fix(runner): silent retry on empty response before finalization

nanobot/agent/runner.py

@@ -30,6 +30,7 @@ from nanobot.utils.runtime import (
 )
 
 _DEFAULT_ERROR_MESSAGE = "Sorry, I encountered an error calling the AI model."
+_MAX_EMPTY_RETRIES = 2
 _SNIP_SAFETY_BUFFER = 1024
 @dataclass(slots=True)
 class AgentRunSpec:
@@ -86,6 +87,7 @@ class AgentRunner:
         stop_reason = "completed"
         tool_events: list[dict[str, str]] = []
         external_lookup_counts: dict[str, int] = {}
+        empty_content_retries = 0
 
         for iteration in range(spec.max_iterations):
             try:
@@ -178,15 +180,30 @@ class AgentRunner:
                         "pending_tool_calls": [],
                     },
                 )
+                empty_content_retries = 0
                 await hook.after_iteration(context)
                 continue
 
             clean = hook.finalize_content(context, response.content)
             if response.finish_reason != "error" and is_blank_text(clean):
+                empty_content_retries += 1
+                if empty_content_retries < _MAX_EMPTY_RETRIES:
+                    logger.warning(
+                        "Empty response on turn {} for {} ({}/{}); retrying",
+                        iteration,
+                        spec.session_key or "default",
+                        empty_content_retries,
+                        _MAX_EMPTY_RETRIES,
+                    )
+                    if hook.wants_streaming():
+                        await hook.on_stream_end(context, resuming=False)
+                    await hook.after_iteration(context)
+                    continue
                 logger.warning(
-                    "Empty final response on turn {} for {}; retrying with explicit finalization prompt",
+                    "Empty response on turn {} for {} after {} retries; attempting finalization",
                     iteration,
                     spec.session_key or "default",
+                    empty_content_retries,
                 )
                 if hook.wants_streaming():
                     await hook.on_stream_end(context, resuming=False)

nanobot/utils/runtime.py

@@ -16,8 +16,7 @@ EMPTY_FINAL_RESPONSE_MESSAGE = (
 )
 
 FINALIZATION_RETRY_PROMPT = (
-    "You have already finished the tool work. Do not call any more tools. "
+    "Please provide your response to the user based on the conversation above."
-    "Using only the conversation and tool results above, provide the final answer for the user now."
 )
 
 

tests/agent/test_runner.py

@@ -458,6 +458,7 @@ async def test_runner_uses_raw_messages_when_context_governance_fails():
 
 @pytest.mark.asyncio
 async def test_runner_retries_empty_final_response_with_summary_prompt():
+    """Empty responses get 2 silent retries before finalization kicks in."""
     from nanobot.agent.runner import AgentRunSpec, AgentRunner
 
     provider = MagicMock()
@@ -465,11 +466,11 @@ async def test_runner_retries_empty_final_response_with_summary_prompt():
 
     async def chat_with_retry(*, messages, tools=None, **kwargs):
         calls.append({"messages": messages, "tools": tools})
-        if len(calls) == 1:
+        if len(calls) <= 2:
             return LLMResponse(
                 content=None,
                 tool_calls=[],
-                usage={"prompt_tokens": 10, "completion_tokens": 1},
+                usage={"prompt_tokens": 5, "completion_tokens": 1},
             )
         return LLMResponse(
             content="final answer",
@@ -486,20 +487,23 @@ async def test_runner_retries_empty_final_response_with_summary_prompt():
         initial_messages=[{"role": "user", "content": "do task"}],
         tools=tools,
         model="test-model",
-        max_iterations=1,
+        max_iterations=3,
         max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
     ))
 
     assert result.final_content == "final answer"
-    assert len(calls) == 2
+    # 2 silent retries (iterations 0,1) + finalization on iteration 1
-    assert calls[1]["tools"] is None
+    assert len(calls) == 3
-    assert "Do not call any more tools" in calls[1]["messages"][-1]["content"]
+    assert calls[0]["tools"] is not None
+    assert calls[1]["tools"] is not None
+    assert calls[2]["tools"] is None
     assert result.usage["prompt_tokens"] == 13
-    assert result.usage["completion_tokens"] == 8
+    assert result.usage["completion_tokens"] == 9
 
 
 @pytest.mark.asyncio
 async def test_runner_uses_specific_message_after_empty_finalization_retry():
+    """After silent retries + finalization all return empty, stop_reason is empty_final_response."""
     from nanobot.agent.runner import AgentRunSpec, AgentRunner
     from nanobot.utils.runtime import EMPTY_FINAL_RESPONSE_MESSAGE
 
@@ -517,7 +521,7 @@ async def test_runner_uses_specific_message_after_empty_finalization_retry():
         initial_messages=[{"role": "user", "content": "do task"}],
         tools=tools,
         model="test-model",
-        max_iterations=1,
+        max_iterations=3,
         max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
     ))
 
@@ -525,6 +529,66 @@ async def test_runner_uses_specific_message_after_empty_finalization_retry():
     assert result.stop_reason == "empty_final_response"
 
 
+@pytest.mark.asyncio
+async def test_runner_empty_response_does_not_break_tool_chain():
+    """An empty intermediate response must not kill an ongoing tool chain.
+
+    Sequence: tool_call → empty → tool_call → final text.
+    The runner should recover via silent retry and complete normally.
+    """
+    from nanobot.agent.runner import AgentRunSpec, AgentRunner
+
+    provider = MagicMock()
+    call_count = 0
+
+    async def chat_with_retry(*, messages, tools=None, **kwargs):
+        nonlocal call_count
+        call_count += 1
+        if call_count == 1:
+            return LLMResponse(
+                content=None,
+                tool_calls=[ToolCallRequest(id="tc1", name="read_file", arguments={"path": "a.txt"})],
+                usage={"prompt_tokens": 10, "completion_tokens": 5},
+            )
+        if call_count == 2:
+            return LLMResponse(content=None, tool_calls=[], usage={"prompt_tokens": 10, "completion_tokens": 1})
+        if call_count == 3:
+            return LLMResponse(
+                content=None,
+                tool_calls=[ToolCallRequest(id="tc2", name="read_file", arguments={"path": "b.txt"})],
+                usage={"prompt_tokens": 10, "completion_tokens": 5},
+            )
+        return LLMResponse(
+            content="Here are the results.",
+            tool_calls=[],
+            usage={"prompt_tokens": 10, "completion_tokens": 10},
+        )
+
+    provider.chat_with_retry = chat_with_retry
+    provider.chat_stream_with_retry = chat_with_retry
+
+    async def fake_tool(name, args, **kw):
+        return "file content"
+
+    tool_registry = MagicMock()
+    tool_registry.get_definitions.return_value = [{"type": "function", "function": {"name": "read_file"}}]
+    tool_registry.execute = AsyncMock(side_effect=fake_tool)
+
+    runner = AgentRunner(provider)
+    result = await runner.run(AgentRunSpec(
+        initial_messages=[{"role": "user", "content": "read both files"}],
+        tools=tool_registry,
+        model="test-model",
+        max_iterations=10,
+        max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
+    ))
+
+    assert result.final_content == "Here are the results."
+    assert result.stop_reason == "completed"
+    assert call_count == 4
+    assert "read_file" in result.tools_used
+
+
 def test_snip_history_drops_orphaned_tool_results_from_trimmed_slice(monkeypatch):
     from nanobot.agent.runner import AgentRunSpec, AgentRunner