diff --git a/backend/internal/garmin/pyscript/tests/test_wrapper.py b/backend/internal/garmin/pyscript/tests/test_wrapper.py
index 6e3fe67..3d20cc3 100644
--- a/backend/internal/garmin/pyscript/tests/test_wrapper.py
+++ b/backend/internal/garmin/pyscript/tests/test_wrapper.py
@@ -269,3 +269,25 @@ def test_authenticate_prevents_later_lazy_startup_login():
         wrapper.dispatch({"id": 34, "cmd": "call", "params": {"method": "x", "args": {}}})
 
     mock_startup.assert_not_called()
+
+
+def test_call_marks_not_found_error_specifically():
+    from garminconnect import GarminConnectNotFoundError
+
+    wrapper._auth_state = "authenticated"
+    wrapper._client = MagicMock()
+    wrapper._client.get_workout_by_id.side_effect = GarminConnectNotFoundError("API Error 404")
+    resp = wrapper.dispatch({
+        "id": 11, "cmd": "call", "params": {"method": "get_workout_by_id", "args": {"workout_id": "999"}},
+    })
+    assert resp == {"id": 11, "error": "API Error 404", "not_found": True}
+
+
+def test_call_does_not_mark_other_errors_as_not_found():
+    wrapper._auth_state = "authenticated"
+    wrapper._client = MagicMock()
+    wrapper._client.get_workout_by_id.side_effect = Exception("rate limited")
+    resp = wrapper.dispatch({
+        "id": 12, "cmd": "call", "params": {"method": "get_workout_by_id", "args": {"workout_id": "999"}},
+    })
+    assert resp == {"id": 12, "error": "rate limited"}
diff --git a/backend/internal/garmin/pyscript/wrapper.py b/backend/internal/garmin/pyscript/wrapper.py
index fc0e823..074f024 100644
--- a/backend/internal/garmin/pyscript/wrapper.py
+++ b/backend/internal/garmin/pyscript/wrapper.py
@@ -9,7 +9,7 @@ import sys
 import threading
 import traceback
 
-from garminconnect import Garmin
+from garminconnect import Garmin, GarminConnectNotFoundError
 
 TOKENSTORE = os.path.expanduser(os.environ.get("GARMIN_TOKENSTORE", "~/.garmin"))
 
@@ -208,7 +208,15 @@ def dispatch(req):
     except Exception as exc:
         _debug(f"{req.get('cmd')} raised {type(exc).__name__}: {exc}")
         _debug(traceback.format_exc())
-        return {"id": req.get("id"), "error": str(exc)}
+        resp = {"id": req.get("id"), "error": str(exc)}
+        # A 404 (e.g. get_workout_by_id for a workout deleted on Garmin's
+        # side after being linked to an activity) is definitive, not a
+        # transient failure worth retrying forever -- marked specifically so
+        # internal/garmin/client.go can tell the two apart (see
+        # docs/superpowers/specs/2026-07-27-workout-not-found-design.md).
+        if isinstance(exc, GarminConnectNotFoundError):
+            resp["not_found"] = True
+        return resp
 
 
 def main():