From 925f5a1294ee464514188fee181cd2fcee2be712 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 27 May 2026 12:57:30 +0100
Subject: [PATCH] Handle MSC4242 in redact

---
 rust/src/events/constants.rs |  2 ++
 rust/src/events/utils.rs     | 13 +++++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/rust/src/events/constants.rs b/rust/src/events/constants.rs
index 26209e4ba0..5f1d8572eb 100644
--- a/rust/src/events/constants.rs
+++ b/rust/src/events/constants.rs
@@ -61,6 +61,8 @@ pub mod event_field {
     pub const REPLACES_STATE: &str = "replaces_state";
     /// Event field: msc4354_sticky
     pub const MSC4354_STICKY: &str = "msc4354_sticky";
+    // Event field: prev_state_events
+    pub const PREV_STATE_EVENTS: &str = "prev_state_events";
 }
 
 pub mod unsigned_field {
diff --git a/rust/src/events/utils.rs b/rust/src/events/utils.rs
index c1a5d0e618..33a4849c53 100644
--- a/rust/src/events/utils.rs
+++ b/rust/src/events/utils.rs
@@ -24,8 +24,10 @@ use super::constants::{
     redaction_field,
     unsigned_field::AGE_TS,
 };
-use crate::canonical_json::CanonicalizationOptions;
 use crate::room_versions::{EventFormatVersions, RoomVersion};
+use crate::{
+    canonical_json::CanonicalizationOptions, events::constants::event_field::PREV_STATE_EVENTS,
+};
 
 /// Calculates the event_id of an event.
 ///
@@ -99,7 +101,6 @@ pub fn redact(event: &Value, room_version: &RoomVersion) -> anyhow::Result<Value
         (STATE_KEY),
         (DEPTH),
         (PREV_EVENTS),
-        (AUTH_EVENTS),
         (ORIGIN_SERVER_TS),
     ]);
 
@@ -108,6 +109,14 @@ pub fn redact(event: &Value, room_version: &RoomVersion) -> anyhow::Result<Value
         allowed_keys.extend([PREV_STATE, MEMBERSHIP, ORIGIN]);
     }
 
+    // Room versions with MSC4242 have `prev_state_events` instead of
+    // `auth_events`.
+    if room_version.msc4242_state_dags {
+        allowed_keys.insert(PREV_STATE_EVENTS);
+    } else {
+        allowed_keys.insert(AUTH_EVENTS);
+    }
+
     let event_type = event
         .get(TYPE)
         .with_context(|| format!("Missing {TYPE} field in json"))?