diff --git a/litellm/proxy/_types.py b/litellm/proxy/_types.py
index 08b3238bc6b..b54f23cdd1e 100644
--- a/litellm/proxy/_types.py
+++ b/litellm/proxy/_types.py
@@ -4123,6 +4123,12 @@ LiteLLM_ManagementEndpoint_MetadataFields_Premium = [
     "allowed_passthrough_routes",
 ]
 
+# Metadata keys that are immutable once set: preserved when an update omits them,
+# and rejected (400) when an update tries to change them.
+LiteLLM_Reserved_Metadata_Fields = [
+    "service_account_id",
+]
+
 
 class ProviderBudgetResponseObject(LiteLLMPydanticObjectBase):
     """
diff --git a/litellm/proxy/management_endpoints/key_management_endpoints.py b/litellm/proxy/management_endpoints/key_management_endpoints.py
index 1fdf62cdb5b..2ebe1a86f96 100644
--- a/litellm/proxy/management_endpoints/key_management_endpoints.py
+++ b/litellm/proxy/management_endpoints/key_management_endpoints.py
@@ -1523,6 +1523,22 @@ def prepare_metadata_fields(
 
     casted_metadata = cast(dict, non_default_values["metadata"])
 
+    # Reserved metadata fields are immutable once set. Preserve the existing value
+    # when omitted, reject any explicit attempt to change it (including null).
+    for reserved_field in LiteLLM_Reserved_Metadata_Fields:
+        existing_value = existing_metadata.get(reserved_field)
+        if existing_value is None:
+            continue
+        if casted_metadata is None or (
+            reserved_field in casted_metadata
+            and casted_metadata[reserved_field] != existing_value
+        ):
+            raise HTTPException(
+                status_code=400,
+                detail=f"{reserved_field} is immutable once set and cannot be changed via update.",
+            )
+        casted_metadata[reserved_field] = existing_value
+
     data_json = data.model_dump(exclude_unset=True, exclude_none=True)
 
     try:
diff --git a/tests/proxy_unit_tests/test_proxy_utils.py b/tests/proxy_unit_tests/test_proxy_utils.py
index 8703331ea83..feca4251a45 100644
--- a/tests/proxy_unit_tests/test_proxy_utils.py
+++ b/tests/proxy_unit_tests/test_proxy_utils.py
@@ -686,12 +686,13 @@ async def test_proxy_config_update_from_db():
 
 @pytest.mark.asyncio
 async def test_prepare_key_update_data():
-    from litellm.proxy._types import UpdateKeyRequest
+    from litellm.proxy._types import LiteLLM_VerificationToken, UpdateKeyRequest
     from litellm.proxy.management_endpoints.key_management_endpoints import (
         prepare_key_update_data,
     )
 
-    existing_key_row = MagicMock()
+    existing_key_row = MagicMock(spec=LiteLLM_VerificationToken)
+    existing_key_row.metadata = {}
     data = UpdateKeyRequest(key="test_key", models=["gpt-4"], duration="120s")
     updated_data = await prepare_key_update_data(data, existing_key_row)
     assert "expires" in updated_data
diff --git a/tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py b/tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py
index 8e6fd78a6f6..647be49e0d1 100644
--- a/tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py
+++ b/tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py
@@ -1211,6 +1211,131 @@ async def test_update_service_account_works_with_team_id():
     await prepare_key_update_data(data=data, existing_key_row=existing_key)
 
