diff --git a/src/interface/abi/Nimiser/ABI/Capstone.idr b/src/interface/abi/Nimiser/ABI/Capstone.idr
new file mode 100644
index 0000000..edd4d9a
--- /dev/null
+++ b/src/interface/abi/Nimiser/ABI/Capstone.idr
@@ -0,0 +1,88 @@
+-- SPDX-License-Identifier: MPL-2.0
+-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+--
+||| Layer 5 — the ABI SOUNDNESS CERTIFICATE for Nimiser.
+|||
+||| This capstone proves nothing new. Instead it ASSEMBLES the four prior
+||| proof layers of the Nimiser ABI into a single inhabited value, so that
+||| "the whole ABI contract is discharged" becomes one typechecked fact:
+|||
+|||   * Layer 2 (flagship semantics) — `Semantics.samplePreserved`, the
+|||     positive-control witness that the code generator's output for the
+|||     canonical `proc add(a,b: int): int` provably PRESERVES the source
+|||     signature (arity + per-argument/return Nim->C lowering).
+|||   * Layer 3 (deeper invariant) — `Invariants.composedArgsWitness`, a
+|||     concrete `ArgsLower` witness BUILT by composing two sub-witnesses
+|||     through the distributivity/composition lemma (`argsLowerAppend`),
+|||     i.e. the generator is a monoid homomorphism on argument lists.
+|||   * Layer 3 (whole-signature injectivity) — `Invariants.genBindingInjective`,
+|||     the theorem that distinct Nim signatures lower to distinct C bindings
+|||     (no codegen-introduced symbol/ABI aliasing).
+|||   * Layer 4 (FFI seam) — `FfiSeam.resultToIntInjective`, soundness of the
+|||     ABI<->C `Result` encoding: distinct ABI outcomes never collide on the
+|||     wire.
+|||
+||| The certificate threads the chain manifest -> ABI proofs (flagship +
+||| invariant) -> FFI seam into one end-to-end soundness statement. The single
+||| inhabited value `abiContractDischarged` cannot be constructed unless EVERY
+||| referenced witness/theorem from the prior layers still typechecks; if any
+||| earlier layer regressed into unsoundness, this module would fail to build.
+||| That is the point of a capstone: one value that stands or falls with the
+||| entire stack.
+
+module Nimiser.ABI.Capstone
+
+import Nimiser.ABI.Types
+import Nimiser.ABI.Semantics
+import Nimiser.ABI.Invariants
+import Nimiser.ABI.FfiSeam
+
+%default total
+
+--------------------------------------------------------------------------------
+-- The certificate record
+--------------------------------------------------------------------------------
+
+||| `ABISound` bundles the key proven facts of the Nimiser ABI. Each field is a
+||| genuine proof object reused verbatim from the layer that established it — no
+||| field is re-proved here, and none can be faked. An inhabitant of this record
+||| is exactly a certificate that all four layers hold simultaneously.
+public export
+record ABISound where
+  constructor MkABISound
+  ||| Layer 2 (flagship): the generated C binding for the canonical sample
+  ||| signature provably preserves it (name + arity + per-type lowering).
+  flagshipPreserved :
+    SigPreserved Semantics.sampleNimSig Semantics.sampleCSig
+  ||| Layer 3 (composition invariant): a concrete composed `ArgsLower` witness
+  ||| for an extended argument list, demonstrating lowering is a homomorphism
+  ||| over list concatenation.
+  invariantComposed :
+    ArgsLower (Invariants.baseSig.sigArgs ++ Invariants.extraArgs)
+              [CIntT, CDoubleT, CCharStarT]
+  ||| Layer 3 (whole-signature injectivity): distinct Nim signatures lower to
+  ||| distinct C bindings — the generator introduces no ABI aliasing.
+  bindingInjective :
+    (s1, s2 : NimSig) -> genBinding s1 = genBinding s2 -> s1 = s2
+  ||| Layer 4 (FFI seam): the ABI<->C `Result` encoding is injective, so
+  ||| distinct ABI outcomes never collide on the wire.
+  ffiSeamInjective :
+    (a, b : Result) -> resultToInt a = resultToInt b -> a = b
+
+--------------------------------------------------------------------------------
+-- The capstone value
+--------------------------------------------------------------------------------
+
+||| THE CAPSTONE. A single inhabited `ABISound`, constructed entirely from the
+||| existing exported witnesses/theorems of Layers 2-4. This value is the
+||| end-to-end ABI soundness certificate for Nimiser: it typechecks iff the
+||| flagship semantic property, the deeper composition invariant, whole-signature
+||| injectivity, and the FFI-seam encoding soundness are ALL discharged together.
+public export
+abiContractDischarged : ABISound
+abiContractDischarged =
+  MkABISound
+    Semantics.samplePreserved
+    Invariants.composedArgsWitness
+    Invariants.genBindingInjective
+    FfiSeam.resultToIntInjective
diff --git a/src/interface/abi/nimiser-abi.ipkg b/src/interface/abi/nimiser-abi.ipkg
index 6bedff8..e7f8eb5 100644
--- a/src/interface/abi/nimiser-abi.ipkg
+++ b/src/interface/abi/nimiser-abi.ipkg
@@ -1,4 +1,4 @@
 -- SPDX-License-Identifier: MPL-2.0
 package nimiser-abi
 sourcedir = "."
-modules = Nimiser.ABI.Types, Nimiser.ABI.Layout, Nimiser.ABI.Foreign, Nimiser.ABI.Proofs, Nimiser.ABI.Semantics, Nimiser.ABI.Invariants, Nimiser.ABI.FfiSeam
+modules = Nimiser.ABI.Types, Nimiser.ABI.Layout, Nimiser.ABI.Foreign, Nimiser.ABI.Proofs, Nimiser.ABI.Semantics, Nimiser.ABI.Invariants, Nimiser.ABI.FfiSeam, Nimiser.ABI.Capstone