diff --git a/src/lib/handlers/api-manager.mjs b/src/lib/handlers/api-manager.mjs
index 0b365986..011bc156 100644
--- a/src/lib/handlers/api-manager.mjs
+++ b/src/lib/handlers/api-manager.mjs
@@ -56,6 +56,16 @@ import { isFrameworkInternal } from "#handlers/framework-internals";
 // the filesystem-backed add/reload/remove paths are Node-only and never run in browser mode.
 import { fsp, path } from "@cldmv/slothlet/helpers/platform";
 
+/**
+ * Mount-path segment names that must never be written through to the object graph.
+ * Assigning to any of these while walking `current[segment] = …` mutates `Object.prototype`
+ * (or `Function.prototype`) globally — classic prototype pollution — instead of the api tree.
+ * They are refused at path-normalization time in any segment position (#302).
+ * @type {ReadonlySet<string>}
+ * @private
+ */
+const UNSAFE_PATH_SEGMENTS = new Set(["__proto__", "constructor", "prototype"]);
+
 /**
  * Manages runtime API component lifecycle (add/remove/reload).
  * @class ApiManager
@@ -165,6 +175,18 @@ export class ApiManager extends ComponentBase {
 				});
 			}
 
+			// Prototype-pollution guard: refuse a __proto__/constructor/prototype segment in any position (#302).
+			const unsafeArrayIndex = apiPath.findIndex((segment) => UNSAFE_PATH_SEGMENTS.has(segment));
+			if (unsafeArrayIndex !== -1) {
+				throw new this.SlothletError("INVALID_CONFIG_API_PATH_INVALID", {
+					apiPath,
+					reason: translate("API_PATH_REASON_UNSAFE_SEGMENT"),
+					index: unsafeArrayIndex,
+					segment: apiPath[unsafeArrayIndex],
+					validationError: true
+				});
+			}
+
 			return { apiPath: apiPath.join("."), parts: apiPath };
 		}
 
@@ -206,6 +228,18 @@ export class ApiManager extends ComponentBase {
 			});
 		}
 
+		// Prototype-pollution guard: refuse a __proto__/constructor/prototype segment in any position (#302).
+		const unsafeIndex = parts.findIndex((segment) => UNSAFE_PATH_SEGMENTS.has(segment));
+		if (unsafeIndex !== -1) {
+			throw new this.SlothletError("INVALID_CONFIG_API_PATH_INVALID", {
+				apiPath: normalized,
+				reason: translate("API_PATH_REASON_UNSAFE_SEGMENT"),
+				index: unsafeIndex,
+				segment: parts[unsafeIndex],
+				validationError: true
+			});
+		}
+
 		return { apiPath: normalized, parts };
 	}
 
@@ -443,6 +477,26 @@ export class ApiManager extends ComponentBase {
 	 * @public
 	 */
 	setOwnedProperty(apiPath, value, callerWrapper) {
+		// Reject prototype-pollution segments BEFORE the canonical normalizer runs. This surface has
+		// its own dedicated LOOSE_SET_RESERVED_KEY error; `normalizeApiPath` now also blocks these
+		// segments, but with a generic INVALID_CONFIG_API_PATH_INVALID, so the check must run first to
+		// keep the loose-set error. `self.__proto__ = obj` would assign onto the API root's prototype
+		// chain; `self.a.__proto__ = obj` (via the dotted form) would do the same on the wrapper at `a`.
+		// Reuses the module-level UNSAFE_PATH_SEGMENTS (same set the add-path guard uses). `String()`
+		// coerces without a branch and without throwing on a Symbol (unlike a template literal) — callers
+		// always pass `String(prop)`, and null/undefined stringify to a non-reserved token that falls
+		// through to the empty-path guard below.
