fix(code_interpreter): preserve binary content in _create_tool_result

src/strands_tools/code_interpreter/agent_core_code_interpreter.py

@@ -477,16 +477,32 @@ def write_files(self, action: WriteFilesAction) -> Dict[str, Any]:
         return self._create_tool_result(response)
 
     def _create_tool_result(self, response) -> Dict[str, Any]:
-        """Create tool result from response."""
+        """Create tool result from response without stringifying binary content.
+
+        When reading binary files (e.g., PNG images) via readFiles, the content
+        may already be properly structured as a list. Stringifying it destroys
+        the binary data. This method preserves content structure when it's
+        already a list, and only wraps non-list content in text format.
+        """
         if "stream" in response:
             event_stream = response["stream"]
             for event in event_stream:
                 if "result" in event:
                     result = event["result"]
+                    content = result.get("content")
                     is_error = response.get("isError", False)
+
+                    # Preserve content structure if already a list (e.g., binary file content)
+                    if isinstance(content, list):
+                        return {
+                            "status": "success" if not is_error else "error",
+                            "content": content,
+                        }
+
+                    # Fall back to text wrapping for non-list content
                     return {
                         "status": "success" if not is_error else "error",
-                        "content": [{"text": str(result.get("content"))}],
+                        "content": [{"text": str(content)}],
                     }
 
             return {"status": "error", "content": [{"text": f"Failed to create tool result: {str(response)}"}]}

tests/code_interpreter/test_agent_core_code_interpreter.py

@@ -823,3 +823,47 @@ def test_module_level_session_mapping():
             mock_client2.get_session.assert_called_once_with(
                 interpreter_id="aws.codeinterpreter.v1", session_id="aws-session-123"
             )
+
+
+def test_create_tool_result_preserves_list_content(interpreter):
+    """Test _create_tool_result preserves list content (for binary files like PNG).
+
+    When reading binary files via readFiles, the content may already be a list.
+    The method should preserve this structure instead of stringifying it.
+    Fixes: https://github.com/strands-agents/tools/issues/370
+    """
+    # Simulate content that's already a properly structured list (like binary file data)
+    binary_content = [{"type": "resource", "resource": {"blob": b"\x89PNG\r\n"}}]
+    response = {"stream": [{"result": {"content": binary_content}}], "isError": False}
+
+    result = interpreter._create_tool_result(response)
+
+    assert result["status"] == "success"
+    # Content should be preserved as-is, not wrapped in str()
+    assert result["content"] == binary_content
+    assert result["content"][0]["type"] == "resource"
+
+
+def test_create_tool_result_wraps_string_content(interpreter):
+    """Test _create_tool_result wraps non-list content in text format.
+
+    For backward compatibility, when content is a string or other non-list type,
+    it should be wrapped in [{"text": str(content)}].
+    """
+    response = {"stream": [{"result": {"content": "Hello, World!"}}], "isError": False}
+
+    result = interpreter._create_tool_result(response)
+
+    assert result["status"] == "success"
+    assert result["content"] == [{"text": "Hello, World!"}]
+
+
+def test_create_tool_result_preserves_list_on_error(interpreter):
+    """Test _create_tool_result preserves list content even on error responses."""
+    error_content = [{"type": "text", "text": "Error details"}]
+    response = {"stream": [{"result": {"content": error_content}}], "isError": True}
+
+    result = interpreter._create_tool_result(response)
+
+    assert result["status"] == "error"
+    assert result["content"] == error_content