BaseTool.from_config drops T | None arguments on Python 3.11-3.13 #7840

🔴 Required Information

Describe the Bug:
On Python 3.11–3.13, BaseTool.from_config drops every constructor argument annotated with PEP 604 syntax (T | None). Before Python 3.14, get_origin(int | None) is types.UnionType, not typing.Union. The Optional[T] unwrapping (if get_origin(param_type) is Union) is therefore skipped, no branch matches, and the value is discarded with only an "Unsupported parsing for argument" warning.

Several built-in tools rely on the default from_config and annotate their arguments this way, so YAML agent configs silently lose them. Among them:

• VertexAiRagRetrieval: rag_corpora, rag_resources, similarity_top_k, vector_distance_threshold
• GoogleSearchTool: model
• ExecuteBashTool: workspace

A RAG tool configured in YAML ends up retrieving from no corpus.

Steps to Reproduce:

1. root_agent.yaml:

name: contracts_agent
model: gemini-2.5-flash
instruction: Answer questions from the contract corpus.
tools:
  - name: google.adk.tools.retrieval.vertex_ai_rag_retrieval.VertexAiRagRetrieval
    args:
      name: retrieve_contracts
      description: Retrieves contract clauses.
      rag_corpora:
        - projects/p/locations/us-central1/ragCorpora/123
      similarity_top_k: 5
      vector_distance_threshold: 0.4

2. agent = config_agent_utils.from_config("root_agent.yaml") and inspect agent.tools[0].vertex_rag_store.

Expected Behavior:
rag_corpora, similarity_top_k and vector_distance_threshold are set from the config.

Observed Behavior:

Unsupported parsing for argument: rag_corpora.
Unsupported parsing for argument: similarity_top_k.
Unsupported parsing for argument: vector_distance_threshold.
rag_corpora: None
similarity_top_k: None
vector_distance_threshold: None

Environment Details:

• ADK Library Version (pip show google-adk): 2.13.0 (also on main at 75fb788)
• Desktop OS: macOS
• Python Version (python -V): 3.11.16

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A (config loading)

🟡 Optional Information

Additional Context:
Optional[T] annotations work, which is why only some arguments are affected. #4371 / #4406 covered PEP 604 unions in function-declaration parsing, not this config path.

How often has this issue occurred?:

• Always (100%) on Python 3.11–3.13