diff --git a/backend/packages/ai_engine/src/windup_ai_engine/impl/character_generator.py b/backend/packages/ai_engine/src/windup_ai_engine/impl/character_generator.py
index ff453e7d..12fc8ab0 100644
--- a/backend/packages/ai_engine/src/windup_ai_engine/impl/character_generator.py
+++ b/backend/packages/ai_engine/src/windup_ai_engine/impl/character_generator.py
@@ -28,6 +28,7 @@
     SequenceGeometry,
 )
 from windup_ai_engine.postprocess import FOOT_LINE, align_bottom_center, frame_durations
+from windup_ai_engine.postprocess.pack import ANCHOR_CENTROID, FILL_H
 from windup_ai_engine.prompt import PROMPT_VERSION
 from windup_ai_engine.slicing import (
     dead_frame_indices,
@@ -40,6 +41,7 @@
     ROUTE_MATRIX,
     DerivationStrategy,
     is_cyclic,
+    vertical_anchor,
 )
 
 # ── 进度刻度:整条生产线只有一个 total ────────────────────────────────────────
@@ -184,8 +186,8 @@ def _finish(
                 "请调小 n_frames 或加长视频。"
             )
 
-        # 最后一公里:脚线对齐成原地序列帧(直接对齐到调用方要的画布尺寸)
-        aligned = self._lastmile(frames, progress, canvas)
+        # 最后一公里:对齐成原地序列帧(直接对齐到调用方要的画布尺寸)
+        aligned = self._lastmile(frames, action, progress, canvas)
 
         # 量交付成色。在**对齐之后**量,量的是用户真正会看到的那组帧:抠图 / 像素化 /
         # 对齐都会改像素,在中间任何一步量出来的数都描述不了交付物。
@@ -199,7 +201,7 @@ def _finish(
         return GeneratedAction(
             frames=[_png(im) for im in aligned],
             durations=frame_durations(action.action.value, len(aligned)),
-            geometry=_geometry_of(aligned[0], canvas),
+            geometry=_geometry_of(aligned[0], canvas, vertical_anchor(action)),
             quality=quality,
             prompt_version=PROMPT_VERSION,
         )
@@ -227,10 +229,11 @@ def _assess(self, frames: list[Image.Image], action: ActionSpec) -> ActionQualit
     def _lastmile(
         self,
         frames: list[bytes],
+        action: ActionSpec,
         progress: ProgressPort,
         canvas: tuple[int, int] | None = None,
     ) -> list[Image.Image]:
-        """脚线对齐:把各帧对齐成原地序列帧(消除逐帧画布漂移,Issue #21)。
+        """对齐成原地序列帧(消除逐帧画布漂移,Issue #21);锚点随动作有无地面接触。
 
         返回 PIL 而不是 PNG bytes:紧接着的成色测量要按图看帧,再编码回 PNG 只为了
         让上一句话好听、下一句话又得解码回来。编码统一在 ``generate`` 出参那一步做。
@@ -243,7 +246,11 @@ def _lastmile(
         原尺寸居中贴进 512 画布,于是这里刚对齐好的脚线 0.92 被挪到 0.709(2026-08-11
         实测),角色不站在地上、跨动作对齐也失效。在这里一次出到位就没有那一步了。
         """
-        progress.step("lastmile", _TICK_LASTMILE, _TOTAL, "脚线对齐(原地)")
+        anchor = vertical_anchor(action)
+        progress.step(
+            "lastmile", _TICK_LASTMILE, _TOTAL,
+            f"{'质心' if anchor == ANCHOR_CENTROID else '脚线'}对齐(原地)",
+        )
         # 空帧不再静默跳过:未实现的路线现在在 strategy / 装配表处就抛错(见 generate),
         # 走到这里还有空帧说明 provider 或抠图吐了坏数据,同样要炸而不是原样放行。
         if not frames:
@@ -262,22 +269,28 @@ def _lastmile(
         # TODO(dev, #21): tail_match 循环闭合(净位移动作先锚点再匹配帧)
         ref = float(np.median(hs)) if hs else None
         if canvas is None:
-            return align_bottom_center(imgs, ref_height=ref)
+            return align_bottom_center(imgs, ref_height=ref, anchor=anchor)
         cw, ch = canvas
-        return align_bottom_center(imgs, cell=cw, cell_h=ch, ref_height=ref)
+        return align_bottom_center(imgs, cell=cw, cell_h=ch, ref_height=ref, anchor=anchor)
 
 
-def _geometry_of(frame: Image.Image, canvas: tuple[int, int] | None) -> SequenceGeometry:
+def _geometry_of(
+    frame: Image.Image, canvas: tuple[int, int] | None, anchor: str
+) -> SequenceGeometry:
     """交付帧的落位几何。取自对齐那一步用的同一组常量,不另立一份。
 
