diff --git a/Complexity.lean b/Complexity.lean
index 82d05da562..ab821ce0d5 100644
--- a/Complexity.lean
+++ b/Complexity.lean
@@ -1268,6 +1268,7 @@ public import Complexity.Computability.Complexity.Classes.CountingSimulation.Int
 public import Complexity.Computability.Complexity.Classes.CountingSimulation.Promise
 public import Complexity.Computability.Complexity.Classes.CountingSimulation.Separation
 public import Complexity.Computability.Complexity.Classes.CountingSimulation.TruncatedEntry
+public import Complexity.Computability.Complexity.Classes.CountingSimulation.TruncatedSum
 public import Complexity.Computability.Complexity.Classes.DP
 public import Complexity.Computability.Complexity.Classes.EXP
 public import Complexity.Computability.Complexity.Classes.FewP
diff --git a/Complexity/Computability/Complexity/Classes/CountingSimulation/TruncatedSum.lean b/Complexity/Computability/Complexity/Classes/CountingSimulation/TruncatedSum.lean
new file mode 100644
index 0000000000..eee4014abc
--- /dev/null
+++ b/Complexity/Computability/Complexity/Classes/CountingSimulation/TruncatedSum.lean
@@ -0,0 +1,325 @@
+/-
+Copyright (c) 2026 Samuel Schlesinger. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Samuel Schlesinger
+-/
+
+module
+-- upstream: candidate
+
+public import Complexity.Computability.Complexity.Classes.CountingSimulation.Accumulate
+public import Complexity.Computability.Circuit.Frontier.TensorNetwork.TruncatedAlgorithm
+import Complexity.Computability.Machines.Turing.MultiTape.Complexity.LinearSpaceArith
+import Complexity.Foundations.Data.List.OfFn
+import Complexity.Computability.Machines.Turing.MultiTape.Complexity.PolyTime.Nat.BinaryDiv
+import Complexity.Computability.Machines.Turing.MultiTape.Complexity.PolyTime.Nat.Binary
+import Mathlib.Algebra.BigOperators.ModEq
+
+/-!
+# Summing the truncated contractions of a computed family of networks
+
+On each branch, the proof of Lemma D.2 of [OpenAI2026ETH] (pp. 56–57) contracts one network per
+degree array by Theorem D.1 at truncation order `ℓ`. It sums the contractions in
+`B = (ℤ / 2 ^ p)[V] / (V ^ ℓ)`, applies the coefficient sum `S_ℓ`, and reduces modulo `2 ^ p`.
+This file is that loop, for any computed family of networks and any algorithm with the interface
+of Theorem D.1 at truncation order `ℓ` (`Cslib.Circuits.Frontier.TruncatedContractionAlgorithm`).
+
+`S_ℓ` is additive. So the loop applies it to each contraction
+(`Cslib.Complexity.CountingSimulation.truncCoeffSum`) and adds the residues modulo the modulus,
+one network at a time (`Cslib.Complexity.CountingSimulation.truncSum`). The streamed modular sum
+`Cslib.Complexity.CountingSimulation.modSum` keeps only the index, the context and the
+accumulator. If the contractions sum, coefficientwise below `V ^ ℓ` and modulo `2 ^ P`, to given
+integers, the loop computes their sum modulo `2 ^ P`
+(`Cslib.Complexity.CountingSimulation.truncSum_modEq`). In Lemma D.2 these integers are the
+coefficients below `V ^ ℓ` of the incidence sum, whose sum is the branch count modulo `2 ^ p` by the
+congruence (D.2) (#2076).
+
+**Resources** (`Cslib.Complexity.CountingSimulation.exists_truncSum_certificate`). Suppose the
+networks are built in polynomial time from the context and the index. Suppose further that, on a
+context `c`, they are admissible at size `s c` with precision and truncation order between `1` and
+`D s c`. Then the loop takes time `(count c + 1) K (n + 1) ^ K 2 ^ (K s / √⌊log₂ s⌋)` and space
+`K (n + 1) ^ K`, for the context's encoded length `n` and a constant `K`. The constant is chosen
+before the context, and the number of networks is an explicit factor.
+
+## Main definitions
+
+* `Cslib.Complexity.CountingSimulation.truncCoeffSum`: `S_ℓ` of a truncated contraction.
