Prompt-improve as system prompt + One Obsession quality defaults

- Prompt-improve: pass the improve instruction as the LLM system prompt and
the user's original prompt as the user message (was a combined user turn).
- One Obsession (NoobXL): prepend a quality prefix (masterpiece, best quality,
…) to the positive prompt, skipping any tag already present; replace the
baseline NOOBXL_NEGATIVE with the new default negative (applied via the
existing negative wiring + UI default).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -99,12 +99,32 @@ def _checkpoint_path(filename):
 
 MAX_SEED = np.iinfo(np.int32).max
 
+# One Obsession (Illustrious/SDXL) quality defaults. The prefix is prepended to
+# the positive prompt (skipping any tag the prompt already has); the negative is
+# the model's baseline negative.
+NOOBXL_PREFIX = (
+    "masterpiece, best quality, amazing quality, very awa, absurdres, newest, "
+    "very aesthetic, depth of field, highres"
+)
 NOOBXL_NEGATIVE = (
-    "lowres, {bad}, error, fewer, extra, missing, worst quality, jpeg artifacts, "
-    "bad quality, watermark, unfinished, displeasing, chromatic aberration, signature, "
-    "extra digits, artistic error, username, scan, [abstract]"
+    "worst quality, normal quality, anatomical nonsense, bad anatomy, "
+    "interlocked fingers, extra fingers, watermark, simple background, transparent, "
+    "low quality, logo, text, signature, face backlighting, backlighting"
 )
 
+
+def _apply_noobxl_prefix(prompt):
+    """Prepend ``NOOBXL_PREFIX`` to ``prompt``, dropping any prefix tag the prompt
+    already contains (case-insensitive, per comma-separated token)."""
+    prompt = (prompt or "").strip()
+    have = {t.strip().lower() for t in prompt.split(",") if t.strip()}
+    add = [t.strip() for t in NOOBXL_PREFIX.split(",")
+           if t.strip() and t.strip().lower() not in have]
+    if not add:
+        return prompt
+    pre = ", ".join(add)
+    return f"{pre}, {prompt}" if prompt else pre
+
 # -----------------------------------------------------------------------------
 # Load both pipelines once at startup. On ZeroGPU the `.to("cuda")` calls are
 # captured by the runtime and the work happens inside the @spaces.GPU function.
@@ -545,9 +565,11 @@ def generate_image(
     effective_prompt = prompt
     instruction = (prompt_improve_instruction or "").strip()
     if instruction:
+        # The improve instruction is the LLM's SYSTEM prompt; the user's original
+        # prompt is the user message it rewrites.
         improved = ""
         for ev in _vlm_chat_core(
-            _compose_instruction(instruction, prompt), None, "Off", 512
+            prompt, None, "Off", 512, system_prompt=instruction
         ):
             if ev[0] == "text":
                 improved = ev[1]
@@ -629,6 +651,8 @@ def _generate_image_inner(
             requires_pooled=[False, True],
             truncate_long_prompts=False,
         )
+        # One Obsession quality prefix (skipping tags already in the prompt).
+        prompt = _apply_noobxl_prefix(prompt)
         # Wire the negative prompt in only when this model supports it and the
         # caller hasn't disabled it; otherwise fall back to no negative prompt.
         if not (use_negative_prompt and _supports_negative(model_name)):