diff --git a/proofs/agda/All.agda b/proofs/agda/All.agda
index 683ebf0..9d0dacc 100644
--- a/proofs/agda/All.agda
+++ b/proofs/agda/All.agda
@@ -121,6 +121,7 @@ open import Ordinal.Brouwer.Phase13
 open import Ordinal.Brouwer.OmegaPow
 open import Ordinal.Brouwer.OrdinalExp
 open import Ordinal.Brouwer.VeblenPhi
+open import Ordinal.Brouwer.VeblenPhiNormal -- φ₁ a normal function; next-ε β LEAST ε-number above β
 open import Ordinal.Brouwer.StrictLeftMonoRefuted
 open import Ordinal.Brouwer.AdditivePrincipalGenericRefuted
 open import Ordinal.Buchholz.Syntax
diff --git a/proofs/agda/Ordinal/Brouwer/VeblenPhiNormal.agda b/proofs/agda/Ordinal/Brouwer/VeblenPhiNormal.agda
new file mode 100644
index 0000000..1c0705a
--- /dev/null
+++ b/proofs/agda/Ordinal/Brouwer/VeblenPhiNormal.agda
@@ -0,0 +1,161 @@
+{-# OPTIONS --safe --without-K #-}
+-- SPDX-License-Identifier: MPL-2.0
+-- SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+
+-- Veblen φ-hierarchy over Brouwer ordinals — RUNG 3 (slice 3): φ₁ is a
+-- NORMAL FUNCTION, and `next-ε β` is the LEAST ε-number above β.  Target-
+-- side climb toward ψ₀(Ω_ω) (BH order-type fidelity, open problem
+-- D-2026-06-14).  Builds directly on `VeblenPhi` (φ₁ / next-ε defined and
+-- proved ε-valued) and `OrdinalExp` (ω^^).  2026-06-19.
+--
+-- ## What this slice adds
+--
+-- The previous slice (`VeblenPhi`) proved that every value of φ₁ IS an
+-- ε-number (`φ₁-ε-number`) and that `next-ε β` is AN ε-number strictly
+-- above β.  This slice upgrades that to the two defining properties of an
+-- ε-number ENUMERATION:
+--
+--   * `next-ε-least` — `next-ε β` is the LEAST ω^^-closed ordinal strictly
+--     above β.  This is what makes `next-ε` the genuine "next ε-number"
+--     operator (not merely *an* ε-number above β).  Definitional, because
+--     `olim f ≤′ γ` reduces to `∀ n → f n ≤′ γ` (olim is the least upper
+--     bound), so the proof is induction on the ω^^-tower index using
+--     ω^^-monotonicity + the ω^^-closure hypothesis on γ.
+--   * `φ₁` is a NORMAL FUNCTION:
+--       - `φ₁-mono`        — monotone               (α ≤′ β → φ₁ α ≤′ φ₁ β)
+--       - `φ₁-strict-mono` — strictly monotone      (α <′ β → φ₁ α <′ φ₁ β)
+--       - `φ₁-continuous`  — continuous at limits    (definitional, `refl`)
+--     A normal function is precisely a strictly-monotone, continuous
+--     ordinal function; φ₁ is now mechanically one.
+--
+-- The one prerequisite the previous slices left open is `ω^^-mono-≤′`
+-- (monotonicity of ω-exponentiation), proved here by structural recursion
+-- on the `_≤′_` shape; it is a pure ω^^ fact and a natural candidate to
+-- migrate into `OrdinalExp` when the binary Veblen rung needs it.
+--
+-- ## Honest scope (still rung 3 of a LONG climb — do not overclaim)
+--
+-- φ₁ being a normal function is the standard precondition for taking its
+-- fixed points (the next Veblen level) and ultimately the binary φ_α and
+-- its diagonal → Γ₀.  It does NOT reach Γ₀, and ψ₀(Ω_ω) sits far above
+-- even Γ₀ and additionally needs the ordinal-collapsing layer.  Order-type
+-- fidelity (ψ₀(Ω_ω)) REMAINS OPEN (D-2026-06-14); this slice neither
+-- reaches Γ₀ nor plugs `Fidelity.AtHeight`.  bi-`≤′` (not `≡`) is used for
+-- the fixed-point facts because Brouwer `olim`s of different ℕ-indexings of
+-- one supremum are not definitionally equal; the monotonicity results are
+-- single-`≤′` / single-`<′` and exact.
+
+module Ordinal.Brouwer.VeblenPhiNormal where
+
+open import Data.Nat.Base using (ℕ; zero; suc)
+open import Data.Product.Base using (Σ; _,_)
+open import Data.Empty using (⊥-elim)
+open import Relation.Binary.PropositionalEquality using (_≡_; refl)
+
+open import Ordinal.Brouwer using (Ord; oz; osuc; olim)
+open import Ordinal.Brouwer.Phase13
+  using (_≤′_; _<′_; ≤′-refl; ≤′-trans; ≤′-lim; ≤′-self-osuc)
+open import Ordinal.Brouwer.OmegaPow using (_·ℕ_; ·ℕ-mono-≤-left)
+open import Ordinal.Brouwer.OrdinalExp using (ω^^_; ε₀; ω^^-pos)
+open import Ordinal.Brouwer.VeblenPhi
+  using (tower-from; next-ε; φ₁; ω^^-next-ε-≤; β<next-ε)
+
+----------------------------------------------------------------------
+-- ω-exponentiation is monotone (the one missing prerequisite).
