From 319d61effdad40ac633425d6504a98f6d2ad0cd2 Mon Sep 17 00:00:00 2001
From: Dubslow <bunslow@gmail.com>
Date: Tue, 19 May 2026 18:38:07 +0200
Subject: [PATCH] Penalize TTEs whose inexact value mismatches the current
 window

Credit shared with @xu-shawn, who has done much of the work to instrument the search and investigate search explosions and endgame paralysis: https://github.com/official-stockfish/Stockfish/issues/6742#issuecomment-4386174472

This patch is a natural result of that commentary. It can also be seen as a refinement of more generalized secondary aging.

Passed STC: https://tests.stockfishchess.org/tests/view/69fb18e8c91f9625ce325427
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 434784 W: 111417 L: 110528 D: 212839
Ptnml(0-2): 1089, 50675, 112949, 51616, 1063

Passed LTC: https://tests.stockfishchess.org/tests/view/6a0379588d9bd4cd7cd68edf
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 81018 W: 20558 L: 20154 D: 40306
Ptnml(0-2): 35, 8554, 22939, 8934, 47

VVLTC *reversion*: https://tests.stockfishchess.org/tests/view/6a0622538d9bd4cd7cd69340
Ptnml(0-2): 2, 5159, 18107, 5050, 0
LLR: -2.94 <0.50,2.50>
LLR: -2.47 <0.00,2.00>
LLR: -2.24 <-0.25,1.75>

closes https://github.com/official-stockfish/Stockfish/pull/6819

Bench: 2676789
---
 src/search.cpp   | 6 ++++++
 src/tt.cpp       | 6 ++++++
 src/tt.h         | 1 +
 tests/testing.py | 1 +
 4 files changed, 14 insertions(+)

diff --git a/src/search.cpp b/src/search.cpp
index babe6f05c..5988c23fd 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -853,6 +853,12 @@ Value Search::Worker::search(
             else
                 return ttData.value;
         }
+    }  // No cutoff, but why? Does the stored inexact value mismatch our aspiration window?
+    else if (!PvNode && !excludedMove && ttData.depth > depth - (ttData.value <= beta)
+             && is_valid(ttData.value) && ttData.bound != BOUND_EXACT
+             && ttData.bound & (ttData.value >= beta ? BOUND_UPPER : BOUND_LOWER) && depth > 5)
+    {  // If a window-bound mismatch is the only reason cutoff failed, penalize the now-useless tte
+        ttWriter.penalize(1);
     }
 
     // Step 6. Tablebases probe
diff --git a/src/tt.cpp b/src/tt.cpp
index 359399347..7f7c5ab4e 100644
--- a/src/tt.cpp
+++ b/src/tt.cpp
@@ -75,6 +75,7 @@ struct TTEntry {
 
    private:
     friend class TranspositionTable;
+    friend struct TTWriter;
 
     uint16_t key16;
     uint8_t  depth8;
@@ -137,6 +138,11 @@ void TTWriter::write(
     entry->save(k, v, pv, b, d, m, ev, curr_generation);
 }
 
+void TTWriter::penalize(int penalty) {
+    assert(entry->depth8 + DEPTH_NONE > penalty);
+    entry->depth8 -= penalty;
+}
+
 
 // A TranspositionTable is an array of Cluster, of size clusterCount. Each cluster consists of ClusterSize number
 // of TTEntry. Each non-empty TTEntry contains information on exactly one position. The size of a Cluster should
diff --git a/src/tt.h b/src/tt.h
index 584b92bff..2f8091725 100644
--- a/src/tt.h
+++ b/src/tt.h
@@ -68,6 +68,7 @@ struct TTData {
 struct TTWriter {
    public:
     void write(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev, uint8_t generation8);
+    void penalize(int penalty);  // decrement stored depth by the penalty
 
    private:
     friend class TranspositionTable;
diff --git a/tests/testing.py b/tests/testing.py
index 619c8a59c..9711a43ba 100644
--- a/tests/testing.py
+++ b/tests/testing.py
@@ -51,6 +51,7 @@ class TSAN:
                 """
 race:Stockfish::TTEntry::read
 race:Stockfish::TTEntry::save
+race:Stockfish::TTWriter::penalize
 race:Stockfish::TranspositionTable::probe
 race:Stockfish::TranspositionTable::hashfull
 """