diff --git a/backend/services/garmin.py b/backend/services/garmin.py
index 8e099c4..0ebbced 100644
--- a/backend/services/garmin.py
+++ b/backend/services/garmin.py
@@ -111,11 +111,20 @@ def _note_rate_limit(user_id):
 
 
 def _is_rate_limited(e):
-    """429 from Garmin, however it happens to be dressed."""
+    """429 from Garmin, however it happens to be dressed.
+
+    Usually it arrives as a JSONDecodeError, because garth calls .json() on a
+    429 whose body is the plain text "Rate limited" — the status code is gone
+    by the time the exception reaches us, and the message is the useless
+    "Expecting value: line 1 column 1". What survives is the body itself, on
+    the exception's `doc` attribute, so that is where to look.
+    """
     response = getattr(e, "response", None)
     if response is not None and getattr(response, "status_code", None) == 429:
         return True
-    return "rate limit" in str(e).lower()
+
+    haystacks = [str(e), str(getattr(e, "doc", "") or "")]
+    return any("rate limit" in h.lower() for h in haystacks)
 
 
 class MFARequired(RuntimeError):
diff --git a/backend/tests/test_garmin_sync.py b/backend/tests/test_garmin_sync.py
index 8f9c0c6..89d4190 100644
--- a/backend/tests/test_garmin_sync.py
+++ b/backend/tests/test_garmin_sync.py
@@ -619,8 +619,16 @@ class TestRateLimiting:
     def teardown_method(self):
         garmin_svc._rate_limited_until.clear()
 
-    def test_plain_text_body_is_recognised(self):
-        assert garmin_svc._is_rate_limited(ValueError("Rate limited"))
+    def test_the_real_shape_is_recognised(self):
+        """garth calls .json() on the 429, so what actually reaches us is a
+        JSONDecodeError whose message says nothing and whose `doc` holds the
+        body. An earlier version of this test asserted on a hand-made
+        ValueError("Rate limited") — a shape that never occurs — and passed
+        while the detector missed every real one."""
+        import json
+        err = json.JSONDecodeError("Expecting value", "Rate limited", 0)
+        assert "rate limit" not in str(err).lower(), "the message alone is not enough"
+        assert garmin_svc._is_rate_limited(err)
 
     def test_status_code_is_recognised(self):
         class Resp: