fix(bedrock): send a placeholder for empty assistant turns on Converse

CHANGELOG.md

@@ -8,6 +8,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 -->
 
 
+## [Unreleased]
+
+### Bug fixes
+
+* `ChatBedrock()` (with the default `api="converse"`) no longer sends assistant turns with an empty `content` array, which Converse rejects. This happens when a response carries no content blocks, for example when a guardrail intervenes before the model produces any. A `"[empty string]"` placeholder is sent instead, matching how empty text content is already normalized. (#426)
+
+
 ## [0.23.0] - 2026-09-04
 
 ### New features

chatlas/_provider_bedrock_converse.py

@@ -324,6 +324,12 @@ def as_converse_messages(turns: list[Turn]) -> list[MessageUnionTypeDef]:
         for c in turn.contents:
             content.append(as_converse_content(c, document_index=index))
             index += 1
+
+        # Converse requires non-empty content, and dropping the turn instead
+        # would leave two consecutive user messages.
+        if role == "assistant" and not content:
+            content = [{"text": "[empty string]"}]
+
         messages.append({"role": role, "content": content})
     return messages
 

tests/test_provider_bedrock_converse.py

@@ -452,6 +452,28 @@ def test_unknown_turn_role_raises(self):
         with pytest.raises(ValueError, match="Unknown role"):
             as_converse_messages([turn])
 
+    def test_empty_assistant_turn_gets_a_placeholder(self):
+        from chatlas._provider_bedrock_converse import as_converse_messages
+        from chatlas._turn import AssistantTurn, UserTurn
+
+        turns = [
+            UserTurn("Don't say anything"),
+            AssistantTurn([]),
+            UserTurn("What did I just say?"),
+        ]
+        messages = as_converse_messages(turns)
+
+        assert [m["role"] for m in messages] == ["user", "assistant", "user"]
+        assert messages[1]["content"] == [{"text": "[empty string]"}]
+
+    def test_empty_user_turn_is_left_alone(self):
+        from chatlas._provider_bedrock_converse import as_converse_messages
+        from chatlas._turn import UserTurn
+
+        messages = as_converse_messages([UserTurn([])])
+
+        assert messages[0]["content"] == []
+
 
 class TestRequestTransport:
     def binary_request(self) -> "ConverseRequestTypeDef":