fix(router): keep retry breadcrumbs small enough to stay in budget

RETRY_BREADCRUMB_LIMIT bounds how many breadcrumbs a request keeps, but each one
stored str(e) whole, and litellm exceptions embed the upstream response body: a
200 KB error message produced an 800 KB retry record, and 300 failing requests
grew the proxy RSS by 86.8 MB against the 48 MB budget of
test_failing_requests_do_not_grow_rss_or_stored_request.

Keep the diagnostic head of the message and append how much was dropped.

litellm/router.py

@@ -694,6 +694,12 @@ set_live_deployment_replay(_replay_live_router_model_cost)
 
 
 RETRY_BREADCRUMB_LIMIT: Final = 4
+# Litellm exceptions embed the upstream response body, so a single retry record
+# could hold hundreds of kilobytes: the release-gate test
+# test_failing_requests_do_not_grow_rss_or_stored_request saw the proxy RSS grow
+# 86.8 MB over 300 failing requests against a 48 MB budget. Only the diagnostic
+# head of the message is kept.
+RETRY_BREADCRUMB_EXCEPTION_LIMIT: Final = 500
 
 
 class FallbackAwareStreamWrapper(CustomStreamWrapper):
@@ -8296,11 +8302,17 @@ class Router:
         model_info: Final = request_metadata.get("model_info")
         deployment_id: Final = model_info.get("id") if isinstance(model_info, Mapping) else None
         attempted_retries: Final = request_metadata.get("attempted_retries")
+        exception_string: Final = str(e)
         attempt_record: Final[RetryAttemptRecord] = {
             "model_group": model_group if isinstance(model_group, str) else None,
             "deployment_id": deployment_id if isinstance(deployment_id, str) else None,
             "exception_type": type(e).__name__,
-            "exception_string": str(e),
+            "exception_string": (
+                exception_string[:RETRY_BREADCRUMB_EXCEPTION_LIMIT]
+                + f"... [truncated, {len(exception_string)} chars]"
+                if len(exception_string) > RETRY_BREADCRUMB_EXCEPTION_LIMIT
+                else exception_string
+            ),
             "attempted_retries": attempted_retries if type(attempted_retries) is int else None,
         }
         earlier_breadcrumbs: Final = request_metadata.get("previous_models")

tests/test_litellm/router/test_retry_breadcrumbs.py

@@ -0,0 +1,45 @@
+"""Retry breadcrumbs must stay small: they are stored on the request record."""
+
+from litellm.router import RETRY_BREADCRUMB_EXCEPTION_LIMIT, Router
+
+
+def _record_for(error: Exception) -> dict:
+    kwargs = {"model": "gpt-4o", "metadata": {}}
+    Router.log_retry(object(), kwargs, error)
+    return kwargs["metadata"]["previous_models"][-1]
+
+
+def test_large_exception_message_is_truncated_in_the_breadcrumb():
+    """Litellm exceptions embed the upstream body, so str(e) can be hundreds of KB.
+
+    The release-gate test test_failing_requests_do_not_grow_rss_or_stored_request
+    saw the proxy RSS grow 86.8 MB over 300 failing requests against a 48 MB budget.
+    """
+    error = Exception("Error code: 500 - " + "x" * 200_000)
+
+    record = _record_for(error)
+
+    assert len(record["exception_string"]) < RETRY_BREADCRUMB_EXCEPTION_LIMIT + 100
+    assert record["exception_string"].startswith("Error code: 500 - ")
+    assert "truncated" in record["exception_string"]
+    assert record["exception_type"] == "Exception"
+
+
+def test_short_exception_message_is_kept_whole():
+    record = _record_for(ValueError("bad request"))
+
+    assert record["exception_string"] == "bad request"
+    assert record["exception_type"] == "ValueError"
+
+
+def test_breadcrumb_metadata_stays_bounded_across_requests():
+    error = Exception("Error code: 500 - " + "x" * 200_000)
+
+    total = 0
+    for _ in range(50):
+        kwargs = {"model": "gpt-4o", "metadata": {}}
+        for _ in range(6):
+            Router.log_retry(object(), kwargs, error)
+        total += len(repr(kwargs["metadata"]))
+
+    assert total < 500_000