+		const coercedPath = String(apiPath);
+		for (const segment of coercedPath.split(".")) {
+			if (UNSAFE_PATH_SEGMENTS.has(segment)) {
+				throw new this.SlothletError("LOOSE_SET_RESERVED_KEY", {
+					apiPath: coercedPath,
+					segment,
+					validationError: true
+				});
+			}
+		}
+
 		// Delegate parsing to the canonical normalizer used by api.add / api.remove:
 		// it rejects empty segments (e.g. "a..b") AND reserved root names (slothlet,
 		// shutdown, destroy) so a runtime `self.slothlet = …` can't overwrite the
@@ -463,21 +517,6 @@ export class ApiManager extends ComponentBase {
 			});
 		}
 
-		// Reject prototype-pollution segments at any position. `self.__proto__ = obj`
-		// would assign onto the API root's prototype chain; `self.a.__proto__ = obj`
-		// (via dotted form) would do the same on the wrapper at `a`. Mirrors the same
-		// blocked set used by metadata.mjs and api_builder.mjs.
-		const RESERVED = new Set(["__proto__", "prototype", "constructor"]);
-		for (const segment of parts) {
-			if (RESERVED.has(segment)) {
-				throw new this.SlothletError("LOOSE_SET_RESERVED_KEY", {
-					apiPath: parts.join("."),
-					segment,
-					validationError: true
-				});
-			}
-		}
-
 		// Ownership root = the caller module's MOUNT POINT, not its function-level apiPath.
 		// The wrapper for `lib.config.foo` may belong to a module whose entire mount
 		// point is `lib.config` — that whole subtree is the module's domain. We look
diff --git a/src/lib/i18n/languages/de-de.json b/src/lib/i18n/languages/de-de.json
index 702a617a..88868a66 100644
--- a/src/lib/i18n/languages/de-de.json
+++ b/src/lib/i18n/languages/de-de.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "Array-Elemente müssen Strings sein",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "Array enthält leere String-Segmente",
 		"API_PATH_REASON_RESERVED_NAME": "kollidiert mit reservierten Namen (slothlet, shutdown, destroy)",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "enthält ein unsicheres Segment (__proto__, constructor, prototype), das die Prototypenkette verunreinigen würde",
 		"API_PATH_REASON_INVALID_TYPE": "muss ein String, ein Array von Strings, ein leerer String (Root) oder null/undefined (Root) sein",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "enthält leere Pfad-Segmente",
 		"API_PATH_REASON_COLLISION_ERROR": "Pfad existiert bereits und Kollisionsmodus ist 'error'",
diff --git a/src/lib/i18n/languages/en-gb.json b/src/lib/i18n/languages/en-gb.json
index 00d718cf..873919c9 100644
--- a/src/lib/i18n/languages/en-gb.json
+++ b/src/lib/i18n/languages/en-gb.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "array elements must be strings",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "array contains empty string segments",
 		"API_PATH_REASON_RESERVED_NAME": "conflicts with reserved names (slothlet, shutdown, destroy)",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "contains an unsafe segment (__proto__, constructor, prototype) that would pollute the prototype chain",
 		"API_PATH_REASON_INVALID_TYPE": "must be a string, array of strings, empty string (root), or null/undefined (root)",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "contains empty path segments",
 		"API_PATH_REASON_COLLISION_ERROR": "path already exists and collision mode is 'error'",
diff --git a/src/lib/i18n/languages/en-us.json b/src/lib/i18n/languages/en-us.json
index 2ba93b98..3e887866 100644
--- a/src/lib/i18n/languages/en-us.json
+++ b/src/lib/i18n/languages/en-us.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "array elements must be strings",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "array contains empty string segments",
