diff --git a/api/cohorts/sync_views.py b/api/cohorts/sync_views.py
index 3c4aa0fadb45..1f8e1c51f588 100644
--- a/api/cohorts/sync_views.py
+++ b/api/cohorts/sync_views.py
@@ -22,7 +22,13 @@
 )
 
 _LIST_RESPONSE = inline_serializer(
-    "AmplitudeListResponse", {"listId": serializers.UUIDField()}
+    "AmplitudeListResponse",
+    {
+        "list_id": serializers.UUIDField(),
+        "response": inline_serializer(
+            "AmplitudeListResponseEnvelope", {"list_id": serializers.UUIDField()}
+        ),
+    },
 )
 
 _MIXPANEL_RESPONSE = inline_serializer(
@@ -73,10 +79,15 @@ def create(self, request: Request) -> Response:
             name=serializer.validated_data["name"],
             source_type=CohortSourceType.AMPLITUDE,
         )
-        # Amplitude's production sync worker reads the list ID from its
-        # documented default key, camelCase "listId", ignoring the response
-        # path configured in the Integration Portal.
-        return Response({"listId": str(cohort.uuid)})
+        # Amplitude reads the list ID from this body at the path configured
+        # in the Integration Portal, which their portal requires to start
+        # with "response." — but the two sides of Amplitude disagree on what
+        # that path is applied to. Their Testing tab wraps this body in a
+        # {"response": ...} envelope first, so it finds the flat copy; their
+        # production sync worker applies the same path to the raw body, so
+        # it needs the nested copy. Verified against both, 2026-08-28.
+        list_id = str(cohort.uuid)
+        return Response({"list_id": list_id, "response": {"list_id": list_id}})
 
     @action(detail=True, methods=["POST"])
     def add(self, request: Request, pk: str) -> Response:
diff --git a/api/tests/unit/cohorts/test_sync_views.py b/api/tests/unit/cohorts/test_sync_views.py
index cbc5a87bac97..a2556bea3458 100644
--- a/api/tests/unit/cohorts/test_sync_views.py
+++ b/api/tests/unit/cohorts/test_sync_views.py
@@ -50,7 +50,9 @@ def test_amplitude_create_list__valid_key__creates_amplitude_cohort(
 
     # Then
     assert response.status_code == status.HTTP_200_OK
-    cohort = Cohort.objects.get(uuid=response.json()["listId"])
+    cohort = Cohort.objects.get(uuid=response.json()["list_id"])
+    # Amplitude's Testing tab reads the flat copy, production the nested one.
+    assert response.json()["response"]["list_id"] == response.json()["list_id"]
     assert cohort.environment == key.environment
     assert cohort.source_type == CohortSourceType.AMPLITUDE
     assert cohort.segment.name == "[Amplitude] Beta users: 1234"
@@ -74,7 +76,7 @@ def test_amplitude_create_list__postgres_environment__creates_cohort(
 
     # Then
     assert response.status_code == status.HTTP_200_OK
-    cohort = Cohort.objects.get(uuid=response.json()["listId"])
+    cohort = Cohort.objects.get(uuid=response.json()["list_id"])
     assert cohort.environment == environment
 
 
@@ -350,7 +352,7 @@ def test_amplitude_create_list__valid_key__audits_and_queues_environment_update(
     # Then - the audit record carries no user, names the source, and is the
     # hook that rebuilds the environment document.
     assert response.status_code == status.HTTP_200_OK
-    cohort = Cohort.objects.get(uuid=response.json()["listId"])
+    cohort = Cohort.objects.get(uuid=response.json()["list_id"])
     audit_log = AuditLog.objects.get(related_object_id=cohort.segment_id)
     assert audit_log.author is None
     assert audit_log.master_api_key is None
@@ -377,7 +379,7 @@ def test_amplitude_create_list__valid_key__history_records_no_user(
     # Then - a machine caller leaves no user on historical records; stamping
     # one would fail, since the sync key is not a Flagsmith user.
     assert response.status_code == status.HTTP_200_OK
-    cohort = Cohort.objects.get(uuid=response.json()["listId"])
+    cohort = Cohort.objects.get(uuid=response.json()["list_id"])
     history_record = cohort.segment.history.get()
     assert history_record.history_user is None
     assert history_record.master_api_key is None
diff --git a/docs/docs/deployment-self-hosting/observability/_events-catalogue.md b/docs/docs/deployment-self-hosting/observability/_events-catalogue.md
index 4e77c78106c0..9c89874b5296 100644
--- a/docs/docs/deployment-self-hosting/observability/_events-catalogue.md
+++ b/docs/docs/deployment-self-hosting/observability/_events-catalogue.md
@@ -166,7 +166,7 @@ Attributes:
 ### `cohorts.sync_webhook.rejected`
 
 Logged at `warning` from:
- - `api/cohorts/sync_views.py:217`
+ - `api/cohorts/sync_views.py:228`
 
 Attributes:
  - `action`
diff --git a/openapi.yaml b/openapi.yaml
index 78578f6e6a84..7481be959734 100644
--- a/openapi.yaml
+++ b/openapi.yaml
@@ -18943,11 +18943,22 @@ components:
     AmplitudeListResponse:
       type: object
       properties:
-        listId:
+        list_id:
           type: string
           format: uuid
+        response:
+          $ref: '#/components/schemas/AmplitudeListResponseEnvelope'
       required:
-        - listId
+        - list_id
+        - response
+    AmplitudeListResponseEnvelope:
+      type: object
+      properties:
+        list_id:
+          type: string
+          format: uuid
+      required:
+        - list_id
     AuditLogList:
       type: object
       properties: