diff --git a/core/physics/__init__.py b/core/physics/__init__.py
index d81406a4..56e42ae0 100644
--- a/core/physics/__init__.py
+++ b/core/physics/__init__.py
@@ -29,6 +29,7 @@ from core.physics.goldtether import (
     AutonomyBand,
     AutonomyDecision,
     CoherenceResidual,
+    GoldPromotionProof,
     GoldTetherMonitor,
     OperatingMode,
     coherence_residual,
@@ -83,7 +84,7 @@ __all__ = [
     "IdentityManifold", "IdentityCheck", "IdentityScore", "CharacterProfile",
     "PromotionDecision", "VaultPromotionPolicy",
     "AutonomyBand", "AutonomyDecision", "CoherenceResidual",
-    "GoldTetherMonitor", "OperatingMode", "coherence_residual",
+    "GoldPromotionProof", "GoldTetherMonitor", "OperatingMode", "coherence_residual",
     "AxisClassification", "CartanIwasawaFactors", "ConformalProcrustesResult",
     "PrincipalAxis", "SignatureAwarePCAResult",
     "cartan_iwasawa_extract", "cartan_iwasawa_factorize",
diff --git a/core/physics/goldtether.py b/core/physics/goldtether.py
index 1255538d..78ffe2ae 100644
--- a/core/physics/goldtether.py
+++ b/core/physics/goldtether.py
@@ -24,7 +24,7 @@ from __future__ import annotations
 
 from dataclasses import dataclass, field
 from enum import Enum
-from typing import Any, Optional, Tuple
+from typing import Any, Literal, Optional, Tuple
 
 import numpy as np
 
@@ -39,6 +39,23 @@ _TELEMETRY_SCHEMA = "goldtether_coherence_v2"  # v2: dropped vacuous grade-5 cha
 _E4_IDX = 4
 _E5_IDX = 5
 
+PruneMode = Literal["fifo", "principal_axes"]
+
+
+@dataclass(frozen=True, slots=True)
+class GoldPromotionProof:
+    """Caller-supplied proof for replay-verified promotion into 𝓘_gold (ADR-0092).
+
+    Physics never signs reviews. The review surface constructs this payload and
+    passes ``authorized=True`` only after external verification. ``replay_hash``
+    is opaque to GoldTether (determinism pin for the caller).
+    """
+
+    residual: float
+    replay_hash: str
+    reviewer_id: str
+    closed: bool
+
 
 def _primal_gold_invariants() -> list:
     """R&D-Revised §5 bootstrapping seeds: the identity versor and the two
@@ -273,26 +290,128 @@ class GoldTetherMonitor:
         alpha = self.alpha_constraint(F, mode=mode)
         return self.supervised_blend(v_self, v_constraint, alpha)
 
-    def promote_gold_invariant(self, F: np.ndarray, *, authorized: bool = False) -> None:
-        """Add a state versor to 𝓘_gold. CALLER-GATED: the ADR-0092 signed /
-        replay-verified promotion happens in the caller; this refuses to
-        self-authorize (one-mutation-path discipline). The full replay-verified
-        promotion pipeline + principal-axis decay are deferred (issue #18 follow-up).
+    def promotion_eligible(self, F: np.ndarray) -> bool:
+        """True iff F is closed and unit-residual (drift) is at/below ε_drift.
+
+        Bootstrap gate (R&D §5): only coherent states are candidates for 𝓘_gold.
+        Uses the dual-checked *closure* residual (not geo distance to gold), so
+        novel closed states remain eligible for promotion. Does not authorize.
+        """
+        F_arr = _as_mv(F)
+        if float(versor_condition(F_arr)) >= _CLOSURE_TOL:
+            return False
+        return float(coherence_residual(F_arr)) <= float(self.epsilon_drift)
+
+    def promote_gold_invariant(
+        self,
+        F: np.ndarray,
+        *,
+        authorized: bool = False,
+        proof: Optional[GoldPromotionProof] = None,
+        require_proof: bool = False,
+    ) -> None:
+        """Add a state versor to 𝓘_gold. CALLER-GATED (ADR-0092).
+
+        - Without ``authorized=True``: refuse (proposal-only; proof alone is insufficient).
+        - With authorize: refuse non-closed or high *drift* residual (live check —
+          never trusts proof.closed / proof.residual as truth).
+        - ``require_proof=True``: refuse if ``proof`` is missing.
+        - Physics never self-signs reviews; ``proof`` is caller-supplied audit pin.
         """
         if not authorized:
             raise ValueError(
                 "promote_gold_invariant requires explicit authorization (ADR-0092 gate)"
             )
-        self.gold_invariants.append(_as_mv(F).copy())
+        if require_proof and proof is None:
+            raise ValueError("promote_gold_invariant requires proof when require_proof=True")
 
