# UNDF: UNDF-2026-000001066
# UNDF: UNDF-2026-XXXXXXXXX
--- a/src/z80.cpp
+++ b/src/z80.cpp
@@ -162,6 +162,9 @@ t_z80regs z80;
 std::vector<Breakpoint> breakpoints;
 std::unordered_set<dword> breakpoint_addresses;
 std::vector<Watchpoint> watchpoints;
+// Fast O(1) lookup sets for watchpoints, separated by access type.
+std::unordered_set<dword> watchpoint_reads;   // addresses watched on READ
+std::unordered_set<dword> watchpoint_writes;  // addresses watched on WRITE
--- a/src/z80.cpp
+++ b/src/z80.cpp
@@ -340,11 +340,11 @@ inline byte read_mem(word addr) {
   if (!watchpoints.empty()) {
-    if (std::any_of(watchpoints.begin(), watchpoints.end(), [&](const auto& w) {
-          return w.address == addr && (w.type & READ);
-          })) {
+    if (watchpoint_reads.count(addr)) {
       z80.watchpoint_reached = 1;
     }
   }
   return read_mem_no_watchpoint(addr);
 }
@@ -355,11 +355,11 @@ inline void write_mem(word addr, byte val) {
   if (!watchpoints.empty()) {
-    if (std::any_of(watchpoints.begin(), watchpoints.end(), [&](const auto& w) {
-          return w.address == addr && (w.type & WRITE);
-          })) {
+    if (watchpoint_writes.count(addr)) {
       z80.watchpoint_reached = 1;
     }
   }
--- a/src/gui/src/CapriceDevTools.cpp
+++ b/src/gui/src/CapriceDevTools.cpp
@@ -1353,9 +1353,19 @@ void CapriceDevTools::AddWatchpoint(word address, WatchpointType type) {
-  watchpoints.emplace_back(address, type);
+  watchpoints.emplace_back(address, type);
+  if (type & READ)  watchpoint_reads.insert(address);
+  if (type & WRITE) watchpoint_writes.insert(address);
 }

 void CapriceDevTools::RemoveWatchpoint(std::size_t idx) {
+  const auto& wp = watchpoints[idx];
+  // Only erase from sets if no other watchpoint covers this address+type.
+  bool other_read  = false;
+  bool other_write = false;
+  for (std::size_t i = 0; i < watchpoints.size(); i++) {
+    if (i == idx) continue;
+    if (watchpoints[i].address == wp.address) {
+      if (watchpoints[i].type & READ)  other_read  = true;
+      if (watchpoints[i].type & WRITE) other_write = true;
+    }
+  }
+  if (!other_read  && (wp.type & READ))  watchpoint_reads.erase(wp.address);
+  if (!other_write && (wp.type & WRITE)) watchpoint_writes.erase(wp.address);
   watchpoints.erase(watchpoints.begin() + idx);
 }