 		"API_PATH_REASON_RESERVED_NAME": "conflicts with reserved names (slothlet, shutdown, destroy)",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "contains an unsafe segment (__proto__, constructor, prototype) that would pollute the prototype chain",
 		"API_PATH_REASON_INVALID_TYPE": "must be a string, array of strings, empty string (root), or null/undefined (root)",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "contains empty path segments",
 		"API_PATH_REASON_COLLISION_ERROR": "path already exists and collision mode is 'error'",
diff --git a/src/lib/i18n/languages/es-es.json b/src/lib/i18n/languages/es-es.json
index 45f266b9..1fc944b0 100644
--- a/src/lib/i18n/languages/es-es.json
+++ b/src/lib/i18n/languages/es-es.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "los elementos del array deben ser cadenas",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "el array contiene segmentos de cadena vacíos",
 		"API_PATH_REASON_RESERVED_NAME": "entra en conflicto con nombres reservados (slothlet, shutdown, destroy)",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "contiene un segmento no seguro (__proto__, constructor, prototype) que contaminaría la cadena de prototipos",
 		"API_PATH_REASON_INVALID_TYPE": "debe ser una cadena, un array de cadenas, cadena vacía (raíz) o null/undefined (raíz)",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "contiene segmentos de ruta vacíos",
 		"API_PATH_REASON_COLLISION_ERROR": "la ruta ya existe y el modo de colisión es 'error'",
diff --git a/src/lib/i18n/languages/es-mx.json b/src/lib/i18n/languages/es-mx.json
index ae1b78d8..bafa14c7 100644
--- a/src/lib/i18n/languages/es-mx.json
+++ b/src/lib/i18n/languages/es-mx.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "los elementos del array deben ser cadenas",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "el array contiene segmentos de cadena vacíos",
 		"API_PATH_REASON_RESERVED_NAME": "entra en conflicto con nombres reservados (slothlet, shutdown, destroy)",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "contiene un segmento no seguro (__proto__, constructor, prototype) que contaminaría la cadena de prototipos",
 		"API_PATH_REASON_INVALID_TYPE": "debe ser una cadena, un array de cadenas, cadena vacía (raíz) o null/undefined (raíz)",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "contiene segmentos de ruta vacíos",
 		"API_PATH_REASON_COLLISION_ERROR": "la ruta ya existe y el modo de colisión es 'error'",
diff --git a/src/lib/i18n/languages/fr-fr.json b/src/lib/i18n/languages/fr-fr.json
index 73a4a57f..77cf3f99 100644
--- a/src/lib/i18n/languages/fr-fr.json
+++ b/src/lib/i18n/languages/fr-fr.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "les éléments du tableau doivent être des chaînes",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "le tableau contient des segments de chaînes vides",
 		"API_PATH_REASON_RESERVED_NAME": "entre en conflit avec les noms réservés (slothlet, shutdown, destroy)",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "contient un segment non sécurisé (__proto__, constructor, prototype) qui polluerait la chaîne de prototypes",
 		"API_PATH_REASON_INVALID_TYPE": "doit être une chaîne, un tableau de chaînes, une chaîne vide (racine) ou null/undefined (racine)",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "contient des segments de chemin vides",
 		"API_PATH_REASON_COLLISION_ERROR": "le chemin existe déjà et le mode de collision est 'error'",
diff --git a/src/lib/i18n/languages/hi-in.json b/src/lib/i18n/languages/hi-in.json
index c1acee29..bbdd801d 100644
--- a/src/lib/i18n/languages/hi-in.json
+++ b/src/lib/i18n/languages/hi-in.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "एरे तत्व स्ट्रिंग्स होने चाहिए",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "एरे में खाली स्ट्रिंग खंड हैं",
