Agents and models

Agent is an immutable, provider-neutral definition containing instructions, model, tools, permissions, settings, and metadata. A ModelProvider emits normalized text, tool-call, completion, usage, or failure events. Runtime code never imports a vendor SDK.

FakeModel.responses(...), FakeModel.script(...), and FakeModel.turns(...) provide deterministic development and tests. Production adapters should translate vendor payloads only at the provider boundary.