diff --git a/unsloth/import_fixes.py b/unsloth/import_fixes.py
index 172ba36f63..ee82715306 100644
--- a/unsloth/import_fixes.py
+++ b/unsloth/import_fixes.py
@@ -97,6 +97,53 @@ class HidePrintMessage:
         return getattr(self._original_stream, name)
 
 
+import contextlib
+import ctypes
+
+try:
+    _libc = ctypes.CDLL(None)
+except Exception:
+    _libc = None
+
+
+@contextlib.contextmanager
+def suppress_cuda_printf():
+    """Suppress CUDA device-side printf by redirecting stdout/stderr fds to /dev/null.
+
+    CUDA device printf (eg CUTLASS "Arch conditional MMA" errors on Blackwell)
+    writes to stdout fd 1 at the C level, bypassing Python sys.stdout entirely.
+    The existing HidePrintMessage filter on sys.stderr cannot catch these since
+    they go to a different fd at a different layer. This context manager redirects
+    both fd 1 and fd 2 at the OS level, syncs CUDA, then restores them.
+    """
+    sys.stdout.flush()
+    sys.stderr.flush()
+    saved_fds = {}
+    try:
+        for fd in (1, 2):
+            saved_fds[fd] = os.dup(fd)
+            devnull = os.open(os.devnull, os.O_WRONLY)
+            os.dup2(devnull, fd)
+            os.close(devnull)
+        yield
+    finally:
+        try:
+            import torch
+
+            if torch.cuda.is_available():
+                torch.cuda.synchronize()
+        except Exception:
+            pass
+        if _libc is not None:
+            try:
+                _libc.fflush(None)
+            except Exception:
+                pass
+        for fd, saved in saved_fds.items():
+            os.dup2(saved, fd)
+            os.close(saved)
+
+
 if not UNSLOTH_ENABLE_LOGGING:
     import sys
 
diff --git a/unsloth/kernels/fp8.py b/unsloth/kernels/fp8.py
index 0b073d59d6..a57f4ffb64 100644
--- a/unsloth/kernels/fp8.py
+++ b/unsloth/kernels/fp8.py
@@ -579,7 +579,14 @@ try:
     if Version(fbgemm_gpu.__version__) >= Version("1.4.0"):
         # We must manually confirm if blockwise FBGEMM works!
         # This check is a must for consumer grade GPUs which fail
-        if test_has_fbgemm():
+        # Suppress CUDA device printf during probe -- on Blackwell (SM100) GPUs,
+        # FBGEMM's CUTLASS blockwise kernel (hardcoded SM90) fires thousands of
+        # "Arch conditional MMA" lines to stdout fd 1 before aborting.
+        from unsloth.import_fixes import suppress_cuda_printf
+
+        with suppress_cuda_printf():
+            _has_fbgemm = test_has_fbgemm()
+        if _has_fbgemm:
             os.environ["UNSLOTH_HAS_FBGEMM"] = "1"
             logger.info(f"Using fbgemm_gpu block quantized FP8 matmul")
             fp8_block_quant_linear = fp8_fbgemm_block_linear