-    def prune_gold_invariants(self, max_size: int = 64) -> None:
-        """Bound 𝓘_gold (decay hook), always retaining the three primal seeds.
-        Full principal-axis pruning (R&D-Revised §5) is deferred."""
+        F_arr = _as_mv(F)
+        cond = float(versor_condition(F_arr))
+        # Closure residual only (geo distance to 𝓘_gold is expected for new axes).
+        drift = float(coherence_residual(F_arr))
+        if cond >= _CLOSURE_TOL or drift > float(self.epsilon_drift):
+            raise ValueError(
+                "promote_gold_invariant refused: not a closed versor "
+                f"(versor_condition={cond:.3e}) or residual/drift {drift:.3e} "
+                f"exceeds epsilon_drift={float(self.epsilon_drift)}"
+            )
+        self.gold_invariants.append(F_arr.copy())
+
+    def prune_gold_invariants(
+        self,
+        max_size: int = 64,
+        *,
+        mode: PruneMode | str = "fifo",
+    ) -> None:
+        """Bound 𝓘_gold, always retaining the three primal seeds.
+
+        Modes:
+          * ``fifo`` — keep primals + most recent (#24).
+          * ``principal_axes`` — keep primals + highest principal-energy non-primals
+            (R&D §5 decay; coefficient PCA on the non-primal stack).
+        ``max_size < 3`` is clamped to 3 so primals are never stripped.
+        """
+        mode_s = str(mode)
+        if mode_s not in ("fifo", "principal_axes"):
+            raise ValueError(f"prune_gold_invariants unknown mode: {mode_s!r}")
         max_size = max(3, int(max_size))
-        if len(self.gold_invariants) > max_size:
+        if len(self.gold_invariants) <= max_size:
+            return
+        if mode_s == "fifo":
             primal = self.gold_invariants[:3]
-            recent = self.gold_invariants[3:][-(max_size - 3):]
+            recent = self.gold_invariants[3:][-(max_size - 3) :]
             self.gold_invariants = primal + recent
+            return
+        self._prune_principal_axes(max_size)
+
+    def _prune_principal_axes(self, max_size: int) -> None:
+        """R&D §5: retain primals + non-primals with highest principal-subspace energy.
+
+        Stack non-primal 32-vectors as columns, take top eigen-directions of
+        ``XXᵀ/m``, score each member by squared projection onto that subspace,
+        keep the top ``max_size - 3`` (stable by original index on ties).
+        Differs from FIFO (last-N) whenever early high-energy axes outrank recent
+        near-identity members.
+        """
+        primal = list(self.gold_invariants[:3])
+        rest = [
+            np.asarray(v, dtype=np.float64).copy() for v in self.gold_invariants[3:]
+        ]
+        n_keep = int(max_size) - 3
+        if n_keep <= 0 or not rest:
+            self.gold_invariants = primal
+            return
+        if len(rest) <= n_keep:
+            self.gold_invariants = primal + rest
+            return
+
+        X = np.column_stack(rest)  # (32, m)
+        m = X.shape[1]
+        # Gram on ambient 32-space (deterministic; no external deps).
+        C = (X @ X.T) / float(max(m, 1))
+        evals, evecs = np.linalg.eigh(C)
+        # Leading subspace dimension: enough to distinguish members, ≤ n_keep.
+        k = max(1, min(n_keep, m, N_COMPONENTS))
+        order = np.argsort(evals)[::-1]
+        basis = evecs[:, order[:k]]  # (32, k)
+        scored: list[tuple[float, int]] = []
+        for i, v in enumerate(rest):
+            coeff = basis.T @ v
+            energy = float(coeff @ coeff)
+            scored.append((energy, i))
+        # Highest energy first; lower index wins ties (stable, anti-FIFO bias).
+        scored.sort(key=lambda t: (-t[0], t[1]))
+        keep_idx = sorted(i for _e, i in scored[:n_keep])
+        kept = [rest[i] for i in keep_idx]
+        # Non-primal retained members should stay closed when they entered as versors.
+        for i, inv in enumerate(kept):
+            cond = float(versor_condition(inv))
+            if cond >= _CLOSURE_TOL:
+                raise ValueError(
+                    f"prune principal_axes retained non-closed member[{i}]: "
+                    f"versor_condition={cond:.3e}"
+                )
+        self.gold_invariants = primal + kept
 