     ``canvas`` 为 None 时 ``_lastmile`` 走 ``align_bottom_center`` 的默认 cell,
     所以这里也回落到实际交付帧的尺寸,而不是把 CELL 再抄一遍。
+
+    锚点要跟着传:质心那档把主体摆在 ``foot_line - fill_h/2``,仍报脚线的话
+    帧是按质心摆的、几何说的是脚线,消费方按它落位就会错开半个身高。
     """
     w, h = canvas if canvas else frame.size
+    y = FOOT_LINE - FILL_H / 2 if anchor == ANCHOR_CENTROID else FOOT_LINE
     return SequenceGeometry(
         canvas_w=w,
         canvas_h=h,
         anchor_x=0.5,          # align_bottom_center 横向恒居中
-        anchor_y=FOOT_LINE,
-        foot_y=int(h * FOOT_LINE),
+        anchor_y=y,
+        foot_y=int(h * y),
     )
diff --git a/backend/packages/ai_engine/src/windup_ai_engine/postprocess/pack.py b/backend/packages/ai_engine/src/windup_ai_engine/postprocess/pack.py
index e0cce3ae..73be104b 100644
--- a/backend/packages/ai_engine/src/windup_ai_engine/postprocess/pack.py
+++ b/backend/packages/ai_engine/src/windup_ai_engine/postprocess/pack.py
@@ -13,9 +13,10 @@
 
 _logger = logging.getLogger(__name__)
 
-__all__ = ["CELL", "CORE_THICKNESS", "DRIFT_CX_TOL", "DRIFT_FOOT_TOL",
-           "FILL_H", "FILL_W", "FOOT_LINE", "drifted_frames",
-           "align_bottom_center", "core_span", "sprite_sheet", "save_gif"]
+__all__ = ["ANCHOR_CENTROID", "ANCHOR_FOOT", "CELL", "CORE_THICKNESS",
+           "DRIFT_CX_TOL", "DRIFT_FOOT_TOL", "FILL_H", "FILL_W", "FOOT_LINE",
+           "drifted_frames", "align_bottom_center", "core_span", "sprite_sheet",
+           "save_gif"]
 
 # 交付画布的几何 —— 提成模块常量而不是只当默认参数,是因为**入口预检要按同一套几何
 # 判母版能不能装下**(见 master_check.REJECT_ASPECT)。抄一份数字过去就等于埋下
@@ -29,6 +30,12 @@
 # 0.25 之下是延展物(尾巴、翅膀、披风、举起的武器)—— 它们细,本体厚。
 CORE_THICKNESS = 0.25
 
+# 垂直对齐锚点。脚线那档对站在地上的动作成立;飞 / 游 / 攀全程没有地面接触,它们的包围盒
+# 底边是尾羽与爪子、逐帧在变,钉死底边等于让身体跟着延展物上下浮动(#534)。
+ANCHOR_FOOT = "foot"
+ANCHOR_CENTROID = "centroid"
+_ANCHORS = (ANCHOR_FOOT, ANCHOR_CENTROID)
+
 
 def core_span(frame: Image.Image, thickness: float = CORE_THICKNESS) -> tuple[float, float] | None:
     """本体的 (高, 宽),单位=该帧像素。空帧返回 ``None``。
@@ -54,12 +61,9 @@ def core_span(frame: Image.Image, thickness: float = CORE_THICKNESS) -> tuple[fl
     #   · 它又让**大量列**只有 10px 高 → 列方向若以中位数为基准,中位数被压到 10、门槛低到
     #     2,翅膀整条算进本体,量出的本体宽 209px(真值 49)。故列用 **max**。
     # 判据不对称是数据形态决定的,不是漏了统一。
-    def span(counts: np.ndarray, base: float) -> float:
-        keep = np.flatnonzero(counts >= base * thickness)
-        return float(keep.max() - keep.min())
-
-    nz_rows = rows[rows > 0]
-    return (span(rows, float(np.median(nz_rows))), span(cols, float(cols.max())))
+    r0, r1 = _core_rows(m, thickness)
+    keep = np.flatnonzero(cols >= float(cols.max()) * thickness)
+    return (float(r1 - r0), float(keep.max() - keep.min()))
 
 
 # 单调漂移的判定门槛(整段首尾相对变化)。低于它的不动 —— 真实身高起伏实测约 4%,
@@ -152,6 +156,7 @@ def align_bottom_center(
     preserve_lift: bool = False,
     ref_height: float | None = None,
     cell_h: int | None = None,
+    anchor: str = ANCHOR_FOOT,
 ) -> list[Image.Image]:
     """按脚线对齐到统一画布,消除逐帧画布漂移(Issue #21)。
 
@@ -160,7 +165,7 @@ def align_bottom_center(
     缩小。统一缩放后帧间只剩真实姿态差,尺度稳定。
 
     水平方向按**主体水平中心**对齐(不含挥出的武器会更好,当前用整体包围盒中心兜底);
-    垂直方向按**脚线**(包围盒底边)对齐到 ``foot_line``。
+    垂直方向按 ``anchor`` 选锚点,默认脚线(包围盒底边)对齐到 ``foot_line``。
 
     ``ref_height``:**跨动作一致性的关键**,单位=传入帧的像素高。给定时按它定标,否则按本
     序列最高帧。按最高帧定标会让"举过头顶"的动作整段被缩小去迁就那一帧 —— 实测攻击时
@@ -170,6 +175,10 @@ def align_bottom_center(
     ``preserve_lift``:腾空位移**默认不烘进像素**(业界:位移交引擎 root motion)。仅在要把
     位移画进序列帧时才开;开启后以序列里最低的脚线为地面基准,保留每帧相对地面的抬升量。
 
