diff --git a/pywikidiff2/pywikidiff2.cpp b/pywikidiff2/pywikidiff2.cpp
index 3b87f14..a46acc2 100644
--- a/pywikidiff2/pywikidiff2.cpp
+++ b/pywikidiff2/pywikidiff2.cpp
@@ -4,6 +4,8 @@
 #include <thread>
 #include <chrono>
 #include <atomic>
+#include <mutex>
+#include <condition_variable>
 #include "structmember.h"
 #undef HAVE_CONFIG_H // this disables the php allocator
 #include "../mediawiki-php-wikidiff2/src/lib/Wikidiff2.h"
@@ -118,11 +120,18 @@ static Wikidiff2::String wikidiff2_inline_json_diff(pywikidiff2Obj *self, char*
   self->config.cancelFlag = &cancelFlag;
   self->timed_out = false;
 
+  std::mutex timerMutex;
+  std::condition_variable timerCv;
+  bool diffDone = false;
+
   std::thread timerThread;
   if (timeout_ms > 0) {
-    timerThread = std::thread([&cancelFlag, timeout_ms]() {
-      std::this_thread::sleep_for(std::chrono::milliseconds(timeout_ms));
-      cancelFlag.store(true, std::memory_order_relaxed);
+    timerThread = std::thread([&cancelFlag, &timerMutex, &timerCv, &diffDone, timeout_ms]() {
+      std::unique_lock<std::mutex> lock(timerMutex);
+      if (!timerCv.wait_for(lock, std::chrono::milliseconds(timeout_ms), [&diffDone]{ return diffDone; })) {
+        // Timeout expired before diff finished
+        cancelFlag.store(true, std::memory_order_relaxed);
+      }
     });
   }
 
@@ -140,8 +149,13 @@ static Wikidiff2::String wikidiff2_inline_json_diff(pywikidiff2Obj *self, char*
   // Check if we timed out before stopping the timer thread
   self->timed_out = cancelFlag.load(std::memory_order_relaxed);
 
+  // Signal timer thread to exit and wait for it
   if (timerThread.joinable()) {
-    cancelFlag.store(true, std::memory_order_relaxed);  // Stop timer if still waiting
+    {
+      std::lock_guard<std::mutex> lock(timerMutex);
+      diffDone = true;
+    }
+    timerCv.notify_one();
     timerThread.join();
   }