diff --git a/.codecov.yml b/.codecov.yml
index 2620c51c..5dc2d51f 100644
--- a/.codecov.yml
+++ b/.codecov.yml
@@ -25,4 +25,5 @@ ignore:
   - "*/examples/*"
   - "*/python/unittest/*"
   - "*/ark/unittest/*"
+  - "*/ark/ops/ops_test_common.*"
   - "*/ark/include/kernels/*"
diff --git a/.github/workflows/ut.yml b/.github/workflows/ut.yml
index 1dd1a854..2fbe2733 100644
--- a/.github/workflows/ut.yml
+++ b/.github/workflows/ut.yml
@@ -76,7 +76,7 @@ jobs:
         if: github.event_name != 'schedule'
         run: |
           cd build
-          lcov --capture --directory . --output-file cpp_coverage.info
+          lcov --ignore-errors negative --capture --directory . --output-file cpp_coverage.info
           lcov --ignore-errors unused --remove cpp_coverage.info \
               '/usr/*' \
               '/tmp/*' \
@@ -86,6 +86,7 @@ jobs:
               '*/examples/*' \
               '*/python/*' \
               '*/ark/unittest/unittest_utils.cpp' \
+              '*/ark/ops/ops_test_common.*' \
               --output-file cpp_coverage.info
           lcov --list cpp_coverage.info
 
diff --git a/ark/gpu/gpu.hpp b/ark/gpu/gpu.hpp
index 1010683c..176e60f7 100644
--- a/ark/gpu/gpu.hpp
+++ b/ark/gpu/gpu.hpp
@@ -125,6 +125,8 @@ ARK_GPU_DEFINE_CONSTANT_ALIAS(gpuPointerAttributeSyncMemops,
                               HIP_POINTER_ATTRIBUTE_SYNC_MEMOPS);
 
 // runtime API
+ARK_GPU_DEFINE_FUNC_ALIAS(gpuGetDeviceCount, cudaGetDeviceCount,
+                          hipGetDeviceCount);
 ARK_GPU_DEFINE_FUNC_ALIAS(gpuGetErrorString, cudaGetErrorString,
                           hipGetErrorString);
 ARK_GPU_DEFINE_FUNC_ALIAS(gpuGetLastError, cudaGetLastError, hipGetLastError);
diff --git a/ark/ops/ops_all_reduce.cpp b/ark/ops/ops_all_reduce.cpp
index 08f06257..49ee60ac 100644
--- a/ark/ops/ops_all_reduce.cpp
+++ b/ark/ops/ops_all_reduce.cpp
@@ -13,6 +13,12 @@ Tensor Model::all_reduce(Tensor input, int gpu_id, int gpu_num, Tensor output,
     }
     if (output.is_null()) {
         output = this->copy(input);
+    } else if (output.ref()->buffer()->id() == input.ref()->buffer()->id()) {
+        // In-place: copy input so the ring loop does not mutate send data.
+        // TODO: This catches the common case (output IS input). Sub-buffer
+        // offset aliasing or aliasing through different buffer objects backed
+        // by the same allocation is not currently detected.
+        input = this->copy(input);
     }
     Tensor prev_recv = NullTensor;
     Tensor cumulate = output;
diff --git a/ark/ops/ops_all_reduce_test.cpp b/ark/ops/ops_all_reduce_test.cpp
index dc0ad821..14b9835c 100644
--- a/ark/ops/ops_all_reduce_test.cpp
+++ b/ark/ops/ops_all_reduce_test.cpp
@@ -4,6 +4,7 @@
 #include "model/model_buffer.hpp"
 #include "model/model_node.hpp"
 #include "model/model_op.hpp"
+#include "model/model_tensor.hpp"
 #include "ops_test_common.hpp"
 