+--
+-- `α ≤′ β → ω^^ α ≤′ ω^^ β`, by structural recursion on the `_≤′_` shape
+-- of (α, β) — the same descent pattern as `Phase13.≤′-trans`.  Uses only
+-- ω^^-positivity (the `oz` base), `·ℕ`-left-monotonicity (the
+-- successor/successor case, where `ω^^ (osuc α) = olim (n ↦ ω^^ α ·ℕ n)`),
+-- and the olim least-upper-bound `≤′-lim`.
+----------------------------------------------------------------------
+
+ω^^-mono-≤′ : ∀ {α β} → α ≤′ β → ω^^ α ≤′ ω^^ β
+ω^^-mono-≤′ {oz}     {β}      _       = ω^^-pos β
+ω^^-mono-≤′ {osuc α} {oz}     p       = ⊥-elim p
+ω^^-mono-≤′ {osuc α} {osuc β} p       =
+  λ n → ≤′-lim {(ω^^ α) ·ℕ n} (λ m → (ω^^ β) ·ℕ m) n
+          (·ℕ-mono-≤-left {ω^^ α} {ω^^ β} n (ω^^-mono-≤′ {α} {β} p))
+ω^^-mono-≤′ {osuc α} {olim f} (n , q) =
+  ≤′-lim {ω^^ (osuc α)} (λ k → ω^^ (f k)) n (ω^^-mono-≤′ {osuc α} {f n} q)
+ω^^-mono-≤′ {olim g} {β}      p       =
+  λ n → ω^^-mono-≤′ {g n} {β} (p n)
+
+----------------------------------------------------------------------
+-- `next-ε β` is the LEAST ω^^-closed ordinal strictly above β.
+--
+-- For any γ with `osuc β ≤′ γ` (β strictly below γ) and `ω^^ γ ≤′ γ`
+-- (γ is closed under ω-exponentiation — the direction of the ε-number
+-- property we need), `next-ε β ≤′ γ`.  `next-ε β = olim (tower-from
+-- (osuc β))`, and `olim … ≤′ γ` is definitionally "every tower approximant
+-- is ≤′ γ", proved by induction on the tower index n:
+--   * n = 0  : `tower-from (osuc β) 0 = osuc β ≤′ γ`           (hypothesis)
+--   * n+1    : `ω^^ (tower-from … n) ≤′ ω^^ γ ≤′ γ`            (ω^^-mono + closure)
+----------------------------------------------------------------------
+
+next-ε-least : ∀ {β γ} → osuc β ≤′ γ → ω^^ γ ≤′ γ → next-ε β ≤′ γ
+next-ε-least {β} {γ} β<γ ω^^γ≤γ = go
+  where
+  go : ∀ n → tower-from (osuc β) n ≤′ γ
+  go zero    = β<γ
+  go (suc m) =
+    ≤′-trans {ω^^ (tower-from (osuc β) m)} {ω^^ γ} {γ}
+      (ω^^-mono-≤′ {tower-from (osuc β) m} {γ} (go m)) ω^^γ≤γ
+
+-- Monotonicity of `next-ε`: a larger base gives a larger next ε-number.
+-- By `next-ε-least` with γ := next-ε β (which is ω^^-closed by
+-- `ω^^-next-ε-≤`), since `osuc α ≤′ osuc β ≤′ next-ε β`.
+next-ε-mono : ∀ {α β} → α ≤′ β → next-ε α ≤′ next-ε β
+next-ε-mono {α} {β} p =
+  next-ε-least {α} {next-ε β}
+    (≤′-trans {osuc α} {osuc β} {next-ε β} p (β<next-ε β))
+    (ω^^-next-ε-≤ β)
+
+----------------------------------------------------------------------
+-- ε₀ is the least value of φ₁ (φ₁ oz), hence ≤′ every φ₁ value.
+-- The `oz` base case of monotonicity; recursion on β alone.
+----------------------------------------------------------------------
+
+ε₀-least : ∀ β → ε₀ ≤′ φ₁ β
+ε₀-least oz       = ≤′-refl {ε₀}
+ε₀-least (osuc β) =
+  ≤′-trans {ε₀} {φ₁ β} {next-ε (φ₁ β)}
+    (ε₀-least β)
+    (≤′-trans {φ₁ β} {osuc (φ₁ β)} {next-ε (φ₁ β)}
+      (≤′-self-osuc (φ₁ β)) (β<next-ε (φ₁ β)))
+ε₀-least (olim f) = ≤′-lim {ε₀} (λ n → φ₁ (f n)) 0 (ε₀-least (f 0))
+
+----------------------------------------------------------------------
+-- φ₁ is a NORMAL FUNCTION.
