diff --git a/crates/matrix-sdk-base/src/sliding_sync/mod.rs b/crates/matrix-sdk-base/src/sliding_sync/mod.rs
index 334336e3ba7..de9f1f7701f 100644
--- a/crates/matrix-sdk-base/src/sliding_sync/mod.rs
+++ b/crates/matrix-sdk-base/src/sliding_sync/mod.rs
@@ -830,7 +830,10 @@ mod tests {
     };
 
     use assert_matches::assert_matches;
-    use matrix_sdk_common::{deserialized_responses::SyncTimelineEvent, ring_buffer::RingBuffer};
+    use matrix_sdk_common::{
+        deserialized_responses::{SyncTimelineEvent, UnableToDecryptInfo, UnableToDecryptReason},
+        ring_buffer::RingBuffer,
+    };
     use matrix_sdk_test::async_test;
     use ruma::{
         api::client::sync::sync_events::UnreadNotificationsCount,
@@ -2372,7 +2375,7 @@ mod tests {
     }
 
     fn make_encrypted_event(id: &str) -> SyncTimelineEvent {
-        SyncTimelineEvent::new(
+        SyncTimelineEvent::new_utd_event(
             Raw::from_json_string(
                 json!({
                     "type": "m.room.encrypted",
@@ -2390,6 +2393,10 @@ mod tests {
                 .to_string(),
             )
             .unwrap(),
+            UnableToDecryptInfo {
+                session_id: Some("".to_owned()),
+                reason: UnableToDecryptReason::MissingMegolmSession,
+            },
         )
     }
 
diff --git a/crates/matrix-sdk-ui/src/timeline/tests/encryption.rs b/crates/matrix-sdk-ui/src/timeline/tests/encryption.rs
index 8e0952d2e13..4b0683529d8 100644
--- a/crates/matrix-sdk-ui/src/timeline/tests/encryption.rs
+++ b/crates/matrix-sdk-ui/src/timeline/tests/encryption.rs
@@ -27,15 +27,13 @@ use matrix_sdk::{
     crypto::{decrypt_room_key_export, types::events::UtdCause, OlmMachine},
     test_utils::test_client_builder,
 };
+use matrix_sdk_base::deserialized_responses::{SyncTimelineEvent, UnableToDecryptReason};
 use matrix_sdk_test::{async_test, BOB};
 use ruma::{
     assign,
-    events::{
-        room::encrypted::{
-            EncryptedEventScheme, MegolmV1AesSha2ContentInit, Relation, Replacement,
-            RoomEncryptedEventContent,
-        },
-        AnySyncTimelineEvent,
+    events::room::encrypted::{
+        EncryptedEventScheme, MegolmV1AesSha2ContentInit, Relation, Replacement,
+        RoomEncryptedEventContent,
     },
     room_id,
     serde::Raw,
@@ -107,7 +105,8 @@ async fn test_retry_message_decryption() {
                 ),
                 None,
             ))
-            .sender(&BOB),
+            .sender(&BOB)
+            .into_utd_sync_timeline_event(),
         )
         .await;
 
@@ -217,7 +216,11 @@ async fn test_retry_edit_decryption() {
         .into(),
     );
     timeline
-        .handle_live_event(f.event(RoomEncryptedEventContent::new(encrypted, None)).sender(&BOB))
+        .handle_live_event(
+            f.event(RoomEncryptedEventContent::new(encrypted, None))
+                .sender(&BOB)
+                .into_utd_sync_timeline_event(),
+        )
         .await;
 
     let event_id =
@@ -244,7 +247,8 @@ async fn test_retry_edit_decryption() {
             f.event(assign!(RoomEncryptedEventContent::new(encrypted, None), {
                 relates_to: Some(Relation::Replacement(Replacement::new(event_id))),
             }))
-            .sender(&BOB),
+            .sender(&BOB)
+            .into_utd_sync_timeline_event(),
         )
         .await;
 
