diff --git a/studio/backend/core/inference/diffusion.py b/studio/backend/core/inference/diffusion.py
index 8037dde174..25244e55c9 100644
--- a/studio/backend/core/inference/diffusion.py
+++ b/studio/backend/core/inference/diffusion.py
@@ -1167,17 +1167,13 @@ class DiffusionBackend:
                 # explicit "off" / "fbcache" are pinned and never toggled.
                 cache_request = normalize_transformer_cache(transformer_cache)
                 cache_auto = transformer_cache is None or cache_request == TC_AUTO
-                cache_quant_active = (
-                    transformer_quant_engaged is not None or bool(gguf_filename)
-                )
+                cache_quant_active = transformer_quant_engaged is not None or bool(gguf_filename)
                 default_steps: Optional[int] = None
                 if cache_auto:
                     default_steps, _ = default_generation_params(
                         gguf_filename, repo_id, base, fam.name
                     )
-                    cache_request = (
-                        TC_FBCACHE if default_steps >= FBCACHE_MIN_STEPS else None
-                    )
+                    cache_request = TC_FBCACHE if default_steps >= FBCACHE_MIN_STEPS else None
                 cache_engaged = apply_step_cache(
                     pipe,
                     mode = cache_request,
diff --git a/studio/backend/core/inference/sd_cpp_backend.py b/studio/backend/core/inference/sd_cpp_backend.py
index d8cb21d409..45ecee64f5 100644
--- a/studio/backend/core/inference/sd_cpp_backend.py
+++ b/studio/backend/core/inference/sd_cpp_backend.py
@@ -89,6 +89,17 @@ _MAX_SERVER_BATCH = 8
 _SERVER_PER_IMAGE_TIMEOUT_S = 1800.0
 
 
+def _default_threads() -> int:
+    """Physical-core thread count for the sd.cpp CPU backend.
+
+    ``threads = None`` lets sd.cpp pick its own default, which is the logical-core
+    count (all hyperthreads). For the compute-bound GGML matmuls the diffusion CPU
+    path runs, oversubscribing the hyperthreads adds scheduling contention without
+    extra throughput, so pin to physical cores (``cpu_count // 2``) instead. Falls
+    back to 8 when the count is unknown, and clamps to at least 1."""
+    return max(1, (os.cpu_count() or 8) // 2)
+
+
 def _server_binary_runnable(binary: str) -> bool:
     """Best-effort probe that ``binary`` can actually execute (not just exist).
 
@@ -531,7 +542,9 @@ class SdCppDiffusionBackend:
                             vae_format = fam.sd_cpp_vae_format,
                             offload = list(offload),
                             native_speed = native_speed,
-                            threads = None,
+                            # Pin the CPU backend to physical cores; sd.cpp's own
+                            # default oversubscribes hyperthreads (see _default_threads).
+                            threads = _default_threads(),
                         )
                     except SdCppCancelled:
                         # Startup was aborted by an unload / superseding load: stop the
@@ -568,7 +581,9 @@ class SdCppDiffusionBackend:
                     vae_format = fam.sd_cpp_vae_format,
                     native_speed = native_speed,
                     offload_flags = offload,
-                    threads = None,
+                    # One-shot sd-cli reads this per generation (state.threads); pin to
+                    # physical cores for the same reason as the server (see _default_threads).
+                    threads = _default_threads(),
                     sampling_method = fam.sd_cpp_sampling_method,
                     flow_shift = fam.sd_cpp_flow_shift,
                     server = server,
diff --git a/studio/backend/tests/test_diffusion_attention.py b/studio/backend/tests/test_diffusion_attention.py
index 48647bb0b4..dce7dd2bfa 100644
--- a/studio/backend/tests/test_diffusion_attention.py
+++ b/studio/backend/tests/test_diffusion_attention.py
@@ -245,7 +245,6 @@ class _Recorder:
 
 def _stub_subprocess(monkeypatch, run):
     import subprocess
-
     monkeypatch.setattr(subprocess, "run", run)
 
 
diff --git a/studio/backend/tests/test_diffusion_dit_trainer.py b/studio/backend/tests/test_diffusion_dit_trainer.py
index b58868c701..6d7736a106 100644
--- a/studio/backend/tests/test_diffusion_dit_trainer.py
+++ b/studio/backend/tests/test_diffusion_dit_trainer.py
@@ -230,10 +230,15 @@ def test_mxfp8_training_config_falls_back_to_the_torchao_0_17_api(monkeypatch):
 
 def _patch_capability(monkeypatch, capability):
     # Drive train_precision_modes' GPU probe: pretend CUDA is present at the given tensor
-    # core capability (fp8 needs sm89+, mxfp8 needs sm100+).
+    # core capability (fp8 needs sm89+, mxfp8 needs sm100+). The torchao probe is stubbed
+    # functional so these tests exercise the CAPABILITY gate on hosts without torchao
+    # (the CPU-only CI runner does not install it).
     import torch
+
+    import core.training.diffusion_train_common as dtc
     monkeypatch.setattr(torch.cuda, "is_available", lambda: True)
     monkeypatch.setattr(torch.cuda, "get_device_capability", lambda *a, **k: capability)
+    monkeypatch.setattr(dtc, "has_functional_torchao", lambda: True)
 
 
 def test_train_precision_modes_blackwell_lists_mxfp8(monkeypatch):