diff --git a/docs/handoff/b4-pr1-frameverdict-type-inv31-brief-2026-06-15.md b/docs/handoff/b4-pr1-frameverdict-type-inv31-brief-2026-06-15.md
new file mode 100644
index 00000000..0f535285
--- /dev/null
+++ b/docs/handoff/b4-pr1-frameverdict-type-inv31-brief-2026-06-15.md
@@ -0,0 +1,142 @@
+# Brief — B4 PR-1: FrameVerdict type + isolated evaluator + INV-31 firewall (gated)
+
+> **Red line.**
+> PR-1 is **not** a closed-world serving feature. It introduces a **sealed type +
+> isolated evaluator** and the **INV-31 firewall** only. **No** runtime caller, **no**
+> response-governance integration, **no** session/vault/chat wire, **no** perception
+> adapter, **no** CWA lane, **no** prose lowering. The evaluator exists for tests and
+> future PR-2/PR-3 callers — never for `chat`/`runtime`/`session`/`governance`.
+
+**Gate:** ADR-0222 ratified (#780) ✅ + ProofWriter-OWA floor landed (#779) ✅ → PR-1 is
+**authorized**. It ships the type **with** INV-31, never after. **No serving wire, no
+lane, no perception adapter, no governance integration** (those are PR-2/3/4).
+
+**Open with:**
+
+```
+git fetch origin main && git worktree add ../core-frameverdict-pr1 origin/main -b feat/b4-pr1-frameverdict-type
+```
+
+## Why
+
+ADR-0222 §10 PR-1 is the *first and only* implementation step authorized now: the
+closed-world `FrameVerdict` envelope + its text-frame **isolated evaluator** + the
+**INV-31** two-part firewall that makes a closed-world `False` unable to reach the
+open-world runtime. It composes existing sound producers (`proof_chain.entail`), adds
+**no** `answer=False`, and leaves `determine.py` untouched.
+
+## Scope (exactly this)
+
+1. **Lift `_basis` → `generate/epistemic_basis.py`** (ADR §3 note, §8 A3). Move `_basis`
+   **byte-identically** out of `generate/determine/determine.py` into a new shared module;
+   `determine.py` imports it from there (no behavior change — its three `Determined` sites
+   and INV-30 stay intact). This removes the open/closed package coupling so `frame_verdict`
+   can import `_basis` without importing `determine`.
+
+2. **New package `generate/frame_verdict/`** with the §3 types verbatim:
+   - `FrameKind` / `WorldAssumption` / `FrameVerdictKind` enums.
+   - `ClosedWorldProof` (incl. `positive_refutation_kind`).
+   - `FrameVerdict` **with the `__post_init__` admissibility invariant** — `entailed_false`
+     requires `(producer, outcome, positive_refutation_kind)` =
+     `proof_chain.entail/REFUTED/robdd_refutation` **or**
+     `sensorium.falsification/FALSIFIED/perception_changed_slot`, and a non-empty
+     `proof_sha256`.
+   - `ClosedFrame` (§3.1).
+   - `evaluate_frame_verdict(frame: ClosedFrame, query: str) -> FrameVerdict` (§4) — the
+     **isolated evaluator**, the **only** `FrameVerdict` constructor. **Text frame only.**
+     Treat `frame.propositions` + `query` as propositional-formula strings (the existing
+     `entail.py` grammar — prose→proposition lowering is **PR-2**), delegate to the
+     **unchanged** `evaluate_entailment_with_trace(premises, query)`, and map per the §5.1
+     table:
+
+     | `entail` outcome | verdict |
+     |---|---|
+     | ENTAILED | `entailed_true` |
+     | REFUTED | `entailed_false` (proof: `proof_chain.entail` / `REFUTED` / `robdd_refutation`; `proof_keys` = conjunction / query / refutation_check keys) |
+     | UNKNOWN | `undetermined` |
+     | REFUSED(INCONSISTENT_PREMISES) | `contradiction` |
+     | REFUSED(OUT_OF_REGIME_OR_MALFORMED) | `scope_boundary` |
+
+     `world_assumption == OPEN`, a missing closure declaration, or `frame_kind != TEXT` →
+     **`scope_boundary`** (never `entailed_false`). Perception (`frame_kind = PERCEPTION`)
+     is **PR-3** → `scope_boundary` here.
+   - **Serialization:** `trace_hash` and `proof_sha256` via the existing `sha256_json`
+     (canonical, order-invariant); `basis` via the lifted `_basis`.
