diff --git a/server/src/crdt_snapshot.rs b/server/src/crdt_snapshot.rs
index cdd8c13c..a7f2b346 100644
--- a/server/src/crdt_snapshot.rs
+++ b/server/src/crdt_snapshot.rs
@@ -439,6 +439,12 @@ mod tests {
 
     use crate::crdt_state::PipelineDoc;
 
+    /// Serialises tests that mutate the global SNAPSHOT_STATE / ALL_OPS / VECTOR_CLOCK
+    /// statics. These statics are shared across test threads (only the per-thread
+    /// CRDT is thread-local), so without this lock parallel tests interleave their
+    /// op writes and snapshot generation.
+    static GLOBAL_STATE_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
+
     // ── Wire message tests ──────────────────────────────────────────────
 
     /// Snapshot wire message serialization round-trip.
@@ -516,12 +522,14 @@ mod tests {
     /// Single node is always leader.
     #[test]
     fn single_node_is_always_leader() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         assert!(is_snapshot_leader("node_a", &[]));
     }
 
     /// Leader is the node with the lowest hash.
     #[test]
     fn leader_is_lowest_hash_node() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         let nodes = vec![
             "node_alpha".to_string(),
             "node_beta".to_string(),
@@ -553,6 +561,7 @@ mod tests {
     /// Leader selection is deterministic.
     #[test]
     fn leader_selection_is_deterministic() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         let peers = vec!["a".to_string(), "b".to_string(), "c".to_string()];
         let result1 = is_snapshot_leader("a", &peers);
         let result2 = is_snapshot_leader("a", &peers);
@@ -564,6 +573,7 @@ mod tests {
     /// Threshold is configurable.
     #[test]
     fn snapshot_threshold_default() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         // When not set, defaults to DEFAULT_SNAPSHOT_THRESHOLD.
         assert_eq!(snapshot_threshold(), DEFAULT_SNAPSHOT_THRESHOLD);
     }
@@ -573,6 +583,7 @@ mod tests {
     /// Record ack returns true when all peers have acked.
     #[test]
     fn coordination_quorum_reached() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         init();
         let snapshot = Snapshot {
             at_seq: 10,
@@ -593,6 +604,7 @@ mod tests {
     /// Ack for wrong at_seq is rejected.
     #[test]
     fn coordination_ack_wrong_seq_rejected() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         init();
         let snapshot = Snapshot {
             at_seq: 20,
@@ -610,6 +622,7 @@ mod tests {
     /// Abort clears pending state.
     #[test]
     fn coordination_abort_clears_state() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         init();
         let snapshot = Snapshot {
             at_seq: 30,
@@ -630,6 +643,7 @@ mod tests {
     /// Unacked peers are reported correctly.
     #[test]
     fn unacked_peers_reported() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         init();
         let snapshot = Snapshot {
             at_seq: 40,
@@ -660,6 +674,7 @@ mod tests {
     /// Snapshot generation from ops includes attribution manifest.
     #[test]
     fn snapshot_generation_includes_manifest() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         crdt_state::init_for_test();
 
         // Write some items to populate ALL_OPS.
@@ -705,6 +720,7 @@ mod tests {
     /// Attribution can be queried by story_id after snapshot.
     #[test]
     fn attribution_query_by_story_id() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         crdt_state::init_for_test();
         init();
 
@@ -741,6 +757,7 @@ mod tests {
     /// After compaction, ALL_OPS size is reduced.
     #[test]
     fn compaction_reduces_ops() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         crdt_state::init_for_test();
         init();
 
@@ -782,6 +799,7 @@ mod tests {
     /// Latest snapshot is available after compaction.
     #[test]
     fn latest_snapshot_available_after_compaction() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         crdt_state::init_for_test();
         init();
 
@@ -813,6 +831,7 @@ mod tests {
     /// the same `at_seq` and pruned ops disappear from each node's log.
     #[test]
     fn three_node_compaction_convergence() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         use bft_json_crdt::json_crdt::BaseCrdt;
 
         // Simulate 3 independent nodes.
@@ -924,6 +943,7 @@ mod tests {
     /// abort the compaction cleanly (no half-applied snapshot state).
     #[test]
     fn failure_mode_node_offline_aborts_cleanly() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         init();
 
         let snapshot = Snapshot {
@@ -969,6 +989,7 @@ mod tests {
     /// + ops with seq >= at_seq.
     #[test]
     fn new_node_onboarding_with_snapshot() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         let kp = make_keypair();
         let mut crdt_existing = BaseCrdt::<PipelineDoc>::new(&kp);
 
@@ -1026,6 +1047,7 @@ mod tests {
     /// Peers without snapshot support fall back to vector-clock-based full sync.
     #[test]
     fn backwards_compat_unknown_snapshot_message_ignored() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         // A peer that doesn't understand snapshot messages should be able to
         // parse them as unknown variants and ignore them gracefully.
         let snapshot_json = r#"{"type":"snapshot","at_seq":100,"state":["op1"],"op_manifest":[]}"#;
@@ -1054,6 +1076,7 @@ mod tests {
     /// After compaction, an archived story's attribution can be reconstructed.
     #[test]
     fn attribution_preserved_after_compaction() {
+        let _g = GLOBAL_STATE_LOCK.lock().unwrap_or_else(|e| e.into_inner());
         crdt_state::init_for_test();
         init();