diff --git a/src/adclaw/meta/client.py b/src/adclaw/meta/client.py
index 4a3e262..3339cfd 100644
--- a/src/adclaw/meta/client.py
+++ b/src/adclaw/meta/client.py
@@ -381,12 +381,24 @@ class MetaClient:
         objective: str,
         status: str = CREATE_STATUS,
         special_ad_categories: list[str] | None = None,
+        adset_budget_sharing: bool = False,
     ) -> str:
         """Create a paused campaign and return its id.
 
         Raises `ValueError` (making no network call) if `status` is
         anything other than "PAUSED" — campaigns are always created paused;
         use `update_object_status` to launch.
+
+        `is_adset_budget_sharing_enabled` is **required** by Graph whenever
+        the campaign carries no budget of its own, which is always the case
+        here: MaskanX puts the budget on the ad set. Omitting it is
+        rejected with code 100 / subcode 4834011.
+
+        It defaults to False on purpose. True lets ad sets lend each other
+        up to 20% of their budget, which makes a single ad set's daily
+        spend exceed the number we set for it — and the guardrails treat
+        that number as a ceiling. Predictable spend beats Meta's
+        optimisation here.
         """
         if status != CREATE_STATUS:
             raise ValueError(
@@ -400,6 +412,11 @@ class MetaClient:
                 "objective": objective,
                 "status": CREATE_STATUS,
                 "special_ad_categories": json.dumps(special_ad_categories or []),
+                # Sent as a lowercase literal: form-encoding a Python bool
+                # yields "True", which Graph does not accept.
+                "is_adset_budget_sharing_enabled": (
+                    "true" if adset_budget_sharing else "false"
+                ),
             },
         )
         return result["id"]
diff --git a/tests/test_meta_writes.py b/tests/test_meta_writes.py
index 2001d74..0aee567 100644
--- a/tests/test_meta_writes.py
+++ b/tests/test_meta_writes.py
@@ -559,3 +559,46 @@ def test_build_creative_payload_falls_back_to_campaign_name_for_headline():
     payload = build_creative_payload(spec, page_id="page_1", image_hash="hash123")
 
     assert payload["headline"] == "Test Campaign"
+
+
+# ---------------------------------------------------------------------------
+# create_campaign: the field Graph requires when the campaign has no budget
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_create_campaign_declares_adset_budget_sharing():
+    """Graph rejects a budget-less campaign that omits this field.
+
+    MaskanX always puts the budget on the ad set, so the campaign never
+    carries one and the field is never optional. Omitting it is what
+    produced code 100 / subcode 4834011 against the live account, with the
+    useless message "Invalid parameter".
+
+    It must be the lowercase literal: form-encoding a Python bool sends
+    "True", which Graph does not accept.
+    """
+    transport = FakeTransport([{"id": "c1"}])
+    client = MetaClient(access_token="tok", transport=transport)
+
+    await client.create_campaign("act_1", name="Q3", objective="OUTCOME_LEADS")
+
+    sent = transport.calls[0]["params"]["is_adset_budget_sharing_enabled"]
+    assert sent == "false"
+
+
+@pytest.mark.asyncio
+async def test_adset_budget_sharing_is_off_by_default_but_overridable():
+    """True lets ad sets lend each other up to 20% of their budget, so an
+    ad set can outspend the number we set for it. The guardrails treat that
+    number as a ceiling, so the default has to be off."""
+    transport = FakeTransport([{"id": "c1"}, {"id": "c2"}])
+    client = MetaClient(access_token="tok", transport=transport)
+
+    await client.create_campaign("act_1", name="A", objective="OUTCOME_LEADS")
+    await client.create_campaign(
+        "act_1", name="B", objective="OUTCOME_LEADS", adset_budget_sharing=True,
+    )
+
+    assert transport.calls[0]["params"]["is_adset_budget_sharing_enabled"] == "false"
+    assert transport.calls[1]["params"]["is_adset_budget_sharing_enabled"] == "true"