fix(compaction): mark end of context summary in role=user fallback

When the head ends with assistant/tool and the tail starts with assistant,
the summary is inserted as a standalone role="user" message. The body's
verbatim "## Active Task" quote then gets read as fresh user input by
weak/local models (#11475, #14521).

The merge-into-tail path already appends an explicit end-of-summary marker
for this reason. Mirror it on the standalone path so both insertion routes
give the model the same "summary above, not new input" signal.

agent/context_compressor.py

@@ -1418,6 +1418,19 @@ The user has requested that this compaction PRIORITISE preserving all informatio
                 # Merge the summary into the first tail message instead
                 # of inserting a standalone message that breaks alternation.
                 _merge_summary_into_tail = True
+
+        # When the summary lands as a standalone role="user" message,
+        # weak models read the verbatim "## Active Task" quote of a past
+        # user request as fresh input (#11475, #14521). Append the explicit
+        # end marker — the same one used in the merge-into-tail path — so
+        # the model has a clear "summary above, not new input" signal.
+        if not _merge_summary_into_tail and summary_role == "user":
+            summary = (
+                summary
+                + "\n\n--- END OF CONTEXT SUMMARY — "
+                "respond to the message below, not the summary above ---"
+            )
+
         if not _merge_summary_into_tail:
             compressed.append({"role": summary_role, "content": summary})
 

tests/agent/test_context_compressor.py

@@ -664,6 +664,44 @@ class TestCompressWithClient:
             "call_123"
         ]
 
+    def test_user_role_summary_carries_end_marker(self):
+        """When the summary lands as standalone role='user' (e.g. head ends
+        with assistant/tool), the message body must include the explicit
+        '--- END OF CONTEXT SUMMARY ---' marker. Without it, weak models
+        read the verbatim past user request quoted in '## Active Task' as
+        fresh input (#11475, #14521).
+        """
+        mock_response = MagicMock()
+        mock_response.choices = [MagicMock()]
+        mock_response.choices[0].message.content = "summary text"
+
+        with patch("agent.context_compressor.get_model_context_length", return_value=100000):
+            c = ContextCompressor(model="test", quiet_mode=True, protect_first_n=2, protect_last_n=2)
+
+        # head_last=assistant, tail_first=assistant (same shape as the
+        # existing consecutive-user test) → role resolves to "user".
+        msgs = [
+            {"role": "user", "content": "msg 0"},
+            {"role": "assistant", "content": "msg 1"},
+            {"role": "user", "content": "msg 2"},
+            {"role": "assistant", "content": "msg 3"},
+            {"role": "user", "content": "msg 4"},
+            {"role": "assistant", "content": "msg 5"},
+            {"role": "user", "content": "msg 6"},
+            {"role": "assistant", "content": "msg 7"},
+        ]
+        with patch("agent.context_compressor.call_llm", return_value=mock_response):
+            result = c.compress(msgs)
+
+        summary_msg = next(
+            m for m in result if (m.get("content") or "").startswith(SUMMARY_PREFIX)
+        )
+        assert summary_msg["role"] == "user"
+        assert "END OF CONTEXT SUMMARY" in summary_msg["content"]
+        assert summary_msg["content"].rstrip().endswith(
+            "respond to the message below, not the summary above ---"
+        )
+
     def test_summary_role_avoids_consecutive_user_messages(self):
         """Summary role should alternate with the last head message to avoid consecutive same-role messages."""
         mock_client = MagicMock()