diff --git a/src/api/core/ciphers.rs b/src/api/core/ciphers.rs
index 74274a3a..237df116 100644
--- a/src/api/core/ciphers.rs
+++ b/src/api/core/ciphers.rs
@@ -159,7 +159,25 @@ async fn sync(data: SyncData, headers: Headers, client_version: Option<ClientVer
     let domains_json = if data.exclude_domains {
         Value::Null
     } else {
-        api::core::_get_eq_domains(headers, true).into_inner()
+        api::core::_get_eq_domains(&headers, true).into_inner()
+    };
+
+    // This is very similar to the the userDecryptionOptions sent in connect/token,
+    // but as of 2025-12-19 they're both using different casing conventions.
+    let has_master_password = !headers.user.password_hash.is_empty();
+    let master_password_unlock = if has_master_password {
+        json!({
+            "kdf": {
+                "kdfType": headers.user.client_kdf_type,
+                "iterations": headers.user.client_kdf_iter,
+                "memory": headers.user.client_kdf_memory,
+                "parallelism": headers.user.client_kdf_parallelism
+            },
+            "masterKeyEncryptedUserKey": headers.user.akey,
+            "salt": headers.user.email
+        })
+    } else {
+        Value::Null
     };
 
     Ok(Json(json!({
@@ -170,6 +188,9 @@ async fn sync(data: SyncData, headers: Headers, client_version: Option<ClientVer
         "ciphers": ciphers_json,
         "domains": domains_json,
         "sends": sends_json,
+        "userDecryption": {
+            "masterPasswordUnlock": master_password_unlock,
+        },
         "object": "sync"
     })))
 }
diff --git a/src/api/core/mod.rs b/src/api/core/mod.rs
index 5d236bf5..dc7f4628 100644
--- a/src/api/core/mod.rs
+++ b/src/api/core/mod.rs
@@ -74,11 +74,11 @@ const GLOBAL_DOMAINS: &str = include_str!("../../static/global_domains.json");
 
 #[get("/settings/domains")]
 fn get_eq_domains(headers: Headers) -> Json<Value> {
-    _get_eq_domains(headers, false)
+    _get_eq_domains(&headers, false)
 }
 
-fn _get_eq_domains(headers: Headers, no_excluded: bool) -> Json<Value> {
-    let user = headers.user;
+fn _get_eq_domains(headers: &Headers, no_excluded: bool) -> Json<Value> {
+    let user = &headers.user;
     use serde_json::from_str;
 
     let equivalent_domains: Vec<Vec<String>> = from_str(&user.equivalent_domains).unwrap();