From 7a4c7dd46d3d90dfd2ddc6ca7efa10188ea1d456 Mon Sep 17 00:00:00 2001
From: stefiosif <si.iosifidis@gmail.com>
Date: Wed, 15 Jul 2026 19:37:29 +0300
Subject: [PATCH] Compare with original alpha to decide node type

---
 src/search/negamax.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/search/negamax.rs b/src/search/negamax.rs
index db6f487..a71f95a 100644
--- a/src/search/negamax.rs
+++ b/src/search/negamax.rs
@@ -106,6 +106,9 @@ pub fn negamax(
         )
     });
 
+    // Keep alpha before the loop raises it, so the TT node type uses
+    // the window we actually searched for this node
+    let alpha_original = alpha;
     let mut legal_moves = 0;
     let mut best_move = entry.and_then(|entry| entry.mv);
     let mut best_score = MIN_SCORE;
@@ -187,7 +190,7 @@ pub fn negamax(
 
     let node_type = match best_score {
         s if s >= beta => NodeType::LowerBound,
-        s if s <= alpha => NodeType::UpperBound,
+        s if s <= alpha_original => NodeType::UpperBound,
         _ => NodeType::Exact,
     };