diff --git a/agent/turn_usage.py b/agent/turn_usage.py
index 131e2cd348..517b216ffe 100644
--- a/agent/turn_usage.py
+++ b/agent/turn_usage.py
@@ -143,6 +143,9 @@ def record_response_usage(
 
     # Stash canonical usage for on_turn_complete(); keep the latest call's.
     agent._last_turn_usage = dict(usage_dict)
+    # The parent's CURRENT prompt size for headroom math (delegate summary budgets): the
+    # aggregator's own prompt, never the MoA-folded total (advisor prompts are not in this context).
+    agent._last_prompt_size_tokens = int(aggregator_usage.prompt_tokens or 0)
 
     # Persist only provider-confirmed context lengths, not probe tiers.
     if getattr(compressor, "_context_probed", False):
diff --git a/tests/tools/test_delegate_summary_budget.py b/tests/tools/test_delegate_summary_budget.py
index f5b81fed85..6a3de40c17 100644
--- a/tests/tools/test_delegate_summary_budget.py
+++ b/tests/tools/test_delegate_summary_budget.py
@@ -88,3 +88,23 @@ def test_budget_uses_current_prompt_size_not_the_session_sum():
     fresh = _FakeParent(context_length=200_000, used_tokens=30_000, max_tokens=8_000)
     assert _parent_summary_char_budget(long_lived, 1) == _parent_summary_char_budget(fresh, 1)
     assert _parent_summary_char_budget(long_lived, 1) > _MIN_SUMMARY_CHARS
+
+
+def test_unknown_parent_usage_means_static_ceiling_not_zero_context():
+    """Independent-review witness: a parent with no usage yet was treated as 0 tokens used, so a 190K/200K
+    prompt got a 384K-char summary budget instead of ~4K."""
+    from types import SimpleNamespace
+    from tools.delegate_tool_results import _parent_summary_char_budget
+    parent = SimpleNamespace(context_compressor=SimpleNamespace(context_length=200_000, max_tokens=0),
+                             _last_turn_usage=None)
+    assert _parent_summary_char_budget(parent, 1) is None
+
+
+def test_moa_fold_does_not_inflate_the_parents_prompt_size():
+    """MoA folds advisor prompts into reported usage; the parent's context holds only the aggregator's."""
+    from types import SimpleNamespace
+    from tools.delegate_tool_results import _parent_summary_char_budget
+    cc = SimpleNamespace(context_length=200_000, max_tokens=0)
+    folded = SimpleNamespace(context_compressor=cc, _last_turn_usage={"prompt_tokens": 190_000}, _last_prompt_size_tokens=50_000)
+    unfolded = SimpleNamespace(context_compressor=cc, _last_turn_usage={"prompt_tokens": 50_000})
+    assert _parent_summary_char_budget(folded, 1) == _parent_summary_char_budget(unfolded, 1)
diff --git a/tools/delegate_tool_results.py b/tools/delegate_tool_results.py
index f45e7c13ee..a921807c9d 100644
--- a/tools/delegate_tool_results.py
+++ b/tools/delegate_tool_results.py
@@ -218,6 +218,20 @@ def _trim_summary_with_footer(summary: str, cap: int, task_index: int) -> tuple[
     footer_lines.append("─" * 37)
     return head + "\n\n[... middle omitted — see footer ...]\n\n" + tail + "\n".join(footer_lines), spill_path
 
+def _parent_prompt_size_tokens(parent_agent) -> Optional[int]:
+    """The parent's current prompt size: the aggregator's own last ``prompt_tokens`` (pre-MoA-fold), else
+    the last provider usage. ``None`` when no request has completed yet: the caller then applies the
+    static ceiling only. Treating "no usage yet" as zero handed a 190K/200K parent a 384K-char budget."""
+    size = getattr(parent_agent, "_last_prompt_size_tokens", None)
+    if isinstance(size, (int, float)) and size > 0:
+        return int(size)
+    last_usage = getattr(parent_agent, "_last_turn_usage", None) or {}
+    used = last_usage.get("prompt_tokens") if isinstance(last_usage, dict) else None
+    if isinstance(used, (int, float)) and used > 0:
+        return int(used)
+    return None
+
+
 def _parent_summary_char_budget(parent_agent, n_summaries: int) -> Optional[int]:
     """Per-summary char budget from the parent's *remaining* context headroom (context length − the parent's
     current prompt size − the compressor's output reserve), a fraction of it split across the batch at ~4
@@ -233,10 +247,9 @@ def _parent_summary_char_budget(parent_agent, n_summaries: int) -> Optional[int]
         context_length = getattr(compressor, "context_length", None)
         if not isinstance(context_length, int) or context_length <= 0:
             return None
-        last_usage = getattr(parent_agent, "_last_turn_usage", None) or {}
-        used_tokens = last_usage.get("prompt_tokens") if isinstance(last_usage, dict) else None
-        if not isinstance(used_tokens, (int, float)) or used_tokens < 0:
-            used_tokens = 0
+        used_tokens = _parent_prompt_size_tokens(parent_agent)
+        if used_tokens is None:
+            return None  # no usage yet and nothing to estimate from: static ceiling only, never "zero context"
         headroom_tokens = context_length - int(used_tokens) - int(getattr(compressor, "max_tokens", 0) or 0)
         if headroom_tokens <= 0:
             return _MIN_SUMMARY_CHARS  # parent already over budget: floor only