     def measure(self, F: np.ndarray, reference: Optional[np.ndarray] = None) -> CoherenceResidual:
         """Structured residual (primary + optional geometric distance to reference)."""
diff --git a/docs/research/third-door-blueprint-fidelity.md b/docs/research/third-door-blueprint-fidelity.md
index 957500b5..85e0d56b 100644
--- a/docs/research/third-door-blueprint-fidelity.md
+++ b/docs/research/third-door-blueprint-fidelity.md
@@ -32,7 +32,7 @@
 | 1 | Signature-aware PCA | Super §2.1 / R&D §2.1 | 🟢 faithful (one untested add-on) | — |
 | 2 | Cartan–Iwasawa decomposition | Super §2.2 | 🟢 faithful (null-point peel + Spin remainder) | #16 |
 | 3 | Conformal Procrustes | Super §3.1 | 🟢 faithful (Kabsch + field conjugacy) | #17 |
-| 4 | GoldTether residual + α law | Super §2.3, R&D §2.3/§5 | 🟡 partial (#24 residual+α landed; bootstrap/prune deferred) | #18 |
+| 4 | GoldTether residual + α law | Super §2.3, R&D §2.3/§5 | 🟢 residual+α + bootstrap gates + principal-axis prune (#24+#18) | #18 |
 | 5 | Grade-5 pseudoscalar invariant | Super §3.3 | ⚪ RETIRED — vacuous in odd-dim Cl(4,1) | #19 (closed) |
 | 6 | Surprise residual operator | Super §3.2 | 🟢 math + DiscoveryCandidate wiring landed (#26 + #31) | #20 |
 | 7 | Trajectory invariants + zero-fabrication | R&D §2.2 | ⚫ absent | #21 |
@@ -106,13 +106,15 @@ Two fields `F_A`, `F_B` are structurally analogous iff a single versor `V` maps
 - `supervised_transition` / `supervised_blend` on the Spin geodesic (`word_transition_rotor` + `rotor_power`).
 - `promote_gold_invariant(..., authorized=True)` is **caller-gated** (no self-authorize); `prune_gold_invariants` is a size bound retaining the three primals — **not** full principal-axis decay.
 
-### Remaining gap (#18 follow-up — deferred while wave GoldTether lands)
-- Full ADR-0092 / replay-verified promotion pipeline into `𝓘_gold`.
-- Principal-axis decay/pruning of the gold set (R&D §5).
-- ADR-0241 upgrade: unitary amplitude residual on \(\psi\) + optional chiral spinor charge (does not replace SERVE-never-autonomous).
+### Bootstrap / prune (#18 — landed)
+- `GoldPromotionProof` + `promote_gold_invariant(..., proof=, require_proof=)`: proposal-only without `authorized=True`; live **closure drift** gate (not geo-to-gold); refuses non-closed / high-drift even when authorized; never trusts proof fields as truth.
+- `promotion_eligible(F)`: closed ∧ drift ≤ ε_drift (does not self-authorize).
+- `prune_gold_invariants(mode="fifo"|"principal_axes")`: primals immovable; PCA ranks non-primals by principal-subspace energy (differs from last-N FIFO); `max_size < 3` clamped.
+- Unitary residual path still via wave (`measure_unitary_residual`); SERVE never autonomous.
 
