Fix GlobalOptimManager.override_config not propagating to optimizer (#1269)

override_config only wrote to pid2config, but get_config only read from
index2config. When override_config was called after register_parameters
(the documented usage order), the override was never seen by the optimizer.

Fix by having get_config also check pid2config as a fallback after
index2config, so overrides work regardless of call order.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: TimDettmers

bitsandbytes/optim/optimizer.py

@@ -352,6 +352,13 @@ def get_config(self, gindex, pindex, group):
 
         if (gindex, pindex) in self.mng.index2config:
             config.update(self.mng.index2config[(gindex, pindex)])
+
+        # Also check pid2config as a fallback so that override_config works
+        # regardless of whether it was called before or after register_parameters.
+        p = self.param_groups[gindex]["params"][pindex]
+        if id(p) in self.mng.pid2config:
+            config.update(self.mng.pid2config[id(p)])
+
         return config
 
     def init_state(self, group, p, gindex, pindex):

tests/test_optim.py

@@ -302,6 +302,39 @@ def test_global_config(dim1, dim2, gtype, device):
         assert adam2.state[p3]["state2"].dtype == torch.uint8
 
 
+@pytest.mark.parametrize("device", get_available_devices(no_cpu=True))
+@pytest.mark.skipif(not get_available_devices(no_cpu=True), reason="No device")
+def test_override_config_after_register(device):
+    """Test that override_config works when called after register_parameters (issue #1269)."""
+    if device not in ["cuda", "xpu"]:
+        pytest.skip("Optimizers are only supported on CUDA and XPU")
+
+    mng = bnb.optim.GlobalOptimManager.get_instance()
+    mng.initialize()
+
+    p1 = torch.randn(64, 64, device="cpu") * 0.1
+    p2 = torch.randn(64, 64, device="cpu") * 0.1
+
+    # Register first, override second (the documented order)
+    mng.register_parameters([p1, p2])
+    p1 = p1.to(device)
+    p2 = p2.to(device)
+
+    # Override p2 to use 8-bit after register_parameters
+    mng.override_config(p2, "optim_bits", 8)
+
+    adam = bnb.optim.Adam([p1, p2], lr=0.001, optim_bits=32)
+
+    # Run a step to trigger init_state
+    p1.grad = torch.randn_like(p1) * 0.1
+    p2.grad = torch.randn_like(p2) * 0.1
+    adam.step()
+
+    # p1 should be 32-bit, p2 should be 8-bit
+    assert adam.state[p1]["state1"].dtype == torch.float32
+    assert adam.state[p2]["state1"].dtype == torch.uint8
+
+
 optimizer_names_8bit = [
     "adam8bit_blockwise",
     "lion8bit_blockwise",