fix: serialize non-standard FunctionResponse dicts in AnthropicLlm

part_to_message_block() only handled response dicts containing
'content' or 'result' keys.  Any other dict structure (e.g.
SkillToolset returning {"skill_name": ..., "instructions": ...})
fell through to an empty string, causing Claude to never see the
tool output.

Add json.dumps() fallback for non-standard response dicts so all
structured data reaches the model.

Fixes #4779

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: giulio-leone

src/google/adk/models/anthropic_llm.py

@@ -18,6 +18,7 @@
 
 import base64
 from functools import cached_property
+import json
 import logging
 import os
 from typing import Any
@@ -121,6 +122,10 @@ def part_to_message_block(
       # ToolResultBlockParam content doesn't support list of dict. Converting
       # to str to prevent anthropic.BadRequestError from being thrown.
       content = str(response_data["result"])
+    elif response_data:
+      # Fallback: serialize any non-standard response dict (e.g. SkillToolset
+      # returns {"skill_name": ..., "instructions": ..., "frontmatter": ...})
+      content = json.dumps(response_data)
 
     return anthropic_types.ToolResultBlockParam(
         tool_use_id=part.function_response.id or "",

tests/unittests/models/test_anthropic_llm.py

@@ -522,6 +522,42 @@ def test_part_to_message_block_with_multiple_content_items():
   assert result["content"] == "First part\nSecond part"
 
 
+def test_part_to_message_block_with_non_standard_response():
+  """Test that part_to_message_block serializes non-standard response dicts.
+
+  Regression test for https://github.com/google/adk-python/issues/4779.
+  SkillToolset returns dicts like {"skill_name": ..., "instructions": ...}
+  that don't contain "content" or "result" keys.  These were silently
+  dropped (empty string), causing Claude to never see the tool output.
+  """
+  import json
+
+  from google.adk.models.anthropic_llm import part_to_message_block
+
+  skill_response = {
+      "skill_name": "search_docs",
+      "instructions": "Use the search API to find documents.",
+      "frontmatter": {"version": "1.0"},
+  }
+  part = types.Part.from_function_response(
+      name="load_skill",
+      response=skill_response,
+  )
+  part.function_response.id = "test_skill_id"
+
+  result = part_to_message_block(part)
+
+  assert isinstance(result, dict)
+  assert result["tool_use_id"] == "test_skill_id"
+  assert result["type"] == "tool_result"
+  assert not result["is_error"]
+  # Content must be non-empty and contain the original data
+  parsed = json.loads(result["content"])
+  assert parsed["skill_name"] == "search_docs"
+  assert parsed["instructions"] == "Use the search API to find documents."
+  assert parsed["frontmatter"] == {"version": "1.0"}
+
+
 content_to_message_param_test_cases = [
     (
         "user_role_with_text_and_image",