fix(memory): prevent oldest entry from dropping in STLTMemory

mesa_llm/memory/st_lt_memory.py

@@ -136,7 +136,7 @@ def _process_step_core(self, pre_step: bool):
             > self.capacity + (self.consolidation_capacity or 0)
             and self.consolidation_capacity
         ):
-            self.short_term_memory.popleft()
+            # Defer popleft() to the caller so _build_consolidation_prompt can see it.
             should_consolidate = True
 
         elif (
@@ -155,6 +155,7 @@ def process_step(self, pre_step: bool = False):
 
         if should_consolidate:
             self._update_long_term_memory()
+            self.short_term_memory.popleft()
 
         if new_entry and self.display:
             new_entry.display()
@@ -167,6 +168,7 @@ async def aprocess_step(self, pre_step: bool = False):
 
         if should_consolidate:
             await self._aupdate_long_term_memory()
+            self.short_term_memory.popleft()
 
         if new_entry and self.display:
             new_entry.display()

tests/test_memory/test_STLT_memory.py

@@ -200,3 +200,39 @@ def test_get_prompt_ready_returns_str_when_empty(self, mock_agent):
         )
         assert "Short term memory:" in result
         assert "Long term memory:" in result
+
+    def test_memory_does_not_drop_oldest_during_consolidation(
+        self, mock_agent, mock_llm, llm_response_factory
+    ):
+        """
+        Verify that the oldest entry in short-term memory is still present when
+        building the consolidation prompt. Fixes Issue #186.
+        """
+        mock_llm.generate.return_value = llm_response_factory(
+            "Consolidated memory summary"
+        )
+
+        memory = STLTMemory(
+            agent=mock_agent,
+            short_term_capacity=2,
+            consolidation_capacity=1,
+            llm_model="provider/test_model",
+        )
+        memory.llm = mock_llm
+
+        # Fill to capacity + consolidation_capacity + 1 to trigger consolidation
+        with patch("rich.console.Console"):
+            for i in range(4):
+                memory.step_content = {"observation": f"critical event at step {i}"}
+                memory.process_step(pre_step=True)
+                mock_agent.model.steps = i + 1
+                memory.step_content = {"action": f"response to step {i}"}
+                memory.process_step()
+
+        # Get the arguments passed to llm.generate during consolidation
+        call_args = mock_llm.generate.call_args[0][0]
+
+        # The oldest entry MUST be in the prompt sent to the LLM
+        assert "critical event at step 0" in call_args, (
+            "Oldest memory entry was silently dropped before consolidation!"
+        )