fix: do not leak set_attention_backend into process-wide registry

src/diffusers/models/modeling_utils.py

@@ -644,8 +644,10 @@ def set_attention_backend(self, backend: str) -> None:
                 continue
             processor._attention_backend = backend
 
-        # Important to set the active backend so that it propagates gracefully throughout.
-        _AttentionBackendRegistry.set_active_backend(backend)
+        # Only pin the backend on this model's attention modules.
+        # Do not flip the process-wide active backend: that would leak into
+        # other models whose per-module backend is still unset (see #14249).
+        # Use `attention_backend(...)` when a temporary global override is needed.
 
     def reset_attention_backend(self) -> None:
         """

tests/models/test_attention_backend_isolation.py

@@ -0,0 +1,44 @@
+# coding=utf-8
+# Copyright 2026 The HuggingFace Team.
+#
+# Licensed under the Apache License, Version 2.0.
+
+"""Unit tests for attention backend process isolation."""
+
+from diffusers.models.attention_dispatch import (
+    AttentionBackendName,
+    _AttentionBackendRegistry,
+)
+from diffusers.models.modeling_utils import ModelMixin
+from diffusers.models.attention_processor import Attention
+
+
+class _TinyAttentionModel(ModelMixin):
+    def __init__(self):
+        super().__init__()
+        # Minimal Attention module so set_attention_backend has a processor to stamp.
+        self.attn = Attention(query_dim=16, heads=2, dim_head=8)
+
+
+def test_set_attention_backend_does_not_mutate_process_global_registry():
+    """model.set_attention_backend must not leak into the process-global registry."""
+    initial_backend, _ = _AttentionBackendRegistry.get_active_backend()
+    model = _TinyAttentionModel()
+
+    try:
+        # Pick a backend different from the current global default when possible.
+        target = AttentionBackendName.NATIVE
+        if initial_backend == target:
+            # Still exercise the API; isolation is what we assert.
+            pass
+        model.set_attention_backend(target.value)
+        active_backend, _ = _AttentionBackendRegistry.get_active_backend()
+        assert active_backend == initial_backend, (
+            "set_attention_backend must not change the process-global active backend; "
+            f"expected {initial_backend}, got {active_backend}"
+        )
+        # Per-module processor should still be stamped.
+        assert model.attn.processor._attention_backend == target
+    finally:
+        model.reset_attention_backend()
+        _AttentionBackendRegistry.set_active_backend(initial_backend)