diff --git a/EvenPerfectBitScanner.Tests/ProgramTests.cs b/EvenPerfectBitScanner.Tests/ProgramTests.cs
index c4e4182..2afa0cf 100644
--- a/EvenPerfectBitScanner.Tests/ProgramTests.cs
+++ b/EvenPerfectBitScanner.Tests/ProgramTests.cs
@@ -360,6 +360,48 @@ public class ProgramTests
         }
     }
 
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void Residue_mode_accepts_known_primes_in_parallel()
+    {
+        var useDivisorField = typeof(Program).GetField("_useDivisor", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var mersenneField = typeof(Program).GetField("MersenneTesters", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var primeField = typeof(Program).GetField("PrimeTesters", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var residueField = typeof(Program).GetField("PResidue", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var gcdFilterField = typeof(Program).GetField("_useGcdFilter", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var forceCpuProp = typeof(GpuContextPool).GetProperty("ForceCpu");
+
+        useDivisorField.SetValue(null, false);
+        gcdFilterField.SetValue(null, false);
+        mersenneField.SetValue(null, new ThreadLocal<MersenneNumberTester>(() => new MersenneNumberTester(
+            useIncremental: true,
+            useResidue: true,
+            maxK: 5_000_000UL,
+            residueDivisorSets: PerfectNumberConstants.ExtraDivisorCycleSearchLimit), trackAllValues: true));
+        primeField.SetValue(null, new ThreadLocal<PrimeTester>(() => new PrimeTester(), trackAllValues: true));
+        residueField.SetValue(null, new ThreadLocal<ModResidueTracker>(() => new ModResidueTracker(ResidueModel.Identity, 2UL, true), trackAllValues: true));
+        forceCpuProp!.SetValue(null, false);
+
+        try
+        {
+            ulong[] primes = [107UL, 127UL, 521UL, 607UL];
+            Parallel.ForEach(primes, prime =>
+            {
+                Program.IsEvenPerfectCandidate(prime, 0UL, out bool searched, out bool detailed).Should().BeTrue();
+                searched.Should().BeTrue();
+                detailed.Should().BeTrue();
+            });
+        }
+        finally
+        {
+            useDivisorField.SetValue(null, false);
+            mersenneField.SetValue(null, null);
+            primeField.SetValue(null, null);
+            residueField.SetValue(null, null);
+            forceCpuProp!.SetValue(null, false);
+        }
+    }
+
     [Fact]
     public void Residue_mode_with_default_limits_accepts_large_known_primes()
     {
diff --git a/PerfectNumbers.Core.Tests/PrimeTester/PrimeTesterIsPrimeTests.cs b/PerfectNumbers.Core.Tests/PrimeTester/PrimeTesterIsPrimeTests.cs
index 9b2b6a9..7f583ac 100644
--- a/PerfectNumbers.Core.Tests/PrimeTester/PrimeTesterIsPrimeTests.cs
+++ b/PerfectNumbers.Core.Tests/PrimeTester/PrimeTesterIsPrimeTests.cs
@@ -36,5 +36,33 @@ public class PrimeTesterIsPrimeTests
 
         GpuContextPool.ForceCpu = false;
     }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void IsPrimeGpu_handles_parallel_calls_for_known_primes()
+    {
+        GpuContextPool.ForceCpu = false;
+        GpuPrimeWorkLimiter.SetLimit(64);
+        PrimeTester.GpuBatchSize = 256_000;
+
+        ulong[] primes = [107UL, 127UL, 521UL, 607UL];
+        var results = new bool[primes.Length];
+
+        var testers = new ThreadLocal<PrimeTester>(() => new PrimeTester(), trackAllValues: true);
+        Parallel.For(0, primes.Length, i =>
+        {
+            results[i] = testers.Value!.IsPrimeGpu(primes[i], CancellationToken.None);
+        });
+
+        results.Should().AllBeEquivalentTo(true);
+
+        Parallel.For(0, Environment.ProcessorCount * 16, _ =>
+        {
+            foreach (ulong prime in primes)
+            {
+                testers.Value!.IsPrimeGpu(prime, CancellationToken.None).Should().BeTrue();
+            }
+        });
+    }
 }
 