+* `Cslib.Complexity.CountingSimulation.truncSum`: the sum of the `truncCoeffSum` of the networks,
+  modulo the modulus.
+
+## Main results
+
+* `Cslib.Complexity.CountingSimulation.truncSum_eq`: the loop is the sum modulo the modulus.
+* `Cslib.Complexity.CountingSimulation.truncSum_modEq`: the congruence to the sum of the summed
+  coefficients.
+* `Cslib.Complexity.CountingSimulation.exists_truncSum_certificate`: time and space.
+
+## References
+
+* [OpenAI, *The Exponential Time Hypothesis is False*][OpenAI2026ETH], Theorem D.1 (p. 53) and the
+  proof of Lemma D.2 (pp. 54–57).
+-/
+
+public section
+
+namespace Cslib.Complexity.CountingSimulation
+
+open Cslib.Circuits.Frontier Cslib.GenericRAM Turing.MultiTapeTM Finset
+
+variable {C T : Type} {encC : C ↪ List Bool} {encT : T ↪ List Bool}
+  {entry : T → List Bool → List ℕ}
+
+/-- **The coefficient sum `S_ℓ` of a truncated contraction**: the sum of the `ℓ` coefficients of
+the contraction of `G` in `(ℤ / 2 ^ P)[V] / (V ^ ℓ)`, for the input `(P, ℓ, r, G)`. -/
+@[expose] noncomputable def truncCoeffSum (entry : T → List Bool → List ℕ) :
+    ℕ × ℕ × ℕ × DescribedNetwork T → ℕ
+  | (P, ℓ, _, G) => (G.truncContraction entry P ℓ).sum
+
+/-- `S_ℓ` of a truncated contraction is the sum of the coefficients below `V ^ ℓ` of the
+contraction over `ℕ[V]`, each reduced modulo `2 ^ P`. -/
+theorem truncCoeffSum_eq (entry : T → List Bool → List ℕ) (P ℓ r : ℕ) (G : DescribedNetwork T) :
+    truncCoeffSum entry (P, ℓ, r, G) =
+      ∑ j ∈ range ℓ, (G.polyContraction entry).coeff j % 2 ^ P := by
+  rw [truncCoeffSum, DescribedNetwork.truncContraction, List.sum_map_range]
+  exact Fin.sum_univ_eq_sum_range (fun j => (G.polyContraction entry).coeff j % 2 ^ P) ℓ
+
+/-- **The loop of Lemma D.2 on one branch**: the sum, modulo `modulus c`, of the coefficient sums
+of the truncated contractions of the networks `build (c, i)` for `i < count c`, streamed one
+network at a time. -/
+@[expose] noncomputable def truncSum (entry : T → List Bool → List ℕ)
+    (build : C × ℕ → ℕ × ℕ × ℕ × DescribedNetwork T) (modulus count : C → ℕ) (c : C) : ℕ :=
+  modSum (fun q => truncCoeffSum entry (build q) % modulus q.1) modulus count c
+
+variable {build : C × ℕ → ℕ × ℕ × ℕ × DescribedNetwork T} {modulus count : C → ℕ}
+
+/-- **The loop is the sum modulo the modulus.** -/
+theorem truncSum_eq (c : C) :
+    truncSum entry build modulus count c =
+      (∑ i ∈ range (count c), truncCoeffSum entry (build (c, i))) % modulus c := by
+  rw [truncSum, modSum_eq_sum_mod]
+  change (∑ i ∈ range (count c), truncCoeffSum entry (build (c, i)) % modulus c) % modulus c = _
+  exact (Finset.sum_nat_mod _ _ _).symm
+
+/-- **The congruence of the loop.** Suppose the networks `G i` of context `c` have precision `P`
+and truncation order `ℓ`, the modulus is `2 ^ P`, and for every `j < ℓ` the `V ^ j` coefficients
+of their contractions over `ℕ[V]` sum to `q j` modulo `2 ^ P`. Then the loop is the sum of the
+`q j` modulo `2 ^ P`. -/
+theorem truncSum_modEq {c : C} {P ℓ : ℕ} (hmod : modulus c = 2 ^ P) (G : ℕ → DescribedNetwork T)
+    (hbuild : ∀ i < count c, ∃ r, build (c, i) = (P, ℓ, r, G i)) (q : ℕ → ℤ)
+    (hq : ∀ j < ℓ, ((∑ i ∈ range (count c), ((G i).polyContraction entry).coeff j : ℕ) : ℤ) ≡