 
+@pytest.mark.asyncio
+async def test_update_preserves_service_account_id_when_metadata_replaced():
+    """
+    Regression: /key/update wholesale-replaced metadata, silently dropping
+    service_account_id. The pre-call check then treated the key as a regular
+    key and bypassed service_account_settings.enforced_params.
+    """
+    data = UpdateKeyRequest(
+        key="sk-1",
+        metadata={"unrelated": "value"},
+        team_id="IJ",
+    )
+    existing_key = LiteLLM_VerificationToken(
+        token="hashed",
+        team_id="IJ",
+        metadata={"service_account_id": "sa-123"},
+    )
+
+    result = await prepare_key_update_data(data=data, existing_key_row=existing_key)
+
+    assert result["metadata"]["service_account_id"] == "sa-123"
+    assert result["metadata"]["unrelated"] == "value"
+
+
+@pytest.mark.asyncio
+async def test_update_rejects_service_account_id_overwrite():
+    """
+    Once assigned, a key's service_account_id is an identity marker — rebinding
+    it via update would break spend attribution. Reject rather than silently
+    ignore so scripted callers surface the bug.
+    """
+    data = UpdateKeyRequest(
+        key="sk-1",
+        metadata={"service_account_id": "sa-new"},
+        team_id="IJ",
+    )
+    existing_key = LiteLLM_VerificationToken(
+        token="hashed",
+        team_id="IJ",
+        metadata={"service_account_id": "sa-old"},
+    )
+
+    with pytest.raises(HTTPException) as exc_info:
+        await prepare_key_update_data(data=data, existing_key_row=existing_key)
+    assert exc_info.value.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_update_allows_matching_service_account_id():
+    """Resending the same value (e.g. UI round-trip) is a no-op, not a conflict."""
+    data = UpdateKeyRequest(
+        key="sk-1",
+        metadata={"service_account_id": "sa-123", "other": "value"},
+        team_id="IJ",
+    )
+    existing_key = LiteLLM_VerificationToken(
+        token="hashed",
+        team_id="IJ",
+        metadata={"service_account_id": "sa-123"},
+    )
+
+    result = await prepare_key_update_data(data=data, existing_key_row=existing_key)
+
+    assert result["metadata"]["service_account_id"] == "sa-123"
+    assert result["metadata"]["other"] == "value"
+
+
+@pytest.mark.asyncio
+async def test_update_rejects_explicit_null_service_account_id():
+    """
+    Explicit null is an attempt to clear — not an omission. Silently ignoring
+    it would let a caller think they cleared the field when they didn't, so
+    treat it the same as any other rebind attempt and return 400.
+    """
+    data = UpdateKeyRequest(
+        key="sk-1",
+        metadata={"service_account_id": None},
+        team_id="IJ",
+    )
+    existing_key = LiteLLM_VerificationToken(
+        token="hashed",
+        team_id="IJ",
+        metadata={"service_account_id": "sa-old"},
+    )
+
+    with pytest.raises(HTTPException) as exc_info:
+        await prepare_key_update_data(data=data, existing_key_row=existing_key)
+    assert exc_info.value.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_update_rejects_whole_metadata_null_on_service_account_key():
+    """
+    `metadata: null` on a key with a reserved field would have dereferenced None
+    inside the reserved-field loop and returned a 500. Must surface as a 400
+    since the effect would be to clear an immutable field.
+    """
+    data = UpdateKeyRequest(key="sk-1", metadata=None, team_id="IJ")
+    existing_key = LiteLLM_VerificationToken(
+        token="hashed",
+        team_id="IJ",
+        metadata={"service_account_id": "sa-123"},
+    )
+
+    with pytest.raises(HTTPException) as exc_info:
+        await prepare_key_update_data(data=data, existing_key_row=existing_key)
+    assert exc_info.value.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_update_without_metadata_still_preserves_existing():
+    """Omitting metadata entirely must not drop existing metadata fields."""
+    data = UpdateKeyRequest(key="sk-1", max_budget=100)
+    existing_key = LiteLLM_VerificationToken(
+        token="hashed",
+        team_id="IJ",
+        metadata={"service_account_id": "sa-123", "other": "kept"},
+    )
+
+    result = await prepare_key_update_data(data=data, existing_key_row=existing_key)
+
+    assert result["metadata"]["service_account_id"] == "sa-123"
+    assert result["metadata"]["other"] == "kept"
+
+
 @pytest.mark.asyncio
 async def test_prepare_key_update_data_duration_never_expires():
     """Test that duration="-1" sets expires to None (never expires)."""
@@ -8007,6 +8132,7 @@ async def test_update_key_non_budget_fields_allowed_for_internal_user(monkeypatc
     mock_existing_key.max_budget = 10.0
     mock_existing_key.key_alias = None
     mock_existing_key.models = []
+    mock_existing_key.metadata = {}
     mock_existing_key.model_dump.return_value = {
         "token": test_hashed_token,
         "user_id": "internal_user",
@@ -8089,9 +8215,7 @@ async def test_update_key_non_budget_rejects_cross_user_modification(monkeypatch
     )
 
     mock_prisma_client = AsyncMock()
-    test_hashed_token = (
-        "cafebabe" * 8
-    )
+    test_hashed_token = "cafebabe" * 8
 
     mock_existing_key = MagicMock()
     mock_existing_key.token = test_hashed_token