+    ``anchor``:``"foot"`` 给有地面接触的动作,``"centroid"`` 给飞 / 游 / 攀 —— 后者的底边
+    是尾羽与爪子、逐帧在变,钉死底边身体反而上下浮动(#534)。质心那档落在
+    ``foot_line - fill_h/2``,即参考姿态包围盒的纵向中心,故换锚点不改变构图。
+
     ``cell``/``cell_h``:交付画布的宽与高,``cell_h=None`` 即方形 ``cell×cell``(默认,
     行为与加这个参数之前逐像素相同)。**要能出非方形画布,是为了让引擎一次就出到项目
     要的 sprite 尺寸、不必在上层再缩一次。** 上层那次二次缩放不是"糊一点"那么简单:
@@ -189,6 +198,14 @@ def align_bottom_center(
         # 不静默出一张 0×0:PIL 允许建 0 边长的图,后面 alpha_composite 也不报错,
         # 错产物要到落库/前端才暴露。
         raise ValueError(f"交付画布尺寸必须为正,收到 cell={cell} cell_h={cell_h}")
+    if anchor not in _ANCHORS:
+        # 拼错一个字母不静默回落到脚线:那正好是本参数要修的那个错,而帧数 / 尺寸 / 成色
+        # 全部正常,要靠人看动图才发现。
+        raise ValueError(f"anchor 只能是 {_ANCHORS} 之一,收到 {anchor!r}")
+    if preserve_lift and anchor == ANCHOR_CENTROID:
+        # 质心对齐把每帧摆到同一条线上,抬升量随之被抹平 —— 两个都开等于 preserve_lift
+        # 是个空操作。
+        raise ValueError("preserve_lift 与 anchor='centroid' 互斥:质心对齐会抹平抬升量")
 
     boxes: list[tuple[int, int, int, int] | None] = []
     for f in frames:
@@ -277,10 +294,16 @@ def align_bottom_center(
         fs = scale * per_frame[idx]
         w = max(1, round(crop.width * fs))
         h = max(1, round(crop.height * fs))
+        if anchor == ANCHOR_CENTROID:
+            # 锚点在缩放之前量:本体判据看的是原始像素,resize 的取整会把细延展物抹掉或加粗。
+            r0, r1 = _core_rows(np.asarray(crop)[:, :, 3] > 128)
+            top = round(ch * (foot_line - fill_h / 2) - (r0 + r1) / 2 * fs)
+        else:
+            lift = round((ground - box[3]) * fs) if preserve_lift else 0
+            top = int(ch * foot_line) - h - lift
         crop = crop.resize((w, h), Image.NEAREST)
-        lift = round((ground - box[3]) * fs) if preserve_lift else 0
         canvas = Image.new("RGBA", (cw, ch), (0, 0, 0, 0))
-        canvas.alpha_composite(crop, (cw // 2 - w // 2, int(ch * foot_line) - h - lift))
+        canvas.alpha_composite(crop, (cw // 2 - w // 2, top))
         out.append(canvas)
     return out
 
@@ -357,6 +380,19 @@ def _core_columns(mask) -> tuple[int, int] | None:
     return int(keep.min()), int(keep.max())
 
 
+def _core_rows(mask, thickness: float = CORE_THICKNESS) -> tuple[int, int]:
+    """本体占据的首尾行;``mask`` 须有主体像素。
+
+    基准取行宽中位数而不是最厚那行,理由见 :func:`core_span` 里"延展物对行、列的污染方向
+    相反"那段 —— 与它共用一把尺子,免得本体的**位置**和**跨度**各按各的判据算。
+    """
+    import numpy as np
+
+    rows = mask.sum(1)
+    keep = np.flatnonzero(rows >= float(np.median(rows[rows > 0])) * thickness)
+    return int(keep.min()), int(keep.max())
+
+
 def _outliers(obs: list[tuple[int, float]], tol: float) -> set[int]:
     """按中位数判离群;整段有净位移时先把这条直线除掉。
 
diff --git a/backend/packages/ai_engine/src/windup_ai_engine/strategy/base.py b/backend/packages/ai_engine/src/windup_ai_engine/strategy/base.py
index 1865ad26..a96ea9e7 100644
--- a/backend/packages/ai_engine/src/windup_ai_engine/strategy/base.py
+++ b/backend/packages/ai_engine/src/windup_ai_engine/strategy/base.py
@@ -17,6 +17,7 @@
 from windup_common.models import ActionSpec, ActionType, CharacterCard, GenRoute
 
 from windup_ai_engine.ports import ProgressPort
+from windup_ai_engine.postprocess.pack import ANCHOR_CENTROID, ANCHOR_FOOT
 
 # 动作类型 → 生成路线(架构决策,写死为契约)
 ROUTE_MATRIX: dict[ActionType, GenRoute] = {
@@ -59,6 +60,18 @@ def is_cyclic(action: ActionSpec) -> bool:
         return bool(action.cyclic)
     return action.action in CYCLIC_ACTIONS
 
