diff --git a/backend/apps/service/client.py b/backend/apps/service/client.py
index 951a79b9..fc537fde 100644
--- a/backend/apps/service/client.py
+++ b/backend/apps/service/client.py
@@ -400,6 +400,8 @@ def submit_diagnostic(diagnostic: dict) -> None:
     # is deliberately capturing, so only the sink-less case is blocked.
     if os.environ.get("PYTEST_CURRENT_TEST") and test_sink is None:
         return
+    # The build that wrote an envelope is the first thing a field read needs, and it used to be inferred from which fields were present.
+    diagnostic.setdefault("app_version", APP_VERSION)
     try:
         from backend.apps.service.ring_buffer import snapshot
         diagnostic["recent_log"] = snapshot()
diff --git a/backend/tests/test_service.py b/backend/tests/test_service.py
index bc9085fb..b7dc4f91 100644
--- a/backend/tests/test_service.py
+++ b/backend/tests/test_service.py
@@ -214,6 +214,16 @@ def test_legacy_submit_diagnostic(sink):
     assert len(sink) == 1
 
 
+def test_every_diagnostic_carries_the_app_version(sink):
+    # Alex's -11 crashes (2026-09-04) had to be dated to a build by which envelope FIELDS they carried; the version rides every envelope now.
+    from backend.apps.service.client import submit_diagnostic
+    from backend.apps.service.version import APP_VERSION
+    submit_diagnostic({"kind": "model_error"})
+    _, body = sink[0]
+    assert body["d"]["diagnostic"]["app_version"] == APP_VERSION
+    assert APP_VERSION and APP_VERSION != "unknown"
+
+
 # --- spool -------------------------------------------------------------------
 
 def test_buffer_enqueue_and_drain(tmp_path):