diff --git a/contracts/split/src/lib.rs b/contracts/split/src/lib.rs
index 3a9c2b1..7e332de 100644
--- a/contracts/split/src/lib.rs
+++ b/contracts/split/src/lib.rs
@@ -100,6 +100,13 @@ fn admin_key() -> Symbol {
 fn admins_key() -> Symbol {
     symbol_short!("admins")
 }
+
+/// Issue #477: One-shot initialiser guard — instance storage.
+/// Set to `true` at the end of `initialize()`; checked at the top to prevent
+/// re-initialisation (front-run protection).
+fn initialised_key() -> Symbol {
+    symbol_short!("init_flg")
+}
 fn paused_key() -> Symbol {
     symbol_short!("paused")
 }
@@ -2385,10 +2392,11 @@ impl SplitContract {
         rate_limit: u32,
         rate_window: u64,
     ) {
-        assert!(
-            !env.storage().instance().has(&admin_key()),
-            "already initialized"
-        );
+        // Issue #477: one-shot initialiser guard using a dedicated key so the
+        // guard cannot be bypassed by front-running the admin write.
+        if env.storage().instance().get::<_, bool>(&initialised_key()).unwrap_or(false) {
+            panic!("AlreadyInitialised");
+        }
         assert!(creation_fee >= 0, "creation_fee must be non-negative");
         assert!(
             platform_fee_bps <= 10_000,
@@ -2431,6 +2439,9 @@ impl SplitContract {
         env.storage()
             .instance()
             .set(&storage_quota_key(), &DEFAULT_INVOICE_STORAGE_QUOTA);
+        // Issue #477: set the initialisation flag atomically at the end so the
+        // contract is marked fully initialised only after all state is written.
+        env.storage().instance().set(&initialised_key(), &true);
     }
 
     /// Add a new admin with a given role. Requires SuperAdmin auth.
diff --git a/contracts/split/src/storage_snapshot.rs b/contracts/split/src/storage_snapshot.rs
index 176d896..eeabf3e 100644
--- a/contracts/split/src/storage_snapshot.rs
+++ b/contracts/split/src/storage_snapshot.rs
@@ -125,6 +125,8 @@ fn storage_key_snapshot() {
         "upgrade_proposal_key",
         hex_xdr(&env, upgrade_proposal_key()),
     ));
+    // Issue #477: one-shot initialiser guard — instance storage
+    keys.push(("initialised_key", hex_xdr(&env, initialised_key())));
 
     // -----------------------------------------------------------------------
     // Persistent-tier keys (per-entity)
diff --git a/tests/snapshots/storage_keys.json b/tests/snapshots/storage_keys.json
index acac5b8..5514243 100644
--- a/tests/snapshots/storage_keys.json
+++ b/tests/snapshots/storage_keys.json
@@ -50,6 +50,7 @@
     "governance_contract_key": "0000000f00000007676f765f63747200",
     "group_key": "0000001000000001000000020000000f0000000367727000000000050000000000000001",
     "group_treasury_key": "0000001000000001000000020000000f000000066772705f74720000000000050000000000000001",
+    "initialised_key": "0000000f00000008696e69745f666c67",
     "invoice_compact_key": "0000001000000001000000020000000f00000007696e765f63707400000000050000000000000001",
     "invoice_count_key": "0000001000000001000000020000000f00000009696e765f636f756e740000000000001200000000000000000000000000000000000000000000000000000000000000000000000000000000",
     "invoice_ext2_key": "0000001000000001000000020000000f00000007696e765f65783200000000050000000000000001",