+
+def vertical_anchor(action: ActionSpec) -> str:
+    """这次生成按什么对齐垂直方向(见 postprocess.pack.align_bottom_center)。
+
+    **jump 不算无地面接触** —— 它腾空但要回地,按质心对齐会让落地帧悬在半空。全程无地面
+    的飞 / 游 / 攀只能以 CUSTOM 进来,故由调用方显式声明:按描述文字猜错是静默的(#534)。
+    """
+    if action.action is ActionType.CUSTOM and action.ground_contact is False:
+        return ANCHOR_CENTROID
+    return ANCHOR_FOOT
+
+
 # 本矩阵的形状本身有个已知边界,记录在此以免后来者按错误前提扩展:
 # 它是「动作类型 → 路线」的一对一映射,隐含前提是"路线由动作的物理性质唯一决定"。
 # 该前提对逐帧 / 视频两条路线成立(有无连续步态是动作固有属性),但对渲染出帧路线不成立
diff --git a/backend/packages/app/src/windup_app/server/orchestrator/executor.py b/backend/packages/app/src/windup_app/server/orchestrator/executor.py
index 380f3dc4..7197e162 100644
--- a/backend/packages/app/src/windup_app/server/orchestrator/executor.py
+++ b/backend/packages/app/src/windup_app/server/orchestrator/executor.py
@@ -344,7 +344,14 @@ def _produce_action(
             # 缺 loop 时兜成一次性,依据是失败代价不对称:一次性误当循环会让末帧接回首帧
             # 抽搐、产物不可用;反之只是不无缝闭环、仍可用。不从描述文字猜。
             cyclic = False if input.loop is None else bool(input.loop)
-            extra = {"custom_action": input.custom_prompt or "", "cyclic": cyclic}
+            # 缺 ground_contact 时兜成"有地面接触":绝大多数自述动作有脚踩地,而误判成
+            # 全程离地会让角色不站在地上 —— 比飞行动作上下浮动严重。
+            grounded = True if input.ground_contact is None else bool(input.ground_contact)
+            extra = {
+                "custom_action": input.custom_prompt or "",
+                "cyclic": cyclic,
+                "ground_contact": grounded,
+            }
         action = ActionSpec(
             action=engine_action,
             poses=[""] * input.num_frames,
diff --git a/backend/packages/app/src/windup_app/server/orchestrator/model.py b/backend/packages/app/src/windup_app/server/orchestrator/model.py
index 8d9495e6..ca8464f2 100644
--- a/backend/packages/app/src/windup_app/server/orchestrator/model.py
+++ b/backend/packages/app/src/windup_app/server/orchestrator/model.py
@@ -99,6 +99,10 @@ class CharacterActionInput:
     # 这个动作是否循环播放。``None`` 原样往下传,由编排层兜成一次性:本层替调用方填默认值
     # 的话,"没给"和"明确给了 False"从这里起就再也分不开了。
     loop: bool | None = None
+    # 这个动作有没有地面接触。``None`` 原样往下传,由编排层兜成"有" —— 本层替调用方填默认
+    # 值的话,"没给"与"明确给了 True"从这里起就分不开了。飞 / 游 / 攀传 False,垂直对齐
+    # 改按躯干中心走(#534);jump 不属此列,它腾空但要回地。
+    ground_contact: bool | None = None
     # 视频模型。``None`` = 用部署配置的默认值。取值域为
     # ``ModelRegistry.chain(CHARACTER_ACTION)``(部署默认 + fallbacks);不在链上 → 入口
     # 报错,不到付费调用才失败。选中的型号表示这次从它开始试,由 Gateway 读 start_from_model。
diff --git a/backend/packages/app/src/windup_app/web/api/generation.py b/backend/packages/app/src/windup_app/web/api/generation.py
index 939abd90..27c22b0b 100644
--- a/backend/packages/app/src/windup_app/web/api/generation.py
+++ b/backend/packages/app/src/windup_app/web/api/generation.py
@@ -200,6 +200,10 @@ class CharacterActionGenerateRequest(BaseModel):
     # 不对称:一次性动作被当成循环会让末帧接回首帧抽搐、产物不可用,反之只是不无缝闭环、
     # 仍可用。而且猜错是静默的,帧数/时长/成色全部正常、没有任何一道会红。
     loop: bool | None = None
+    # 这个动作有没有地面接触。飞 / 游 / 攀全程离地,它们的包围盒底边是尾羽与爪子、逐帧在变,
+    # 按脚线对齐反而让身体上下浮动(#534)。不给按"有"处理:误判成离地会让角色不站在地上,
+    # 比浮动严重。jump 不走这个字段 —— 它腾空但要回地。
+    ground_contact: bool | None = None
     # 视频模型。None = 用部署默认。取值域见 ModelRegistry.chain(CHARACTER_ACTION);
     # 非法值在入口就报错,不到付费调用才失败。选中的型号表示这次从它开始试。
     video_model: str | None = None
@@ -222,6 +226,16 @@ def require_custom_prompt(self):
             self.custom_prompt = prompt
         return self
 
