validate_schema skips validation for non-BaseModel schemas (list[str], dict[…], raw JSON Schema)

[michaelkillgta]

Description

validate_schema (google/adk/utils/_schema_utils.py) and SchemaType advertise support for list[str], list[int], dict[str, T], raw dict JSON schemas, and types.Schema.

Only BaseModel / list[BaseModel] go through Pydantic. The else branch is a bare safe_json_loads — no TypeAdapter / jsonschema check — so invalid values are silently accepted into session state / tool results.

Environment

• google-adk==2.9.2
• Offline unit repro (no live API / GCP)

Repro

from google.adk.utils._schema_utils import validate_schema
from pydantic import BaseModel, ValidationError

class M(BaseModel):
    a: str

# Control: BaseModel still validates
try:
    validate_schema(M, '{"a": 1}')
except ValidationError:
    print("basemodel rejects bad: OK")

print(validate_schema(list[str], "[1, 2, 3]"))       # → [1, 2, 3]  WRONG
print(validate_schema(dict[str, int], '{"a": "x"}')) # → {'a': 'x'} WRONG
schema = {"type": "object", "properties": {"a": {"type": "string"}}, "required": ["a"]}
print(validate_schema(schema, '{"a": 1}'))           # → {'a': 1}   WRONG

Observed (google-adk==2.9.2)

• list[str] + "[1, 2, 3]" → [1, 2, 3]
• dict[str, int] + '{"a": "x"}' → {'a': 'x'}
• Raw JSON Schema requiring string a + '{"a": 1}' → {'a': 1}
• BaseModel path still raises ValidationError (control OK)

Expected

Non-BaseModel schemas should be validated (e.g. TypeAdapter(schema).validate_json(...) for GenericAlias types; jsonschema for raw dict / types.Schema), or unsupported types should fail loudly — not silently accept invalid values.

Impact

Agents with output_schema=list[str] (etc.) + output_key write invalid values into session state. AgentTool returns unvalidated payloads to the parent. Downstream agents/tools treat garbage as schema-valid.

Suggested fix

For GenericAlias / non-dict schema types, use TypeAdapter(schema).validate_json(json_text) (same as list[BaseModel]). For raw dict / types.Schema JSON schemas, validate with jsonschema (already a dependency) or reject unsupported types loudly.

Notes

Nearby #5054 (output_schema=str + tools loop) and #6747 (python-mode dump) are different root causes. Happy to open a PR.

[chelsealong]

Picking this one up now — opening a PR shortly. Flagging it here so nobody duplicates the work; if someone is already on it, say so and I will drop mine.

[michaelkillgta]

Thanks for picking this up so quickly — and for the PRs (#7285 / #7284 / #7283). Happy to review or clarify anything from the original repros if useful.