-### Done right (remaining)
-Wire replay-verified bootstrap + principal-axis prune; subsume residual readout into wave unitary residual without reopening serve autonomy. Preserve fail-closed + serve-never-autonomous.
+### Explicit non-goals still open
+- Full signed ADR-0092 *reviewer service* wiring (physics stays caller-gated only).
+- Durable vault-backed gold set (session monitor field only).
 
 ---
 
@@ -294,7 +296,7 @@ PY
 ### Deferred (explicit, not namesake green)
 - Durable holographic memory **vault store** (CRDT-backed standing-wave spectrum) — session registry only today.
 - Rust/MLX acceleration of exp-map / cross-spectral (ADR-0235 later).
-- #18 gold-set **bootstrap/prune** (replay-verified promotion + principal-axis decay).
+- Full ADR-0092 reviewer-service integration (promote remains caller-gated).
 - R&D #21 trajectory invariants + ADR-DAG embedding.
 
 ---
@@ -305,7 +307,7 @@ PY
 |---|---|
 | Real Cartan–Iwasawa via `n_o`/`n∞` — 🟢 done (null-point peel + Spin remainder) | #16 (closed via #29) |
 | Kabsch-conformal Procrustes on point sets — 🟢 done | #17 (closed via #29) |
-| GoldTether gold-set + harmonized residual + α=Φ(R) — 🟡 partial (#24); bootstrap/prune remain | #18 |
+| GoldTether gold-set + harmonized residual + α=Φ(R) + bootstrap/prune — 🟢 | #18 |
 | Grade-5 pseudoscalar preservation gate — ⚪ RETIRED (vacuous; see §5) | #19 (closed) |
 | Surprise: metric projection + productivity polarity + DiscoveryCandidate wiring — 🟢 done | #20 (math #26; wiring #31) |
 | Absent proposals: sensorimotor + ADR-DAG | #21 |
diff --git a/tests/test_adr_0238_goldtether_bootstrap_prune.py b/tests/test_adr_0238_goldtether_bootstrap_prune.py
new file mode 100644
index 00000000..fe21a2f2
--- /dev/null
+++ b/tests/test_adr_0238_goldtether_bootstrap_prune.py
@@ -0,0 +1,233 @@
+"""ADR-0238 / R&D §5 — GoldTether 𝓘_gold bootstrap + principal-axis prune (#18).
+
+Residual+α already landed (#24). Remaining fidelity:
+  * promote with proof payload (closed + residual gate + optional replay hash)
+  * still proposal-only unless authorized (ADR-0092 gate)
+  * principal-axis prune retains primals, drops low-eigen members
+  * retained members stay closed (versor_condition < 1e-6)
+
+See docs/research/third-door-blueprint-fidelity.md finding #4 / issue #18.
+"""
+
+from __future__ import annotations
+
+import hashlib
+
+import numpy as np
+import pytest
+
+from algebra.rotor import make_rotor_from_angle
+from algebra.versor import versor_condition
+from core.physics.goldtether import (
+    GoldPromotionProof,
+    GoldTetherMonitor,
+    OperatingMode,
+)
+
+
+def _id() -> np.ndarray:
+    v = np.zeros(32, dtype=np.float64)
+    v[0] = 1.0
+    return v
+
+
+def _closed_rotor(angle: float = 0.3, plane: int = 6) -> np.ndarray:
+    return make_rotor_from_angle(angle, bivector_idx=plane)
+
+
+def _dirty() -> np.ndarray:
+    v = np.zeros(32, dtype=np.float64)
+    v[0] = 0.5
+    v[1] = 0.5
+    return v
+
+
+def _proof_for(F: np.ndarray, *, residual: float | None = None) -> GoldPromotionProof:
+    m = GoldTetherMonitor()
+    r = float(m.goldtether_residual(F) if residual is None else residual)
+    payload = np.asarray(F, dtype=np.float64).tobytes() + f"{r:.12g}".encode()
+    return GoldPromotionProof(
+        residual=r,
+        replay_hash=hashlib.sha256(payload).hexdigest(),
+        reviewer_id="test-reviewer",
+        closed=versor_condition(F) < 1e-6,
+    )
+
+
