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.

Harness execution primitives

Harness organizes execution, tools, context, state, lifecycle, and verification as runtime constraints. Compiler diagnostics and execution semantics are checked separately.

Primitive	2.4 behavior
autoloop	Step/deadline limits; exact comparison of exit_when with the last assigned text or status field, ignoring surrounding whitespace and case
with_context	Exits on exceptions; trims Chat by whole user turns while preserving tool-call/result pairs
snapshot / restore	Deep-copy restoration of flow variables, Agent sessions, tool results, and provider state
fork / merge	Bounded execution, isolated sessions, stable result order
verify	A failed assertion stops the subsequent success path
reflect	Injects reflection text into later Agent input
Lifecycle hooks	Bound to model/tool events in the current flow, then removed when it exits

before_tool(name) and after_tool(name) observe only the named tool. In on_error(problem: Exception), problem receives the error-summary string stored in the event, suitable for logging. The hook preserves the run's failed status.

Snapshot scope

This complete offline program restores the result to 3.

flow main {
    result = 3;
    point = snapshot();
    result = 9;
    restore(point);
}

The implementation uses deep copies; cost depends on state size. External file and service effects remain. Use checkpoints for durable run recovery.

Executable verification

This complete offline program fails at type verification. unexpected is never returned successfully.

flow main {
    result = 3;
    verify result satisfies string;
    result = "unexpected";
}

Deterministic predicates support types, numeric comparisons, containment, and length. Other semantic checks require an explicit evaluator. .nxintent uses pass/fail/skip/error; only pass counts as passing. Missing runtime, schema, or semantic evaluator produces skip; empty output fails, and evaluator exceptions produce error.

Compiler diagnostic modes

build/run default to --harness warn; harness-check defaults to strict. strict errors block compilation, warn emits diagnostics, and off disables static Harness checks. The runtime continues enforcing tool approval and output validation.

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Source and validation · harness / verification / tools