+      q j [ZMOD 2 ^ P]) :
+    (truncSum entry build modulus count c : ℤ) ≡ ∑ j ∈ range ℓ, q j [ZMOD 2 ^ P] := by
+  have hterm : ∀ i ∈ range (count c), truncCoeffSum entry (build (c, i)) =
+      ∑ j ∈ range ℓ, ((G i).polyContraction entry).coeff j % 2 ^ P := by
+    intro i hi
+    obtain ⟨r, hr⟩ := hbuild i (mem_range.mp hi)
+    rw [hr, truncCoeffSum_eq]
+  rw [truncSum_eq, hmod, sum_congr rfl hterm, sum_comm]
+  push_cast
+  refine (Int.mod_modEq _ _).trans ((Int.ModEq.sum fun j _ =>
+    Int.ModEq.sum fun i _ => Int.mod_modEq _ _).trans ?_)
+  refine Int.ModEq.sum fun j hj => ?_
+  have h := hq j (mem_range.mp hj)
+  push_cast at h
+  exact h
+
+/-! ### Resources -/
+
+private theorem sum_polyTime : ComputableInPolyTime List.sum (encList encNat) encNat := by
+  fun_prop
+
+private theorem mod_polyTime :
+    ComputableInPolyTime (fun p : ℕ × ℕ => p.1 % p.2) (encPair encNat encNat) encNat := by
+  fun_prop
+
+private theorem modulusFst_polyTime (hmodulus : ComputableInPolyTime modulus encC encNat) :
+    ComputableInPolyTime (fun q : C × ℕ => modulus q.1) (encPair encC encNat) encNat :=
+  hmodulus.comp (ComputableInPolyTime.fst _ _)
+
+/-- The native certificate of the terms: build the network, contract it with `cA`, add its
+coefficients and reduce modulo the modulus. -/
+private noncomputable def termCertificate
+    (cA : NativeCertificate
+      (fun x : ℕ × ℕ × ℕ × DescribedNetwork T => x.2.2.2.truncContraction entry x.1 x.2.1)
+      (encTrunc encT) (encList encNat))
+    (hbuild : ComputableInPolyTime build (encPair encC encNat) (encTrunc encT))
+    (hmodulus : ComputableInPolyTime modulus encC encNat) :
+    NativeCertificate (fun q : C × ℕ => truncCoeffSum entry (build q) % modulus q.1)
+      (encPair encC encNat) encNat :=
+  ((NativeCertificate.ofPolyTime mod_polyTime).comp
+    (((NativeCertificate.ofPolyTime sum_polyTime).comp
+        (cA.comp (NativeCertificate.ofPolyTime hbuild))).prod
+      (NativeCertificate.ofPolyTime (modulusFst_polyTime hmodulus)))).congr fun _ => rfl
+
+private theorem termCertificate_time
+    (cA : NativeCertificate
+      (fun x : ℕ × ℕ × ℕ × DescribedNetwork T => x.2.2.2.truncContraction entry x.1 x.2.1)
+      (encTrunc encT) (encList encNat))
+    (hbuild : ComputableInPolyTime build (encPair encC encNat) (encTrunc encT))
+    (hmodulus : ComputableInPolyTime modulus encC encNat) (q : C × ℕ) :
+    (termCertificate cA hbuild hmodulus).time q =
+      (NativeCertificate.ofPolyTime hbuild).time q + cA.time (build q) +
+        (NativeCertificate.ofPolyTime sum_polyTime).time
+          ((build q).2.2.2.truncContraction entry (build q).1 (build q).2.1) +
+        (NativeCertificate.ofPolyTime (modulusFst_polyTime hmodulus)).time q +
+        (NativeCertificate.ofPolyTime mod_polyTime).time
+          (truncCoeffSum entry (build q), modulus q.1) := rfl
+
+private theorem termCertificate_space
+    (cA : NativeCertificate
+      (fun x : ℕ × ℕ × ℕ × DescribedNetwork T => x.2.2.2.truncContraction entry x.1 x.2.1)
+      (encTrunc encT) (encList encNat))
+    (hbuild : ComputableInPolyTime build (encPair encC encNat) (encTrunc encT))
