fix(wan rollout)(5/n): drop the FlowGRPO negative-prompt fallback, use the engine default

miles/rollout/sglang_diffusion_rollout.py

@@ -40,14 +40,16 @@ def build_rollout_sampling_params(
 
     sampling_params: dict[str, Any] = {
         "generator_device": args.diffusion_generator_device,
-        "negative_prompt": neg,
         "width": args.diffusion_width,
         "height": args.diffusion_height,
         "num_inference_steps": num_steps,
         "guidance_scale": args.diffusion_guidance_scale,
         "true_cfg_scale": args.diffusion_true_cfg_scale,
     }
 
+    if neg is not None:
+        sampling_params["negative_prompt"] = neg
+
     if evaluation:
         sampling_params["rollout"] = False
     else:
@@ -87,8 +89,6 @@ def build_rollout_generate_payload(
 ) -> dict[str, Any]:
     """Build full JSON payload for ``POST /rollout/generate`` (``RolloutImageRequest``)."""
     sampling_params["prompt"] = prompt
-    if sampling_params.get("negative_prompt") is None and float(sampling_params.get("guidance_scale", 1.0)) != 1.0:
-        sampling_params["negative_prompt"] = " "  # FlowGRPO default when CFG is on
     sampling_params["num_outputs_per_prompt"] = num_outputs_per_prompt
     return sampling_params
 

scripts/run_diffusion_grpo_pickscore_5gpu_flowgrpo_aligned.py

@@ -58,6 +58,7 @@ def execute(args: ScriptArgs, data_dir: str) -> None:
         "--diffusion-train-iter-order sample_major "
         "--diffusion-num-steps 10 "
         "--diffusion-guidance-scale 4.0 "
+        "--diffusion-negative-prompt ' ' "
         "--diffusion-true-cfg-scale 4.0 "
         "--diffusion-noise-level 1.2 "
         "--diffusion-height 512 "

scripts/run_diffusion_grpo_sd3_ocr_sglang.py

@@ -58,6 +58,7 @@ def execute(args: ScriptArgs, data_dir: str) -> None:
         "--train-dp-split-mode stride "
         "--diffusion-num-steps 10 "
         "--diffusion-guidance-scale 4.5 "
+        "--diffusion-negative-prompt ' ' "
         "--diffusion-noise-level 0.7 "
         "--diffusion-height 512 "
         "--diffusion-width 512 "

tests/fast/rollout/test_rollout_negative_prompt.py

@@ -0,0 +1,45 @@
+from tests.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=5, suite="stage-a-cpu", labels=[])
+
+# The negative_prompt contract of the rollout payload:
+#
+#   --diffusion-negative-prompt <unset>  ─►  key absent from the payload
+#                                            ─► engine strips nothing, per-model default applies
+#   --diffusion-negative-prompt " "      ─►  " " passes through verbatim (FlowGRPO recipes)
+
+from argparse import Namespace
+
+from miles.rollout.sglang_diffusion_rollout import build_rollout_generate_payload, build_rollout_sampling_params
+
+
+def _args(**overrides):
+    values = dict(
+        diffusion_negative_prompt=None,
+        diffusion_generator_device=None,
+        diffusion_eval_num_steps=None,
+        diffusion_num_steps=10,
+        diffusion_width=832,
+        diffusion_height=480,
+        diffusion_guidance_scale=4.0,
+        diffusion_guidance_scale_2=None,
+        diffusion_true_cfg_scale=None,
+        diffusion_output_num_frames=None,
+        diffusion_fps=None,
+        diffusion_sde_type="sde",
+        diffusion_noise_level=0.7,
+        diffusion_log_prob_no_const=False,
+        diffusion_debug_mode=False,
+    )
+    values.update(overrides)
+    return Namespace(**values)
+
+
+def test_unset_negative_prompt_is_omitted():
+    payload = build_rollout_generate_payload(build_rollout_sampling_params(_args()), "p")
+    assert "negative_prompt" not in payload
+
+
+def test_explicit_negative_prompt_passes_through():
+    payload = build_rollout_generate_payload(build_rollout_sampling_params(_args(diffusion_negative_prompt=" ")), "p")
+    assert payload["negative_prompt"] == " "