fix(agent): refund the API call when an empty response activates fallback (#77305)

Fallback activation after empty-response retries re-entered the outer loop
without refunding the provisional call, so a provider hop consumed an
iteration. Refund via _refund_api_call and carry the count back through
EmptyResponseVerdict -> FinalResponseVerdict -> s.api_call_count.

Salvaged from #88867.

Co-authored-by: RelaxJonh <92573950+RelaxJonh@users.noreply.github.com>

agent/turn_empty_response.py

@@ -40,6 +40,7 @@ class EmptyResponseVerdict:
     turn_exit_reason: Any
     active_system_prompt: Any
     preflight_compression_blocked: bool
+    api_call_count: int
 
 
 def _retry_empty(
@@ -154,6 +155,7 @@ def recover_empty_response(
             action=action, result=result, final_response=final_response,
             turn_exit_reason=_turn_exit_reason, active_system_prompt=active_system_prompt,
             preflight_compression_blocked=_preflight_compression_blocked,
+            api_call_count=api_call_count,
         )
 
     # Partial stream recovery: content streamed before the connection died becomes the
@@ -279,6 +281,10 @@ def recover_empty_response(
             # OUTER loop: `continue` re-runs preflight against the fallback's window;
             # `break` would end the turn without calling the fallback.
             _preflight_compression_blocked = False
+            # The fallback hop is a provider switch, not a model turn: refund the empty
+            # call so a mid-turn fallback doesn't eat the iteration budget (#77305).
+            from agent.turn_context_compaction import _refund_api_call
+            api_call_count = _refund_api_call(agent, api_call_count)
             return _verdict("continue")
 
     _turn_exit_reason = "empty_response_exhausted"

agent/turn_final_response.py

@@ -40,6 +40,7 @@ class FinalResponseVerdict:
     length_continue_retries: Any
     _pending_verification_response: Any
     _pending_verification_response_previewed: Any
+    api_call_count: Any
     result: Optional[Dict[str, Any]] = None
 
 
@@ -70,6 +71,7 @@ def finish_text_response(
             length_continue_retries=length_continue_retries,
             _pending_verification_response=_pending_verification_response,
             _pending_verification_response_previewed=_pending_verification_response_previewed,
+            api_call_count=api_call_count,
             result=result,
         )
 
@@ -119,6 +121,7 @@ def finish_text_response(
         _turn_exit_reason = _ev.turn_exit_reason
         active_system_prompt = _ev.active_system_prompt
         _preflight_compression_blocked = _ev.preflight_compression_blocked
+        api_call_count = _ev.api_call_count
         if _ev.action == "return":
             return _verdict("return", _ev.result)
         if _ev.action == "break":

tests/agent/test_empty_response_fallback_refund.py

@@ -0,0 +1,48 @@
+"""Empty-response fallback hop refunds its API call (#77305)."""
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+from agent import turn_empty_response as ter
+
+
+def _agent():
+    a = MagicMock()
+    a._current_streamed_assistant_text = ""
+    a._has_content_after_think_block.return_value = False
+    a._strip_think_blocks.side_effect = lambda t: t or ""
+    a._last_content_with_tools = None
+    a._thinking_prefill_retries = 2
+    a._fallback_chain = ["fallback"]
+    a._api_call_count = 5
+    return a
+
+
+def _recover(agent):
+    msg = SimpleNamespace(reasoning=None, reasoning_content=None, reasoning_details=None)
+    with patch.object(ter, "_retry_empty", return_value=(None, None, False)), \
+            patch("agent.conversation_loop._sync_failover_system_message", return_value="sys"):
+        return ter.recover_empty_response(
+            agent, msg, None, "stop", final_response="", messages=[{"role": "user", "content": "hi"}],
+            api_messages=[], conversation_history=[], active_system_prompt="sys", api_call_count=5,
+            turn_exit_reason=None, preflight_compression_blocked=False,
+        )
+
+
+def test_fallback_activation_refunds_call_and_budget():
+    agent = _agent()
+    agent._try_activate_fallback.return_value = True
+    v = _recover(agent)
+    assert v.action == "continue"
+    assert v.api_call_count == 4
+    assert agent._api_call_count == 4
+    agent.iteration_budget.refund.assert_called_once()
+
+
+def test_exhausted_without_fallback_keeps_count():
+    agent = _agent()
+    agent._try_activate_fallback.return_value = False
+    with patch.object(ter, "_terminal_empty", return_value="(empty)"):
+        v = _recover(agent)
+    assert v.action == "break"
+    assert v.api_call_count == 5
+    agent.iteration_budget.refund.assert_not_called()