+    (hmodulus : ComputableInPolyTime modulus encC encNat) (q : C × ℕ) :
+    (termCertificate cA hbuild hmodulus).space q =
+      (NativeCertificate.ofPolyTime hbuild).space q + cA.space (build q) +
+        (NativeCertificate.ofPolyTime sum_polyTime).space
+          ((build q).2.2.2.truncContraction entry (build q).1 (build q).2.1) +
+        (NativeCertificate.ofPolyTime (modulusFst_polyTime hmodulus)).space q +
+        (NativeCertificate.ofPolyTime mod_polyTime).space
+          (truncCoeffSum entry (build q), modulus q.1) := rfl
+
+/-- **Time and space of the loop.** Let `A` be an algorithm with the interface of Theorem D.1 at
+truncation order `ℓ`, with admissibility `Adm`. Let the networks, the modulus (positive) and the
+number of networks be computable in polynomial time from the context. Then there is a native
+certificate of the loop and a constant `K` with the following property. On every context `c` with
+`2 ≤ s c` all of whose networks are admissible at size `s c` with constant `Cadm`, with precision
+and truncation order between `1` and `D s c`, the loop takes time at most
+`(count c + 1) K (n + 1) ^ K 2 ^ (K s c / √⌊log₂ s c⌋)` and space at most `K (n + 1) ^ K`, where
+`n` is the encoded length of `c`. -/
+theorem exists_truncSum_certificate {Adm : ℕ → ℕ → ℕ × DescribedNetwork T → Prop}
+    (A : TruncatedContractionAlgorithm encT entry Adm)
+    (hbuild : ComputableInPolyTime build (encPair encC encNat) (encTrunc encT))
+    (hmodulus : ComputableInPolyTime modulus encC encNat)
+    (hcount : ComputableInPolyTime count encC encNat) (hpos : ∀ c, 0 < modulus c)
+    (Cadm D : ℕ) (s : C → ℕ) :
+    ∃ (cs : NativeCertificate (truncSum entry build modulus count) encC encNat) (K : ℕ),
+      ∀ c, 2 ≤ s c →
+        (∀ i < count c,
+          let (P, ℓ, r, G) := build (c, i)
+          1 ≤ P ∧ P ≤ D * s c ∧ 1 ≤ ℓ ∧ ℓ ≤ D * s c ∧ Adm Cadm (s c) (r, G)) →
+        cs.time c ≤ (count c + 1) * (K * ((encC c).length + 1) ^ K *
+            2 ^ (K * s c / Nat.sqrt (Nat.log 2 (s c)))) ∧
+          cs.space c ≤ K * ((encC c).length + 1) ^ K := by
+  set cr := termCertificate (entry := entry) A.certificate hbuild hmodulus with hcr
+  let R : C → ℕ := fun c => (range (count c)).sup fun i => cr.time (c, i)
+  let Q : C → ℕ := fun c => (range (count c)).sup fun i => cr.space (c, i)
+  obtain ⟨a, d, h⟩ := exists_computableInO_modSum hmodulus hcount cr hpos
+    (fun c i _ => Nat.mod_lt _ (hpos c)) R Q
+    (fun c i hi => le_sup (f := fun i => cr.time (c, i)) (mem_range.mpr hi))
+    (fun c i hi => le_sup (f := fun i => cr.space (c, i)) (mem_range.mpr hi))
+  refine ⟨⟨_, _, h⟩, ?_⟩
+  -- The polynomial-time pieces of a term.
+  obtain ⟨pB, hpB, mB, bB⟩ := exists_mono_poly_cert hbuild
+  obtain ⟨pS, hpS, mS, bS⟩ := exists_mono_poly_cert sum_polyTime
+  obtain ⟨pF, hpF, mF, bF⟩ := exists_mono_poly_cert (modulusFst_polyTime hmodulus)
+  obtain ⟨pM, hpM, mM, bM⟩ := exists_mono_poly_cert mod_polyTime
+  obtain ⟨pBL, hpBL, mBL, bBL⟩ := exists_mono_poly hbuild.polyBounded_length
+  obtain ⟨pSL, hpSL, mSL, bSL⟩ := exists_mono_poly sum_polyTime.polyBounded_length
+  obtain ⟨K₀, hK₀⟩ := A.bound Cadm D
+  obtain ⟨cO, -, hcO⟩ := A.certificate.computes
+  -- Sizes, as functions of the context.