 template <typename T, int NumGpus>
@@ -27,6 +28,7 @@ template <int NumGpus>
 void test_all_reduce_internal(ark::DimType nelem) {
     for (int gpu_id = 0; gpu_id < NumGpus; ++gpu_id) {
         ark::unittest::spawn_process([gpu_id, nelem]() {
+            UNITTEST_SKIP(ark::unittest::get_gpu_count() < NumGpus);
             // Each GPU's data is equal to its GPU ID + 1.
             ark::Model m(gpu_id, NumGpus);
             ark::Tensor ones = m.tensor({nelem}, ark::FP16);
@@ -118,6 +120,7 @@ template <int NumGpus>
 void test_all_reduce_packet_internal(ark::DimType nelem) {
     for (int gpu_id = 0; gpu_id < NumGpus; ++gpu_id) {
         ark::unittest::spawn_process([gpu_id, nelem]() {
+            UNITTEST_SKIP(ark::unittest::get_gpu_count() < NumGpus);
             // Each GPU's data is equal to its GPU ID + 1.
             ark::Model m(gpu_id, NumGpus);
             ark::Tensor ones = m.tensor({nelem}, ark::FP16);
@@ -224,6 +227,7 @@ void test_all_reduce_sm_internal(ark::DimType nelem) {
     };
     for (int gpu_id = 0; gpu_id < NumGpus; ++gpu_id) {
         ark::unittest::spawn_process([gpu_id, nelem, config_rule]() {
+            UNITTEST_SKIP(ark::unittest::get_gpu_count() < NumGpus);
             // Each GPU's data is equal to its GPU ID + 1.
             ark::Model m(gpu_id, NumGpus);
             ark::Tensor ones = m.tensor({nelem}, ark::FP16);
@@ -244,6 +248,56 @@ void test_all_reduce_sm_internal(ark::DimType nelem) {
     ark::unittest::wait_all_processes();
 }
 
+template <int NumGpus>
+void test_all_reduce_inplace_internal(ark::DimType nelem) {
+    for (int gpu_id = 0; gpu_id < NumGpus; ++gpu_id) {
+        ark::unittest::spawn_process([gpu_id, nelem]() {
+            UNITTEST_SKIP(ark::unittest::get_gpu_count() < NumGpus);
+            // Each GPU's data is equal to its GPU ID + 1.
+            ark::Model m(gpu_id, NumGpus);
+            ark::Tensor ones = m.tensor({nelem}, ark::FP16);
+            ark::Tensor data = m.mul(ones, float(gpu_id + 1));
+            // In-place: pass the same tensor as both input and output.
+            ark::Tensor output = m.all_reduce(data, gpu_id, NumGpus, data);
+
+            // Verify the output is truly in-place (same buffer as input).
+            UNITTEST_EQ(output.ref()->buffer()->id(),
+                        data.ref()->buffer()->id());
+
+            std::vector<ark::half_t> ones_vec(ones.shape().nelems(),
+                                              ark::half_t(1.0f));
+            auto result = ark::op_test(
+                "all_reduce_inplace", m, {ones}, {output},
+                baseline_all_reduce<ark::half_t, NumGpus>, {ones_vec.data()});
+            UNITTEST_LOG(result);
+            UNITTEST_EQ(result.max_diff[0], 0.0f);
+            return ark::unittest::SUCCESS;
+        });
+    }
+    ark::unittest::wait_all_processes();
+}
+
+// The corruption was most visible with gpu_num >= 3 (multiple ring
+// iterations); this 2-GPU case validates the in-place copy path still
+// works for the simpler ring.
+ark::unittest::State test_all_reduce_inplace_2gpus() {
+    test_all_reduce_inplace_internal<2>(64);
+    test_all_reduce_inplace_internal<2>(8192);
+    return ark::unittest::SUCCESS;
+}
+
+ark::unittest::State test_all_reduce_inplace_3gpus() {
+    test_all_reduce_inplace_internal<3>(64);
+    test_all_reduce_inplace_internal<3>(8192);
+    return ark::unittest::SUCCESS;
+}
+
+ark::unittest::State test_all_reduce_inplace_4gpus() {
+    test_all_reduce_inplace_internal<4>(64);
+    test_all_reduce_inplace_internal<4>(8192);
+    return ark::unittest::SUCCESS;
+}
+
 ark::unittest::State test_all_reduce_4gpus() {
     test_all_reduce_internal<4>(64);
     test_all_reduce_internal<4>(8192);
@@ -287,5 +341,8 @@ int main() {
     UNITTEST(test_all_reduce_packet_8gpus);
     UNITTEST(test_all_reduce_sm_4gpus);
     UNITTEST(test_all_reduce_sm_8gpus);
+    UNITTEST(test_all_reduce_inplace_2gpus);
+    UNITTEST(test_all_reduce_inplace_3gpus);
+    UNITTEST(test_all_reduce_inplace_4gpus);
     return 0;
 }
diff --git a/ark/ops/ops_test_common.cpp b/ark/ops/ops_test_common.cpp
index f4139a0e..12922f45 100644
--- a/ark/ops/ops_test_common.cpp
+++ b/ark/ops/ops_test_common.cpp
@@ -11,7 +11,6 @@
 #include "ark/planner.hpp"
 #include "ark/random.hpp"
 #include "cpu_timer.h"
-#include "env.h"
 #include "gpu/gpu_logging.hpp"
 #include "logging.hpp"
 #include "model/model_data_type.hpp"
@@ -40,11 +39,7 @@ OpsTestResult op_test(const std::string &test_name_prefix, const Model &model,
                       const std::vector<void *> &inputs_data,
                       const std::vector<Planner::ConfigRule> &config_rules,
                       bool print_on_error) {
-    const char *env = std::getenv("ARK_UT_CORRECTNESS");
-    if (!env || std::string(env) != "1") {
-        // Zero values let callers pass without GPU work.
-        LOG(INFO, "[SKIP] %s: ARK_UT_CORRECTNESS not set",
-            test_name_prefix.c_str());
+    auto make_skipped = [&]() {
         OpsTestResult skipped;
         skipped.test_name = test_name_prefix;
         skipped.iter = 0;
@@ -56,6 +51,17 @@ OpsTestResult op_test(const std::string &test_name_prefix, const Model &model,
         skipped.num_wrong.resize(n, 0);
         skipped.num_total.resize(n, 0);
         return skipped;
+    };
+    const char *env = std::getenv("ARK_UT_CORRECTNESS");
+    if (!env || std::string(env) != "1") {
+        // Zero values let callers pass without GPU work.
+        LOG(INFO, "[SKIP] %s: ARK_UT_CORRECTNESS not set",
+            test_name_prefix.c_str());
+        return make_skipped();
+    }
+    if (ark::unittest::get_gpu_count() < 1) {
+        LOG(INFO, "[SKIP] %s: no GPU available", test_name_prefix.c_str());
+        return make_skipped();
     }
     DefaultExecutor exe(model, -1, nullptr, config_rules);
 
