a model card could cross the line the request path refuses to

_apply_sampler turns a cross-family sampler into a 400 that explains itself:
flow-matching and DDPM schedulers parameterise the trajectory differently, so
one cannot stand in for the other. The acceleration config applied its
`scheduler` with no such check, and a card is exactly where that mistake gets
made and never noticed.

Krea-2-Turbo's entry asked for EulerDiscreteScheduler on a flow-matching
model. The swap succeeded silently and the generation then died with

    EulerDiscreteScheduler.set_timesteps() got an unexpected keyword
    argument 'mu'

which names neither the card nor the model, arrives from inside diffusers, and
reads like a library bug. Now it is refused and logged, naming both
schedulers, and the model keeps its own -- acceleration is an optimisation and
has always been allowed to decline.

One set of family names, imported by both paths, so they cannot drift.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZnLhydX9QqUcfBbqLu

codai/api/images.py

@@ -445,10 +445,9 @@ SAMPLERS = {
 
 #: Schedulers for flow-matching models. A pipeline whose scheduler is one of
 #: these cannot be given a classic DDPM-family sampler, and vice versa.
-_FLOW_SCHEDULERS = {"FlowMatchEulerDiscreteScheduler",
-                    "FlowMatchHeunDiscreteScheduler",
-                    "FlowMatchLCMScheduler",
-                    "LTXEulerAncestralRFScheduler"}
+#: Shared with the model-card path, which applies a scheduler from the
+#: acceleration config and must draw the family line in the same place.
+from codai.models.acceleration import FLOW_SCHEDULERS as _FLOW_SCHEDULERS
 
 
 def _apply_sampler(pipeline, sampler: str) -> str:

codai/models/acceleration.py

@@ -225,6 +225,17 @@ def _split_lora_ref(ref: str):
     return ref, None
 
 
+#: Schedulers that parameterise a flow-matching trajectory. Swapping one of
+#: these for a DDPM-family scheduler (or the reverse) does not make a worse
+#: picture, it makes noise -- so both the request path (images._apply_sampler)
+#: and the model-card path below refuse to cross the line. One set, so they
+#: cannot disagree about where it is.
+FLOW_SCHEDULERS = {"FlowMatchEulerDiscreteScheduler",
+                   "FlowMatchHeunDiscreteScheduler",
+                   "FlowMatchLCMScheduler",
+                   "LTXEulerAncestralRFScheduler"}
+
+
 def apply_accel_to_pipeline(pipe, accel: Optional[dict]) -> None:
     """Fuse the distill LoRA into a diffusers pipeline and apply scheduler tweaks.
 
@@ -252,6 +263,21 @@ def apply_accel_to_pipeline(pipe, accel: Optional[dict]) -> None:
             import diffusers as _diffusers
             sched_cls = getattr(_diffusers, sched_name, None)
             cur = getattr(pipe, "scheduler", None)
+            cur_name = type(cur).__name__ if cur is not None else ""
+            if (sched_cls is not None and cur is not None
+                    and (cur_name in FLOW_SCHEDULERS)
+                    != (sched_name in FLOW_SCHEDULERS)):
+                # The request path refuses this swap with an explanation; a
+                # model card could do it silently, and the model then fails at
+                # call time with something that names neither scheduler --
+                # Krea 2 with EulerDiscreteScheduler configured here answered
+                # "EulerDiscreteScheduler.set_timesteps() got an unexpected
+                # keyword argument 'mu'", which reads like a diffusers bug.
+                log.warning(
+                    "[accel] ignoring scheduler %s: this model's scheduler is "
+                    "%s, and flow-matching and DDPM-family samplers cannot be "
+                    "substituted for one another", sched_name, cur_name)
+                sched_cls = None
             if sched_cls is not None and cur is not None:
                 # `scheduler_config` carries the sampler's own settings, because
                 # the class name alone does not identify a sampler. The samplers

tests/test_accel_scheduler_family.py

+"""A model card must not be able to cross the flow/DDPM line either.
+
+The request path (`images._apply_sampler`) refuses a cross-family sampler with
+an explanation. The acceleration config had no such guard, so a card could
+swap a flow-matching model's scheduler for a DDPM one silently — and the model
+then failed at call time with
+
+    EulerDiscreteScheduler.set_timesteps() got an unexpected keyword
+    argument 'mu'
+
+which names neither the card nor the model and reads like a diffusers bug.
+Measured on Krea-2-Turbo, whose entry asked for EulerDiscreteScheduler.
+"""
+import logging
+
+import pytest
+
+from codai.models import acceleration as accel_mod
+
+
+class _Sched:
+    def __init__(self, name, cfg=None):
+        self.__class__.__name__ = name
+        self.config = cfg or {}
+
+
+class _FlowSched:
+    config = {"num_train_timesteps": 1000}
+
+
+class _Pipe:
+    def __init__(self, scheduler):
+        self.scheduler = scheduler
+
+
+def test_the_two_paths_share_one_definition_of_the_families():
+    """Two copies of this set would drift, and the drift is invisible until a
+    model produces noise."""
+    from codai.api import images
+
+    assert images._FLOW_SCHEDULERS is accel_mod.FLOW_SCHEDULERS
+    assert "FlowMatchEulerDiscreteScheduler" in accel_mod.FLOW_SCHEDULERS
+
+
+def test_a_ddpm_scheduler_is_refused_on_a_flow_matching_model(caplog):
+    """Krea 2's own scheduler is FlowMatchEulerDiscreteScheduler; the card
+    asked for EulerDiscreteScheduler."""
+    pipe = _Pipe(_FlowSched())
+    type(pipe.scheduler).__name__ = "FlowMatchEulerDiscreteScheduler"
+    with caplog.at_level(logging.WARNING):
+        accel_mod.apply_accel_to_pipeline(
+            pipe, {"scheduler": "EulerDiscreteScheduler", "lora": None})
+    assert type(pipe.scheduler).__name__ == "FlowMatchEulerDiscreteScheduler"
+    assert any("cannot be substituted" in r.getMessage() for r in caplog.records)
+
+
+def test_a_same_family_scheduler_is_still_applied():
+    """LCM on an SD model is the reason this feature exists."""
+    pipe = _Pipe(_FlowSched())
+    type(pipe.scheduler).__name__ = "EulerDiscreteScheduler"
+    accel_mod.apply_accel_to_pipeline(
+        pipe, {"scheduler": "LCMScheduler", "lora": None})
+    assert type(pipe.scheduler).__name__ == "LCMScheduler"