clip_skip was mapped onto diffusers' argument of the same name, and that
argument counts something else. Its SDXL path reads

    hidden_states[-2]                 # clip_skip is None
    hidden_states[-(clip_skip + 2)]   # otherwise

so diffusers' clip_skip is the number of layers skipped BEYOND the penultimate
one SDXL already uses. Every model card, A1111, ComfyUI and InvokeAI mean
something simpler by the name: 1 is the final layer, 2 the penultimate, n the
n-th from the end. So a request asking for 2 -- which is what the booru-tag
SDXL families are trained at, and what their model cards all say -- read
hidden_states[-4] instead: two layers too deep, the equivalent of clip-skip 4,
which on a Pony-family model is the difference between a photograph and mush.
It was inconsistent as well as wrong: the same 2 meant [-4] on SDXL and [-3] on
SD 1.x, two depths from one number.

The mapping now happens here, once, in the sense users mean, and clip_skip 2
reaches the same layer whichever family the model belongs to. "Mirrors
diffusers exactly" was the mistake in 0.4.89: mirroring is the wrong goal for a
field a caller fills in.

SDXL's pooled conditioning vector is also handled properly now. The UNet cannot
run without it and diffusers refuses prompt_embeds that arrive without one, so
handing it half the set is a ValueError raised in check_inputs -- an instant
500, before any denoising. Where a pipeline does not offer the vector, it is
rebuilt the way CLIP produces it (pooled at the end-of-text token, then
projected through text_projection) rather than giving up the weighting and the
window count this module exists for; only when even that is impossible does the
encoder stand down and let the pipeline encode the prompt itself. The invalid
combination is no longer constructible: either all four keys or None.

A 500 from /v1/images/edits, /inpaint or /outfit now logs its traceback. The
cause used to reach the caller alone, so the log showed the request and then
the status, and a failure there could not be diagnosed afterwards at all.

Test note: test_a_400_is_not_relabelled_as_a_server_fault scanned a fixed
250-character window for the HTTPException pass-through above each catch-all;
the new log line pushed that to 316-382 and broke a passing check on correct
code. It now asserts structure instead of proximity -- the pass-through must be
the nearest handler inside the same function, with the logging between it and
the raise -- which also catches it drifting into a different function.

58 tests in the prompt file; full suite 4521 passed with the same 15
pre-existing failures as 0.4.91 (diff empty).
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S8LAP3ueNzQJtwzhgVbq66