diff --git a/server/src/bridge/text_renderer.rs b/server/src/bridge/text_renderer.rs
index 79d746029..ea0929978 100644
--- a/server/src/bridge/text_renderer.rs
+++ b/server/src/bridge/text_renderer.rs
@@ -299,6 +299,7 @@ mod tests {
             dialogue_response: None,
             blocked_entities: vec![],
             scan_events: vec![],
+            rng_seed: None,
         }
     }
 
@@ -422,6 +423,7 @@ mod tests {
             dialogue_response: None,
             blocked_entities: vec![],
             scan_events: vec![],
+            rng_seed: None,
         };
         let text = format_snapshot_text(&snap);
         assert!(text.contains("Tick 0"));
diff --git a/server/src/bridge/types.rs b/server/src/bridge/types.rs
index dee8ed37d..7395e0505 100644
--- a/server/src/bridge/types.rs
+++ b/server/src/bridge/types.rs
@@ -15,7 +15,7 @@ pub use crate::simulation::time::{DayPhase, TickRate};
 /// negotiation is unnecessary. Client should reject snapshots with version !=
 /// PROTOCOL_VERSION. New fields use #[serde(default)] only during the migration
 /// period, then the default is removed once both sides are updated.
-pub const PROTOCOL_VERSION: u8 = 9;
+pub const PROTOCOL_VERSION: u8 = 10;
 
 /// The ONLY data structure crossing the client-server boundary (D-020)
 /// Contains all information visible to the observer at a given tick.
@@ -28,6 +28,7 @@ pub const PROTOCOL_VERSION: u8 = 9;
 /// v7 adds: pending_recognitions (#423, D-060 cognitive delay).
 /// v8 adds: dialogue_response (#305, D-028 dialogue pipeline).
 /// v9 adds: blocked_entities (#514, debug field for LOS-blocked entities).
+/// v10 adds: rng_seed (#527, deterministic replay — completes WRONG button loop).
 /// Future fields: ambient sound events, HUD state (D-020 expansion).
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ObserverSnapshot {
@@ -79,6 +80,11 @@ pub struct ObserverSnapshot {
     /// Sorted ascending for deterministic output. Client can safely ignore.
     #[serde(default)]
     pub blocked_entities: Vec<u64>,
+    /// RNG seed active at this tick for deterministic replay (#527).
+    /// The WRONG button writes this to seed.txt so replays reproduce observed bugs.
+    /// None when the RNG resource is unavailable (should not occur in practice).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub rng_seed: Option<u64>,
 }
 
 /// Game time data for client display (D-031)
diff --git a/server/src/perception/observer/mod.rs b/server/src/perception/observer/mod.rs
index aa7887317..1b196310d 100644
--- a/server/src/perception/observer/mod.rs
+++ b/server/src/perception/observer/mod.rs
@@ -21,6 +21,7 @@ use crate::simulation::interaction::NearbyInteractionBuffer;
 use crate::simulation::inventory::{CarriedBy, InventorySlot, ItemName};
 use crate::simulation::monologue::{MonologueBuffer, SprintAnomalyQueue};
 use crate::simulation::movement::{PlayerCharacter, TilePosition, WalkabilityMap};
+use crate::simulation::rng::SimRng;
 use crate::simulation::stance::Stance;
 use crate::simulation::time::SimulationTime;
 
@@ -81,6 +82,7 @@ pub fn compute_observer_snapshot(
     )>,
     inventory_items: Query<(Entity, &CarriedBy, &ItemName, &InventorySlot)>,
     mut buffer: ResMut<SnapshotBuffer>,
+    sim_rng: Option<Res<SimRng>>,
 ) {
     let Ok((
         observer_entity,
@@ -215,6 +217,7 @@ pub fn compute_observer_snapshot(
         dialogue_response,
         blocked_entities,
         scan_events,
+        rng_seed: sim_rng.as_deref().map(|r| r.seed()),
     });
 }