 		"API_PATH_REASON_RESERVED_NAME": "आरक्षित नामों (slothlet, shutdown, destroy) के साथ संघर्ष",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "एक असुरक्षित सेगमेंट (__proto__, constructor, prototype) शामिल है जो प्रोटोटाइप श्रृंखला को दूषित करेगा",
 		"API_PATH_REASON_INVALID_TYPE": "स्ट्रिंग, स्ट्रिंग का एरे, खाली स्ट्रिंग (रूट), या null/undefined (रूट) होना चाहिए",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "इसमें खाली पथ खंड हैं",
 		"API_PATH_REASON_COLLISION_ERROR": "पथ पहले से मौजूद है और टकराव मोड 'error' है",
diff --git a/src/lib/i18n/languages/ja-jp.json b/src/lib/i18n/languages/ja-jp.json
index dc6d515a..67df20ab 100644
--- a/src/lib/i18n/languages/ja-jp.json
+++ b/src/lib/i18n/languages/ja-jp.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "配列の要素は文字列である必要があります",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "配列に空の文字列セグメントが含まれています",
 		"API_PATH_REASON_RESERVED_NAME": "予約名 (slothlet, shutdown, destroy) と衝突しています",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "プロトタイプチェーンを汚染する安全でないセグメント (__proto__, constructor, prototype) が含まれています",
 		"API_PATH_REASON_INVALID_TYPE": "文字列、文字列の配列、空の文字列 (ルート)、または null/undefined (ルート) である必要があります",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "空のパスセグメントが含まれています",
 		"API_PATH_REASON_COLLISION_ERROR": "パスが既に存在し、衝突モードが 'error' です",
diff --git a/src/lib/i18n/languages/ko-kr.json b/src/lib/i18n/languages/ko-kr.json
index 24d9d22d..df234588 100644
--- a/src/lib/i18n/languages/ko-kr.json
+++ b/src/lib/i18n/languages/ko-kr.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "배열 요소는 문자열이어야 함",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "배열에 빈 문자열 세그먼트가 포함되어 있음",
 		"API_PATH_REASON_RESERVED_NAME": "예약어(slothlet, shutdown, destroy)와 충돌함",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "프로토타입 체인을 오염시킬 수 있는 안전하지 않은 세그먼트(__proto__, constructor, prototype)를 포함함",
 		"API_PATH_REASON_INVALID_TYPE": "문자열, 문자열 배열, 빈 문자열(루트), 또는 null/undefined(루트)여야 함",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "빈 경로 세그먼트를 포함하고 있음",
 		"API_PATH_REASON_COLLISION_ERROR": "경로가 이미 존재하고 충돌 모드가 'error'임",
diff --git a/src/lib/i18n/languages/pt-br.json b/src/lib/i18n/languages/pt-br.json
index ced94f40..9591db42 100644
--- a/src/lib/i18n/languages/pt-br.json
+++ b/src/lib/i18n/languages/pt-br.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "elementos do array devem ser strings",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "o array contém segmentos de string vazios",
 		"API_PATH_REASON_RESERVED_NAME": "conflita com nomes reservados (slothlet, shutdown, destroy)",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "contém um segmento inseguro (__proto__, constructor, prototype) que poluiria a cadeia de protótipos",
 		"API_PATH_REASON_INVALID_TYPE": "deve ser uma string, array de strings, string vazia (raiz) ou null/undefined (raiz)",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "contém segmentos de caminho vazios",
 		"API_PATH_REASON_COLLISION_ERROR": "o caminho já existe e o modo de colisão é 'error'",
diff --git a/src/lib/i18n/languages/ru-ru.json b/src/lib/i18n/languages/ru-ru.json
index 50375723..5aed1931 100644
--- a/src/lib/i18n/languages/ru-ru.json
+++ b/src/lib/i18n/languages/ru-ru.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "Элементы массива должны быть строками",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "Массив содержит пустые сегменты",
 		"API_PATH_REASON_RESERVED_NAME": "Конфликт с зарезервированными именами (slothlet, shutdown, destroy)",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "Содержит небезопасный сегмент (__proto__, constructor, prototype), который загрязнил бы цепочку прототипов",
