Verifications (Deterministic Checks)

Updated 2026-02-14 — Expanded .eventually() verifications (adaptive resnapshotting) and a first-class scroll verification primitive.

Predicate is a verification layer: the agent acts, and Predicate asserts with evidence.

What “deterministic verification” means

In browser agents, it’s common to see:

• the model says it clicked / scrolled / finished
• but the UI state doesn’t actually reflect that (drift), or the result isn’t grounded (hallucination)

Predicate’s approach is to make progress provable:

• snapshot the rendered UI
• run machine-checkable assertions
• retry in a bounded way when appropriate (.eventually())
• fail with trace evidence if required checks can’t be proven

If you’re new to the assertion APIs, start here:

• Jest-Style Assertions →
• Agent Runtime →

Expanded deterministic verifications: adaptive resnapshotting

When you use .eventually() for deterministic checks, you can now grow the snapshot element limit across retries. This helps on long / virtualized pages where a small snapshot can miss the target element, causing a false failure.

from predicate.verification import exists

# Grow snapshot limit on each retry until the element appears.
ok = await runtime.check(
    exists("text~'Checkout'"),
    label="checkout_visible",
    required=True,
).eventually(
    timeout_s=12,
    snapshot_limit_growth={
        "start_limit": 60,
        "step": 40,
        "max_limit": 220,
        "apply_on": "only_on_fail",  # default; or "all"
    },
)

Scroll verification: detect no-op scroll drift

A common failure mode is: the agent “scrolls” but the UI doesn’t move (overlay blocks input, nested scroller, focus issues). This causes silent drift / loops.

Use the runtime scroll helper to scroll and verify the scroll had real effect using a bounded before/after scroll position (scrollTop).

runtime.begin_step("Scroll and verify it moved")
ok = await runtime.scroll_by(
    600,
    verify=True,
    min_delta_px=50,
    label="scroll_effective",
    required=True,
    timeout_s=5.0,
)
if not ok:
    raise RuntimeError("Scroll had no effect (likely blocked by overlay or nested scroller).")