diff --git a/src/interface/abi/Julianiser/ABI/Capstone.idr b/src/interface/abi/Julianiser/ABI/Capstone.idr
new file mode 100644
index 0000000..2b542bc
--- /dev/null
+++ b/src/interface/abi/Julianiser/ABI/Capstone.idr
@@ -0,0 +1,120 @@
+-- SPDX-License-Identifier: MPL-2.0
+-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+--
+||| Layer 5 CAPSTONE: a single end-to-end ABI soundness certificate.
+|||
+||| This module proves NO new domain theorem. Its sole job is to ASSEMBLE the
+||| already-proven facts from every prior proof layer into ONE inhabited value,
+||| `abiContractDischarged : ABISound`. Because that value is constructed only
+||| from genuine exported witnesses/theorems, it typechecks IF AND ONLY IF every
+||| layer it depends on is itself sound — collapsing the whole ABI contract into
+||| a single yes/no compile event.
+|||
+||| The certificate ties the manifest's correctness promise through the ABI
+||| proofs into the FFI seam:
+|||
+|||   * Layer 2 (flagship) — `Julianiser.ABI.Semantics.sampleAgreesGeneric`:
+|||     on the canonical positive-control pipeline (`sampleExpr` over `sampleEnv`)
+|||     the generated Julia code computes exactly the same value as the source.
+|||     This is the headline manifest promise: translation preserves semantics.
+|||
+|||   * Layer 3 (deeper invariant) — two distinct facts reused verbatim:
+|||       - `Julianiser.ABI.Invariants.compositeAgrees`: correctness is CLOSED
+|||         UNDER COMPOSITION (the modular/compositional theorem, witnessed on a
+|||         concrete linked program), and
+|||       - `Julianiser.ABI.Invariants.translateInjective`: the codegen is
+|||         INJECTIVE (distinct source ASTs never collapse) — the structural
+|||         backbone of deterministic, information-preserving translation.
+|||
+|||   * Layer 4 (FFI seam) — `Julianiser.ABI.FfiSeam.resultToIntInjective`:
+|||     the C-ABI wire encoding of `Result` is unambiguous, so the boundary back
+|||     out to Zig/C is sound.
+|||
+||| Together these say: manifest promise -> ABI proofs (flagship + invariant +
+||| injective codegen) -> FFI seam, all discharged in one place. If ANY prior
+||| layer were unsound, the field supplying its witness would fail to resolve and
+||| this module would not build. The adversarial control (in /tmp during CI)
+||| confirms the certificate is non-vacuous: a FALSE field value is rejected.
+
+module Julianiser.ABI.Capstone
+
+import Julianiser.ABI.Types
+import Julianiser.ABI.Semantics
+import Julianiser.ABI.Invariants
+import Julianiser.ABI.FfiSeam
+import Data.Vect
+import Data.Fin
+
+%default total
+
+--------------------------------------------------------------------------------
+-- The end-to-end ABI soundness certificate
+--------------------------------------------------------------------------------
+
+||| `ABISound` is a conjunction of the KEY proven facts of this ABI, one field
+||| per layer. Each field's TYPE is the exact proposition proved in the layer it
+||| names; an inhabitant therefore certifies all layers simultaneously.
+public export
+record ABISound where
+  constructor MkABISound
+
+  ||| Layer 2 flagship, on the canonical positive control: source and generated
+  ||| Julia agree on the sample pipeline. (= `Semantics.sampleAgreesGeneric`.)
+  flagshipControl :
+    evalSrc Semantics.sampleEnv Semantics.sampleExpr
+      = evalJulia Semantics.sampleEnv (translate Semantics.sampleExpr)
+
+  ||| Layer 3 invariant (compositionality): correctness is closed under
+  ||| composition, witnessed on the concrete composite program.
+  ||| (= `Invariants.compositeAgrees`.)
+  invariantCompose :
+    evalSrc Semantics.sampleEnv Invariants.compositeExpr
+      = evalJulia Semantics.sampleEnv (translate Invariants.compositeExpr)
+
+  ||| Layer 3 invariant (deterministic, information-preserving codegen):
+  ||| `translate` is injective for every array length. (=
+  ||| `Invariants.translateInjective`, held as a function field so the full
+  ||| universally-quantified theorem is carried; `n` is bound explicitly here so
+  ||| the field type is closed.)
+  invariantInjective :
+    (n : Nat) -> (e1, e2 : Expr n) -> translate e1 = translate e2 -> e1 = e2
+
+  ||| Layer 4 FFI seam: the `Result` wire encoding is injective / unambiguous.
+  ||| (= `FfiSeam.resultToIntInjective`.)
+  ffiSeamInjective :
+    (a, b : Result) -> resultToInt a = resultToInt b -> a = b
+
+--------------------------------------------------------------------------------
+-- THE CAPSTONE: one inhabited value assembled from the real witnesses
+--------------------------------------------------------------------------------
+
+||| The capstone certificate. Every field is an existing exported theorem/witness
+||| — nothing is re-proved here, only composed. If any layer were unsound, the
+||| corresponding witness would not resolve and this definition would not build.
+public export
+abiContractDischarged : ABISound
+abiContractDischarged = MkABISound
+  Semantics.sampleAgreesGeneric
+  Invariants.compositeAgrees
+  (\n, e1, e2, prf => Invariants.translateInjective e1 e2 prf)
+  FfiSeam.resultToIntInjective
+
+--------------------------------------------------------------------------------
+-- Positive control: the certificate's facts are usable end-to-end
+--------------------------------------------------------------------------------
+
+||| Positive control: project the flagship field back out of the assembled
+||| certificate and confirm it is the genuine sample-agreement equality.
+||| Demonstrates the capstone is not an opaque token but carries real proofs.
+public export
+capstoneFlagship :
+  evalSrc Semantics.sampleEnv Semantics.sampleExpr
+    = evalJulia Semantics.sampleEnv (translate Semantics.sampleExpr)
+capstoneFlagship = flagshipControl abiContractDischarged
+
+||| Positive control: the carried FFI-seam injectivity, applied to a concrete
+||| pair (`Ok`, `Ok`) with `Refl`, yields the expected `Ok = Ok`. Exercises the
+||| function-valued field of the assembled certificate.
+public export
+capstoneSeamOnOk : Ok = Ok
+capstoneSeamOnOk = ffiSeamInjective abiContractDischarged Ok Ok Refl
diff --git a/src/interface/abi/julianiser-abi.ipkg b/src/interface/abi/julianiser-abi.ipkg
index 406e92e..923bd37 100644
--- a/src/interface/abi/julianiser-abi.ipkg
+++ b/src/interface/abi/julianiser-abi.ipkg
@@ -12,3 +12,4 @@ modules = Julianiser.ABI.Types
         , Julianiser.ABI.Semantics
         , Julianiser.ABI.Invariants
         , Julianiser.ABI.FfiSeam
+        , Julianiser.ABI.Capstone