diff --git a/ark/unittest/unittest_utils.cpp b/ark/unittest/unittest_utils.cpp
index 4b74f951..24b47d4b 100644
--- a/ark/unittest/unittest_utils.cpp
+++ b/ark/unittest/unittest_utils.cpp
@@ -11,6 +11,7 @@
 #include <vector>
 
 #include "file_io.h"
+#include "gpu/gpu.hpp"
 #include "logging.hpp"
 
 // Grep SIGALRM and exit.
@@ -22,6 +23,14 @@ static void sigalrm_timeout_handler(int) {
 namespace ark {
 namespace unittest {
 
+int get_gpu_count() {
+    int count = 0;
+    if (gpuGetDeviceCount(&count) != gpuSuccess) {
+        return 0;
+    }
+    return count;
+}
+
 // Temporal unittest states.
 struct TempStates {
     std::vector<int> pids;
diff --git a/ark/unittest/unittest_utils.h b/ark/unittest/unittest_utils.h
index 383f49b6..b0a31f95 100644
--- a/ark/unittest/unittest_utils.h
+++ b/ark/unittest/unittest_utils.h
@@ -22,6 +22,9 @@ namespace unittest {
 
 typedef enum { SUCCESS = 0, FAILURE, UNEXPECTED } State;
 
+// Return the number of visible GPUs (calls gpuGetDeviceCount).
+int get_gpu_count();
+
 void exit(State s, const std::string &errmsg);
 void fexit(const std::string &errmsg = "");
 void uexit(const std::string &errmsg = "");
@@ -240,4 +243,15 @@ std::string get_kernel_code(const std::string &name);
 // Log a message.
 #define UNITTEST_LOG(...) LOG(ark::INFO, __VA_ARGS__)
 
+// Skip the current test (exit with SUCCESS) if @p cond is true.
+// WARNING: calls std::exit(), which skips local destructors.
+// Only safe inside spawn_process() child lambdas.
+#define UNITTEST_SKIP(cond)                          \
+    do {                                             \
+        if (cond) {                                  \
+            LOG(ark::INFO, "unittest skip: " #cond); \
+            std::exit(ark::unittest::SUCCESS);       \
+        }                                            \
+    } while (0)
+
 #endif  // ARK_UNITTEST_UNITTEST_UTILS_H_