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.

Error propagation and recovery

Language-level error handling manages local control flow. Runtime contracts govern durable state and uncertain side effects. Identify the failing layer before choosing a recovery entry.

Mechanism	Purpose
try / catch	Catch exceptions within a statement scope
try_agent / catch_correction	Enter an explicit correction branch
?	Unwrap NexaResult and propagate failure
otherwise	Provide a local fallback for an error result
defer	Parser/scheduler support exists; generated programs currently fail, as detailed below

Complete correction example

flow main {
    result = "before";
    try_agent {
        assert false;
    } catch_correction(e: Exception) {
        result = "corrected";
    }
}

The offline result is corrected. NexaAgent.run_result() returns an explicit NexaResult for Python callers. execute() returns a RunState with status and error.

Failures with side effects

Raise ToolOutcomeUnknown when a tool sent a request but its outcome is uncertain. Returning an ordinary success string marks the result as known and changes recovery behavior. Preserve the checkpoint and let an operator verify the result before calling resolve_tool; see unknown outcome recovery.

Execution states distinguish budget exhaustion, cancellation, and timeout. Use continue_run for a new conversation turn.

Current lowering limitations

The parser and standalone scheduler have defer tests. The complete program flow main { defer print("closed"); result = 1; } currently raises name 'defer' is not defined. Put critical resource cleanup in a trusted Python tool's try/finally until the generated path passes acceptance.

Flows with requires / ensures currently fail because generated ContractClause arguments do not match its constructor. Use executed verify checks, tool-argument schemas, and explicit error states for current task constraints.

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Source and validation · language / execution / checkpoint