+    @model_validator(mode="after")
+    def ground_contact_belongs_to_custom_only(self):
+        # 写死的那几个动作都有地面接触,收下这个字段等于让调用方以为自己能改它。
+        if self.action_type is not ActionType.CUSTOM and self.ground_contact is not None:
+            raise ValueError(
+                f"action_type={self.action_type.value} 不该带 ground_contact:"
+                "它只对 custom 动作有意义,写死的动作都有地面接触"
+            )
+        return self
+
     @model_validator(mode="after")
     def drop_blank_reference_image_urls(self):
         # 执行阶段只取 reference_image_urls[0]。留着空串会让"有母版"的判定成立,
@@ -427,6 +441,7 @@ def submit_action_generation(
         action_type=body.action_type,
         custom_prompt=body.custom_prompt,
         loop=body.loop,
+        ground_contact=body.ground_contact,
         video_model=body.video_model,
         reference_video_url=body.reference_video_url,
         reference_image_urls=body.reference_image_urls,
diff --git a/backend/packages/app/src/windup_app/worker/handlers.py b/backend/packages/app/src/windup_app/worker/handlers.py
index 53668cd7..ddc96631 100644
--- a/backend/packages/app/src/windup_app/worker/handlers.py
+++ b/backend/packages/app/src/windup_app/worker/handlers.py
@@ -87,6 +87,7 @@ def _action_input(payload: dict) -> CharacterActionInput:
         reference_image_urls=list(payload.get("reference_image_urls") or []),
         num_frames=int(raw_frames) if raw_frames is not None else None,
         loop=payload.get("loop"),
+        ground_contact=payload.get("ground_contact"),
         video_model=payload.get("video_model"),
         outfit_id=payload.get("outfit_id"),
         model_3d_url=payload.get("model_3d_url"),
diff --git a/backend/packages/common/src/windup_common/models/character.py b/backend/packages/common/src/windup_common/models/character.py
index b00ab34c..3c3ea894 100644
--- a/backend/packages/common/src/windup_common/models/character.py
+++ b/backend/packages/common/src/windup_common/models/character.py
@@ -241,6 +241,12 @@ class ActionSpec(BaseModel):
     # 这里不给默认值,否则同一个缺省被两处各写一份,改一处另一处静默不动。
     archetype: AttackArchetype | None = None
 
+    # 这个动作有没有地面接触。``False`` = 飞 / 游 / 攀这类**全程离地**,垂直对齐改按躯干
+    # 中心走(见 ai_engine.strategy.base.vertical_anchor);jump 不属此列 —— 它腾空但要回地。
+    # 不给按有地面接触处理:代价不对称,把有脚的动作当离地会让角色不站在地上,而反过来
+    # 只是身体跟着尾羽上下浮动、产物仍可用。
+    ground_contact: bool | None = None
+
     @model_validator(mode="after")
     def _archetype_belongs_to_attack_only(self) -> ActionSpec:
         """非攻击动作带 archetype 要炸:它只被攻击提示词消费,传了不会生效。"""
@@ -275,6 +281,11 @@ def _custom_needs_its_own_settings(self) -> ActionSpec:
                     f"action={self.action.value} 不该带 cyclic;循环性由 CYCLIC_ACTIONS 写死,"
                     "传了不会生效"
                 )
+            if self.ground_contact is not None:
+                raise ValueError(
+                    f"action={self.action.value} 不该带 ground_contact;写死的那几个动作都有"
+                    "地面接触(jump 腾空但要回地),传了不会生效"
+                )
         return self
 
     # 这里**没有** ``route`` 字段:路线选择整个在 server —— 走不走三渲二取决于"这个造型
diff --git a/backend/tests/test_generation_api.py b/backend/tests/test_generation_api.py
index b360b2e9..c7e2490a 100644
--- a/backend/tests/test_generation_api.py
+++ b/backend/tests/test_generation_api.py
@@ -467,6 +467,57 @@ def test_stance_omitted_stays_none_not_biped(auth_client, monkeypatch):
     assert inputs and inputs[0].stance is None
 