@@ -323,7 +327,8 @@ async fn test_retry_edit_and_more() {
                  mBZdKIaqDTUBFvcvbn2gQaWtUipQdJQRKyv2h0AWveVkv75lp5hRb7jolCi08oMX8cM+V3Zzyi7\
                  mlPAzZjDz0PaRbQwfbMTTHkcL7TZybBi4vLX4f5ZR2Iiysc7gw",
             ))
-            .sender(&BOB),
+            .sender(&BOB)
+            .into_utd_sync_timeline_event(),
         )
         .await;
 
@@ -339,7 +344,9 @@ async fn test_retry_edit_and_more() {
     );
     timeline
         .handle_live_event(
-            f.event(assign!(msg2, { relates_to: Some(Relation::Replacement(Replacement::new(event_id))) })).sender(&BOB),
+            f.event(assign!(msg2, { relates_to: Some(Relation::Replacement(Replacement::new(event_id))) }))
+                .sender(&BOB)
+                .into_utd_sync_timeline_event(),
         )
         .await;
 
@@ -351,7 +358,8 @@ async fn test_retry_edit_and_more() {
                  2r/fEvAW/9QB+N6fX4g9729bt5ftXRqa5QI7NA351RNUveRHxVvx+2x0WJArQjYGRk7tMS2rUto\
                  IYt2ZY17nE1UJjN7M87STnCF9c9qy4aGNqIpeVIht6XbtgD7gQ",
             ))
-            .sender(&BOB),
+            .sender(&BOB)
+            .into_utd_sync_timeline_event(),
         )
         .await;
 
@@ -424,7 +432,8 @@ async fn test_retry_message_decryption_highlighted() {
                 ),
                 None,
             ))
-            .sender(&BOB),
+            .sender(&BOB)
+            .into_utd_sync_timeline_event(),
         )
         .await;
 
@@ -475,7 +484,7 @@ async fn test_utd_cause_for_nonmember_event_is_found() {
     let mut stream = timeline.subscribe().await;
 
     // When we add an event with "membership: leave"
-    timeline.handle_live_event(raw_event_with_unsigned(json!({ "membership": "leave" }))).await;
+    timeline.handle_live_event(utd_event_with_unsigned(json!({ "membership": "leave" }))).await;
 
     // Then its UTD cause is membership
     let item = assert_next_matches!(stream, VectorDiff::PushBack { value } => value);
@@ -495,7 +504,7 @@ async fn test_utd_cause_for_nonmember_event_is_found_unstable_prefix() {
 
     // When we add an event with "io.element.msc4115.membership: leave"
     timeline
-        .handle_live_event(raw_event_with_unsigned(
+        .handle_live_event(utd_event_with_unsigned(
             json!({ "io.element.msc4115.membership": "leave" }),
         ))
         .await;
@@ -517,7 +526,7 @@ async fn test_utd_cause_for_member_event_is_unknown() {
     let mut stream = timeline.subscribe().await;
 
     // When we add an event with "membership: join"
-    timeline.handle_live_event(raw_event_with_unsigned(json!({ "membership": "join" }))).await;
+    timeline.handle_live_event(utd_event_with_unsigned(json!({ "membership": "join" }))).await;
 
     // Then its UTD cause is membership
     let item = assert_next_matches!(stream, VectorDiff::PushBack { value } => value);
@@ -536,7 +545,7 @@ async fn test_utd_cause_for_missing_membership_is_unknown() {
     let mut stream = timeline.subscribe().await;
 
     // When we add an event with no membership in unsigned
-    timeline.handle_live_event(raw_event_with_unsigned(json!({}))).await;
+    timeline.handle_live_event(utd_event_with_unsigned(json!({}))).await;
 
     // Then its UTD cause is membership
     let item = assert_next_matches!(stream, VectorDiff::PushBack { value } => value);
@@ -548,8 +557,8 @@ async fn test_utd_cause_for_missing_membership_is_unknown() {
     assert_eq!(*cause, UtdCause::Unknown);
 }
 
