diff --git a/airflow-core/src/airflow/models/serialized_dag.py b/airflow-core/src/airflow/models/serialized_dag.py
index fbff450a9939b..b7c73ee4f5ffc 100644
--- a/airflow-core/src/airflow/models/serialized_dag.py
+++ b/airflow-core/src/airflow/models/serialized_dag.py
@@ -19,6 +19,7 @@
 
 from __future__ import annotations
 
+import copy
 import logging
 import zlib
 from collections.abc import Callable, Iterable, Iterator, Sequence
@@ -645,6 +646,12 @@ def write_dag(
         name_updated = False
         reused_deadline_data: dict[str, dict] | None = None
         if dag.data.get("dag", {}).get("deadline"):
+            # The deadline handling below rewrites data["dag"]["deadline"] from a list of
+            # encoded dicts into a list of UUID references. Work on a copy so we never mutate
+            # the caller's LazyDeserializedDAG in place.
+            from airflow.serialization.serialized_objects import LazyDeserializedDAG
+
+            dag = LazyDeserializedDAG(data=copy.deepcopy(dag.data), last_loaded=dag.last_loaded)
             # Try to reuse existing deadline UUIDs if the deadline definitions haven't changed.
             # This preserves the hash and avoids unnecessary SerializedDagModel recreations.
             existing_serialized_dag = session.scalar(
diff --git a/airflow-core/src/airflow/serialization/schema.json b/airflow-core/src/airflow/serialization/schema.json
index 872c3a1331ee3..dc7d871d79794 100644
--- a/airflow-core/src/airflow/serialization/schema.json
+++ b/airflow-core/src/airflow/serialization/schema.json
@@ -201,6 +201,11 @@
                     "type": "array",
                     "items": { "$ref": "#/definitions/dict" }
                 },
+                {
+                    "$comment": "Once persisted, a Dag's deadline alerts live as rows in the deadline_alert table and the serialized Dag keeps only a list of UUID strings referencing them (see SerializedDagModel._generate_deadline_uuids). This branch lets the stored form validate at any lifecycle stage, not only before the dict->UUID rewrite.",
+                    "type": "array",
+                    "items": { "type": "string" }
+                },
                 { "type": "null" }
             ]
         },
diff --git a/airflow-core/tests/unit/models/test_serialized_dag.py b/airflow-core/tests/unit/models/test_serialized_dag.py
index fcb49805b4c18..86f548a58b6ad 100644
--- a/airflow-core/tests/unit/models/test_serialized_dag.py
+++ b/airflow-core/tests/unit/models/test_serialized_dag.py
@@ -19,6 +19,7 @@
 
 from __future__ import annotations
 
+import copy
 import logging
 from datetime import timedelta
 from unittest import mock
@@ -1155,3 +1156,68 @@ def test_deadline_reuse_skips_write_when_hash_matches(self, testing_dag_bundle,
         alert = session.scalar(select(DAM).where(DAM.serialized_dag_id == orig_serdag.id))
         assert alert is not None
         assert alert.id == orig_alert.id
+
+    def test_write_dag_with_deadline_passes_schema_validation(self, testing_dag_bundle, session):
+        """The persisted serialized Dag for a deadline-bearing Dag must satisfy the JSON schema.
+
+        write_dag stores ``data["dag"]["deadline"]`` as a list of UUID strings referencing
+        deadline_alert rows, so the schema has to accept that persisted form and not only the
+        list-of-dicts form produced before the dict->UUID rewrite.
+        """
+        dag_id = "test_deadline_schema_valid"
+        dag = DAG(
+            dag_id=dag_id,
+            deadline=DeadlineAlert(
+                reference=DeadlineReference.DAGRUN_QUEUED_AT,
+                interval=timedelta(minutes=5),
+                callback=AsyncCallback(empty_callback_for_deadline),
+            ),
+        )
+        EmptyOperator(task_id="task1", dag=dag)
+        sync_dag_to_db(dag, session=session)
+        session.commit()
+
+        result = session.scalar(select(SDM).where(SDM.dag_id == dag_id))
+        persisted_deadline = result.data["dag"]["deadline"]
+        assert isinstance(persisted_deadline, list)
+        assert persisted_deadline
+        assert all(isinstance(ref, str) for ref in persisted_deadline)
+
+        # Must not raise: the stored UUID-reference form has to satisfy the serialized Dag schema.
+        DagSerialization.validate_schema(result.data)
+
+    def test_write_dag_does_not_mutate_caller_deadline_data(self, testing_dag_bundle, session):
+        """write_dag must not rewrite the caller's LazyDeserializedDAG deadline in place.
+
+        The dict->UUID replacement in ``_generate_deadline_uuids`` has to happen on a copy so a
+        LazyDeserializedDAG the caller still references keeps its original list-of-dicts deadline.
+        """
+        dag_id = "test_deadline_no_mutation"
+        dag = DAG(
+            dag_id=dag_id,
+            deadline=DeadlineAlert(
+                reference=DeadlineReference.DAGRUN_QUEUED_AT,
+                interval=timedelta(minutes=5),
+                callback=AsyncCallback(empty_callback_for_deadline),
+            ),
+        )
+        EmptyOperator(task_id="task1", dag=dag)
+        sync_dag_to_db(dag, session=session)
+        session.commit()
+
+        # Change the interval so write_dag regenerates UUIDs (the dict->UUID rewrite path)
+        # rather than reusing the existing ones.
+        dag.deadline = DeadlineAlert(
+            reference=DeadlineReference.DAGRUN_QUEUED_AT,
+            interval=timedelta(minutes=10),
+            callback=AsyncCallback(empty_callback_for_deadline),
+        )
+        lazy_dag = LazyDeserializedDAG.from_dag(dag)
+        original_deadline = copy.deepcopy(lazy_dag.data["dag"]["deadline"])
+        assert original_deadline
+        assert all(isinstance(item, dict) for item in original_deadline)
+
+        SDM.write_dag(lazy_dag, bundle_name="testing", session=session)
+        session.commit()
+
+        assert lazy_dag.data["dag"]["deadline"] == original_deadline