 
+def test_ground_contact_from_request_reaches_the_task_input(auth_client, monkeypatch):
+    """飞 / 游 / 攀的声明断在请求层的话,对齐那一步永远走脚线分支(#534)。"""
+    from windup_app.web.api import generation as gen_api
+    from windup_app.server.orchestrator.model import CharacterActionInput
+
+    dispatched = _capture_action_input(gen_api, monkeypatch)
+    project = _create_project(auth_client)
+    character = _create_character(auth_client, project["id"])
+
+    auth_client.post(
+        "/generation/action",
+        json=_action_payload(
+            project["id"], character["id"],
+            action_type="custom", custom_prompt="flies forward", ground_contact=False,
+        ),
+    )
+
+    inputs = [a for args in dispatched for a in args if isinstance(a, CharacterActionInput)]
+    assert inputs, "任务没被收下"
+    assert inputs[0].ground_contact is False
+
+
+def test_ground_contact_omitted_stays_none_not_true(auth_client, monkeypatch):
+    """不给就原样传 None —— 在这层填默认值,"没给"与"明确有地面接触"就分不开了。"""
+    from windup_app.web.api import generation as gen_api
+    from windup_app.server.orchestrator.model import CharacterActionInput
+
+    dispatched = _capture_action_input(gen_api, monkeypatch)
+    project = _create_project(auth_client)
+    character = _create_character(auth_client, project["id"])
+
+    auth_client.post(
+        "/generation/action", json=_action_payload(project["id"], character["id"]),
+    )
+
+    inputs = [a for args in dispatched for a in args if isinstance(a, CharacterActionInput)]
+    assert inputs and inputs[0].ground_contact is None
+
+
+def test_ground_contact_on_a_fixed_action_is_rejected_at_the_entrance(auth_client):
+    """walk 收下这个字段等于让调用方以为自己能改它,而它不会生效。"""
+    project = _create_project(auth_client)
+    character = _create_character(auth_client, project["id"])
+    body = auth_client.post(
+        "/generation/action",
+        json=_action_payload(project["id"], character["id"], ground_contact=False),
+    ).json()
+    assert body["code"] == 400, body
+    assert "ground_contact" in body["message"], body["message"]
+
+
 def test_illegal_stance_is_rejected_at_the_entrance(auth_client):
     project = _create_project(auth_client)
     character = _create_character(auth_client, project["id"])
diff --git a/backend/tests/test_ground_contact_anchor.py b/backend/tests/test_ground_contact_anchor.py
new file mode 100644
index 00000000..08d55fe9
--- /dev/null
+++ b/backend/tests/test_ground_contact_anchor.py
@@ -0,0 +1,286 @@
+"""无地面接触的动作按躯干中心对齐(#534)。
+
+这一片锁的是一种静默错误:飞行序列的包围盒底边是尾羽与爪子、逐帧在变,把底边钉死在
+脚线上,身体反过来跟着延展物上下浮动 —— 而帧数、时长、成色、脚线 std 全部正常,
+只有人盯着动图看才发现。
+
+覆盖三段:① 几何(换锚点真的止住浮动,且不动定标);② 映射(jump 腾空但要回地,
+不算无地面接触);③ 贯通(声明从 HTTP 入参一路走到对齐那一步,中间不许掉队)。
+"""
+from __future__ import annotations
+
+import dataclasses
+import io
+
+import numpy as np
+import pytest
+from PIL import Image
+from pydantic import ValidationError
+
+from windup_ai_engine.impl import CharacterGenerator
+from windup_ai_engine.postprocess.pack import (
+    ANCHOR_CENTROID,
+    ANCHOR_FOOT,
+    FOOT_LINE,
+    align_bottom_center,
+    core_span,
+)
+from windup_ai_engine.strategy.base import DerivationStrategy, vertical_anchor
+from windup_app.server.orchestrator.executor import ActionTaskExecutor, ProjectConstraints
+from windup_app.server.orchestrator.model import ActionType as ApiActionType
+from windup_app.server.orchestrator.model import CharacterActionInput
+from windup_app.worker.handlers import _action_input
+from windup_common.models import (
+    ActionSpec,
+    ActionType,
+    CharacterCard,
+    GenRoute,
+    Stylize,
+)
+
+CELL = 256
+REF_HEIGHT = 120.0        # 参考姿态高(生产由 _lastmile 按各帧包围盒中位数给)
+_CORE = (200, 80, 80, 255)
+_EXT = (180, 140, 90, 255)
+
+
+def _flying(n: int = 12, size: int = 256, bw: int = 48, bh: int = 44, top: int = 90):
+    """飞行序列:躯干在原帧里一动不动,尾羽与爪子逐帧伸缩。
+
+    躯干不动是**故意**的 —— 交付帧里量到的任何纵向浮动都只能来自锚点选择,不掺姿态。
+    """
+    out = []
+    for i in range(n):
+        a = np.zeros((size, size, 4), np.uint8)
+        x0 = (size - bw) // 2
+        a[top:top + bh, x0:x0 + bw] = _CORE
+        reach = int(4 + 22 * abs(np.sin(i / n * 2 * np.pi)))
+        a[top + bh:top + bh + reach, x0 + 6:x0 + 10] = _EXT
+        a[top + bh:top + bh + reach // 2, x0 + bw - 10:x0 + bw - 6] = _EXT
+        out.append(Image.fromarray(a, "RGBA"))
+    return out
+
+
+def _walking(n: int = 12, size: int = 256, bw: int = 44, bh: int = 96):
+    """有地面接触的序列:脚线钉在同一行,身高按步态自然起伏。"""
+    out = []
+    for i in range(n):
+        a = np.zeros((size, size, 4), np.uint8)
+        bob = i % 3
+        x0, y1 = (size - bw) // 2, 200
+        a[y1 - bh + bob:y1, x0:x0 + bw] = _CORE
+        a[y1 - bh - 12:y1 - bh, x0 + 8:x0 + 12] = _EXT       # 举起的武器
+        out.append(Image.fromarray(a, "RGBA"))
+    return out
+
+
+def _core_center(img: Image.Image) -> float:
+    """交付帧里躯干的纵向中心。按颜色认躯干,免得又被延展物带走。"""
+    a = np.asarray(img)
+    m = (a[:, :, 0] > 150) & (a[:, :, 1] < 120) & (a[:, :, 3] > 128)
+    ys, _ = np.where(m)
+    return float(ys.min() + ys.max()) / 2
+
+
+def _foot_ratio(img: Image.Image) -> float:
+    a = np.asarray(img)[:, :, 3]
+    ys, _ = np.where(a > 128)
+    return (float(ys.max()) + 1) / img.size[1]
+
+
+def _png(img: Image.Image) -> bytes:
+    buf = io.BytesIO()
+    img.save(buf, "PNG")
+    return buf.getvalue()
+
+
+def _custom(ground_contact=None, **kw) -> ActionSpec:
+    kw.setdefault("n_frames", 8)
+    kw.setdefault("stylize", Stylize.NONE)
+    return ActionSpec(
+        action=ActionType.CUSTOM,
+        custom_action="flies forward with slow wing beats",
+        cyclic=True,
+        ground_contact=ground_contact,
+        **kw,
+    )
+
+
+# ── ① 几何:换锚点止住浮动,定标一动不动 ────────────────────────────────────
+
+
+def test_core_stops_bobbing_when_anchored_to_the_core():
+    """验收①:同一条飞行序列,躯干中心的纵向 std 明显低于按脚线对齐。"""
+    src = _flying()
+    foot = [_core_center(f) for f in align_bottom_center(src, cell=CELL, ref_height=REF_HEIGHT)]
+    core = [
+        _core_center(f)
+        for f in align_bottom_center(src, cell=CELL, ref_height=REF_HEIGHT, anchor=ANCHOR_CENTROID)
+    ]
+    foot_std, core_std = float(np.std(foot)), float(np.std(core))
+    # 先验素材:脚线那档本来就该被延展物带着浮动,量不到说明这组帧根本没复现问题
+    assert foot_std > 5.0, f"素材不成立:按脚线对齐也没浮动(std={foot_std:.2f})"
+    assert core_std <= 1.0, (
+        f"躯干仍在浮动:脚线 std={foot_std:.2f} → 质心 std={core_std:.2f};"
