diff --git a/projects/rccl/src/bootstrap.cc b/projects/rccl/src/bootstrap.cc
index a51ce790cd3f..e539af1cc568 100644
--- a/projects/rccl/src/bootstrap.cc
+++ b/projects/rccl/src/bootstrap.cc
@@ -1609,10 +1609,12 @@ static ncclResult_t socketRingAllGather(struct ncclSocket* nextSock, struct nccl
   BOOTSTRAP_PROF_OPEN(tFirst);
   for (int step = 0; step < totalSteps; step++) {
     bool isFinalUnidirectional = (step == totalSteps - 1) && (nranks % 2 == 0);
-    int sendSliceRing0 = (rank - step + nranks) % nranks;
-    int recvSliceRing0 = (rank - step - 1 + nranks) % nranks;
-    int sendSliceRing1 = (rank + step) % nranks;
-    int recvSliceRing1 = (rank + step + 1) % nranks;
+    // Use size_t slice indices so that `slice * size` stays 64-bit. An int multiply overflows
+    // with a large payload (the ~100 MB XML at 64 ranks), giving a bad pointer / EFAULT "Bad address".
+    size_t sendSliceRing0 = (rank - step + nranks) % nranks;
+    size_t recvSliceRing0 = (rank - step - 1 + nranks) % nranks;
+    size_t sendSliceRing1 = (rank + step) % nranks;
+    size_t recvSliceRing1 = (rank + step + 1) % nranks;
     if (isFinalUnidirectional) {
       NCCLCHECKGOTO(socketSendRecv(nextSock, data + sendSliceRing0 * size, size, prevSock, data + recvSliceRing0 * size, size), res, exit);
     } else {
@@ -1680,10 +1682,12 @@ static ncclResult_t netBiDirRingAllGather(ncclNet_t* net,
   BOOTSTRAP_PROF_OPEN(tFirst);
   for (int step = 0; step < totalSteps; step++) {
     bool isFinalUnidirectional = (step == totalSteps - 1) && (nranks % 2 == 0);
-    int sendSliceRing0 = (rank - step + nranks) % nranks;     // Ring0 send to next
-    int recvSliceRing0 = (rank - step - 1 + nranks) % nranks; // Ring0 recv from prev
-    int sendSliceRing1 = (rank + step) % nranks;              // Ring1 send to prev
-    int recvSliceRing1 = (rank + step + 1) % nranks;          // Ring1 recv from next
+    // Use size_t slice indices so that `slice * size` is computed in 64-bit, for the
+    // same overflow reason described in socketRingAllGather.
+    size_t sendSliceRing0 = (rank - step + nranks) % nranks;     // Ring0 send to next
+    size_t recvSliceRing0 = (rank - step - 1 + nranks) % nranks; // Ring0 recv from prev
+    size_t sendSliceRing1 = (rank + step) % nranks;              // Ring1 send to prev
+    size_t recvSliceRing1 = (rank + step + 1) % nranks;          // Ring1 recv from next
     if (isFinalUnidirectional) {
       // Final step on even N: only ring0 over the forward QP pair.
       res = netSendRecv(net, sendComm, data + sendSliceRing0 * size, size, sendH,
diff --git a/projects/rccl/src/graph/connect.cc b/projects/rccl/src/graph/connect.cc
index af05908bec64..fa047225fb22 100644
--- a/projects/rccl/src/graph/connect.cc
+++ b/projects/rccl/src/graph/connect.cc
@@ -1214,7 +1214,9 @@ ncclResult_t ncclTopoPostset(struct ncclComm* comm, int* firstRanks, int* treePa
     // If user didn't override, use requested channels; otherwise keep capped max.
     if (!userUpdatedMaxChannels) {
       maxNchannels = nc * comm->nChannels * channelMultiplier;
-      nc = singleNode? maxNchannels : std::min(maxNchannels, maxChannels);
+      // Cap the single-node count at MAXCHANNELS/2, the maximum WarpSpeed supports. Left unbounded it
+      // overruns the fixed comm->channels[MAXCHANNELS] array (for example nc reaches 1760 in CPX mode at 64 ranks).
