diff --git a/packages/ai/src/api/google-shared.ts b/packages/ai/src/api/google-shared.ts
index 3f889170a..d68d1c4f2 100644
--- a/packages/ai/src/api/google-shared.ts
+++ b/packages/ai/src/api/google-shared.ts
@@ -372,7 +372,34 @@ function sanitizeForOpenApi(schema: unknown): unknown {
 }
 
 /** Schema keywords whose values map names to subschemas, so their keys are not schema keywords. */
-const JSON_SCHEMA_NAME_MAPS = new Set(["properties", "patternProperties", "$defs", "definitions", "dependentSchemas"]);
+const JSON_SCHEMA_NAME_MAPS = new Set([
+	"properties",
+	"patternProperties",
+	"$defs",
+	"definitions",
+	"dependentSchemas",
+	"dependencies",
+]);
+
+/** Keywords containing a subschema or an array of subschemas, not literal data. */
+const JSON_SCHEMA_SUBSCHEMAS = new Set([
+	"additionalProperties",
+	"unevaluatedProperties",
+	"propertyNames",
+	"items",
+	"prefixItems",
+	"additionalItems",
+	"unevaluatedItems",
+	"contains",
+	"allOf",
+	"anyOf",
+	"oneOf",
+	"not",
+	"if",
+	"then",
+	"else",
+	"contentSchema",
+]);
 
 /**
  * Rewrite string and number `const` values to single-value `enum`s.
@@ -382,10 +409,14 @@ function rewriteConstAsEnum(schema: unknown, isNameMap = false): unknown {
 	if (Array.isArray(schema)) return schema.map((item) => rewriteConstAsEnum(item));
 	if (typeof schema !== "object" || schema === null) return schema;
 
-	const result: Record<string, unknown> = {};
-	for (const [key, value] of Object.entries(schema)) {
-		result[key] = isNameMap ? rewriteConstAsEnum(value) : rewriteConstAsEnum(value, JSON_SCHEMA_NAME_MAPS.has(key));
-	}
+	// fromEntries preserves names such as __proto__ as own properties.
+	const result: Record<string, unknown> = Object.fromEntries(
+		Object.entries(schema).map(([key, value]) => {
+			if (isNameMap || JSON_SCHEMA_SUBSCHEMAS.has(key)) return [key, rewriteConstAsEnum(value)];
+			if (JSON_SCHEMA_NAME_MAPS.has(key)) return [key, rewriteConstAsEnum(value, true)];
+			return [key, value];
+		}),
+	);
 	if (!isNameMap && "const" in result && result.enum === undefined) {
 		const value = result.const;
 		if (typeof value === "string" || typeof value === "number") {
diff --git a/packages/ai/test/google-shared-convert-tools.test.ts b/packages/ai/test/google-shared-convert-tools.test.ts
index d9d4d8b36..02ba47d0a 100644
--- a/packages/ai/test/google-shared-convert-tools.test.ts
+++ b/packages/ai/test/google-shared-convert-tools.test.ts
@@ -202,7 +202,92 @@ describe("google-shared convertTools", () => {
 	});
 });
 
+// Regression coverage for #11308.
 describe("google-shared convertTools const handling", () => {
+	it.each(["default", "examples", "enum", "const", "x-custom"])("preserves literal data under %s", (key) => {
+		const literal = { const: "literal-data", properties: { nested: { const: 1 } } };
+		const parameters = {
+			type: "object",
+			properties: { value: { type: "object", [key]: key === "enum" || key === "examples" ? [literal] : literal } },
+		};
+		const original = structuredClone(parameters);
+
+		expect(convertTools([makeTool(parameters)])?.[0]?.functionDeclarations?.[0]?.parametersJsonSchema).toEqual(
+			original,
+		);
+		expect(parameters).toEqual(original);
+	});
+
+	it.each(["properties", "patternProperties", "$defs", "definitions", "dependentSchemas", "dependencies"])(
+		"preserves names while rewriting schemas in %s",
+		(key) => {
+			const parameters = {
+				type: "object",
+				[key]: {
+					["__proto__"]: { const: "x" },
+					const: { const: "y" },
+					properties: { const: "z" },
+				},
+			};
+			const original = structuredClone(parameters);
+			const expected = {
+				type: "object",
+				[key]: {
+					["__proto__"]: { enum: ["x"] },
+					const: { enum: ["y"] },
+					properties: { enum: ["z"] },
+				},
+			};
+
+			const result = convertTools([makeTool(parameters)])?.[0]?.functionDeclarations?.[0]?.parametersJsonSchema;
+			expect(result).toEqual(expected);
+			expect(JSON.stringify(result)).toBe(JSON.stringify(expected));
+			expect(parameters).toEqual(original);
+		},
+	);
+
+	it("rewrites only schema-bearing keywords", () => {
+		const parameters = {
+			type: "object",
+			additionalProperties: { const: "x" },
+			unevaluatedProperties: false,
+			propertyNames: { const: "name" },
+			// biome-ignore lint/suspicious/noThenProperty: JSON Schema conditional keyword, not a thenable.
+			allOf: [{ if: { const: "x" }, then: { const: "y" }, else: { not: { const: "z" } } }],
+			properties: {
+				list: {
+					type: "array",
+					items: [{ const: 1 }],
+					prefixItems: [{ const: 2 }],
+					additionalItems: { const: 3 },
+					contains: { const: 4 },
+					unevaluatedItems: { const: 5 },
+				},
+			},
+			dependencies: { list: ["other"] },
+		};
+
+		expect(convertTools([makeTool(parameters)])?.[0]?.functionDeclarations?.[0]?.parametersJsonSchema).toEqual({
+			type: "object",
+			additionalProperties: { enum: ["x"] },
+			unevaluatedProperties: false,
+			propertyNames: { enum: ["name"] },
+			// biome-ignore lint/suspicious/noThenProperty: JSON Schema conditional keyword, not a thenable.
+			allOf: [{ if: { enum: ["x"] }, then: { enum: ["y"] }, else: { not: { enum: ["z"] } } }],
+			properties: {
+				list: {
+					type: "array",
+					items: [{ enum: [1] }],
+					prefixItems: [{ enum: [2] }],
+					additionalItems: { enum: [3] },
+					contains: { enum: [4] },
+					unevaluatedItems: { enum: [5] },
+				},
+			},
+			dependencies: { list: ["other"] },
+		});
+	});
+
 	it("rewrites string and number const values to single-value enums in parametersJsonSchema", () => {
 		const parameters = {
 			type: "object",