diff --git a/client/scripts/autoloads/session_manager.gd b/client/scripts/autoloads/session_manager.gd
index cba63a5d9..d480ae0ce 100644
--- a/client/scripts/autoloads/session_manager.gd
+++ b/client/scripts/autoloads/session_manager.gd
@@ -33,9 +33,11 @@ func new_game() -> String:
 	GameState.current_game_id = game_id
 
 	# #175: Generate world_seed for deterministic simulation (D-010, D-029).
-	# Combines two randi() calls (u32 each) into full u64 entropy range.
-	# Without this, upper 32 bits are always zero — halving the seed space.
-	GameState.world_seed = (rng.randi() << 32) | rng.randi()
+	# Combines two randi() calls (u32 each) into 63-bit entropy range.
+	# Mask bit 31 of the upper word before shifting to prevent signed overflow:
+	# GDScript int is i64 — if bit 63 is set, MessagePack encodes as negative,
+	# and Rust rmp_serde rejects negative values when deserializing as u64.
+	GameState.world_seed = ((rng.randi() & 0x7FFFFFFF) << 32) | rng.randi()
 
 	# Persist world_seed to save directory so resume_game() can restore it.
 	# Without this, loaded sessions would send seed=0, breaking D-010 determinism.
@@ -134,12 +136,14 @@ func _write_seed_file(save_path: String, seed: int) -> void:
 
 
 ## Read world_seed from save directory. Returns 0 if file missing (legacy saves).
+## Masks the sign bit on read: save files written before the signed-overflow fix
+## may contain negative i64 values that Rust rmp_serde rejects as u64.
 func _read_seed_file(save_path: String) -> int:
 	var file := FileAccess.open(save_path + "world_seed", FileAccess.READ)
 	if file == null:
 		push_warning("SessionManager: no seed file in %s — using seed=0 (legacy save)" % save_path)
 		return 0
-	return file.get_64()
+	return file.get_64() & 0x7FFFFFFFFFFFFFFF
 
 
 func _find_newest_save(dir_path: String) -> String: