diff --git a/src/test/fuzz/cmpctblock.cpp b/src/test/fuzz/cmpctblock.cpp
index 497389a67de..d5e6e0d19de 100644
--- a/src/test/fuzz/cmpctblock.cpp
+++ b/src/test/fuzz/cmpctblock.cpp
@@ -34,7 +34,6 @@
 #include <txmempool.h>
 #include <uint256.h>
 #include <util/check.h>
-#include <util/task_runner.h>
 #include <util/time.h>
 #include <validation.h>
 #include <validationinterface.h>
@@ -48,7 +47,6 @@
 #include <memory>
 #include <optional>
 #include <string>
-#include <thread>
 #include <utility>
 #include <vector>
 
@@ -104,15 +102,6 @@ public:
 };
 
 
-//! Used to run tasks in a std::thread to avoid DEBUG_LOCKORDER false positives.
-class ImmediateBackgroundTaskRunner : public util::TaskRunnerInterface
-{
-public:
-    void insert(std::function<void()> func) override { std::thread(std::move(func)).join(); }
-    void flush() override {}
-    size_t size() override { return 0; }
-};
-
 } // namespace
 
 extern void MakeRandDeterministicDANGEROUS(const uint256& seed) noexcept;
diff --git a/src/test/fuzz/process_messages.cpp b/src/test/fuzz/process_messages.cpp
index f893ebe43d8..f4a93df8dc1 100644
--- a/src/test/fuzz/process_messages.cpp
+++ b/src/test/fuzz/process_messages.cpp
@@ -50,6 +50,8 @@ void initialize_process_messages()
             {}),
     };
     g_setup = testing_setup.get();
+    // Replace validation_signals before creating chainman and mempool so they use it.
+    g_setup->m_node.validation_signals = std::make_unique<ValidationSignals>(std::make_unique<ImmediateBackgroundTaskRunner>());
     ResetChainmanAndMempool(*g_setup, init_clock);
 }
 
diff --git a/src/test/util/validation.h b/src/test/util/validation.h
index 91930b3dd73..f5d05ec4888 100644
--- a/src/test/util/validation.h
+++ b/src/test/util/validation.h
@@ -7,8 +7,12 @@
 
 #include <consensus/amount.h>
 #include <primitives/transaction.h>
+#include <util/task_runner.h>
 #include <validation.h>
 
+#include <cstddef>
+#include <functional>
+#include <thread>
 #include <utility>
 #include <vector>
 
@@ -19,6 +23,15 @@ class CValidationInterface;
 class FakeNodeClock;
 struct TestingSetup;
 
+/// Runs callbacks synchronously and deterministically, while avoiding DEBUG_LOCKORDER false positives.
+class ImmediateBackgroundTaskRunner : public util::TaskRunnerInterface
+{
+public:
+    void insert(std::function<void()> func) override { std::thread(std::move(func)).join(); }
+    void flush() override {}
+    size_t size() override { return 0; }
+};
+
 struct TestBlockManager : public node::BlockManager {
     /** Test-only method to clear internal state for fuzzing */
     void CleanupForFuzzing();