-fn raw_event_with_unsigned(unsigned: serde_json::Value) -> Raw<AnySyncTimelineEvent> {
-    Raw::from_json(
+fn utd_event_with_unsigned(unsigned: serde_json::Value) -> SyncTimelineEvent {
+    let raw = Raw::from_json(
         to_raw_value(&json!({
             "event_id": "$myevent",
             "sender": "@u:s",
@@ -566,5 +575,13 @@ fn raw_event_with_unsigned(unsigned: serde_json::Value) -> Raw<AnySyncTimelineEv
 
         }))
         .unwrap(),
+    );
+
+    SyncTimelineEvent::new_utd_event(
+        raw,
+        matrix_sdk::deserialized_responses::UnableToDecryptInfo {
+            session_id: Some("SESSION_ID".into()),
+            reason: UnableToDecryptReason::MissingMegolmSession,
+        },
     )
 }
diff --git a/crates/matrix-sdk-ui/src/timeline/tests/read_receipts.rs b/crates/matrix-sdk-ui/src/timeline/tests/read_receipts.rs
index 574a8bec6a4..6f0c2e02a17 100644
--- a/crates/matrix-sdk-ui/src/timeline/tests/read_receipts.rs
+++ b/crates/matrix-sdk-ui/src/timeline/tests/read_receipts.rs
@@ -400,7 +400,8 @@ async fn test_read_receipts_updates_on_message_decryption() {
                 ),
                 None,
             ))
-            .sender(&BOB),
+            .sender(&BOB)
+            .into_utd_sync_timeline_event(),
         )
         .await;
 
diff --git a/crates/matrix-sdk-ui/src/timeline/tests/shields.rs b/crates/matrix-sdk-ui/src/timeline/tests/shields.rs
index ad86b21c913..b0a18ff9253 100644
--- a/crates/matrix-sdk-ui/src/timeline/tests/shields.rs
+++ b/crates/matrix-sdk-ui/src/timeline/tests/shields.rs
@@ -151,7 +151,8 @@ async fn test_utd_shield() {
                 ),
                 None,
             ))
-            .sender(&ALICE),
+            .sender(&ALICE)
+            .into_utd_sync_timeline_event(),
         )
         .await;
 
diff --git a/crates/matrix-sdk/src/test_utils/events.rs b/crates/matrix-sdk/src/test_utils/events.rs
index 9ace4ccd49f..3ecaebb5b47 100644
--- a/crates/matrix-sdk/src/test_utils/events.rs
+++ b/crates/matrix-sdk/src/test_utils/events.rs
@@ -16,7 +16,9 @@
 
 use std::sync::atomic::{AtomicU64, Ordering::SeqCst};
 
+use as_variant::as_variant;
 use matrix_sdk_base::deserialized_responses::{SyncTimelineEvent, TimelineEvent};
+use matrix_sdk_common::deserialized_responses::UnableToDecryptReason;
 use ruma::{
     events::{
         message::TextContentBlock,
@@ -31,6 +33,7 @@ use ruma::{
         reaction::ReactionEventContent,
         relation::{Annotation, InReplyTo, Replacement, Thread},
         room::{
+            encrypted::{EncryptedEventScheme, RoomEncryptedEventContent},
             message::{Relation, RoomMessageEventContent, RoomMessageEventContentWithoutRelation},
             redaction::RoomRedactionEventContent,
         },
@@ -185,6 +188,23 @@ where
     }
 }
 
+impl EventBuilder<RoomEncryptedEventContent> {
+    /// Turn this event into a SyncTimelineEvent representing a decryption
+    /// failure
+    pub fn into_utd_sync_timeline_event(self) -> SyncTimelineEvent {
+        let session_id = as_variant!(&self.content.scheme, EncryptedEventScheme::MegolmV1AesSha2)
+            .map(|content| content.session_id.clone());
+
+        SyncTimelineEvent::new_utd_event(
+            self.into(),
+            crate::deserialized_responses::UnableToDecryptInfo {
+                session_id,
+                reason: UnableToDecryptReason::MissingMegolmSession,
+            },
+        )
+    }
+}
+
 impl EventBuilder<RoomMessageEventContent> {
     /// Adds a reply relation to the current event.
     pub fn reply_to(mut self, event_id: &EventId) -> Self {