+  set L : C → ℕ := fun c => (encC c).length with hL
+  set lm : C → ℕ := fun c => (encNat (modulus c)).length with hlm
+  set lk : C → ℕ := fun c => (encNat (count c)).length with hlk
+  have hlmp : PolyBounded L lm := hmodulus.polyBounded_length
+  have hlkp : PolyBounded L lk := hcount.polyBounded_length
+  -- The input of a term, the network, the contraction and its coefficient sum.
+  let Lq : C → ℕ := fun c => 2 * L c + 1 + lk c
+  let Lb : C → ℕ := fun c => pBL (Lq c)
+  let Sp : C → ℕ := fun c => K₀ * (Lb c + 1) ^ K₀
+  let Lo : C → ℕ := fun c => cO * (Sp c + 1)
+  let Lp : C → ℕ := fun c => 2 * pSL (Lo c) + 1 + lm c
+  let U : C → ℕ := fun c => pB (Lq c) + pS (Lo c) + pF (Lq c) + pM (Lp c) + Sp c
+  have hLq : PolyBounded L Lq :=
+    (((PolyBounded.const 2).mul PolyBounded.self).add (PolyBounded.const 1)).add hlkp
+  have hLb : PolyBounded L Lb := (PolyBounded.of_polynomiallyBounded id hpBL).comp hLq
+  have hSp : PolyBounded L Sp :=
+    (PolyBounded.const K₀).mul ((hLb.add (PolyBounded.const 1)).pow K₀)
+  have hLo : PolyBounded L Lo := (PolyBounded.const cO).mul (hSp.add (PolyBounded.const 1))
+  have hLp : PolyBounded L Lp :=
+    (((PolyBounded.const 2).mul ((PolyBounded.of_polynomiallyBounded id hpSL).comp hLo)).add
+      (PolyBounded.const 1)).add hlmp
+  have hU : PolyBounded L U :=
+    (((((PolyBounded.of_polynomiallyBounded id hpB).comp hLq).add
+      ((PolyBounded.of_polynomiallyBounded id hpS).comp hLo)).add
+      ((PolyBounded.of_polynomiallyBounded id hpF).comp hLq)).add
+      ((PolyBounded.of_polynomiallyBounded id hpM).comp hLp)).add hSp
+  let W : C → ℕ := fun c => a * (L c + 1) ^ d + U c
+  have hW : PolyBounded L W :=
+    ((PolyBounded.const a).mul ((PolyBounded.self.add (PolyBounded.const 1)).pow d)).add hU
+  obtain ⟨a', d', hWad⟩ := hW.exists_le
+  refine ⟨a' + d' + K₀, fun c hs hgood => ?_⟩
+  set E := 2 ^ (K₀ * s c / Nat.sqrt (Nat.log 2 (s c))) with hE
+  have hE1 : 1 ≤ E := Nat.one_le_two_pow
+  -- A term at a summed index costs at most `U c` plus the contraction.
+  have hterm : ∀ i < count c, cr.time (c, i) ≤ U c * E ∧ cr.space (c, i) ≤ U c := by
+    intro i hi
+    obtain ⟨hP1, hPD, hℓ1, hℓD, hadm⟩ := hgood i hi
+    have hq : (encPair encC encNat (c, i)).length ≤ Lq c := by
+      rw [length_encPair]
+      have : (encNat i).length ≤ lk c := length_encNat_mono (le_of_lt hi)
+      simp only [Lq, L]
+      omega
+    have hxlen : (encTrunc encT (build (c, i))).length ≤ Lb c := (bBL (c, i)).trans (mBL hq)
+    obtain ⟨hAt, hAs⟩ := hK₀ (s c) (build (c, i)).1 (build (c, i)).2.1 (build (c, i)).2.2.1
+      (build (c, i)).2.2.2 hs hP1 hPD hℓ1 hℓD hadm
+    have hpow : (encTrunc encT (build (c, i))).length + 1 ≤ Lb c + 1 := by omega
+    have hAs' : A.certificate.space (build (c, i)) ≤ Sp c :=
+      hAs.trans (Nat.mul_le_mul_left _ (Nat.pow_le_pow_left hpow _))
+    have hAt' : A.certificate.time (build (c, i)) ≤ Sp c * E :=
+      hAt.trans (Nat.mul_le_mul_right _ (Nat.mul_le_mul_left _ (Nat.pow_le_pow_left hpow _)))
+    have hout : (encList encNat ((build (c, i)).2.2.2.truncContraction entry (build (c, i)).1
+        (build (c, i)).2.1)).length ≤ Lo c :=
+      (hcO (build (c, i))).trans (Nat.mul_le_mul_left _ (by omega))
