diff --git a/agent/agent_runtime_helpers.py b/agent/agent_runtime_helpers.py
index bac21f14061..b5c70392946 100644
--- a/agent/agent_runtime_helpers.py
+++ b/agent/agent_runtime_helpers.py
@@ -819,6 +819,14 @@ def restore_primary_runtime(agent) -> bool:
     ``gateway/run.py``), so this restoration IS needed there too.
     """
     if not agent._fallback_activated:
+        # Reset the chain index even when no fallback was activated this
+        # turn.  Without this, a turn where _try_activate_fallback() was
+        # called but returned False (chain exhausted or provider not
+        # configured) leaves _fallback_index >= len(_fallback_chain) while
+        # _fallback_activated stays False.  The next turn skips this block
+        # entirely, stranding the index and silently blocking all future
+        # fallback attempts for the session.  Fixes #20465.
+        agent._fallback_index = 0
         return False
 
     if getattr(agent, "_rate_limited_until", 0) > time.monotonic():