursor, leafKey)) {
     return false;
   }
   delete cursor[leafKey];
@@ -82,7 +99,7 @@ export function unsetConfigValueAtPath(root: PathNode, path: string[]): boolean
 export function getConfigValueAtPath(root: PathNode, path: string[]): unknown {
   let cursor: unknown = root;
   for (const key of path) {
-    if (!isPlainObject(cursor)) {
+    if (!isPlainObject(cursor) || !Object.hasOwn(cursor, key)) {
       return undefined;
     }
     cursor = cursor[key];