diff --git a/PerfectNumbers.Core/PrimeTester.cs b/PerfectNumbers.Core/PrimeTester.cs
index 97efd4d..5d83d5a 100644
--- a/PerfectNumbers.Core/PrimeTester.cs
+++ b/PerfectNumbers.Core/PrimeTester.cs
@@ -83,48 +83,53 @@ public sealed class PrimeTester(bool useInternal = false)
         // Limit concurrency and declare variables outside loops for performance and reuse
         var limiter = GpuPrimeWorkLimiter.Acquire();
         var gpu = GpuContextPool.Rent();
-        var accelerator = gpu.Accelerator;
 
-        // Get per-accelerator cached kernel and device primes buffer.
-        var state = GpuKernelState.GetOrCreate(accelerator);
-
-        MemoryBuffer1D<ulong, Stride1D.Dense> input;
-        MemoryBuffer1D<byte, Stride1D.Dense> output;
-        int totalLength, batchSize, pos, remaining, count;
-        ulong[] temp;
-
-        totalLength = values.Length;
-        batchSize = Math.Max(1, GpuBatchSize);
-
-        // Rent per-call scratch buffers sized to current batch (thread-safe)
-        var scratch = state.RentScratch(batchSize, accelerator);
-        input = scratch.Input;
-        output = scratch.Output;
-
-        // Reusable host buffer per call
-        temp = ArrayPool<ulong>.Shared.Rent(batchSize);
-
-        pos = 0;
-        while (pos < totalLength)
+        try
         {
-            remaining = totalLength - pos;
-            count = remaining > batchSize ? batchSize : remaining;
+            var accelerator = gpu.Accelerator;
 
-            // Copy chunk to host temp then to device
-            values.Slice(pos, count).CopyTo(temp);
-            input.View.CopyFromCPU(ref temp[0], count);
+            // Get per-accelerator cached kernel and device primes buffer.
+            var state = GpuKernelState.GetOrCreate(accelerator);
+            int totalLength = values.Length;
+            int batchSize = Math.Max(1, GpuBatchSize);
 
-            state.Kernel(count, input.View, state.DevicePrimes.View, output.View);
-            accelerator.Synchronize();
-            output.View.CopyToCPU(ref results[pos], count);
+            lock (gpu.ExecutionLock)
+            {
+                var scratch = state.RentScratch(batchSize, accelerator);
+                var input = scratch.Input;
+                var output = scratch.Output;
+                ulong[] temp = ArrayPool<ulong>.Shared.Rent(batchSize);
 
-            pos += count;
+                try
+                {
+                    int pos = 0;
+                    while (pos < totalLength)
+                    {
+                        int remaining = totalLength - pos;
+                        int count = remaining > batchSize ? batchSize : remaining;
+
+                        values.Slice(pos, count).CopyTo(temp);
+                        input.View.CopyFromCPU(ref temp[0], count);
+
+                        state.Kernel(count, input.View, state.DevicePrimes.View, output.View);
+                        accelerator.Synchronize();
+                        output.View.CopyToCPU(ref results[pos], count);
+
+                        pos += count;
+                    }
+                }
+                finally
+                {
+                    ArrayPool<ulong>.Shared.Return(temp);
+                    state.ReturnScratch(scratch);
+                }
+            }
+        }
+        finally
+        {
+            gpu.Dispose();
+            limiter.Dispose();
         }
-
-        ArrayPool<ulong>.Shared.Return(temp);
-        state.ReturnScratch(scratch);
-        gpu.Dispose();
-        limiter.Dispose();
     }
 
     // Per-accelerator GPU state for prime sieve (kernel + uploaded primes).