+    have hsumlen : (encNat (truncCoeffSum entry (build (c, i)))).length ≤ pSL (Lo c) :=
+      (bSL _).trans (mSL hout)
+    have hpair : (encPair encNat encNat (truncCoeffSum entry (build (c, i)), modulus c)).length ≤
+        Lp c := by
+      rw [length_encPair]
+      simp only [Lp, lm]
+      omega
+    have h1 := (bB (c, i)).1.trans (mB hq)
+    have h1' := (bB (c, i)).2.trans (mB hq)
+    have h2 := (bS _).1.trans (mS hout)
+    have h2' := (bS _).2.trans (mS hout)
+    have h3 := (bF (c, i)).1.trans (mF hq)
+    have h3' := (bF (c, i)).2.trans (mF hq)
+    have h4 := (bM (truncCoeffSum entry (build (c, i)), modulus c)).1.trans (mM hpair)
+    have h4' := (bM (truncCoeffSum entry (build (c, i)), modulus c)).2.trans (mM hpair)
+    rw [hcr, termCertificate_time, termCertificate_space]
+    dsimp only
+    refine ⟨?_, ?_⟩
+    · have hle : pB (Lq c) + pS (Lo c) + pF (Lq c) + pM (Lp c) ≤
+          (pB (Lq c) + pS (Lo c) + pF (Lq c) + pM (Lp c)) * E := Nat.le_mul_of_pos_right _ hE1
+      have hUE : U c * E = (pB (Lq c) + pS (Lo c) + pF (Lq c) + pM (Lp c)) * E + Sp c * E := by
+        simp only [U]
+        ring
+      rw [hUE]
+      omega
+    · simp only [U]
+      omega
+  -- The trajectory maxima.
+  have hR : R c ≤ U c * E := Finset.sup_le fun i hi => (hterm i (mem_range.mp hi)).1
+  have hQ : Q c ≤ U c := Finset.sup_le fun i hi => (hterm i (mem_range.mp hi)).2
+  have hWc := hWad c
+  have hK : a' * (L c + 1) ^ d' ≤ (a' + d' + K₀) * (L c + 1) ^ (a' + d' + K₀) :=
+    Nat.mul_le_mul (by omega) (Nat.pow_le_pow_right (by omega) (by omega))
+  have hEK : E ≤ 2 ^ ((a' + d' + K₀) * s c / Nat.sqrt (Nat.log 2 (s c))) :=
+    Nat.pow_le_pow_right (by omega)
+      (Nat.div_le_div_right (Nat.mul_le_mul_right _ (by omega)))
+  refine ⟨?_, ?_⟩
+  · change (count c + 1) * (a * (L c + 1) ^ d + R c) ≤ _
+    refine Nat.mul_le_mul_left _ ?_
+    calc a * (L c + 1) ^ d + R c ≤ W c * E := by
+          have : a * (L c + 1) ^ d ≤ a * (L c + 1) ^ d * E := Nat.le_mul_of_pos_right _ hE1
+          simp only [W, add_mul]
+          omega
+      _ ≤ ((a' + d' + K₀) * (L c + 1) ^ (a' + d' + K₀)) *
+          2 ^ ((a' + d' + K₀) * s c / Nat.sqrt (Nat.log 2 (s c))) :=
+        Nat.mul_le_mul (hWc.trans hK) hEK
+  · change a * (L c + 1) ^ d + Q c ≤ _
+    have : a * (L c + 1) ^ d + Q c ≤ W c := by simp only [W]; omega
+    exact this.trans (hWc.trans hK)
+
+end Cslib.Complexity.CountingSimulation
diff --git a/ComplexityTests/Computability/Complexity/Classes/CountingSimulation/TruncatedSum.lean b/ComplexityTests/Computability/Complexity/Classes/CountingSimulation/TruncatedSum.lean
new file mode 100644
index 0000000000..eb1e1e5e4a
--- /dev/null
+++ b/ComplexityTests/Computability/Complexity/Classes/CountingSimulation/TruncatedSum.lean
@@ -0,0 +1,76 @@
+/-
+Copyright (c) 2026 Samuel Schlesinger. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Samuel Schlesinger
+-/
+
+module
+-- upstream: local-only (test driver)
+
+public import Complexity.Computability.Complexity.Classes.CountingSimulation.TruncatedSum
+public import ComplexityTests.Computability.Circuit.Frontier.TensorNetworkTruncatedAlgorithm
+import Complexity.Computability.Machines.Turing.MultiTape.Complexity.PolyTime.Nat.Binary
+
+/-!
