The runtime interface

A runtime is how an agent’s “brain” is loaded and driven for one iteration. Where providers and tooling are I/O, a runtime is the program the agent runs: it is handed the agent’s brain (a wasm component, a rhai script, or a js script) and asked to execute it against the AgentContext.

Named runtimes live in the global [runtime.<name>] config map. Each agent wiring pins itself to one by name.

The abstraction

A runtime exposes, through the WIT runtime interface (exported by the guest component and called by the host):

• kind() — which brain implementation this is (wasm, rhai, or js).
• run(script) — run one iteration. Returns the terminal message if the brain chose to exit, otherwise nothing.

The host’s Runtime::run(&AgentContext) drives this off the tokio worker on a spawn_blocking thread (the wasm engine and the rhai / js interpreter components built on it are synchronous). The script argument is the agent’s brain. For a wasm brain the program is baked into the component and the script is unused, while for Rhai it is the Rhai source text and for JS it is the JavaScript source text.

How a run happens

build(kind) dispatches to wasm, rhai, or js. Each runtime loads its engine, and pushes a blocking task that:

1. builds a Store whose data is the runtime Host (the agent context, a resource table, and a WASI context),
2. wires the imports — the WASI wasip2 imports plus the provider, tooling, and host interfaces — into a Linker,
3. instantiates the component,
4. calls the exported runtime.run(script) and
5. surfaces the terminal message as Exited(message) or Completed if the brain ran to a finish.

The agent context

Every runtime call receives an AgentContext: the agent name, the brain script path, the named providers and tooling registries, the shared MessageBus, the agent’s own StreamRegistry (chat streams), the timer CancelRegistry, the resource-subscription CancelRegistry, and the dedicated tokio rt used to bridge synchronous host calls to the async provider/tooling.

Provider and tooling registries, and the message bus, are shared across all agents in one process — the StreamRegistry, timer/resource registries, and bridge runtime are per-agent.