Claude API on Vertex AI (Cloud Code Assist / Antigravity) enforces strict
JSON Schema 2020-12 validation and rejects root-level anyOf without a
top-level type field.
TypeBox Type.Union compiles to { anyOf: [...] } which Anthropic's direct
API accepts but Vertex rejects with:
tools.11.custom.input_schema: JSON schema is invalid
This follows the same pattern used in browser-tool.ts which has the same
fix with an explanatory comment.
Flatten the schema to Type.Object with an action enum, matching how
browser tool handles this constraint.