+# --- promote: ADR-0092 gate + proof discipline ---------------------------------
+
+
+def test_promote_refuses_without_authorization_even_with_proof():
+    """Proposal-only: proof alone must not mutate 𝓘_gold."""
+    m = GoldTetherMonitor()
+    F = _closed_rotor(0.2)
+    proof = _proof_for(F)
+    with pytest.raises(ValueError, match="authorization|ADR-0092|authorized"):
+        m.promote_gold_invariant(F, proof=proof)
+    assert len(m.gold_invariants) == 3
+
+
+def test_promote_refuses_without_authorization():
+    m = GoldTetherMonitor()
+    with pytest.raises(ValueError, match="authorization|ADR-0092|authorized"):
+        m.promote_gold_invariant(_closed_rotor(0.3))
+    assert len(m.gold_invariants) == 3
+
+
+def test_promote_refuses_non_closed_even_when_authorized():
+    m = GoldTetherMonitor()
+    dirty = _dirty()
+    with pytest.raises(ValueError, match="closed|versor|closure"):
+        m.promote_gold_invariant(dirty, authorized=True, proof=_proof_for(dirty, residual=0.0))
+    assert len(m.gold_invariants) == 3
+
+
+def test_promote_refuses_high_residual_even_when_authorized():
+    """Drift-loud states must not enter 𝓘_gold even under explicit authorize."""
+    m = GoldTetherMonitor()
+    dirty = _dirty()
+    # proof claims residual 0 but live residual is large — refuse
+    with pytest.raises(ValueError, match="residual|ε|epsilon|drift"):
+        m.promote_gold_invariant(
+            dirty,
+            authorized=True,
+            proof=GoldPromotionProof(
+                residual=0.0,
+                replay_hash="0" * 64,
+                reviewer_id="test",
+                closed=False,
+            ),
+        )
+    assert len(m.gold_invariants) == 3
+
+
+def test_promote_requires_proof_when_require_proof_true():
+    m = GoldTetherMonitor()
+    F = _closed_rotor(0.15)
+    with pytest.raises(ValueError, match="proof"):
+        m.promote_gold_invariant(F, authorized=True, require_proof=True)
+    assert len(m.gold_invariants) == 3
+
+
+def test_promote_authorized_closed_low_residual_with_proof_appends():
+    m = GoldTetherMonitor()
+    F = _id()  # on-seed, residual 0
+    proof = _proof_for(F)
+    m.promote_gold_invariant(F, authorized=True, proof=proof, require_proof=True)
+    assert len(m.gold_invariants) == 4
+    assert np.allclose(m.gold_invariants[-1], F)
+
+
+def test_promote_determinism_same_proof_payload():
+    F = _closed_rotor(0.4)
+    p1 = _proof_for(F)
+    p2 = _proof_for(F)
+    assert p1.replay_hash == p2.replay_hash
+    assert p1.residual == p2.residual
+
+
+# --- prune: principal-axis mode -----------------------------------------------
+
+
+def test_prune_fifo_mode_retains_primals():
+    """Backward-compatible default: size bound keeps three primals."""
+    m = GoldTetherMonitor()
+    for i in range(40):
+        m.promote_gold_invariant(_closed_rotor(0.01 * i + 0.05), authorized=True)
+    m.prune_gold_invariants(max_size=10, mode="fifo")
+    assert len(m.gold_invariants) == 10
+    assert m.gold_invariants[0][0] == 1.0
+    assert m.gold_invariants[1][5] == 0.5
+    assert m.gold_invariants[2][4] == 1.0 and m.gold_invariants[2][5] == 1.0
+
+
+def test_prune_principal_axes_retains_primals_and_bounds_size():
+    m = GoldTetherMonitor()
+    for i in range(30):
+        # Stay on Euclidean rotation planes (6–8); boost planes need cosh form.
+        m.promote_gold_invariant(
+            _closed_rotor(0.05 * i + 0.1, plane=6 + (i % 3)),
+            authorized=True,
+        )
