diff --git a/packages/core/src/db/adapter/get-id-field.test.ts b/packages/core/src/db/adapter/get-id-field.test.ts
new file mode 100644
index 0000000000..5a47f714ea
--- /dev/null
+++ b/packages/core/src/db/adapter/get-id-field.test.ts
@@ -0,0 +1,222 @@
+import { describe, expect, it } from "vitest";
+import type { BetterAuthOptions } from "../../types";
+import type { BetterAuthDBSchema } from "../type";
+import { initGetIdField } from "./get-id-field";
+
+const minimalSchema: BetterAuthDBSchema = {
+	user: {
+		modelName: "user",
+		fields: {
+			name: { type: "string" },
+			email: { type: "string" },
+		},
+	},
+};
+
+const uuidRegex =
+	/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
+function getField(
+	options: BetterAuthOptions,
+	initExtra?: Partial<Parameters<typeof initGetIdField>[0]>,
+	fieldExtra?: { customModelName?: string; forceAllowId?: boolean },
+) {
+	const idField = initGetIdField({
+		schema: minimalSchema,
+		options,
+		...initExtra,
+	});
+	return idField({
+		customModelName: fieldExtra?.customModelName ?? "user",
+		forceAllowId: fieldExtra?.forceAllowId,
+	});
+}
+
+describe("defaultValue priority", () => {
+	it("should return undefined when disableIdGeneration is true", () => {
+		const field = getField(
+			{ database: {} as any },
+			{ disableIdGeneration: true },
+		);
+		expect(field.defaultValue).toBeUndefined();
+	});
+
+	it("should return undefined when generateId is false", () => {
+		const value = getField({
+			database: {} as any,
+			advanced: { database: { generateId: false } },
+		}).defaultValue?.();
+		expect(value).toBeUndefined();
+	});
+
+	it("should return undefined when generateId is 'serial'", () => {
+		const value = getField({
+			database: {} as any,
+			advanced: { database: { generateId: "serial" } },
+		}).defaultValue?.();
+		expect(value).toBeUndefined();
+	});
+
+	it("should use generateId function over 'uuid' and customIdGenerator", () => {
+		const value = getField(
+			{
+				database: {} as any,
+				advanced: { database: { generateId: () => "fn-id" } },
+			},
+			{ customIdGenerator: () => "adapter-id" },
+		).defaultValue?.();
+		expect(value).toBe("fn-id");
+	});
+
+	it("should use 'uuid' over customIdGenerator", () => {
+		const value = getField(
+			{
+				database: {} as any,
+				advanced: { database: { generateId: "uuid" } },
+			},
+			{ customIdGenerator: () => "adapter-id", supportsUUIDs: false },
+		).defaultValue?.();
+		expect(value).toMatch(uuidRegex);
+	});
+
+	it("should use customIdGenerator when generateId is not set", () => {
+		const value = getField(
+			{ database: {} as any },
+			{ customIdGenerator: () => "adapter-id" },
+		).defaultValue?.();
+		expect(value).toBe("adapter-id");
+	});
+
+	it("should fall back to default id generation", () => {
+		const value = getField({ database: {} as any }).defaultValue?.();
+		expect(typeof value).toBe("string");
+		expect(value).not.toMatch(uuidRegex);
+	});
+});
+
+describe("type and required", () => {
+	it("should have type 'number' when generateId is 'serial'", () => {
+		const field = getField({
+			database: {} as any,
+			advanced: { database: { generateId: "serial" } },
+		});
+		expect(field.type).toBe("number");
+		expect(field.required).toBe(false);
+	});
+
+	it("should have type 'string' by default", () => {
+		const field = getField({ database: {} as any });
+		expect(field.type).toBe("string");
+		expect(field.required).toBe(true);
+	});
+
+	it("should not generate id when useUUIDs and supportsUUIDs", () => {
+		const field = getField(
+			{
+				database: {} as any,
+				advanced: { database: { generateId: "uuid" } },
+			},
+			{ supportsUUIDs: true },
+		);
+		expect(field.required).toBe(false);
+		expect(field.defaultValue).toBeUndefined();
+	});
+});
+
+describe("transform.input", () => {
+	it("should return undefined for falsy value", () => {
+		const field = getField({ database: {} as any });
+		expect(field.transform.input(undefined)).toBeUndefined();
+		expect(field.transform.input(null)).toBeUndefined();