 		"API_PATH_REASON_INVALID_TYPE": "Должно быть строкой, массивом строк, пустой строкой (root) или null/undefined (root)",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "Содержит пустые сегменты пути",
 		"API_PATH_REASON_COLLISION_ERROR": "Путь уже существует и режим столкновения 'error'",
diff --git a/src/lib/i18n/languages/zh-cn.json b/src/lib/i18n/languages/zh-cn.json
index 9136c9ab..6d737424 100644
--- a/src/lib/i18n/languages/zh-cn.json
+++ b/src/lib/i18n/languages/zh-cn.json
@@ -376,6 +376,7 @@
 		"API_PATH_REASON_ARRAY_ELEMENTS": "数组元素必须为字符串",
 		"API_PATH_REASON_ARRAY_EMPTY_SEGMENTS": "数组包含空字符串片段",
 		"API_PATH_REASON_RESERVED_NAME": "与保留名称冲突（slothlet、shutdown、destroy）",
+		"API_PATH_REASON_UNSAFE_SEGMENT": "包含会污染原型链的不安全段（__proto__、constructor、prototype）",
 		"API_PATH_REASON_INVALID_TYPE": "必须是字符串、字符串数组、空字符串（根）或 null/undefined（根）",
 		"API_PATH_REASON_EMPTY_SEGMENTS": "包含空的路径段",
 		"API_PATH_REASON_COLLISION_ERROR": "路径已存在且冲突模式为 'error'",
diff --git a/tests/vitests/suites/api-manager/api-manager-reserved-paths.test.vitest.mjs b/tests/vitests/suites/api-manager/api-manager-reserved-paths.test.vitest.mjs
index 5e5bebe4..e141c64c 100644
--- a/tests/vitests/suites/api-manager/api-manager-reserved-paths.test.vitest.mjs
+++ b/tests/vitests/suites/api-manager/api-manager-reserved-paths.test.vitest.mjs
@@ -97,3 +97,46 @@ describe.each(CONFIGS)("normalizeApiPath reserved names — $name", ({ config })
 		await expect(api.slothlet.api.add("a..b", TEST_DIRS.API_TEST)).rejects.toMatchObject({ code: "INVALID_CONFIG_API_PATH_INVALID" });
 	});
 });
+
+// #302 – a mount-path segment of __proto__/constructor/prototype must be refused, never
+// written through to the real prototype chain (prototype pollution). Any segment position
+// is dangerous, and both the dotted-string and array path forms funnel through normalizeApiPath.
+describe.each(CONFIGS)("normalizeApiPath prototype-pollution guard — $name", ({ config }) => {
+	let api;
+
+	afterEach(async () => {
+		// Fail loudly if any assertion below leaked a write onto Object.prototype.
+		delete Object.prototype.x;
+		delete Object.prototype.pwn;
+		if (api) {
+			await api.shutdown();
+			api = null;
+		}
+	});
+
+	const UNSAFE = [
+		{ label: "__proto__ (string)", path: "__proto__.x", probe: "x" },
+		{ label: "constructor.prototype (string)", path: "constructor.prototype.pwn", probe: "pwn" },
+		{ label: "__proto__ (array)", path: ["__proto__", "x"], probe: "x" },
+		{ label: "prototype at a deeper segment (array)", path: ["safe", "prototype", "pwn"], probe: "pwn" }
+	];
+
+	it.each(UNSAFE)("rejects a $label mount path and does not pollute Object.prototype", async ({ path, probe }) => {
+		api = await makeApi(config);
+		await expect(api.slothlet.api.add(path, () => "polluted", { moduleID: "proto-guard" })).rejects.toMatchObject({
+			code: "INVALID_CONFIG_API_PATH_INVALID"
+		});
+		// The write must not have reached the prototype chain.
+		expect(Object.prototype[probe]).toBeUndefined();
+		expect({}[probe]).toBeUndefined();
+	});
+
+	it("accepts a __-prefixed / reserved-substring segment (precise guard, not a blanket '__' ban)", async () => {
+		// Only the exact segments __proto__/constructor/prototype are refused. A name that merely
+		// starts with "__" (the module-private convention) or contains a reserved word as a substring
+		// must still mount — the guard matches whole segments, not prefixes or substrings.
+		api = await makeApi(config);
+		await expect(api.slothlet.api.add("__config.value", () => 1, { moduleID: "safe-underscore" })).resolves.toBeDefined();
+		await expect(api.slothlet.api.add(["prototypeName", "leaf"], () => 2, { moduleID: "safe-substring" })).resolves.toBeDefined();
+	});
+});