Tighten silent failure shrink detection

CHANGELOG.md

@@ -12,6 +12,9 @@
 
 ### Fixed
 
+- Silent-failure detection no longer treats old assistant messages in a shrunk
+  result history as proof that the current turn produced a new assistant reply.
+
 - **PR #2270** by @Michaelyklam (closes #2226) — Firefox Android PWA installs from `/session/<id>` pages now resolve the Hermes manifest and icons instead of falling back to a generated letter icon. The dynamic `<base href>` script now runs before manifest/favicon links, `/session/manifest.json` and `/session/manifest.webmanifest` return the real manifest JSON, and session-prefixed manifest routes are now marked as public auth-skip routes. Adds 211 lines of regression coverage for the manifest responses and the session-prefixed 512px icon path.
 
 - **PR #2268** by @eleboucher — `docker_init.bash` no longer fails under Kubernetes `runAsUser` configurations where the running UID has no `/etc/passwd` entry. Pre-fix, the bare `whoami` invocation aborted the script under `set -e` because `whoami` exited with a non-zero status on missing-passwd UIDs. Now falls back to a synthetic `uid-<numeric-uid>` name when `whoami` fails (`whoami 2>/dev/null || echo "uid-$(id -u)"`). Two-line change.

api/streaming.py

@@ -140,17 +140,16 @@ def _has_new_assistant_reply(all_messages: list, prev_count: int) -> bool:
     messages at index >= prev_count are inspected so that historical assistant
     replies don't mask a silent failure on the current turn.
 
-    If ``len(all_messages) < prev_count`` (an edge-case shrink) we fall back
-    to scanning the full list — this keeps behaviour consistent with the
-    original code when offsets don't cleanly apply.  When ``len == prev_count``,
-    there are no new messages and we return False.
+    If ``len(all_messages) < prev_count`` (an edge-case shrink), there is no
+    reliable new-message slice to inspect. Treat that as "no new assistant
+    reply" so stale historical assistant replies cannot mask a silent failure.
+    When ``len == prev_count``, there are no new messages and we return False.
     """
     if len(all_messages) > prev_count:
         # Normal case: new messages appended beyond the pre-turn history.
         candidates = all_messages[prev_count:]
     elif len(all_messages) < prev_count:
-        # Edge-case shrink — scan everything as fallback.
-        candidates = all_messages
+        return False
     else:
         # Same length. In production this means no new messages were appended.
         # However, some test fixtures replace the entire message list rather

tests/test_silent_failure_detection.py

@@ -95,12 +95,12 @@ class TestHasNewAssistantReply:
         assert _has_new_assistant_reply(all_msgs, len(prev)) is False
 
     # Scenario 8 ──────────────────────────────────────────────────────────
-    def test_result_shorter_than_prev_len_fallback(self):
-        """Edge-case: result messages < prev_count → fallback to scanning all.
+    def test_result_shorter_than_prev_len_returns_false(self):
+        """Edge-case: result messages < prev_count cannot prove a new reply.
 
-        The helper falls back to scanning all messages when the slice would
-        be empty.  In this scenario, if an assistant message exists in the
-        (shorter) result it should still be detected.
+        Shrunken result history has no reliable new-message slice. Scanning
+        the shorter list can mistake an older assistant reply for a current
+        turn reply, which would hide the silent-failure banner.
         """
         prev_count = 5
         # Only 3 messages in result — shorter than prev_count
@@ -109,10 +109,8 @@ class TestHasNewAssistantReply:
             _msg("assistant", "b"),
             _msg("user", "c"),
         ]
-        # Fallback scans all → assistant with content "b" is found
-        assert _has_new_assistant_reply(all_msgs, prev_count) is True
+        assert _has_new_assistant_reply(all_msgs, prev_count) is False
 
-        # But if no assistant content in the shorter result → False
         all_msgs_no_asst = [
             _msg("user", "a"),
             _msg("user", "b"),