diff --git a/proofs/agda/All.agda b/proofs/agda/All.agda
index 0cc243a..d0ee553 100644
--- a/proofs/agda/All.agda
+++ b/proofs/agda/All.agda
@@ -130,6 +130,7 @@ open import Ordinal.Brouwer.VeblenBinary    -- binary Veblen φ_α(β) + the dia
 open import Ordinal.Brouwer.VeblenBinaryNormal -- every φ_α a normal function; φ_{α+1} enumerates fixed points of φ_α
 open import Ordinal.Brouwer.VeblenBinaryMono -- first-arg monotonicity; Γ₀ ≤′ φ_Γ₀(0) (diagonal pre-fixed point)
 open import Ordinal.Brouwer.VeblenBinaryLeast -- nextFix is the LEAST pre-fixed point; reverse-Γ₀ reduced to one closure
+open import Ordinal.Brouwer.VeblenBinaryMonoG -- the engine is monotone in its iterated function (deriv/nextFix mono in g)
 open import Ordinal.Brouwer.StrictLeftMonoRefuted
 open import Ordinal.Brouwer.AdditivePrincipalGenericRefuted
 open import Ordinal.Buchholz.Syntax
diff --git a/proofs/agda/Ordinal/Brouwer/VeblenBinaryMonoG.agda b/proofs/agda/Ordinal/Brouwer/VeblenBinaryMonoG.agda
new file mode 100644
index 0000000..47f693d
--- /dev/null
+++ b/proofs/agda/Ordinal/Brouwer/VeblenBinaryMonoG.agda
@@ -0,0 +1,114 @@
+{-# 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>
+
+-- Binary Veblen — RUNG 8: the generic fixed-point engine is MONOTONE IN
+-- ITS ITERATED FUNCTION.  Target-side climb toward ψ₀(Ω_ω) (BH order-type
+-- fidelity, open problem D-2026-06-14).  Builds on `VeblenBinary` (the
+-- engine) and `VeblenBinaryNormal` (`nextFix-mono` — monotone in the
+-- base).  2026-06-21.
+--
+-- ## What this slice adds
+--
+-- The engine's monotonicity was known in the BASE argument (`nextFix-mono`,
+-- `deriv-mono`).  The missing axis is monotonicity in the iterated FUNCTION
+-- `g` itself: a pointwise-smaller (continuous) function has a pointwise-
+-- smaller fixed-point enumeration.
+--
+--   * `g-tower-mono-in-g` — `g ≤ h` pointwise (and `h` monotone) ⇒ the
+--     iteration towers are ordered at every index.
+--   * `nextFix-mono-in-g` — hence `nextFix g x ≤′ nextFix h x`.
+--   * `deriv-mono-in-g`   — hence `deriv g β ≤′ deriv h β` for every β
+--     (osuc case also uses `nextFix-mono` in the base).
+--
+-- This is the engine-side tool that *general first-argument monotonicity*
+-- of `φ` (`a ≤′ b ⇒ φ_a x ≤′ φ_b x`) and the Γ₀ diagonal-closure
+-- (`commonStep (n ↦ φ_{Γ-tower n}) Γ₀ ≤′ Γ₀`, the obligation
+-- `VeblenBinaryLeast.Γ₀-fixed-from-closure` reduces the reverse Γ₀ fixed
+-- point to) are built from: when one level's defining function dominates
+-- another's, their `deriv` enumerations are ordered.
+--
+-- ## Honest scope (the Γ₀ fixed point is NOT closed here)
+--
+-- These are unconditional engine lemmas.  They do NOT by themselves close
+-- `φ_Γ₀(0) ≤′ Γ₀`.  That closure is a COUPLED CLUSTER — it additionally
+-- needs (i) level-inflationarity `α <′ φ_α(0)`, (ii) strict Γ-tower
+-- monotonicity (from i), (iii) the general level-fixed-point
+-- `a <′ b ⇒ φ_a(φ_b(y)) ≤′ φ_b(y)` (a value of a higher level is a fixed
+-- point of every lower level), and (iv) general first-argument
+-- monotonicity — which are mutually entangled and have delicate
+-- degenerate-`olim` cases under the recursive `_≤′_`.  This slice supplies
+-- the engine-monotonicity axis (iv's engine half); the rest remains the
+-- open frontier.  Order-type fidelity ψ₀(Ω_ω) REMAINS OPEN
+-- (D-2026-06-14).  No postulate is closed.  bi-`≤′` throughout.
+
+module Ordinal.Brouwer.VeblenBinaryMonoG where
+
+open import Data.Nat.Base using (ℕ; zero; suc)
+
+open import Ordinal.Brouwer using (Ord; oz; osuc; olim)
+open import Ordinal.Brouwer.Phase13
+  using (_≤′_; ≤′-refl; ≤′-trans; ≤′-lim; f-in-lim′)
+open import Ordinal.Brouwer.VeblenBinary using (g-tower; nextFix; deriv)
+open import Ordinal.Brouwer.VeblenBinaryNormal using (nextFix-mono)
+
+----------------------------------------------------------------------
+-- The iteration tower is monotone in the iterated function.