+# Tests for the loop summing truncated contractions
+
+The resource bound of the loop is not vacuous. It applies to direct summation on networks of
+boundedly many edges (the test instance `boundedAlgorithm` of the interface of Theorem D.1 at
+truncation order `ℓ`) with the empty network at precision and truncation order `n + 1`, and every
+context `n` satisfies its hypotheses. The loop over two empty networks is `2` modulo
+`2 ^ (n + 1)`: the empty network contracts to `1`, whose coefficients below `V ^ (n + 1)` sum to
+`1`.
+-/
+
+@[expose] public section
+
+namespace ComplexityTests.TruncatedSum
+
+open Cslib Cslib.GenericRAM Cslib.Circuits.Frontier Cslib.Complexity.CountingSimulation
+  Turing.MultiTapeTM Finset ComplexityTests.TensorNetworkTruncatedAlgorithm
+
+/-- The empty network, at precision and truncation order `n + 1`, for every index. -/
+def build (q : ℕ × ℕ) : ℕ × ℕ × ℕ × DescribedNetwork (List ℕ) :=
+  (q.1 + 1, q.1 + 1, 0, ⟨0, []⟩)
+
+theorem build_polyTime :
+    ComputableInPolyTime build (encPair encUnary encNat) (encTrunc (encList encNat)) := by
+  unfold build
+  fun_prop
+
+theorem modulus_polyTime :
+    ComputableInPolyTime (fun n : ℕ => 2 ^ (n + 1)) encUnary encNat :=
+  ComputableInPolyTime.binary_two_pow.comp
+    (by fun_prop : ComputableInPolyTime (fun n : ℕ => n + 1) encUnary encUnary)
+
+theorem count_polyTime : ComputableInPolyTime (fun _ : ℕ => 2) encUnary encNat := by
+  fun_prop
+
+/-- **The resource bound applies to every context**: the empty networks have no edges, and
+precision and truncation order `n + 1` lie between `1` and `n + 2`. -/
+example : ∃ (cs : NativeCertificate
+      (truncSum constEntry build (fun n => 2 ^ (n + 1)) fun _ => 2) encUnary encNat) (K : ℕ),
+    ∀ n : ℕ, cs.time n ≤ 3 * (K * ((encUnary n).length + 1) ^ K *
+        2 ^ (K * (n + 2) / Nat.sqrt (Nat.log 2 (n + 2)))) ∧
+      cs.space n ≤ K * ((encUnary n).length + 1) ^ K := by
+  obtain ⟨cs, K, hK⟩ := exists_truncSum_certificate boundedAlgorithm build_polyTime
+    modulus_polyTime count_polyTime (fun n => Nat.two_pow_pos _) 0 1 (fun n => n + 2)
+  refine ⟨cs, K, fun n => hK n (by omega) fun i _ => ?_⟩
+  simp only [build, BoundedEdges, le_refl, and_true]
+  omega
+
+/-- The empty network contracts to `1`, so its truncated contraction is `[1, 0, …, 0]` and each
+term is `1`; the loop over two networks is `2` modulo `2 ^ (n + 1)`. -/
+example (n : ℕ) :
+    truncSum constEntry build (fun n => 2 ^ (n + 1)) (fun _ => 2) n = 2 % 2 ^ (n + 1) := by
+  have hpoly : DescribedNetwork.polyContraction constEntry
+      (⟨0, []⟩ : DescribedNetwork (List ℕ)) = 1 := by
+    simp [DescribedNetwork.polyContraction, DescribedNetwork.contraction]
+  have hterm : ∀ i, truncCoeffSum constEntry (build (n, i)) = 1 := by
+    intro i
+    rw [build, truncCoeffSum_eq, hpoly, sum_range_succ']
+    simp [Polynomial.coeff_one, Nat.one_mod_two_pow]
+  rw [truncSum_eq]
+  simp [hterm]
+
+end ComplexityTests.TruncatedSum