diff --git a/agents/drivers/mongodb/src/main/java/com/dbx/agent/mongodb/MongoAgent.java b/agents/drivers/mongodb/src/main/java/com/dbx/agent/mongodb/MongoAgent.java
index 073b9d5789..df124f0503 100644
--- a/agents/drivers/mongodb/src/main/java/com/dbx/agent/mongodb/MongoAgent.java
+++ b/agents/drivers/mongodb/src/main/java/com/dbx/agent/mongodb/MongoAgent.java
@@ -257,7 +257,7 @@ public final class MongoAgent {
         if (authSource != null && !authSource.isBlank()) {
             return authSource;
         }
-        return defaultString(stringOrNull(connObj, "database"), "admin");
+        return "admin";
     }
 
     private static String urlParam(String urlParams, String key) {
diff --git a/agents/drivers/mongodb/src/test/java/com/dbx/agent/mongodb/MongoAgentTest.java b/agents/drivers/mongodb/src/test/java/com/dbx/agent/mongodb/MongoAgentTest.java
index 03c32f4042..b6a3a14e4a 100644
--- a/agents/drivers/mongodb/src/test/java/com/dbx/agent/mongodb/MongoAgentTest.java
+++ b/agents/drivers/mongodb/src/test/java/com/dbx/agent/mongodb/MongoAgentTest.java
@@ -135,11 +135,11 @@ class MongoAgentTest {
     }
 
     @Test
-    void fallsBackToDefaultDatabaseWhenAuthSourceIsMissing() {
+    void fallsBackToAdminWhenAuthSourceIsMissing() {
         JsonObject connection = new JsonObject();
         connection.addProperty("database", "gray_lite_twin_fat");
 
-        assertEquals("gray_lite_twin_fat", MongoAgent.authenticationDatabase(connection));
+        assertEquals("admin", MongoAgent.authenticationDatabase(connection));
     }
 
     // ─── TLS: configureBuilder JSON parsing ───
diff --git a/crates/dbx-core/src/models/connection.rs b/crates/dbx-core/src/models/connection.rs
index b66ec2cdda..ffdb4ea96e 100644
--- a/crates/dbx-core/src/models/connection.rs
+++ b/crates/dbx-core/src/models/connection.rs
@@ -1080,7 +1080,7 @@ impl ConnectionConfig {
             }
             DatabaseType::Databend => normalize_bare_mysql_url_params(value),
             DatabaseType::Postgres | DatabaseType::Redshift => normalize_postgres_url_params(value, self.ssl),
-            DatabaseType::MongoDb => normalize_mongo_url_params(value, self.ssl),
+            DatabaseType::MongoDb => normalize_mongo_url_params(value, self.ssl, !self.username.trim().is_empty()),
             _ => value.trim_start_matches('?').to_string(),
         }
     }
@@ -1170,7 +1170,7 @@ fn normalize_mysql_url_params(value: &str, force_tls: bool, accept_invalid_certs
     parts.join("&")
 }
 
-fn normalize_mongo_url_params(value: &str, force_tls: bool) -> String {
+fn normalize_mongo_url_params(value: &str, force_tls: bool, default_auth_source: bool) -> String {
     let value = value.trim_start_matches('?');
     let mut parts: Vec<String> = value.split('&').filter(|part| !part.is_empty()).map(str::to_string).collect();
 
@@ -1179,6 +1179,10 @@ fn normalize_mongo_url_params(value: &str, force_tls: bool) -> String {
         parts.insert(0, "tls=true".to_string());
     }
 
+    if default_auth_source && !parts.iter().any(|part| url_param_key_is(part, "authSource")) {
+        parts.push("authSource=admin".to_string());
+    }
+
     parts.join("&")
 }
 
@@ -2137,10 +2141,32 @@ mod tests {
     }
 
     #[test]
-    fn mongodb_form_url_without_params_does_not_force_topology_or_auth() {
+    fn mongodb_form_url_without_params_defaults_auth_source_to_admin() {
         let config = mongodb_config("root", "secret", Some("admin"));
 
-        assert_eq!(config.connection_url(), "mongodb://root:secret@10.1.2.3:17000/admin");
+        assert_eq!(config.connection_url(), "mongodb://root:secret@10.1.2.3:17000/admin?authSource=admin");
+    }
+
+    #[test]
+    fn mongodb_form_url_default_database_does_not_change_auth_source() {
+        let config = mongodb_config("root", "secret", Some("app"));
+
+        assert_eq!(config.connection_url(), "mongodb://root:secret@10.1.2.3:17000/app?authSource=admin");
+    }
+
+    #[test]
+    fn mongodb_form_url_without_username_does_not_default_auth_source() {
+        let config = mongodb_config("", "", Some("app"));
+
+        assert_eq!(config.connection_url(), "mongodb://10.1.2.3:17000/app");
+    }
+
+    #[test]
+    fn mongodb_form_url_preserves_explicit_auth_source() {
+        let mut config = mongodb_config("root", "secret", Some("app"));
+        config.url_params = Some("authSource=app".to_string());
+
+        assert_eq!(config.connection_url(), "mongodb://root:secret@10.1.2.3:17000/app?authSource=app");
     }
 
     #[test]
@@ -2228,8 +2254,8 @@ mod tests {
         let mut config = mongodb_config("root", "secret", Some("admin"));
         config.ssl = true;
 
-        assert_eq!(config.connection_url(), "mongodb://root:secret@10.1.2.3:17000/admin?tls=true");
-        assert_eq!(config.redacted_connection_url(), "mongodb://10.1.2.3:17000/admin?tls=true");
+        assert_eq!(config.connection_url(), "mongodb://root:secret@10.1.2.3:17000/admin?tls=true&authSource=admin");
+        assert_eq!(config.redacted_connection_url(), "mongodb://10.1.2.3:17000/admin?tls=true&authSource=admin");
     }
 
     #[test]