From 4efb5efa5d419fdacb67bf323ebca55b301e4b9d Mon Sep 17 00:00:00 2001
From: KinfeMichael Tariku <65047246+Kinfe123@users.noreply.github.com>
Date: Tue, 2 Sep 2025 19:24:47 +0300
Subject: [PATCH] fix: prevent undefined from passed to adapter in username
 plugin (#4361)

---
 packages/better-auth/src/adapters/create-adapter/index.ts | 4 +++-
 packages/better-auth/src/plugins/username/index.ts        | 8 ++++++--
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/packages/better-auth/src/adapters/create-adapter/index.ts b/packages/better-auth/src/adapters/create-adapter/index.ts
index 50b1a062bd..aa68df92f6 100644
--- a/packages/better-auth/src/adapters/create-adapter/index.ts
+++ b/packages/better-auth/src/adapters/create-adapter/index.ts
@@ -409,7 +409,9 @@ export const createAdapter =
 					});
 				}
 
-				transformedData[newFieldName] = newValue;
+				if (newValue !== undefined) {
+					transformedData[newFieldName] = newValue;
+				}
 			}
 			return transformedData;
 		};
diff --git a/packages/better-auth/src/plugins/username/index.ts b/packages/better-auth/src/plugins/username/index.ts
index 7fe58f03db..5545ab0ac2 100644
--- a/packages/better-auth/src/plugins/username/index.ts
+++ b/packages/better-auth/src/plugins/username/index.ts
@@ -488,8 +488,12 @@ export const username = (options?: UsernameOptions) => {
 						);
 					},
 					handler: createAuthMiddleware(async (ctx) => {
-						ctx.body.displayUsername ||= ctx.body.username;
-						ctx.body.username ||= ctx.body.displayUsername;
+						if (ctx.body.username && !ctx.body.displayUsername) {
+							ctx.body.displayUsername = ctx.body.username;
+						}
+						if (ctx.body.displayUsername && !ctx.body.username) {
+							ctx.body.username = ctx.body.displayUsername;
+						}
 					}),
 				},
 			],