+        f"逐帧中心 {[round(v, 1) for v in core]}"
+    )
+
+
+def test_grounded_frames_are_pixel_identical_to_the_default():
+    """验收②:默认锚点一个像素都不许变。"""
+    src = _walking()
+    a = align_bottom_center(src, cell=CELL, ref_height=REF_HEIGHT)
+    b = align_bottom_center(src, cell=CELL, ref_height=REF_HEIGHT, anchor=ANCHOR_FOOT)
+    for x, y in zip(a, b, strict=True):
+        assert np.array_equal(np.asarray(x), np.asarray(y))
+
+
+def test_walking_still_lands_on_the_foot_line():
+    """验收②的读数版:走路 / 待机的脚线仍落在 0.92。"""
+    out = align_bottom_center(_walking(), cell=CELL, ref_height=REF_HEIGHT)
+    feet = [_foot_ratio(f) for f in out]
+    assert max(abs(v - FOOT_LINE) for v in feet) <= 0.01, feet
+
+
+def test_switching_the_anchor_does_not_resize_the_character():
+    """只换垂直锚点,不动 fill_h / ref_height 那套定标 —— 交付本体高必须一样。"""
+    src = _flying()
+    foot = align_bottom_center(src, cell=CELL, ref_height=REF_HEIGHT)
+    core = align_bottom_center(src, cell=CELL, ref_height=REF_HEIGHT, anchor=ANCHOR_CENTROID)
+    for a, b in zip(foot, core, strict=True):
+        assert abs(core_span(a)[0] - core_span(b)[0]) <= 1
+
+
+def test_unknown_anchor_raises_instead_of_falling_back():
+    """拼错一个字母不静默回落到脚线:那正是本参数要修的错,而产出看起来完全正常。"""
+    with pytest.raises(ValueError, match="anchor"):
+        align_bottom_center(_flying(), cell=CELL, anchor="centroide")
+
+
+def test_preserve_lift_and_the_core_anchor_are_mutually_exclusive():
+    """质心对齐会把每帧摆到同一条线上,抬升量随之被抹平 —— 同开等于空操作。"""
+    with pytest.raises(ValueError, match="preserve_lift"):
+        align_bottom_center(_flying(), cell=CELL, preserve_lift=True, anchor=ANCHOR_CENTROID)
+
+
+# ── ② 映射:哪些动作算无地面接触 ───────────────────────────────────────────
+
+
+@pytest.mark.parametrize("action", [t for t in ActionType if t is not ActionType.CUSTOM])
+def test_fixed_actions_all_keep_the_foot_anchor(action):
+    assert vertical_anchor(ActionSpec(action=action)) == ANCHOR_FOOT
+
+
+def test_jump_is_airborne_but_still_anchors_to_the_foot_line():
+    """jump 腾空但要回地,与飞 / 游 / 攀不是一回事:按质心对齐会让落地帧悬在半空。"""
+    assert vertical_anchor(ActionSpec(action=ActionType.JUMP)) == ANCHOR_FOOT
+
+
+def test_custom_switches_only_when_the_caller_declares_no_ground_contact():
+    assert vertical_anchor(_custom(ground_contact=False)) == ANCHOR_CENTROID
+    assert vertical_anchor(_custom(ground_contact=True)) == ANCHOR_FOOT
+    assert vertical_anchor(_custom()) == ANCHOR_FOOT
+
+
+def test_fixed_actions_must_not_carry_ground_contact():
+    """给 walk 传这个字段要炸:写死的动作都有地面接触,收下它等于让调用方以为能改。"""
+    with pytest.raises(ValidationError, match="ground_contact"):
+        ActionSpec(action=ActionType.WALK, ground_contact=False)
+
+
+# ── ③ 贯通:声明从 HTTP 入参一路走到对齐 ────────────────────────────────────
+
+
+class _FlyingStrategy(DerivationStrategy):
+    """顶替真实 i2v:返回同一条飞行序列,让最后一公里真跑。"""
+
+    route = GenRoute.VIDEO_I2V
+
+    def derive(self, card, action, master, progress) -> list[bytes]:
+        return [_png(f) for f in _flying(action.n_frames)]
+
+
+class _NullProgress:
+    def step(self, stage: str, i: int, total: int, note: str = "") -> None:
+        pass
+
+
+def _generate(action: ActionSpec) -> list[Image.Image]:
+    gen = CharacterGenerator({GenRoute.VIDEO_I2V: _FlyingStrategy()})
+    out = gen.generate(
+        CharacterCard(name="鹰", desc="羽族斥候"),
+        action,
+        master=_png(_flying(1)[0]),
+        progress=_NullProgress(),
+    )
+    return [Image.open(io.BytesIO(f)).convert("RGBA") for f in out.frames]
+
+
+def test_engine_entrypoint_carries_the_declaration_into_the_last_mile():
+    """只测 ``vertical_anchor`` 的话,``_lastmile`` 不把它传下去照样全绿。
+
+    所以这条从引擎入口发起:同一组帧、同一条链,只有声明不同,量交付帧的躯干中心。
+    """
+    walk = [_core_center(f) for f in _generate(ActionSpec(action=ActionType.WALK, n_frames=8))]
+    fly = [_core_center(f) for f in _generate(_custom(ground_contact=False))]
+    assert float(np.std(walk)) > 5.0, "对照组不浮动的话这条用例证明不了任何事"
+    assert float(np.std(fly)) <= 1.0, f"声明了无地面接触,躯干仍在浮动: {fly}"
+
+
+class _Stop(Exception):
+    """捕到 ActionSpec 就停,后面那些上传 / 记账与本条无关。"""
+
+
+class _CaptureGenerator:
+    def __init__(self) -> None:
+        self.specs: list[ActionSpec] = []
+
+    def generate(self, card, action, master, progress, canvas=None):
+        self.specs.append(action)
+        raise _Stop
+
+    def generate_rendered(self, card, action, rigged_model, progress, canvas=None):
+        raise AssertionError("没有 3D 资产却走了三渲二")
+
+
+def _capture_spec(**input_kw) -> ActionSpec:
+    gen = _CaptureGenerator()
+    executor = ActionTaskExecutor(
+        generator=gen,
+        upload=lambda _png: "https://cdn.example.com/f.png",
+        fetch_master=lambda _input: b"master-bytes",
+        fetch_constraints=lambda *_: ProjectConstraints(sprite_w=64, sprite_h=64),
+    )
+    with pytest.raises(_Stop):
+        executor._produce_action(
+            CharacterActionInput(
+                character_id=1,
+                action_type=ApiActionType.CUSTOM,
+                custom_prompt="flies forward",
+                num_frames=4,
+                **input_kw,
+            ),
+            ProjectConstraints(sprite_w=64, sprite_h=64),
+        )
+    return gen.specs[0]
+
+
+def test_orchestrator_puts_the_declaration_on_the_action_spec():
+    assert _capture_spec(ground_contact=False).ground_contact is False
+    assert _capture_spec(ground_contact=True).ground_contact is True
+
+
+def test_missing_declaration_falls_back_to_having_ground_contact():
+    """不给按"有地面接触":误判成全程离地会让角色不站在地上,比浮动严重。"""
+    assert _capture_spec().ground_contact is True
+
+
+def test_worker_rebuilds_the_declaration_from_the_task_payload():
+    """入参经 MQ 落库再取回 —— 生产就是这条路,漏在这里等于整条链恒走默认分支。"""
+    src = CharacterActionInput(
+        character_id=1,
+        action_type=ApiActionType.CUSTOM,
+        custom_prompt="flies forward",
+        ground_contact=False,
+    )
+    assert _action_input(dataclasses.asdict(src)).ground_contact is False
diff --git a/backend/tests/test_render3d_route_and_assets.py b/backend/tests/test_render3d_route_and_assets.py
index 00779559..2dc27ea4 100644
--- a/backend/tests/test_render3d_route_and_assets.py
+++ b/backend/tests/test_render3d_route_and_assets.py
@@ -607,6 +607,32 @@ def test_after_approval_it_proceeds_and_reuses_the_stored_model(tmp_path):
     assert (m.calls, r.calls) == (1, 1)
 
