fix(anthropic): handle plain string content in part_to_message_block

When a tool returns a response where the "content" key holds a plain
string, part_to_message_block() was iterating over it directly. Since
strings are iterable in Python, the loop produced one character per
iteration instead of the full text.

LoadSkillResourceTool is a concrete case: it returns a response dict
with a "content" key that is a string, not a list.

The fix adds an isinstance check before the loop. When content is a
string it is used directly. When it is a list, the existing join
logic runs unchanged. A unit test covers both paths.

Fixes #5358

Author: AliMuhammadAslam

src/google/adk/models/anthropic_llm.py

@@ -197,20 +197,23 @@ def part_to_message_block(
     content = ""
     response_data = part.function_response.response
 
-    # Handle response with content array
+    # Handle response with content array or plain string
     if "content" in response_data and response_data["content"]:
-      content_items = []
-      for item in response_data["content"]:
-        if isinstance(item, dict):
-          # Handle text content blocks
-          if item.get("type") == "text" and "text" in item:
-            content_items.append(item["text"])
+      if isinstance(response_data["content"], str):
+        content = response_data["content"]
+      else:
+        content_items = []
+        for item in response_data["content"]:
+          if isinstance(item, dict):
+            # Handle text content blocks
+            if item.get("type") == "text" and "text" in item:
+              content_items.append(item["text"])
+            else:
+              # Handle other structured content
+              content_items.append(str(item))
           else:
-            # Handle other structured content
             content_items.append(str(item))
-        else:
-          content_items.append(str(item))
-      content = "\n".join(content_items) if content_items else ""
+        content = "\n".join(content_items) if content_items else ""
     # We serialize to str here
     # SDK ref: anthropic.types.tool_result_block_param
     # https://github.com/anthropics/anthropic-sdk-python/blob/main/src/anthropic/types/tool_result_block_param.py

tests/unittests/models/test_anthropic_llm.py

@@ -776,6 +776,29 @@ def test_part_to_message_block_with_multiple_content_items():
   assert result["content"] == "First part\nSecond part"
 
 
+def test_part_to_message_block_with_string_content():
+  """Test that a plain string in the content key is passed through as-is.
+
+  LoadSkillResourceTool returns {"content": "<file text>"} where content is
+  a string, not a list. Iterating over a string yields individual characters,
+  so without an isinstance check the result would be "H\ne\nl\nl\no" instead
+  of "Hello".
+  """
+  part = types.Part.from_function_response(
+      name="load_skill_resource",
+      response={
+          "skill_name": "my-skill",
+          "file_path": "references/doc.md",
+          "content": "Hello world",
+      },
+  )
+  part.function_response.id = "test_id_str"
+
+  result = part_to_message_block(part)
+
+  assert result["content"] == "Hello world"
+
+
 def test_part_to_message_block_with_pdf_document():
   """Test that part_to_message_block handles PDF document parts."""
   pdf_data = b"%PDF-1.4 fake pdf content"