diff --git a/tests/ccl/test_all_gather.py b/tests/ccl/test_all_gather.py
index f2c50e2f..7858ed18 100644
--- a/tests/ccl/test_all_gather.py
+++ b/tests/ccl/test_all_gather.py
@@ -21,14 +21,16 @@
     ],
 )
 @pytest.mark.parametrize(
-    "M, N",
+    "M, N, block_size_m, block_size_n",
     [
-        (128, 64),  # Small
-        (1024, 256),  # Medium
-        (8192, 8192),  # Large
+        (128, 64, 32, 64),  # Small
+        (128, 128, 32, 32),  # BLOCK_N < N/world_size (partial-width, multi-block per rank)
+        (256, 128, 32, 16),  # Minimum BLOCK_N=16 (16-bit vectorization path)
+        (1024, 256, 32, 64),  # Medium
+        (8192, 8192, 32, 64),  # Large
     ],
 )
-def test_all_gather(dtype, M, N):
+def test_all_gather(dtype, M, N, block_size_m, block_size_n):
     """Test all-gather functionality by comparing against PyTorch's implementation."""
     # Ensure torch.distributed is initialized (should be done by test runner)
     if not dist.is_initialized():
@@ -62,7 +64,7 @@ def test_all_gather(dtype, M, N):
 
     # Run Iris all_gather
     shmem.barrier()
-    config = Config()
+    config = Config(block_size_m=block_size_m, block_size_n=block_size_n)
     shmem.ccl.all_gather(iris_output_tensor, iris_input_tensor, config=config)
     torch.cuda.synchronize()
 
@@ -97,14 +99,16 @@ def test_all_gather(dtype, M, N):
     ],
 )
 @pytest.mark.parametrize(
-    "M, N",
+    "M, N, block_size_m, block_size_n",
     [
-        (128, 64),  # Small
-        (1024, 256),  # Medium
-        (8192, 8192),  # Large
+        (128, 64, 32, 64),  # Small
+        (128, 128, 32, 32),  # BLOCK_N < N/world_size (partial-width, multi-block per rank)
+        (256, 128, 32, 16),  # Minimum BLOCK_N=16 (16-bit vectorization path)
+        (1024, 256, 32, 64),  # Medium
+        (8192, 8192, 32, 64),  # Large
     ],
 )
-def test_all_gather_partitioned(dtype, M, N):
+def test_all_gather_partitioned(dtype, M, N, block_size_m, block_size_n):
     """Test all-gather with partitioned variant by comparing against PyTorch's implementation."""
     # Ensure torch.distributed is initialized (should be done by test runner)
     if not dist.is_initialized():
@@ -140,7 +144,9 @@ def test_all_gather_partitioned(dtype, M, N):
     # COMM_SMS must be divisible by world_size for partitioned variant
     comm_sms = 64  # Assuming world_size divides 64 (e.g., 2, 4, 8)
     shmem.barrier()
-    config = Config(all_gather_variant="partitioned", comm_sms=comm_sms)
+    config = Config(
+        block_size_m=block_size_m, block_size_n=block_size_n, all_gather_variant="partitioned", comm_sms=comm_sms
+    )
     shmem.ccl.all_gather(iris_output_tensor, iris_input_tensor, config=config)
     torch.cuda.synchronize()
 
diff --git a/tests/ccl/test_all_reduce.py b/tests/ccl/test_all_reduce.py
index ffd55e9d..1862e0e3 100644
--- a/tests/ccl/test_all_reduce.py
+++ b/tests/ccl/test_all_reduce.py
@@ -32,14 +32,16 @@
     ],
 )
 @pytest.mark.parametrize(
-    "M, N",
+    "M, N, block_size_m, block_size_n",
     [
-        (128, 64),  # Small
-        (1024, 256),  # Medium
-        (8192, 8192),  # Large
+        (128, 64, 32, 64),  # Small
+        (128, 128, 32, 32),  # BLOCK_N < N/world_size (partial-width, multi-block per rank)
+        (256, 128, 32, 16),  # Minimum BLOCK_N=16 (16-bit vectorization path)
+        (1024, 256, 32, 64),  # Medium
+        (8192, 8192, 32, 64),  # Large
     ],
 )
-def test_all_reduce(variant, dtype, M, N):
+def test_all_reduce(variant, dtype, M, N, block_size_m, block_size_n):
     """Test all-reduce functionality by comparing against PyTorch's implementation."""
     # Ensure torch.distributed is initialized (should be done by test runner)
     if not dist.is_initialized():
@@ -70,7 +72,7 @@ def test_all_reduce(variant, dtype, M, N):
 
     # Run Iris all_reduce with specified variant
     shmem.barrier()
-    config = Config(all_reduce_variant=variant)
+    config = Config(all_reduce_variant=variant, block_size_m=block_size_m, block_size_n=block_size_n)
     if variant == "two_shot":
         # Test both distribution modes for two_shot
         config.all_reduce_distribution = 0  # striding
diff --git a/tests/ccl/test_all_to_all.py b/tests/ccl/test_all_to_all.py
index 76478f5a..99e9bf19 100644
--- a/tests/ccl/test_all_to_all.py
+++ b/tests/ccl/test_all_to_all.py
@@ -21,14 +21,16 @@
     ],
 )
 @pytest.mark.parametrize(
-    "M, N",
+    "M, N, block_size_m, block_size_n",
     [
-        (128, 64),  # Small
-        (1024, 256),  # Medium
-        (8192, 8192),  # Large
+        (128, 64, 32, 64),  # Small
+        (128, 128, 32, 32),  # BLOCK_N < N/world_size (partial-width, multi-block per rank)
+        (256, 128, 32, 16),  # Minimum BLOCK_N=16 (16-bit vectorization path)
+        (1024, 256, 32, 64),  # Medium
+        (8192, 8192, 32, 64),  # Large
     ],
 )
-def test_all_to_all(dtype, M, N):
+def test_all_to_all(dtype, M, N, block_size_m, block_size_n):
     """Test all-to-all functionality by comparing against PyTorch's implementation."""
     # Ensure torch.distributed is initialized (should be done by test runner)
     if not dist.is_initialized():
@@ -74,7 +76,7 @@ def test_all_to_all(dtype, M, N):
 
     # Run Iris all_to_all
     shmem.barrier()
-    config = Config()
+    config = Config(block_size_m=block_size_m, block_size_n=block_size_n)
     shmem.ccl.all_to_all(iris_output_concat, iris_input_concat, config=config)
     torch.cuda.synchronize()
 
diff --git a/tests/x/test_all_reduce.py b/tests/x/test_all_reduce.py
index 30549a50..6e8934c1 100644
--- a/tests/x/test_all_reduce.py
+++ b/tests/x/test_all_reduce.py
@@ -223,6 +223,8 @@ def x_all_reduce_spinlock_kernel(
     "M, N, BLOCK_SIZE_M, BLOCK_SIZE_N",
     [
         (128, 64, 64, 32),  # Small
+        (128, 128, 64, 32),  # BLOCK_N < N/world_size (partial-width, multi-block per rank)
+        (256, 128, 64, 16),  # Minimum BLOCK_N=16 (16-bit vectorization path)
         (1024, 256, 128, 128),  # Medium
         (2048, 2048, 256, 256),  # Large
         # (100, 100, 64, 64),  # Non-aligned dimensions - DISABLED: other=0.0 not supported