 
+def test_geometry_follows_the_anchor_it_was_aligned_with():
+    """质心对齐的动作要报质心那条线,不能仍报脚线。
+
+    帧按质心摆、几何说的是脚线的话,消费方按它落位会错开半个身高,而帧数、时长、成色
+    全部正常,没有一道会红 —— 与 #534 换锚点是同一件事的两半。
+    """
+    from windup_ai_engine.postprocess import FOOT_LINE
+    from windup_ai_engine.postprocess.pack import FILL_H
+
+    canvas = (256, 320)
+    gen = _real_generator(_FakeRenderer())
+    foot = gen.generate_rendered(
+        _card(), _spec(), b"RIGGED", _NullProgress(), canvas=canvas,
+    ).geometry
+    fly = gen.generate_rendered(
+        _card(),
+        _spec(action=ActionType.CUSTOM, custom_action="飞", cyclic=True, ground_contact=False),
+        b"RIGGED", _NullProgress(), canvas=canvas,
+    ).geometry
+
+    assert foot.anchor_y == FOOT_LINE
+    assert fly.anchor_y == pytest.approx(FOOT_LINE - FILL_H / 2)
+    assert fly.foot_y == int(canvas[1] * fly.anchor_y)
+    assert fly.anchor_y < foot.anchor_y, "质心那条线必须在脚线之上"
+
+
 def test_generated_action_reports_the_alignment_geometry_instead_of_a_constant():
     """交付几何由引擎报出,不让消费方按常数推。
 
diff --git a/openapi.json b/openapi.json
index 28d5b27e..cc1444ae 100644
--- a/openapi.json
+++ b/openapi.json
@@ -220,6 +220,17 @@
             "$ref": "#/components/schemas/ActionDirection",
             "default": "east"
           },
+          "ground_contact": {
+            "anyOf": [
+              {
+                "type": "boolean"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "title": "Ground Contact"
+          },
           "loop": {
             "anyOf": [
               {