# C2 — Causality / no look-ahead

**Guarantee.** A node sees only the past. Input history is a read-only window
that ends at the current cursor; a resampler emits a bar only once it is
complete.

**Forbids.** Any node access to data with `timestamp > now`; emitting a partial
/ still-forming bar.

**Why.** Look-ahead is the cardinal backtester bug — a fast backtester that
leaks the future is worse than none. Making the future *physically absent* from
what a node receives beats merely discouraging it.

## Current state

A node's `eval` receives `Ctx::new(&inputs, ts)` (`crates/aura-engine/src/harness.rs`):
the input windows are read-only slices whose head (`window[0]`) is the value
pushed at or before the current `cycle_id`. The engine forwards exactly one
merged record per cycle in ascending timestamp order, so a node structurally
cannot reach a record with `timestamp > now` — the future is absent from what it
receives, not merely off-limits.

The resampler (`crates/aura-market/src/resample.rs`) emits a completed coarse bar
only on bucket rollover; a still-forming bar is never emitted, and an incomplete
final bucket with no following rollover tick simply never emits. The completed
bar carries no timestamp of its own — it takes the rollover instant, the close
instant of the bar just emitted (C4).

## See also

- [C3](c03-single-merge.md) — the single ingestion merge is what makes a
  read-only, monotone cursor possible.
- [C4](c04-cycle-granularity.md) — the data-driven cycle clock the cursor
  advances along.
- [C5](c05-freshness-gating.md) — held (stale) values are real past values, not
  a look-ahead escape hatch.