fix(anthropic): strip trailing assistant messages to prevent prefill error

Anthropic does not support assistant-message prefill and returns a 400
error when the conversation ends with an assistant turn. This commonly
happens when heartbeat/system messages accumulate trailing assistant
replies in the session history.

The _merge_consecutive method already handles same-role merging but did
not strip trailing assistant messages. The base provider's
_enforce_role_alternation (used by OpenAI-compat) does strip them, but
AnthropicProvider uses its own _merge_consecutive instead.

Add a trailing-assistant stripping loop to _merge_consecutive, matching
the behavior already present in _enforce_role_alternation.

Includes 7 new tests covering merge + strip behavior.

nanobot/providers/anthropic_provider.py

@@ -247,7 +247,12 @@ class AnthropicProvider(LLMProvider):
 
     @staticmethod
     def _merge_consecutive(msgs: list[dict[str, Any]]) -> list[dict[str, Any]]:
-        """Anthropic requires alternating user/assistant roles."""
+        """Anthropic requires alternating user/assistant roles.
+
+        Also strips trailing assistant messages since Anthropic does not
+        support assistant-message prefill and will reject the request with
+        a 400 error if the conversation ends with an assistant turn.
+        """
         merged: list[dict[str, Any]] = []
         for msg in msgs:
             if merged and merged[-1]["role"] == msg["role"]:
@@ -262,6 +267,12 @@ class AnthropicProvider(LLMProvider):
                 merged[-1]["content"] = prev_c
             else:
                 merged.append(msg)
+
+        # Drop trailing assistant messages to avoid Anthropic's
+        # "does not support assistant message prefill" 400 error.
+        while merged and merged[-1].get("role") == "assistant":
+            merged.pop()
+
         return merged
 
     # ------------------------------------------------------------------

tests/providers/test_anthropic_merge_consecutive.py

@@ -0,0 +1,66 @@
+"""Tests for AnthropicProvider._merge_consecutive."""
+
+from nanobot.providers.anthropic_provider import AnthropicProvider
+
+
+class TestMergeConsecutive:
+    """Verify role alternation and trailing-assistant stripping."""
+
+    def test_basic_alternation(self):
+        msgs = [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": "hi"},
+            {"role": "user", "content": "bye"},
+        ]
+        result = AnthropicProvider._merge_consecutive(msgs)
+        assert len(result) == 3
+        assert [m["role"] for m in result] == ["user", "assistant", "user"]
+
+    def test_consecutive_same_role_merged(self):
+        msgs = [
+            {"role": "user", "content": "a"},
+            {"role": "user", "content": "b"},
+            {"role": "assistant", "content": "reply"},
+        ]
+        result = AnthropicProvider._merge_consecutive(msgs)
+        # Two user messages merged into one, trailing assistant stripped
+        assert len(result) == 1
+        assert result[0]["role"] == "user"
+
+    def test_trailing_assistant_stripped(self):
+        """Anthropic rejects prefill — trailing assistant must be removed."""
+        msgs = [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": "hi"},
+        ]
+        result = AnthropicProvider._merge_consecutive(msgs)
+        assert len(result) == 1
+        assert result[0]["role"] == "user"
+        assert result[0]["content"] == "hello"
+
+    def test_multiple_trailing_assistant_stripped(self):
+        msgs = [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": "a"},
+            {"role": "user", "content": "ok"},
+            {"role": "assistant", "content": "b"},
+            {"role": "assistant", "content": "c"},
+        ]
+        result = AnthropicProvider._merge_consecutive(msgs)
+        # b+c merged into one assistant, then stripped as trailing
+        assert len(result) == 3
+        assert result[-1]["role"] == "user"
+        assert result[-1]["content"] == "ok"
+
+    def test_empty_messages(self):
+        assert AnthropicProvider._merge_consecutive([]) == []
+
+    def test_single_user_message(self):
+        msgs = [{"role": "user", "content": "hi"}]
+        result = AnthropicProvider._merge_consecutive(msgs)
+        assert len(result) == 1
+
+    def test_single_assistant_stripped(self):
+        msgs = [{"role": "assistant", "content": "hi"}]
+        result = AnthropicProvider._merge_consecutive(msgs)
+        assert len(result) == 0