+
+3. **INV-31 in `tests/test_architectural_invariants.py`** (ADR §8) — lands **with** the type:
+   - **Part A (import containment).** A1: `determine.py` neither imports nor constructs
+     `FrameVerdict`. A2: `FrameVerdict` (and every alternate constructor — factory /
+     `dataclasses.replace` into `verdict`) is built only inside an **exact**
+     `ALLOWED_FRAME_VERDICT_SITES` frozenset; **the construction scan must include `evals/`**
+     (INV-30's walk excludes it). A3: `generate/frame_verdict` is unreachable from the
+     **transitive** first-party import closure of each spine file (`chat/runtime.py`,
+     `session/context.py`, `vault/store.py`) — reuse INV-27's
+     `_transitive_first_party_imports`; forbid re-export through `generate/proof_chain`; the
+     scan is **directional** (spine ↛ frame_verdict; frame_verdict → entail / epistemic_basis
+     is sanctioned).
+   - **Part B (data-flow).** B1: feeding a `ClosedFrame`-derived provenance into `determine()`
+     raises/refuses. B2: the open-world render/disposition path type-rejects a `FrameVerdict`
+     absent an explicit closed-world tag.
+   - **Six non-vacuity anchors** (§8 table) — each fails loudly under its own violation,
+     mirroring INV-30b / INV-30c.
+
+## Acceptance gates (ADR §12 — the obligations PR-1 discharges)
+
+- §12.1 — each of the 6 INV-31 anchors **meaningfully fails** (a fixture-injected violation
+  per scan); not one fixture covering all.
+- §12.2 — `entailed_false` is proof-backed: a mutation test constructs
+  `FrameVerdict(ENTAILED_FALSE, …)` with a mismatched/None proof (incl. a **generic
+  FALSIFIED with `positive_refutation_kind=None`**) → asserts `ValueError`.
+- §12.3 — inconsistent premises → `contradiction`, **never** `entailed_false`.
+- §12.4 — absence never yields `entailed_false`: a text `UNKNOWN` query → `undetermined`;
+  `OPEN` / unlicensed frame → `scope_boundary`.
+- §12.5 — replay determinism: identical `trace_hash` across two runs **and** under premise
+  reordering.
+- INV-30 still exactly its pinned `Determined` count, all `answer=True`; ProofWriter-OWA
+  floor green; full smoke green.
+
+## Do NOT
+
+- touch `generate/determine/determine.py` except the one-line `_basis` import.
+- add any `answer=False` / `Determined` site anywhere.
+- wire `evaluate_frame_verdict` into `chat/runtime`, the session loop, governance, or the vault.
+- build a CWA lane (PR-2), a perception adapter (PR-3), or a disposition/renderer mapping (PR-4).
+- implement prose→proposition lowering or NAF.
+
+## Budget / verify
+
+~45–60 tool calls. **Run in the worktree with `python -m pytest`** (the `core` script
+resolves to the main checkout). Verify: the new `frame_verdict` tests +
+`tests/test_architectural_invariants.py` (INV-30 **and** INV-31) +
+`tests/test_proofwriter_owa_lane.py` + `tests/test_determine_*` (the `_basis` lift is inert)
++ full smoke.
+
+## Honest flags for the operator
+
+- **B2 anchor scoping.** The open-world disclosure path isn't wired to take a `FrameVerdict`
+  until PR-4. Install B2 as a **pre-emptive defensive guard** (the render/disposition entry
+  type-rejects a `FrameVerdict`) and make its anchor non-vacuous against that guard. **If B2
+  cannot be made meaningfully-failing without PR-4's wiring, do NOT ship a vacuous anchor** —
+  flag it, scope B2 to PR-4, and add a one-line ADR-0222 §8 amendment noting the deferral.
+  A1/A2/A3/B1 are the firm PR-1 firewall.
+- **Propositional-input contract.** PR-1's isolated evaluator consumes propositional-formula
+  strings; document this on the evaluator so PR-2's prose front-end has a clean seam.
+- **EpistemicState / governance is out of scope** — the `FrameVerdict` carries `basis` only;
+  the `INFERRED` / `DisclosureClaim.NONE` mapping (§7 / §14) is PR-4.
+
+## Ratified design choices this brief locks in
+
+- `_basis` lift **first**, behavior-preserving.
+- `FrameVerdict` **distinct** from `Determined` (no `answer` field; INV-30 scan can't match it).
+- `entailed_false` **requires** `positive_refutation_kind`.
+- a **generic `FALSIFIED` cannot** prove false.
+- `OPEN` / missing closure / non-`TEXT` frame ⇒ `scope_boundary`.
+- perception frame ⇒ `scope_boundary` in PR-1.
+- **INV-31 ships with the type**, not later.
+- the **B2 data-flow anchor may be deferred** if it cannot be made non-vacuous in PR-1.
+
+---
+
+**Source of truth:** `docs/decisions/ADR-0222-frame-verdict-closed-world.md` (ratified #780).
+Where this brief and the ADR ever disagree, the ADR wins — open an amendment rather than
+diverging silently.