fix(bedrock): normalize empty toolResult content arrays in _format_bedrock_messages (#2123)

Author: ghhamel

src/strands/models/bedrock.py

@@ -601,8 +601,15 @@ def _format_request_message_content(self, content: ContentBlock) -> dict[str, An
         # https://docs.aws.amazon.com/bedrock/latest/APIReference/API_runtime_ToolResultBlock.html
         if "toolResult" in content:
             tool_result = content["toolResult"]
+            # Normalize empty toolResult content arrays.
+            # Some model providers (e.g., Nemotron) reject toolResult blocks with
+            # content: [] via the Converse API, while others (e.g., Claude) accept
+            # them. Replace empty content with a minimal text block to ensure
+            # cross-model compatibility. This follows the same pattern as the
+            # TypeScript SDK's _formatMessages in bedrock.ts.
+            tool_result_content_list = tool_result.get("content") or [{"text": ""}]
             formatted_content: list[dict[str, Any]] = []
-            for tool_result_content in tool_result["content"]:
+            for tool_result_content in tool_result_content_list:
                 if "json" in tool_result_content:
                     # Handle json field since not in ContentBlock but valid in ToolResultContent
                     formatted_content.append({"json": tool_result_content["json"]})

tests/strands/models/test_bedrock.py

@@ -1607,6 +1607,87 @@ def test_format_request_cleans_tool_result_content_blocks(model, model_id):
     assert "status" not in tool_result
 
 
+def test_format_request_message_content_normalizes_empty_tool_result_content(model, model_id):
+    """Test that _format_request_message_content replaces empty toolResult content with a minimal text block.
+
+    Some model providers (e.g., Nemotron) reject toolResult blocks with content: [] via the
+    Converse API, while others (e.g., Claude) accept them. The SDK should normalize empty
+    content arrays to ensure cross-model compatibility.
+
+    See: https://github.com/strands-agents/sdk-python/issues/2122
+    """
+    messages = [
+        {"role": "user", "content": [{"text": "List tables"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {"text": "Querying...\n"},
+                {"toolUse": {"toolUseId": "tool_001", "name": "run_query", "input": {"sql": "SELECT 1"}}},
+            ],
+        },
+        {
+            "role": "user",
+            "content": [
+                {"toolResult": {"toolUseId": "tool_001", "content": []}},
+            ],
+        },
+    ]
+
+    formatted_request = model._format_request(messages)
+
+    tool_result = formatted_request["messages"][2]["content"][0]["toolResult"]
+    assert tool_result["content"] == [{"text": ""}], "Empty toolResult content should be normalized to [{'text': ''}]"
+
+
+def test_format_request_message_content_does_not_mutate_empty_tool_result(model, model_id):
+    """Test that normalizing empty toolResult content does not mutate the original messages."""
+    messages = [
+        {"role": "user", "content": [{"text": "List tables"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {"toolUse": {"toolUseId": "tool_001", "name": "run_query", "input": {"sql": "SELECT 1"}}},
+            ],
+        },
+        {
+            "role": "user",
+            "content": [
+                {"toolResult": {"toolUseId": "tool_001", "content": []}},
+            ],
+        },
+    ]
+
+    original_content = messages[2]["content"][0]["toolResult"]["content"]
+    model._format_request(messages)
+
+    assert original_content == [], "Original empty content list should not be mutated"
+
+
+def test_format_request_message_content_preserves_nonempty_tool_result_content(model, model_id):
+    """Test that _format_request_message_content does not modify non-empty toolResult content."""
+    messages = [
+        {"role": "user", "content": [{"text": "List tables"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {"text": "Querying...\n"},
+                {"toolUse": {"toolUseId": "tool_001", "name": "run_query", "input": {"sql": "SELECT 1"}}},
+            ],
+        },
+        {
+            "role": "user",
+            "content": [
+                {"toolResult": {"toolUseId": "tool_001", "content": [{"text": "some result"}]}},
+            ],
+        },
+    ]
+
+    formatted_request = model._format_request(messages)
+
+    tool_result = formatted_request["messages"][2]["content"][0]["toolResult"]
+    assert tool_result["content"] == [{"text": "some result"}]
+
+
 def test_format_request_removes_status_field_when_configured(model, model_id):
     model.update_config(include_tool_result_status=False)