+----------------------------------------------------------------------
+
+-- Monotone.  Structural recursion on the `_≤′_` shape of (α, β):
+--   * oz base reduces to `ε₀-least`;
+--   * successor/successor is `next-ε-mono` of the inductive step;
+--   * the two olim cases thread through `≤′-lim` / the pointwise olim rule.
+φ₁-mono : ∀ {α β} → α ≤′ β → φ₁ α ≤′ φ₁ β
+φ₁-mono {oz}     {β}      _       = ε₀-least β
+φ₁-mono {osuc α} {oz}     p       = ⊥-elim p
+φ₁-mono {osuc α} {osuc β} p       = next-ε-mono {φ₁ α} {φ₁ β} (φ₁-mono {α} {β} p)
+φ₁-mono {osuc α} {olim f} (n , q) =
+  ≤′-lim {φ₁ (osuc α)} (λ k → φ₁ (f k)) n (φ₁-mono {osuc α} {f n} q)
+φ₁-mono {olim g} {β}      p       = λ n → φ₁-mono {g n} {β} (p n)
+
+-- Strictly increasing at successors: φ₁ α <′ φ₁ (osuc α).  Immediate,
+-- since `φ₁ (osuc α) = next-ε (φ₁ α)` and `β<next-ε` places `φ₁ α` strictly
+-- below `next-ε (φ₁ α)`.
+φ₁-lt-succ : ∀ α → φ₁ α <′ φ₁ (osuc α)
+φ₁-lt-succ α = β<next-ε (φ₁ α)
+
+-- Strictly monotone.  `α <′ β` is `osuc α ≤′ β`; chain the successor jump
+-- `φ₁ α <′ φ₁ (osuc α)` with `φ₁ (osuc α) ≤′ φ₁ β` (monotonicity).
+φ₁-strict-mono : ∀ {α β} → α <′ β → φ₁ α <′ φ₁ β
+φ₁-strict-mono {α} {β} α<β =
+  ≤′-trans {osuc (φ₁ α)} {φ₁ (osuc α)} {φ₁ β}
+    (φ₁-lt-succ α) (φ₁-mono {osuc α} {β} α<β)
+
+-- Continuous at limits.  φ₁ commutes with `olim` definitionally
+-- (the `olim` clause of φ₁), so continuity is `refl` — exactly the
+-- supremum-preservation a normal function requires.
+φ₁-continuous : ∀ f → φ₁ (olim f) ≡ olim (λ n → φ₁ (f n))
+φ₁-continuous f = refl
diff --git a/proofs/agda/Smoke.agda b/proofs/agda/Smoke.agda
index 4650d83..4ad28c3 100644
--- a/proofs/agda/Smoke.agda
+++ b/proofs/agda/Smoke.agda
@@ -1241,6 +1241,26 @@ open import Ordinal.Brouwer.VeblenPhi using
   ; φ₁-ε-number
   )
 
+-- Veblen φ-hierarchy slice 3 (2026-06-19, own block per CLAUDE.md
+-- Working rules): φ₁ is a NORMAL FUNCTION and `next-ε β` is the LEAST
+-- ω^^-closed ordinal above β (slice 2 only proved it was AN ε-number
+-- above β).  `ω^^-mono-≤′` is the missing monotonicity prerequisite;
+-- `next-ε-least` is the headline; `φ₁-strict-mono` + `φ₁-continuous`
+-- make φ₁ mechanically a normal function — the standard precondition for
+-- the next Veblen level / binary φ_α / Γ₀.  Order-type fidelity ψ₀(Ω_ω)
+-- REMAINS OPEN (D-2026-06-14); this neither reaches Γ₀ nor plugs
+-- `Fidelity.AtHeight`.
+open import Ordinal.Brouwer.VeblenPhiNormal using
+  ( ω^^-mono-≤′                     -- ω-exponentiation is monotone (prerequisite)
+  ; next-ε-least                    -- next-ε β is the LEAST ω^^-closed ordinal above β
+  ; next-ε-mono                     -- next-ε is monotone
+  ; ε₀-least                        -- ε₀ = φ₁ oz is the least φ₁ value
+  ; φ₁-mono                         -- φ₁ monotone
+  ; φ₁-lt-succ                      -- φ₁ strictly increasing at successors
+  ; φ₁-strict-mono                  -- φ₁ strictly monotone (normal-function half 1)
+  ; φ₁-continuous                   -- φ₁ continuous at limits (normal-function half 2)
+  )
+
 -- Recommended rank function for unbudgeted `wf-<ᵇʳᶠ_` per Echidna's
 -- design search; transport theorem deferred until Phase 1.3 lemmas land.
 open import Ordinal.Buchholz.RankBrouwer using