diff --git a/pgx/_src/games/chess.py b/pgx/_src/games/chess.py
index 433fdb5e0..493d2adf9 100644
--- a/pgx/_src/games/chess.py
+++ b/pgx/_src/games/chess.py
@@ -134,7 +134,20 @@
 ZOBRIST_SIDE = jax.random.randint(keys[1], shape=(2,), minval=0, maxval=2**31 - 1, dtype=jnp.uint32)
 ZOBRIST_CASTLING = jax.random.randint(keys[2], shape=(4, 2), minval=0, maxval=2**31 - 1, dtype=jnp.uint32)
 ZOBRIST_EN_PASSANT = jax.random.randint(keys[3], shape=(65, 2), minval=0, maxval=2**31 - 1, dtype=jnp.uint32)
-INIT_ZOBRIST_HASH = jnp.uint32([1455170221, 1478960862])
+
+
+def _init_zobrist_hash() -> Array:
+    # Derived from the tables above instead of hardcoded: the tables come from jax.random,
+    # whose outputs are not stable across JAX versions — a stale constant silently breaks
+    # repetition counting for the initial position.
+    hash_ = ZOBRIST_SIDE  # color == 0
+    hash_ ^= lax.reduce(ZOBRIST_BOARD[jnp.arange(64), INIT_BOARD + 6], 0, lax.bitwise_xor, (0,))
+    hash_ ^= lax.reduce(ZOBRIST_CASTLING, 0, lax.bitwise_xor, (0,))  # all castling rights
+    hash_ ^= ZOBRIST_EN_PASSANT[-1]  # en_passant == -1
+    return hash_
+
+
+INIT_ZOBRIST_HASH = _init_zobrist_hash()
 
 
 class GameState(NamedTuple):