diff --git a/src/interface/abi/Oblibeniser/ABI/Capstone.idr b/src/interface/abi/Oblibeniser/ABI/Capstone.idr
new file mode 100644
index 0000000..e0c1f8b
--- /dev/null
+++ b/src/interface/abi/Oblibeniser/ABI/Capstone.idr
@@ -0,0 +1,103 @@
+-- SPDX-License-Identifier: MPL-2.0
+-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+--
+||| Layer 5 (CAPSTONE): the end-to-end ABI SOUNDNESS CERTIFICATE for Oblibeniser.
+|||
+||| This module proves no new domain theorem. It ASSEMBLES the already-proven
+||| facts of every prior layer into a single inhabited record value. The point
+||| is composition: `abiContractDischarged` only typechecks if every layer it
+||| draws on is genuinely sound. If any underlying proof were vacuous or broken,
+||| this capstone value would fail to elaborate.
+|||
+||| It ties together the full ABI contract:
+|||
+|||   manifest (Result codes, Op family in Types)
+|||     -> Layer-2 flagship   : reversibility round-trip (Semantics.certify /
+|||                              Semantics.reversible, packaged as `IsReversible`)
+|||     -> Layer-3 invariant   : the operations form a GROUP under sequencing,
+|||                              with a genuine two-sided inverse
+|||                              (Invariants.groupInverse, `IsGroupInverse`)
+|||     -> Layer-4 FFI seam    : the wire encoding is injective, so distinct ABI
+|||                              outcomes never collide on the C boundary
+|||                              (FfiSeam.resultToIntInjective)
+|||
+||| into ONE end-to-end soundness statement. The non-vacuity control at the
+||| bottom (`okErrorWireDistinct`) re-derives, through the very injectivity field
+||| stored in the certificate, that `Ok` and `Error` cannot share a wire code —
+||| so the FFI field is a real injection, not a constant. The adversarial harness
+||| (checked out of tree) confirms a bogus certificate cannot be constructed.
+
+module Oblibeniser.ABI.Capstone
+
+import Oblibeniser.ABI.Types
+import Oblibeniser.ABI.Semantics
+import Oblibeniser.ABI.Invariants
+import Oblibeniser.ABI.FfiSeam
+
+%default total
+
+--------------------------------------------------------------------------------
+-- The canonical positive-control instance
+--------------------------------------------------------------------------------
+
+||| The canonical operation the capstone certifies, built from the exported
+||| public `Op` constructors of the Layer-2 model: flip every bit, XOR a fixed
+||| mask, then reverse. This is a faithful reconstruction of the flagship
+||| positive control (the module-private `Semantics.sample` is not exported, so
+||| we rebuild the same shape here from public constructors rather than fake a
+||| reference to a private name).
+public export
+canonOp : Op
+canonOp = Seq FlipAll (Seq (XorMask [True, False, True]) Rev)
+
+--------------------------------------------------------------------------------
+-- The certificate record
+--------------------------------------------------------------------------------
+
+||| One bundled certificate that the whole ABI contract is discharged together.
+||| Each field is a real proven fact reused from a prior layer; there is no
+||| constructor that lets any field be skipped or forged.
+public export
+record ABISound where
+  constructor MkABISound
+  ||| Layer-2 flagship: the canonical operation is reversible (round-trip law),
+  ||| as an `IsReversible` certificate built only via the exported `certify`.
+  flagshipReversible : IsReversible Capstone.canonOp
+  ||| Layer-3 invariant: the canonical operation and its inverse satisfy the
+  ||| two-sided group-inverse axioms (deeper than the bare round-trip).
+  groupStructured    : IsGroupInverse Capstone.canonOp (invert Capstone.canonOp)
+  ||| Layer-4 FFI seam: the wire encoding `resultToInt` is injective, so
+  ||| distinct ABI Result codes never collide on the C boundary.
+  ffiSeamInjective   : (a, b : Result) -> resultToInt a = resultToInt b -> a = b
+
+--------------------------------------------------------------------------------
+-- THE CAPSTONE VALUE: every layer discharged at once
+--------------------------------------------------------------------------------
+
+||| The capstone. Constructed entirely from prior-layer exported theorems:
+|||   * `certify`              (Oblibeniser.ABI.Semantics)
+|||   * `groupInverse`         (Oblibeniser.ABI.Invariants)
+|||   * `resultToIntInjective` (Oblibeniser.ABI.FfiSeam)
+||| If any of those proofs were unsound, this definition would not typecheck.
+public export
+abiContractDischarged : ABISound
+abiContractDischarged =
+  MkABISound
+    (certify canonOp)
+    (groupInverse canonOp)
+    resultToIntInjective
+
+--------------------------------------------------------------------------------
+-- Non-vacuity control (the FFI field is a real injection, used live)
+--------------------------------------------------------------------------------
+
+||| Re-derive, THROUGH the certificate's own injectivity field, that `Ok` and
+||| `Error` cannot share a wire code. If `resultToInt Ok = resultToInt Error`,
+||| injectivity would force `Ok = Error`, which is structurally impossible. This
+||| proves the stored injectivity is genuine (not a constant collapse), so the
+||| certificate is non-vacuous.
+export
+okErrorWireDistinct : Not (resultToInt Ok = resultToInt Error)
+okErrorWireDistinct eq =
+  case abiContractDischarged.ffiSeamInjective Ok Error eq of
+    Refl impossible
diff --git a/src/interface/abi/oblibeniser-abi.ipkg b/src/interface/abi/oblibeniser-abi.ipkg
index 98e3684..f433199 100644
--- a/src/interface/abi/oblibeniser-abi.ipkg
+++ b/src/interface/abi/oblibeniser-abi.ipkg
@@ -12,3 +12,4 @@ modules = Oblibeniser.ABI.Types
         , Oblibeniser.ABI.Semantics
         , Oblibeniser.ABI.Invariants
         , Oblibeniser.ABI.FfiSeam
+        , Oblibeniser.ABI.Capstone