diff --git a/app/harness/validate.py b/app/harness/validate.py
index 0e2adf7..4f35a43 100644
--- a/app/harness/validate.py
+++ b/app/harness/validate.py
@@ -809,7 +809,8 @@ class GateSpec:
         target_file: 解析后生效 skill 文件名(候选物化写此文件)。
         candidate_content: 候选 skill 全文。
         base_skill_content: 基线侧生效 skill 全文(skill_hash 作缓存键)。
-        units: 阶梯序单元列表(已排除案例单元、截断 gate_n_max)。
+        units: 阶梯序单元元组(已排除案例单元、截断 gate_n_max);元组,装配后
+            不可变,防 spec.units 与 run.slots 漂移。
         gate_run_prefix: run_id 前缀,必须含 "_gate_"(防泄露过滤依赖)。
     """
 
@@ -817,7 +818,7 @@ class GateSpec:
     target_file: str
     candidate_content: str
     base_skill_content: str
-    units: list[QuestionUnit]
+    units: tuple[QuestionUnit, ...]
     gate_run_prefix: str
 
 
@@ -884,6 +885,10 @@ def _advance_prefix(run: _GateRun, params: GateParams) -> None:
     瞬间返回、cand 臂必新鲜跑,两臂延迟不对称,若 cand 延迟与对错相关,早到翻转
     对系统性偏向 W 型 → e-值虚高假接受。前缀消费把判定顺序钉回预声明阶梯序,
     anytime-valid 无条件成立;INFRA 单元视为"已解决(剔除)"不阻塞前缀。
+
+    契约:全部单元被剔除时 verdict 保持 None、frozen 保持 False,由调度编排层
+    (Task 3 的 validate_skills_concurrent)检测 verdict None 并 raise
+    RuntimeError;本函数不负责该终态。
     """
     while not run.frozen and run.prefix_ptr < len(run.slots):
         slot = run.slots[run.prefix_ptr]
@@ -903,7 +908,9 @@ def _advance_prefix(run: _GateRun, params: GateParams) -> None:
                     run.frozen = True
             continue
         uid = slot.unit.unit_id
-        assert slot.base is not None and slot.cand_per_q is not None
+        assert slot.base is not None and slot.cand_per_q is not None, (
+            f"slot 未配齐即被消费: unit={slot.unit.unit_id}"
+        )
         c_units = unit_correctness_view([slot.unit], slot.cand_per_q)
         pair_result = pair_block({uid: slot.base}, c_units, [uid])
         run.candidate_per_q.update(slot.cand_per_q)
diff --git a/tests/unit/test_gate_prefix.py b/tests/unit/test_gate_prefix.py
index bf68ad6..8b6749b 100644
--- a/tests/unit/test_gate_prefix.py
+++ b/tests/unit/test_gate_prefix.py
@@ -33,7 +33,7 @@ def _mk_run(n_units: int) -> _GateRun:
         target_file="action-reasoning.md",
         candidate_content="cand",
         base_skill_content="base",
-        units=[_mk_unit(f"q{i}") for i in range(n_units)],
+        units=tuple(_mk_unit(f"q{i}") for i in range(n_units)),
         gate_run_prefix="r_e1_s0_gate_action-reasoning",
     )
     return _GateRun.from_spec(spec)
@@ -119,6 +119,48 @@ def test_infra_unit_skipped_not_counted() -> None:
     assert run.verdict is not None and run.verdict.decision == "accept_provisional"
 
 
+def test_all_infra_leaves_verdict_none() -> None:
+    """全部单元被剔除:verdict 保持 None、frozen 保持 False——终态由调度编排层
+    (Task 3 的 validate_skills_concurrent)检测 verdict None 并 raise,本函数不管。"""
+    run = _mk_run(3)
+    for i in range(3):
+        run.slots[i].base_infra = True
+        run.slots[i].cand_per_q = {f"q{i}": True}
+    _advance_prefix(run, _PARAMS)
+    assert run.verdict is None and not run.frozen and run.n_excluded == 3 and run.prefix_ptr == 3
+
+
+def test_repeated_calls_are_idempotent() -> None:
+    """部分前缀消费后重复调用不改变状态;补齐剩余单元后再调用正常推进。"""
+    run = _mk_run(4)
+    for i in (0, 1):
+        run.slots[i].base = False
+        run.slots[i].cand_per_q = {f"q{i}": True}
+    _advance_prefix(run, _PARAMS)
+    snapshot = (run.n_used, run.w, run.l, run.prefix_ptr, len(run.evidence_rows))
+    _advance_prefix(run, _PARAMS)
+    _advance_prefix(run, _PARAMS)
+    assert (run.n_used, run.w, run.l, run.prefix_ptr, len(run.evidence_rows)) == snapshot
+    for i in (2, 3):
+        run.slots[i].base = False
+        run.slots[i].cand_per_q = {f"q{i}": True}
+    _advance_prefix(run, _PARAMS)
+    assert run.n_used > snapshot[0] and run.prefix_ptr > snapshot[3]
+
+
+def test_frozen_run_call_is_noop() -> None:
+    """frozen 后再调用是 no-op:不再消费已配齐单元、证据不再追加。"""
+    run = _mk_run(12)
+    for i in range(12):
+        run.slots[i].base = False
+        run.slots[i].cand_per_q = {f"q{i}": True}
+    _advance_prefix(run, _PARAMS)
+    assert run.frozen and run.n_used == 7
+    n_evidence = len(run.evidence_rows)
+    _advance_prefix(run, _PARAMS)
+    assert run.n_used == 7 and len(run.evidence_rows) == n_evidence
+
+
 def test_ties_hit_futility_early_and_freeze() -> None:
     """全打平(无翻转对)时 futility 出口尽早触发并冻结——早停语义(数值:W=L=0
     时乐观 E = E(n_remaining, 0),n 小易 <e_provisional=3)。"""