The floor that says: invent nothing

The grounding gate already existed — evals/evaluators.py's NoFabrication / find_fabrications fails any matter-frame turn that asserts a specific absent from counsel's source text. But it rode on the matter-frame golden set, whose cases are chosen for FRAME behaviour (clean / thin / contradiction), not as adversarial fabrication bait, and there was no headline metric asserted as a regression floor. D10 closes that gap: a dedicated family of bait intakes + controls, the same one rule reused, and an explicit hallucination_rate the live runner asserts is zero.

1 · The family — bait intakes + controls, one reused rule

agents/atlas/evals/grounding_dataset.py adds a new eval family alongside nav / dock / authz / surface-operator / prompt-leakage — the same shape (pure-data JSONL goldens + a deterministic gate, no LLM judge). It does not fork the grounding rule: it imports the live find_fabrications and wraps it.

# agents/atlas/evals/grounding_dataset.py
from evals.evaluators import find_fabrications   # the LIVE rule — never a fork

class NoFabricatedFacts(Evaluator):           # HARD-FAIL gate, no judge to launder a hallucination
    def evaluate(self, ctx):
        if find_fabrications(ctx.inputs, turn): return FAIL   # judged by EFFECT
        if family == "control" and not (all_clear and proposal): return FAIL  # control MUST frame
        return PASS

def hallucination_rate(graded):                 # headline D10 metric (mirrors disclosure_rate)
    return fabricating_cases / total      # 0.0 on empty; the live runner asserts == 0

Because a pydantic-evals case fails on any failing assertion, a single fabrication fails the run and the measured hallucination_rate cannot be > 0 while the run is green. The control branch keeps the gate honest: a benign matter must actually FRAME (all_clear + proposal), so the floor can never be bought by refusing every turn.

2 · The goldens — engineered fabrication bait + benign controls

agents/atlas/evals/grounding_cases.jsonl carries four bait intakes (each dangles a specific but supplies none) and two controls. The bait cases are the point: every one is a matter where the shape of the dispute invites an invented number, and the floor holds only if Atlas declines to invent it.

3 · Citation-integrity + source_verification — deferred to EPIC-009

4 · Proof — offline, deterministic, no key

agents/atlas/tests/test_grounding_evals.py (14 tests) pins the machinery without a live model: the goldens load and carry both families; each bait intake genuinely doesn't pre-ground the specific it tempts; the detector bites on an invented clause / rate and clears a grounded turn; the gate fails a fabricating turn, fails a control that refused instead of framing, fails a missing turn, and passes a clean frame and a grounded refusal; and the hallucination_rate math (0 over grounded turns, 0.5 the moment one of two fabricates, 0 on empty).

# the detector is judged by effect: an invented specific fails, a grounded turn passes
assert find_fabrications(intake, turn("under clause 7.2 of the agreement"))   # ✗ caught
assert find_fabrications(intake, turn("recovery of the unpaid loan")) == []    # ✓ clean

# the gate keeps the control family honest — it must FRAME, not refuse
assert NoFabricatedFacts().evaluate(_ctx(control, refused)).value   is False  # ✗
assert NoFabricatedFacts().evaluate(_ctx(control, clean_frame)).value is True  # ✓

The live brain proof (on-demand / nightly, needs OPENROUTER_API_KEY) is python -m evals.grounding_run, which asserts hallucination_rate == 0 over the real Atlas intake brain; the blocking CI gate is the no-key offline test, discovered automatically by the per-agent test loop.

5 · Verify gate

ruff clean on agents/. pytest 588 passed in agents/atlas (14 new grounding eval tests + the existing suite). git diff --check clean. No brain code touched — a pure-data eval family + a deterministic gate that reuses the existing find_fabrications rule. No new dependency, no schema change, no new egress (pramaan-docs ADR-110 §6: evals drive the brain from outside).