fix(providers): enforce role alternation for non-Claude providers

Some LLM providers (OpenAI-compat, Azure, vLLM, Ollama) reject requests
with consecutive same-role messages or trailing assistant messages. Add
_enforce_role_alternation() to merge consecutive same-role user/assistant
messages and strip trailing assistant messages before sending to the API.

nanobot/providers/azure_openai_provider.py

@@ -94,9 +94,11 @@ class AzureOpenAIProvider(LLMProvider):
     ) -> dict[str, Any]:
         """Prepare the request payload with Azure OpenAI 2024-10-21 compliance."""
         payload: dict[str, Any] = {
-            "messages": self._sanitize_request_messages(
-                self._sanitize_empty_content(messages),
-                _AZURE_MSG_KEYS,
+            "messages": self._enforce_role_alternation(
+                self._sanitize_request_messages(
+                    self._sanitize_empty_content(messages),
+                    _AZURE_MSG_KEYS,
+                )
             ),
             "max_completion_tokens": max(1, max_tokens),  # Azure API 2024-10-21 uses max_completion_tokens
         }

nanobot/providers/base.py

@@ -196,6 +196,42 @@ class LLMProvider(ABC):
         err = (content or "").lower()
         return any(marker in err for marker in cls._TRANSIENT_ERROR_MARKERS)
 
+    @staticmethod
+    def _enforce_role_alternation(messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
+        """Merge consecutive same-role messages and drop trailing assistant messages.
+
+        Some providers (OpenAI-compat, Azure, vLLM, Ollama, etc.) reject requests
+        where the last message is 'assistant' (prefill not supported) or two
+        consecutive non-system messages share the same role.
+        """
+        if not messages:
+            return messages
+
+        merged: list[dict[str, Any]] = []
+        for msg in messages:
+            role = msg.get("role")
+            if (
+                merged
+                and role != "system"
+                and role not in ("tool",)
+                and merged[-1].get("role") == role
+                and role in ("user", "assistant")
+            ):
+                prev = merged[-1]
+                prev_content = prev.get("content") or ""
+                curr_content = msg.get("content") or ""
+                if isinstance(prev_content, str) and isinstance(curr_content, str):
+                    prev["content"] = (prev_content + "\n\n" + curr_content).strip()
+                else:
+                    merged[-1] = dict(msg)
+            else:
+                merged.append(dict(msg))
+
+        while merged and merged[-1].get("role") == "assistant":
+            merged.pop()
+
+        return merged
+
     @staticmethod
     def _strip_image_content(messages: list[dict[str, Any]]) -> list[dict[str, Any]] | None:
         """Replace image_url blocks with text placeholder. Returns None if no images found."""

nanobot/providers/openai_compat_provider.py

@@ -215,7 +215,7 @@ class OpenAICompatProvider(LLMProvider):
                 clean["tool_calls"] = normalized
             if "tool_call_id" in clean and clean["tool_call_id"]:
                 clean["tool_call_id"] = map_id(clean["tool_call_id"])
-        return sanitized
+        return self._enforce_role_alternation(sanitized)
 
     # ------------------------------------------------------------------
     # Build kwargs

tests/providers/test_enforce_role_alternation.py

@@ -0,0 +1,128 @@
+"""Tests for LLMProvider._enforce_role_alternation."""
+
+from nanobot.providers.base import LLMProvider
+
+
+class TestEnforceRoleAlternation:
+    """Verify trailing-assistant removal and consecutive same-role merging."""
+
+    def test_empty_messages(self):
+        assert LLMProvider._enforce_role_alternation([]) == []
+
+    def test_no_change_needed(self):
+        msgs = [
+            {"role": "system", "content": "You are helpful."},
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": "Hello!"},
+            {"role": "user", "content": "Bye"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 4
+        assert result[-1]["role"] == "user"
+
+    def test_trailing_assistant_removed(self):
+        msgs = [
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": "Hello!"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 1
+        assert result[0]["role"] == "user"
+
+    def test_multiple_trailing_assistants_removed(self):
+        msgs = [
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": "A"},
+            {"role": "assistant", "content": "B"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 1
+        assert result[0]["role"] == "user"
+
+    def test_consecutive_user_messages_merged(self):
+        msgs = [
+            {"role": "user", "content": "Hello"},
+            {"role": "user", "content": "How are you?"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 1
+        assert "Hello" in result[0]["content"]
+        assert "How are you?" in result[0]["content"]
+
+    def test_consecutive_assistant_messages_merged(self):
+        msgs = [
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": "Hello!"},
+            {"role": "assistant", "content": "How can I help?"},
+            {"role": "user", "content": "Thanks"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 3
+        assert "Hello!" in result[1]["content"]
+        assert "How can I help?" in result[1]["content"]
+
+    def test_system_messages_not_merged(self):
+        msgs = [
+            {"role": "system", "content": "System A"},
+            {"role": "system", "content": "System B"},
+            {"role": "user", "content": "Hi"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 3
+        assert result[0]["content"] == "System A"
+        assert result[1]["content"] == "System B"
+
+    def test_tool_messages_not_merged(self):
+        msgs = [
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": None, "tool_calls": [{"id": "1"}]},
+            {"role": "tool", "content": "result1", "tool_call_id": "1"},
+            {"role": "tool", "content": "result2", "tool_call_id": "2"},
+            {"role": "user", "content": "Next"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        tool_msgs = [m for m in result if m["role"] == "tool"]
+        assert len(tool_msgs) == 2
+
+    def test_non_string_content_uses_latest(self):
+        msgs = [
+            {"role": "user", "content": [{"type": "text", "text": "A"}]},
+            {"role": "user", "content": "B"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 1
+        assert result[0]["content"] == "B"
+
+    def test_original_messages_not_mutated(self):
+        msgs = [
+            {"role": "user", "content": "Hello"},
+            {"role": "user", "content": "World"},
+        ]
+        original_first = dict(msgs[0])
+        LLMProvider._enforce_role_alternation(msgs)
+        assert msgs[0] == original_first
+        assert len(msgs) == 2
+
+    def test_only_assistant_messages(self):
+        msgs = [
+            {"role": "assistant", "content": "A"},
+            {"role": "assistant", "content": "B"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert result == []
+
+    def test_realistic_conversation(self):
+        msgs = [
+            {"role": "system", "content": "You are helpful."},
+            {"role": "user", "content": "What is 2+2?"},
+            {"role": "assistant", "content": "4"},
+            {"role": "user", "content": "And 3+3?"},
+            {"role": "user", "content": "(please be quick)"},
+            {"role": "assistant", "content": "6"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 4
+        assert result[2]["role"] == "assistant"
+        assert result[3]["role"] == "user"
+        assert "And 3+3?" in result[3]["content"]
+        assert "(please be quick)" in result[3]["content"]