fix(context_compressor): prevent consecutive same-role messages after compression (#1743)

compress() checks both the head and tail neighbors when choosing the
summary message role.  When only the tail collides, the role is flipped.
When BOTH roles would create consecutive same-role messages (e.g.
head=assistant, tail=user), the summary is merged into the first tail
message instead of inserting a standalone message that breaks role
alternation and causes API 400 errors.

The previous code handled head-side collision but left the tail-side
uncovered — long conversations would crash mid-reply with no useful
error, forcing the user to /reset and lose session history.

Based on PR #1186 by @alireza78a, with improved double-collision
handling (merge into tail instead of unconditional 'user' fallback).

Co-authored-by: alireza78a <alireza78.crypto@gmail.com>

tests/agent/test_context_compressor.py

@@ -111,7 +111,11 @@ class TestCompress:
         # First 2 messages should be preserved (protect_first_n=2)
         # Last 2 messages should be preserved (protect_last_n=2)
         assert result[-1]["content"] == msgs[-1]["content"]
-        assert result[-2]["content"] == msgs[-2]["content"]
+        # The second-to-last tail message may have the summary merged
+        # into it when a double-collision prevents a standalone summary
+        # (head=assistant, tail=user in this fixture).  Verify the
+        # original content is present in either case.
+        assert msgs[-2]["content"] in result[-2]["content"]
 
 
 class TestGenerateSummaryNoneContent:
@@ -329,6 +333,146 @@ class TestCompressWithClient:
         assert len(summary_msg) == 1
         assert summary_msg[0]["role"] == "assistant"
 
+    def test_summary_role_flips_to_avoid_tail_collision(self):
+        """When summary role collides with the first tail message but flipping
+        doesn't collide with head, the role should be flipped."""
+        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 ends with tool (index 1), tail starts with user (index 6).
+        # Default: tool → summary_role="user" → collides with tail.
+        # Flip to "assistant" → tool→assistant is fine.
+        msgs = [
+            {"role": "user", "content": "msg 0"},
+            {"role": "assistant", "content": "", "tool_calls": [
+                {"id": "call_1", "type": "function", "function": {"name": "t", "arguments": "{}"}},
+            ]},
+            {"role": "tool", "tool_call_id": "call_1", "content": "result 1"},
+            {"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)
+        # Verify no consecutive user or assistant messages
+        for i in range(1, len(result)):
+            r1 = result[i - 1].get("role")
+            r2 = result[i].get("role")
+            if r1 in ("user", "assistant") and r2 in ("user", "assistant"):
+                assert r1 != r2, f"consecutive {r1} at indices {i-1},{i}"
+
+    def test_double_collision_merges_summary_into_tail(self):
+        """When neither role avoids collision with both neighbors, the summary
+        should be merged into the first tail message rather than creating a
+        standalone message that breaks role alternation.
+
+        Common scenario: head ends with 'assistant', tail starts with 'user'.
+        summary='user' collides with tail, summary='assistant' collides with head.
+        """
+        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=3, protect_last_n=3)
+
+        # Head: [system, user, assistant]  →  last head = assistant
+        # Tail: [user, assistant, user]    →  first tail = user
+        # summary_role="user" collides with tail, "assistant" collides with head → merge
+        msgs = [
+            {"role": "system", "content": "system prompt"},
+            {"role": "user", "content": "msg 1"},
+            {"role": "assistant", "content": "msg 2"},
+            {"role": "user", "content": "msg 3"},      # compressed
+            {"role": "assistant", "content": "msg 4"},  # compressed
+            {"role": "user", "content": "msg 5"},       # compressed
+            {"role": "user", "content": "msg 6"},       # tail start
+            {"role": "assistant", "content": "msg 7"},
+            {"role": "user", "content": "msg 8"},
+        ]
+        with patch("agent.context_compressor.call_llm", return_value=mock_response):
+            result = c.compress(msgs)
+
+        # Verify no consecutive user or assistant messages
+        for i in range(1, len(result)):
+            r1 = result[i - 1].get("role")
+            r2 = result[i].get("role")
+            if r1 in ("user", "assistant") and r2 in ("user", "assistant"):
+                assert r1 != r2, f"consecutive {r1} at indices {i-1},{i}"
+
+        # The summary text should be merged into the first tail message
+        first_tail = [m for m in result if "msg 6" in (m.get("content") or "")]
+        assert len(first_tail) == 1
+        assert "summary text" in first_tail[0]["content"]
+
+    def test_double_collision_user_head_assistant_tail(self):
+        """Reverse double collision: head ends with 'user', tail starts with 'assistant'.
+        summary='assistant' collides with tail, 'user' collides with head → merge."""
+        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: [system, user]        → last head = user
+        # Tail: [assistant, user]     → first tail = assistant
+        # summary_role="assistant" collides with tail, "user" collides with head → merge
+        msgs = [
+            {"role": "system", "content": "system prompt"},
+            {"role": "user", "content": "msg 1"},
+            {"role": "assistant", "content": "msg 2"},   # compressed
+            {"role": "user", "content": "msg 3"},        # compressed
+            {"role": "assistant", "content": "msg 4"},   # compressed
+            {"role": "assistant", "content": "msg 5"},   # tail start
+            {"role": "user", "content": "msg 6"},
+        ]
+        with patch("agent.context_compressor.call_llm", return_value=mock_response):
+            result = c.compress(msgs)
+
+        # Verify no consecutive user or assistant messages
+        for i in range(1, len(result)):
+            r1 = result[i - 1].get("role")
+            r2 = result[i].get("role")
+            if r1 in ("user", "assistant") and r2 in ("user", "assistant"):
+                assert r1 != r2, f"consecutive {r1} at indices {i-1},{i}"
+
+        # The summary should be merged into the first tail message (assistant)
+        first_tail = [m for m in result if "msg 5" in (m.get("content") or "")]
+        assert len(first_tail) == 1
+        assert "summary text" in first_tail[0]["content"]
+
+    def test_no_collision_scenarios_still_work(self):
+        """Verify that the common no-collision cases (head=assistant/tail=assistant,
+        head=user/tail=user) still produce a standalone summary message."""
+        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=assistant, Tail=assistant → summary_role="user", no collision
+        msgs = [
+            {"role": "user", "content": "msg 0"},
+            {"role": "assistant", "content": "msg 1"},
+            {"role": "user", "content": "msg 2"},
+            {"role": "assistant", "content": "msg 3"},
+            {"role": "assistant", "content": "msg 4"},
+            {"role": "user", "content": "msg 5"},
+        ]
+        with patch("agent.context_compressor.call_llm", return_value=mock_response):
+            result = c.compress(msgs)
+        summary_msgs = [m for m in result if (m.get("content") or "").startswith(SUMMARY_PREFIX)]
+        assert len(summary_msgs) == 1, "should have a standalone summary message"
+        assert summary_msgs[0]["role"] == "user"
+
     def test_summarization_does_not_start_tail_with_tool_outputs(self):
         mock_response = MagicMock()
         mock_response.choices = [MagicMock()]