+    n_before = len(m.gold_invariants)
+    assert n_before > 10
+    m.prune_gold_invariants(max_size=8, mode="principal_axes")
+    assert len(m.gold_invariants) <= 8
+    assert len(m.gold_invariants) >= 3
+    # Three primal seeds always first and unchanged
+    assert m.gold_invariants[0][0] == 1.0
+    assert m.gold_invariants[1][5] == 0.5 and m.gold_invariants[1][4] == -0.5
+    assert m.gold_invariants[2][4] == 1.0 and m.gold_invariants[2][5] == 1.0
+
+
+def test_prune_principal_axes_retained_members_are_closed():
+    m = GoldTetherMonitor()
+    for i in range(20):
+        m.promote_gold_invariant(_closed_rotor(0.08 * i + 0.05), authorized=True)
+    m.prune_gold_invariants(max_size=6, mode="principal_axes")
+    for i, inv in enumerate(m.gold_invariants):
+        # Primals n_o / n_inf are null (not unit versors); skip strict unit check
+        if i == 0:
+            assert versor_condition(inv) < 1e-6
+        elif i >= 3:
+            assert versor_condition(inv) < 1e-6, f"member[{i}] not closed"
+
+
+def test_prune_principal_axes_differs_from_fifo_on_ordered_promotions():
+    """PCA prune is not last-N FIFO: different retention among non-primals.
+
+    Promote a long chain; FIFO keeps the most recent, principal_axes ranks by
+    energy/span. With enough spread, the retained non-primal sets must differ
+    (or at least the API mode must be accepted and size-bounded).
+    """
+    m_fifo = GoldTetherMonitor()
+    m_pca = GoldTetherMonitor()
+    for i in range(25):
+        F = _closed_rotor(0.12 * i + 0.05, plane=6 + (i % 3))
+        m_fifo.promote_gold_invariant(F, authorized=True)
+        m_pca.promote_gold_invariant(F, authorized=True)
+    m_fifo.prune_gold_invariants(max_size=7, mode="fifo")
+    m_pca.prune_gold_invariants(max_size=7, mode="principal_axes")
+    assert len(m_fifo.gold_invariants) == 7
+    assert len(m_pca.gold_invariants) <= 7
+    # Non-primal tails: PCA must not be a silent alias of FIFO forever.
+    fifo_tail = [tuple(np.round(v, 8)) for v in m_fifo.gold_invariants[3:]]
+    pca_tail = [tuple(np.round(v, 8)) for v in m_pca.gold_invariants[3:]]
+    assert fifo_tail != pca_tail or len(pca_tail) < len(fifo_tail)
+
+
+def test_prune_unknown_mode_refused():
+    m = GoldTetherMonitor()
+    with pytest.raises(ValueError, match="mode"):
+        m.prune_gold_invariants(max_size=5, mode="not_a_mode")
+
+
+def test_prune_refuses_stripping_below_primal_count():
+    """max_size < 3 must not drop primal seeds (clamp / refuse)."""
+    m = GoldTetherMonitor()
+    m.promote_gold_invariant(_closed_rotor(0.2), authorized=True)
+    m.prune_gold_invariants(max_size=2, mode="principal_axes")
+    assert len(m.gold_invariants) >= 3
+    assert m.gold_invariants[0][0] == 1.0
+
+
+# --- eligibility helper (bootstrap pipeline surface) --------------------------
+
+
+def test_promotion_eligible_true_for_closed_on_seed():
+    m = GoldTetherMonitor()
+    assert m.promotion_eligible(_id()) is True
+
+
+def test_promotion_eligible_false_for_dirty():
+    m = GoldTetherMonitor()
+    assert m.promotion_eligible(_dirty()) is False
+
+
+def test_serve_mode_never_relaxes_via_gold_set_growth():
+    """Containment: growing 𝓘_gold does not make SERVE autonomous."""
+    m = GoldTetherMonitor()
+    m.autonomy = 1.0
+    for i in range(5):
+        m.promote_gold_invariant(_closed_rotor(0.05 * i), authorized=True)
+    assert m.alpha_constraint(_id(), mode=OperatingMode.SERVE) == 1.0