+		expect(field.transform.input("")).toBeUndefined();
+	});
+
+	it("should return value as-is by default", () => {
+		const field = getField({ database: {} as any });
+		expect(field.transform.input("some-id")).toBe("some-id");
+	});
+
+	describe("serial", () => {
+		it("should convert string to number", () => {
+			const field = getField({
+				database: {} as any,
+				advanced: { database: { generateId: "serial" } },
+			});
+			expect(field.transform.input("42")).toBe(42);
+		});
+
+		it("should return undefined for non-numeric string", () => {
+			const field = getField({
+				database: {} as any,
+				advanced: { database: { generateId: "serial" } },
+			});
+			expect(field.transform.input("not-a-number")).toBeUndefined();
+		});
+	});
+
+	describe("uuid", () => {
+		it("should return value as-is when shouldGenerateId and not forceAllowId", () => {
+			const field = getField(
+				{
+					database: {} as any,
+					advanced: { database: { generateId: "uuid" } },
+				},
+				{ supportsUUIDs: false },
+			);
+			const uuid = crypto.randomUUID();
+			expect(field.transform.input(uuid)).toBe(uuid);
+		});
+
+		it("should return undefined when supportsUUIDs (DB handles it)", () => {
+			const field = getField(
+				{
+					database: {} as any,
+					advanced: { database: { generateId: "uuid" } },
+				},
+				{ supportsUUIDs: true },
+			);
+			expect(field.transform.input("some-value")).toBeUndefined();
+		});
+
+		it("should accept valid UUID when forceAllowId is true", () => {
+			const uuid = crypto.randomUUID();
+			const field = getField(
+				{
+					database: {} as any,
+					advanced: { database: { generateId: "uuid" } },
+				},
+				{ supportsUUIDs: false },
+				{ forceAllowId: true },
+			);
+			expect(field.transform.input(uuid)).toBe(uuid);
+		});
+
+		it("should generate new UUID for non-string value when DB doesn't support UUIDs", () => {
+			const field = getField(
+				{
+					database: {} as any,
+					advanced: { database: { generateId: "uuid" } },
+				},
+				{ supportsUUIDs: false },
+				{ forceAllowId: true },
+			);
+			const result = field.transform.input(123);
+			expect(result).toMatch(uuidRegex);
+		});
+	});
+});
+
+describe("transform.output", () => {
+	it("should return undefined for falsy value", () => {
+		const field = getField({ database: {} as any });
+		expect(field.transform.output(undefined)).toBeUndefined();
+		expect(field.transform.output(null)).toBeUndefined();
+		expect(field.transform.output("")).toBeUndefined();
+	});
+
+	it("should convert value to string", () => {
+		const field = getField({ database: {} as any });
+		expect(field.transform.output(123)).toBe("123");
+		expect(field.transform.output("abc")).toBe("abc");
+	});
+});
diff --git a/packages/core/src/db/adapter/get-id-field.ts b/packages/core/src/db/adapter/get-id-field.ts
index 5b3e9b192c..c4f76a5d2c 100644
--- a/packages/core/src/db/adapter/get-id-field.ts
+++ b/packages/core/src/db/adapter/get-id-field.ts
@@ -57,18 +57,29 @@ export const initGetIdField = ({
 						defaultValue() {
 							if (disableIdGeneration) return undefined;
 							const generateId = options.advanced?.database?.generateId;
-							if (generateId === false || useNumberId) return undefined;
+
+							// let the database handle id generation
+							if (generateId === false || generateId === "serial")
+								return undefined;
+
+							// user-provided function takes highest priority
 							if (typeof generateId === "function") {
 								return generateId({
 									model,
 								});
 							}
-							if (customIdGenerator) {
-								return customIdGenerator({ model });
-							}
+
+							// user-provided "uuid" option
 							if (generateId === "uuid") {
 								return crypto.randomUUID();
 							}
+
+							// database adapter-level custom id generator
+							if (customIdGenerator) {
+								return customIdGenerator({ model });
+							}
+
+							// fallback to default id generation
 							return defaultGenerateId();
 						},
 					}