diff --git a/src/interface/abi/Chapeliser/ABI/Capstone.idr b/src/interface/abi/Chapeliser/ABI/Capstone.idr
new file mode 100644
index 0000000..2e1c8b4
--- /dev/null
+++ b/src/interface/abi/Chapeliser/ABI/Capstone.idr
@@ -0,0 +1,102 @@
+-- SPDX-License-Identifier: MPL-2.0
+-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+--
+||| Chapeliser ABI Soundness CAPSTONE (Layer 5)
+|||
+||| This module is the END-TO-END certificate: it ASSEMBLES the already-proven
+||| facts of every prior ABI layer into a SINGLE inhabited value. It proves no
+||| new domain theorem of its own — its entire content is that the prior layers
+||| compose, and that one record can be built from their real exported witnesses.
+||| If any earlier layer were unsound (its witness retracted or weakened), the
+||| value `abiContractDischarged` below would simply fail to typecheck and the
+||| proof build would go red.
+|||
+||| The contract it ties together, manifest -> ABI proofs -> FFI seam:
+|||
+|||   * Layer 2 (flagship property, `Proofs.idr`): the canonical positive-control
+|||     partition `tenAcrossTwo` is a VALID partition — complete AND disjoint
+|||     (`Proofs.tenAcrossTwoValid : ValidPartition Proofs.tenAcrossTwo`). This is
+|||     the manifest-level "no item lost or double-placed" guarantee on a concrete
+|||     instance.
+|||
+|||   * Layer 3 (deeper invariant, `Invariants.idr`): the REAL block-partition
+|||     strategy (the div/mod counts the codegen actually emits) is complete for
+|||     all n and k>0 — discharged here on the canonical 10-over-3 instance via
+|||     `Invariants.blockPartitionIsComplete`, the packaged `PartitionComplete`
+|||     object. We also carry the GENERAL theorem `blockCountsComplete` as a field
+|||     so the certificate witnesses universal, not merely pointwise, completeness.
+|||
+|||   * Layer 4 (FFI seam, `FfiSeam.idr`): the `resultToInt` encoder crossing the
+|||     Zig/C boundary is INJECTIVE — distinct ABI outcomes never collide on the
+|||     wire (`FfiSeam.resultToIntInjective`). This is the seam soundness the FFI
+|||     wrappers depend on.
+|||
+||| One value below inhabits all of these at once: that is the soundness
+||| certificate. A deliberately-FALSE field is rejected by the type checker (see
+||| the adversarial control in /tmp during the build procedure), so the record is
+||| non-vacuous: it cannot be built from a bogus component.
+|||
+||| All composition is genuine: no `believe_me`, `idris_crash`, `assert_total`,
+||| `postulate`, `sorry`, or `%hint` hacks. Every field is filled with a name
+||| actually exported by the module it comes from.
+
+module Chapeliser.ABI.Capstone
+
+import Chapeliser.ABI.Types
+import Chapeliser.ABI.Proofs
+import Chapeliser.ABI.Partition
+import Chapeliser.ABI.Invariants
+import Chapeliser.ABI.FfiSeam
+
+import Data.Vect
+
+%default total
+
+--------------------------------------------------------------------------------
+-- The capstone certificate type
+--------------------------------------------------------------------------------
+
+||| `ABISound` is the end-to-end ABI soundness certificate. Each field is a KEY
+||| proven fact of a prior layer, reused verbatim — the record is inhabited iff
+||| every layer it names is itself sound.
+public export
+record ABISound where
+  constructor MkABISound
+  ||| Layer 2 flagship: the canonical positive-control partition is VALID
+  ||| (complete and non-overlapping). Reuses `Proofs.tenAcrossTwoValid`.
+  flagshipValid : ValidPartition Proofs.tenAcrossTwo
+
+  ||| Layer 3 invariant, concrete: the REAL block partition of 10 items over 3
+  ||| locales is complete (`PartitionComplete` object). Reuses
+  ||| `Invariants.blockPartitionIsComplete`.
+  blockComplete :
+    PartitionComplete
+      (MkPartition {n = 10} {k = 3} (contiguousFrom 0 (blockCounts 10 2)))
+
+  ||| Layer 3 invariant, GENERAL: the block partition's per-locale counts sum to
+  ||| exactly `n` for every item count `n` and every positive locale count
+  ||| `S k'`. Reuses `Invariants.blockCountsComplete` (universal, not pointwise).
+  blockCompleteAll : (n, k' : Nat) -> sumNat (blockCounts n k') = n
+
+  ||| Layer 4 FFI seam: the `resultToInt` encoder is injective — distinct ABI
+  ||| outcomes never collide on the wire. Reuses `FfiSeam.resultToIntInjective`.
+  ffiInjective : (a, b : Result) -> resultToInt a = resultToInt b -> a = b
+
+--------------------------------------------------------------------------------
+-- The capstone value: the contract, discharged
+--------------------------------------------------------------------------------
+
+||| THE CAPSTONE. A single inhabited `ABISound`, assembled entirely from the
+||| existing exported witnesses/theorems of Layers 2, 3 and 4. Its mere
+||| typechecking is the proof that the full ABI contract — manifest-level
+||| partition validity, real-strategy completeness, and FFI-seam injectivity —
+||| is discharged together, end to end. Weaken any contributing proof and this
+||| value stops typechecking.
+export
+abiContractDischarged : ABISound
+abiContractDischarged =
+  MkABISound
+    Proofs.tenAcrossTwoValid
+    (Invariants.blockPartitionIsComplete 10 2)
+    Invariants.blockCountsComplete
+    FfiSeam.resultToIntInjective
diff --git a/src/interface/abi/chapeliser-abi.ipkg b/src/interface/abi/chapeliser-abi.ipkg
index c043ae3..e508ac0 100644
--- a/src/interface/abi/chapeliser-abi.ipkg
+++ b/src/interface/abi/chapeliser-abi.ipkg
@@ -14,3 +14,4 @@ modules = Chapeliser.ABI.Types
         , Chapeliser.ABI.Partition
         , Chapeliser.ABI.Invariants
         , Chapeliser.ABI.FfiSeam
+        , Chapeliser.ABI.Capstone