+--
+-- With `g y ≤′ h y` everywhere and `h` monotone, each tower step keeps
+-- the order: `g-tower g x (suc n) = g (g-tower g x n) ≤′ h (g-tower g x n)`
+-- [pointwise] `≤′ h (g-tower h x n) = g-tower h x (suc n)` [h monotone on
+-- the IH].
+----------------------------------------------------------------------
+
+g-tower-mono-in-g :
+  (g h : Ord → Ord)
+  (g≤h : ∀ y → g y ≤′ h y)
+  (h-mono : ∀ {a b} → a ≤′ b → h a ≤′ h b)
+  (x : Ord) → ∀ n → g-tower g x n ≤′ g-tower h x n
+g-tower-mono-in-g g h g≤h h-mono x zero    = ≤′-refl {x}
+g-tower-mono-in-g g h g≤h h-mono x (suc n) =
+  ≤′-trans {g (g-tower g x n)} {h (g-tower g x n)} {h (g-tower h x n)}
+    (g≤h (g-tower g x n))
+    (h-mono {g-tower g x n} {g-tower h x n} (g-tower-mono-in-g g h g≤h h-mono x n))
+
+----------------------------------------------------------------------
+-- nextFix is monotone in the iterated function.
+--
+-- `nextFix g x = olim (g-tower g (osuc x))`; `olim T ≤′ olim S` unfolds to
+-- `∀ n → T n ≤′ olim S`, discharged per index by the tower order above
+-- routed through `≤′-lim`.
+----------------------------------------------------------------------
+
+nextFix-mono-in-g :
+  (g h : Ord → Ord)
+  (g≤h : ∀ y → g y ≤′ h y)
+  (h-mono : ∀ {a b} → a ≤′ b → h a ≤′ h b)
+  (x : Ord) → nextFix g x ≤′ nextFix h x
+nextFix-mono-in-g g h g≤h h-mono x = λ n →
+  ≤′-lim {g-tower g (osuc x) n} (g-tower h (osuc x)) n
+    (g-tower-mono-in-g g h g≤h h-mono (osuc x) n)
+
+----------------------------------------------------------------------
+-- deriv is monotone in the iterated function.
+--
+-- Structural recursion on β.  The `osuc` case composes monotonicity in
+-- the function (`nextFix-mono-in-g` at the smaller base `deriv g β`) with
+-- monotonicity in the base (`nextFix-mono h` along the IH); the `olim`
+-- case is branchwise through `≤′-lim`.
+----------------------------------------------------------------------
+
+deriv-mono-in-g :
+  (g h : Ord → Ord)
+  (g≤h : ∀ y → g y ≤′ h y)
+  (h-mono : ∀ {a b} → a ≤′ b → h a ≤′ h b)
+  (β : Ord) → deriv g β ≤′ deriv h β
+deriv-mono-in-g g h g≤h h-mono oz       = nextFix-mono-in-g g h g≤h h-mono oz
+deriv-mono-in-g g h g≤h h-mono (osuc β) =
+  ≤′-trans {nextFix g (deriv g β)} {nextFix h (deriv g β)} {nextFix h (deriv h β)}
+    (nextFix-mono-in-g g h g≤h h-mono (deriv g β))
+    (nextFix-mono h h-mono {deriv g β} {deriv h β}
+      (deriv-mono-in-g g h g≤h h-mono β))
+deriv-mono-in-g g h g≤h h-mono (olim k) = λ n →
+  ≤′-lim {deriv g (k n)} (λ m → deriv h (k m)) n
+    (deriv-mono-in-g g h g≤h h-mono (k n))
diff --git a/proofs/agda/Smoke.agda b/proofs/agda/Smoke.agda
index b35ff88..107e4b8 100644
--- a/proofs/agda/Smoke.agda
+++ b/proofs/agda/Smoke.agda
@@ -1404,6 +1404,19 @@ open import Ordinal.Brouwer.VeblenBinaryLeast using
   ; Γ₀-fixed-from-closure            -- reverse-Γ₀ reduced: closure ⇒ φ_Γ₀(0) ≤′ Γ₀
   )
 
+-- VeblenBinaryMonoG (rung 8) — the generic fixed-point engine is monotone
+-- in its ITERATED FUNCTION (not just its base): a pointwise-smaller
+-- continuous g has a pointwise-smaller deriv/nextFix enumeration.  This is
+-- the engine-side tool that general first-argument monotonicity of φ and
+-- the Γ₀ diagonal-closure are built from.  The full Γ₀ fixed point remains
+-- OPEN (gated on the general level-fixed-point); order-type fidelity
+-- ψ₀(Ω_ω) REMAINS OPEN (D-2026-06-14).
+open import Ordinal.Brouwer.VeblenBinaryMonoG using
+  ( g-tower-mono-in-g                -- towers ordered when g ≤ h pointwise (h monotone)
+  ; nextFix-mono-in-g               -- nextFix g x ≤′ nextFix h x
+  ; deriv-mono-in-g                 -- deriv g β ≤′ deriv h β
+  )
+
 -- 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