diff --git a/packages/better-auth/src/plugins/open-api/__snapshots__/open-api.test.ts.snap b/packages/better-auth/src/plugins/open-api/__snapshots__/open-api.test.ts.snap
index a97e7d0a52..728d339023 100644
--- a/packages/better-auth/src/plugins/open-api/__snapshots__/open-api.test.ts.snap
+++ b/packages/better-auth/src/plugins/open-api/__snapshots__/open-api.test.ts.snap
@@ -3405,9 +3405,10 @@ exports[`open-api > should generate OpenAPI schema > openAPISchema 1`] = `
                     "type": "string",
                   },
                   "rememberMe": {
-                    "description": undefined,
+                    "default": true,
+                    "description": "If this is false, the session will not be remembered. Default is \`true\`.",
                     "type": [
-                      "string",
+                      "boolean",
                       "null",
                     ],
                   },
diff --git a/packages/better-auth/src/plugins/open-api/generator.ts b/packages/better-auth/src/plugins/open-api/generator.ts
index 6c4976152e..89bdc48be6 100644
--- a/packages/better-auth/src/plugins/open-api/generator.ts
+++ b/packages/better-auth/src/plugins/open-api/generator.ts
@@ -80,6 +80,10 @@ export interface Path {
 type AllowedType = "string" | "number" | "boolean" | "array" | "object";
 const allowedType = new Set(["string", "number", "boolean", "array", "object"]);
 function getTypeFromZodType(zodType: z.ZodType<any>) {
+	// unwrap ZodDefault to get the inner type
+	if (zodType instanceof z.ZodDefault) {
+		return getTypeFromZodType(zodType.unwrap() as any);
+	}
 	const type = zodType.type;
 	return allowedType.has(type) ? (type as AllowedType) : "string";
 }
@@ -192,7 +196,7 @@ function getRequestBody(options: EndpointOptions): any {
 function processZodType(zodType: z.ZodType<any>): any {
 	// optional unwrapping
 	if (zodType instanceof z.ZodOptional) {
-		const innerType = (zodType as any)._def.innerType;
+		const innerType = zodType.unwrap() as any;
 		const innerSchema = processZodType(innerType);
 		if (innerSchema.type) {
 			const type = Array.isArray(innerSchema.type)
@@ -207,6 +211,20 @@ function processZodType(zodType: z.ZodType<any>): any {
 			anyOf: [innerSchema, { type: "null" }],
 		};
 	}
+	// default unwrapping
+	if (zodType instanceof z.ZodDefault) {
+		const innerType = zodType.unwrap() as any;
+		const innerSchema = processZodType(innerType);
+		const defaultValueDef = (zodType as any)._def.defaultValue;
+		const defaultValue =
+			typeof defaultValueDef === "function"
+				? defaultValueDef()
+				: defaultValueDef;
+		return {
+			...innerSchema,
+			default: defaultValue,
+		};
+	}
 	// object unwrapping
 	if (zodType instanceof z.ZodObject) {
 		const shape = (zodType as any).shape;
diff --git a/packages/better-auth/src/plugins/open-api/open-api.test.ts b/packages/better-auth/src/plugins/open-api/open-api.test.ts
index 477ff53d25..936431d409 100644
--- a/packages/better-auth/src/plugins/open-api/open-api.test.ts
+++ b/packages/better-auth/src/plugins/open-api/open-api.test.ts
@@ -182,4 +182,61 @@ describe("open-api", async (it) => {
 			checkNoNullable(requestBodySchema, "signInSocialRequestBody"),
 		).not.toThrow();
 	});
+
+	it("should correctly unwrap ZodDefault and infer inner type", async () => {
+		const schema = await auth.api.generateOpenAPISchema();
+		const paths = schema.paths as Record<string, any>;
+
+		// Helper to extract base type from type array
+		const getBaseType = (type: any) => {
+			if (Array.isArray(type)) {
+				return type.filter((t) => t !== "null");
+			}
+			return [type];
+		};
+
+		// Helper to check if field has correct type and default value
+		const validateDefaultField = (
+			properties: any,
+			fieldName: string,
+			expectedType: string,
+			expectedDefault?: any,
+		) => {
+			expect(properties[fieldName]).toBeDefined();
+			const fieldSchema = properties[fieldName];
+
+			// Check type is correctly inferred - not fallback to "string"
+			const baseTypes = getBaseType(fieldSchema.type);
+			expect(baseTypes).toContain(expectedType);
+			expect(baseTypes).not.toContain("string");
+
+			// Check default value is included if expected
+			if (expectedDefault !== undefined) {
+				expect(fieldSchema.default).toBe(expectedDefault);
+			}
+		};
+
+		// Test sign-in endpoint: z.boolean().default(true).optional()
+		const signInPath = paths["/sign-in/email"];
+		expect(signInPath).toBeDefined();
+		expect(signInPath.post).toBeDefined();
+		expect(signInPath.post.requestBody).toBeDefined();
+
+		const signInProps =
+			signInPath.post.requestBody.content["application/json"].schema.properties;
+		validateDefaultField(signInProps, "rememberMe", "boolean", true);
+
+		// Test sign-up endpoint: z.boolean().optional() - no default
+		const signUpPath = paths["/sign-up/email"];
+		expect(signUpPath).toBeDefined();
+		expect(signUpPath.post).toBeDefined();
+		expect(signUpPath.post.requestBody).toBeDefined();
+
+		const signUpProps =
+			signUpPath.post.requestBody.content["application/json"].schema.properties;
+		// Should still be boolean, just without default
+		expect(signUpProps.rememberMe).toBeDefined();
+		const baseTypes = getBaseType(signUpProps.rememberMe.type);
+		expect(baseTypes).toContain("boolean");
+	});
 });