+      nc = singleNode? std::min(maxNchannels, MAXCHANNELS/2) : std::min(maxNchannels, maxChannels);
     } else {
       nc = maxNchannels = std::min(adjustedMaxNchannels * channelMultiplier, MAXCHANNELS);
     }
diff --git a/projects/rccl/test/WarpSpeedMPITests.cpp b/projects/rccl/test/WarpSpeedMPITests.cpp
index 2d3667ce371b..67435a0dbc04 100644
--- a/projects/rccl/test/WarpSpeedMPITests.cpp
+++ b/projects/rccl/test/WarpSpeedMPITests.cpp
@@ -228,4 +228,53 @@ TEST_F(WarpSpeedMPITest, MixedThresholdRace)
     TEST_INFO("All %d iterations passed", kTrainIterations);
 }
 
+// High-rank single-node comm init + AllGather; exercises the bootstrap topology
+// AllGather and single-node WarpSpeed channel setup (a regression aborts init).
+TEST_F(WarpSpeedMPITest, HighRankCommInitAndAllGather)
+{
+    // Single node, all available ranks (repro uses 64 on CPX+NPS2).
+    ASSERT_TRUE(validateTestPrerequisites(kMinProcessesForMPI,
+                                          kNoProcessLimit,
+                                          kNoPowerOfTwoRequired,
+                                          kRequireSingleNode,
+                                          kRequireSingleNode))
+        << "Requires a single-node run";
+
+    // Init exercises the bootstrap topology AllGather and WarpSpeed channel setup.
+    ASSERT_EQ(ncclSuccess, createTestCommunicator());
+    skipUnlessWarpSpeedAutoEnabled();
+    HIP_TEST_CHECK_GTEST_FAIL(hipStreamCreate(&stream_));
+
+    const int    nranks    = MPIEnvironment::world_size;
+    const int    rank      = MPIEnvironment::world_rank;
+    const size_t sendCount = 1u << 20;  // 1M floats (4 MB) per rank
+    const size_t recvCount = sendCount * static_cast<size_t>(nranks);
+
+    void* d_send = nullptr;
+    void* d_recv = nullptr;
+    HIP_TEST_CHECK_GTEST_FAIL(hipMalloc(&d_send, sendCount * sizeof(float)));
+    HIP_TEST_CHECK_GTEST_FAIL(hipMalloc(&d_recv, recvCount * sizeof(float)));
+    // Free on any exit path (including early ASSERT/FAIL returns) to avoid leaks.
+    SCOPE_EXIT(if(d_send) { (void)hipFree(d_send); });
+    SCOPE_EXIT(if(d_recv) { (void)hipFree(d_recv); });
+
+    // Each rank fills its send buffer with its own rank value.
+    HIP_TEST_CHECK_GTEST_FAIL(initializeBufferWithPattern<float>(
+        d_send, sendCount, [rank](size_t) { return static_cast<float>(rank); }));
+
+    ncclComm_t comm = getActiveCommunicator();
+    ASSERT_MPI_EQ(ncclSuccess,
+                  ncclAllGather(d_send, d_recv, sendCount, ncclFloat, comm, stream_));
+    HIP_TEST_CHECK_GTEST_FAIL(hipStreamSynchronize(stream_));
+
+    // After the AllGather, rank i's chunk (indices [i*sendCount, (i+1)*sendCount))
+    // must contain the value i.
+    const bool ok = verifyBufferData<float>(
+        d_recv, recvCount,
+        [sendCount](size_t idx) { return static_cast<float>(idx / sendCount); });
+    EXPECT_TRUE(ok) << "AllGather result mismatch across " << nranks << " ranks";
+
+    TEST_INFO("HighRankCommInitAndAllGather passed on %d ranks (single node)", nranks);
+}
+
 #endif // ENABLE_WARP_SPEED