diff --git a/packages/tools/src/openai/tools.ts b/packages/tools/src/openai/tools.ts
index 9655550b..fec1536f 100644
--- a/packages/tools/src/openai/tools.ts
+++ b/packages/tools/src/openai/tools.ts
@@ -501,8 +501,10 @@ export function createToolCallExecutor(
 		const args = parsed.value
 
 		// Blank arguments are only valid for tools with no required fields.
-		const schema =
-			memoryToolSchemas[functionName as keyof typeof memoryToolSchemas]
+		// hasOwn so names like "constructor" fall through to the unknown-function error.
+		const schema = Object.hasOwn(memoryToolSchemas, functionName)
+			? memoryToolSchemas[functionName as keyof typeof memoryToolSchemas]
+			: undefined
 		const missing = schema?.parameters.required.filter(
 			(field: string) =>
 				typeof args[field] !== "string" || args[field].trim() === "",
diff --git a/packages/tools/src/tool-operations.test.ts b/packages/tools/src/tool-operations.test.ts
index ca48d547..d0b46eb1 100644
--- a/packages/tools/src/tool-operations.test.ts
+++ b/packages/tools/src/tool-operations.test.ts
@@ -404,6 +404,21 @@ describe("openai executeToolCall argument parsing", () => {
 		expect(sdk.add).not.toHaveBeenCalled()
 	})
 
+	it.each([
+		"constructor",
+		"toString",
+		"__proto__",
+	])("reports inherited name %p as an unknown function", async (name) => {
+		const execute = openAi.createToolCallExecutor(API_KEY)
+
+		const result = JSON.parse(await execute(toolCall(name, "{}")))
+
+		expect(result).toEqual({
+			success: false,
+			error: `Unknown function: ${name}`,
+		})
+	})
+
 	it("still reports malformed JSON as a tool error", async () => {
 		const execute = openAi.createToolCallExecutor(API_KEY)