diff --git a/proptest-regressions/event_preview.txt b/proptest-regressions/event_preview.txt
new file mode 100644
index 000000000..ff5d33e65
--- /dev/null
+++ b/proptest-regressions/event_preview.txt
@@ -0,0 +1,7 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc 0c081b28fcee5c8c4b5439561a52585269c1cfb4d75510d9001f584fb8ac3aea # shrinks to body = "", detail = ""
diff --git a/specs/task-agent-chat-owner-approval.spec.md b/specs/task-agent-chat-owner-approval.spec.md
index a6d6e2339..fb3bc9343 100644
--- a/specs/task-agent-chat-owner-approval.spec.md
+++ b/specs/task-agent-chat-owner-approval.spec.md
@@ -41,6 +41,8 @@ expiration, and one-shot state before releasing a coding-agent operation.
   prevents an approval click from becoming undecryptable when the bridge device
   was registered after an older room session was established.
 - Keep the existing `org.octos.approval_request` protocol backward compatible.
+- Custom room-message previews show only the HTML-escaped fallback body, never
+  the Rust debug representation or extra protocol fields.
 
 ## Boundaries
 
@@ -50,6 +52,8 @@ expiration, and one-shot state before releasing a coding-agent operation.
 - src/shared/mod.rs
 - src/home/room_screen.rs
 - src/sliding_sync.rs
+- src/event_preview.rs
+- proptest-regressions/event_preview.txt
 - resources/i18n/en.json
 - resources/i18n/zh-CN.json
 
@@ -69,6 +73,16 @@ expiration, and one-shot state before releasing a coding-agent operation.
 
 ## Completion Criteria
 
+### Rule: custom-preview — preview depends only on escaped fallback body
+
+Scenario: Custom message summaries do not expose protocol internals
+  Test: custom_message_preview_shows_escaped_body_without_protocol_details
+  Test: prop_custom_preview_depends_only_on_escaped_body
+  Given a custom room message with a fallback body and arbitrary extra fields
+  When text_preview_of_message creates a room or reply summary
+  Then the preview is exactly the HTML-escaped fallback body
+  And protocol fields and Rust debug output do not appear
+
 Scenario: Public project-room status is read-only
   Test: test_agentchat_public_status_has_no_actions
   Given a timeline event has msgtype `com.agentchat.approval.status.v1`
diff --git a/src/event_preview.rs b/src/event_preview.rs
index a80bf5810..4bbb0b721 100644
--- a/src/event_preview.rs
+++ b/src/event_preview.rs
@@ -478,10 +478,7 @@ fn text_preview_of_message(
                 htmlize::escape_text(&video.body)
             }
         ),
-        MessageType::_Custom(custom) => format!(
-            "[Custom message]: {:?}",
-            custom,
-        ),
+        MessageType::_Custom(_) => htmlize::escape_text(msg.body()).to_string(),
         other => format!(
             "[Unknown message type]: {}",
             htmlize::escape_text(other.body()),
@@ -834,3 +831,34 @@ pub fn text_preview_of_room_membership_change(
     };
     Some(TextPreview::from((text, BeforeText::UsernameWithoutColon)))
 }
+
+#[cfg(test)]
+mod preview_tests {
+    use super::*;
+    use proptest::prelude::*;
+
+    fn custom_message(body: &str, detail: &str) -> MessageType {
+        serde_json::from_value(serde_json::json!({
+            "msgtype": "com.agentchat.approval.status.v1",
+            "body": body,
+            "com.agentchat.approval": { "kind": "status", "internal_detail": detail },
+        })).expect("valid custom room message")
+    }
+
+    #[test]
+    fn custom_message_preview_shows_escaped_body_without_protocol_details() {
+        let message = custom_message("等待 <owner> & approval", "PRIVATE_SENTINEL");
+        let preview = text_preview_of_message(&message, "agent").text;
+        assert_eq!(preview, "等待 &lt;owner&gt; &amp; approval");
+        assert!(!preview.contains("PRIVATE_SENTINEL"));
+        assert!(!preview.contains("CustomEventContent"));
+    }
+
+    proptest! {
+        #[test]
+        fn prop_custom_preview_depends_only_on_escaped_body(body in any::<String>(), detail in any::<String>()) {
+            let preview = text_preview_of_message(&custom_message(&body, &detail), "agent").text;
+            prop_assert_eq!(preview, htmlize::escape_text(&body).to_string());
+        }
+    }
+}