2.4 compatibility in Nexa 3.0

This page describes the retained 2.4 language/runtime path. Sources without a version declaration use this path. Start new Open Programs with Nexa 3.0; compare contracts in the migration guide.

Actor lifecycle

A flow owns the Actors it creates. Each Actor has an isolated Agent session and a sequential mailbox. Later messages retain its history and accumulate model/tool usage and the total deadline.

Spawn, send, and join

Configure a model, save this program as actor.nx, and run uv run nexa run actor.nx. Worker processes both messages in order; the second message can use the first turn’s history. The flow returns the second reply.

agent Worker {
    prompt: "Process messages in order and retain prior context."
}

flow main {
    worker = spawn Worker("Inspect the evidence");
    pass "Summarize the evidence" to worker;
    result = await worker;
}

spawn starts an Actor, pass queues another input, and await closes its mailbox and returns the last result. A flow allows up to 8 active Actors at once. Awaited Actors release their concurrency slots.

Completion and failure

Every Actor must be awaited before its flow returns. On a flow exception or missing await, the runtime cancels remaining work, waits for its threads, and reports failure. Tool cancellation still follows the cooperative cancellation contract.

Model Actors consume their mailboxes automatically. Standalone receive() requires a custom message handler and currently raises an explicit error on the language runtime path. Actor sessions live within one flow; use execute() or nexa agent for cross-process approval and recovery.

Separate Actors suit parallel work, while successive messages to one Actor suit ordered tasks. For bounded parallel work followed by aggregation, see fork/merge.

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Source and validation · actors / execution