diff --git a/src/interface/abi/Halideiser/ABI/Capstone.idr b/src/interface/abi/Halideiser/ABI/Capstone.idr
new file mode 100644
index 0000000..800c403
--- /dev/null
+++ b/src/interface/abi/Halideiser/ABI/Capstone.idr
@@ -0,0 +1,108 @@
+-- SPDX-License-Identifier: MPL-2.0
+-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+--
+||| Layer 5 — the CAPSTONE ABI SOUNDNESS CERTIFICATE for halideiser.
+|||
+||| Every prior layer proves one slice of the ABI contract in isolation:
+|||
+|||   * Layer 2 (`Halideiser.ABI.Semantics`) — the FLAGSHIP semantic property:
+|||     a loop SPLIT schedule computes exactly what the flat run computes
+|||     (`SplitEquivalent`), with the canonical positive control
+|||     `doubleSplit3x4Equivalent`.
+|||   * Layer 3 (`Halideiser.ABI.Invariants`) — the DEEPER invariant: schedule
+|||     equivalence is a genuine equivalence relation, and TILING (split of an
+|||     outer split) preserves the result by transitive composition, witnessed
+|||     on the canonical positive control `doubleTile2x3x2Equivalent`.
+|||   * Layer 4 (`Halideiser.ABI.FfiSeam`) — the FFI SEAM: the on-the-wire
+|||     `resultToInt` encoding is INJECTIVE (`resultToIntInjective`), so the
+|||     ABI's `Result` values never collide on their C integer codes.
+|||
+||| This module ties those three together into ONE inhabited value. The record
+||| `ABISound` has one field per layer, each typed as the EXACT proven fact
+||| exported by that layer. `abiContractDischarged : ABISound` is constructed
+||| solely from the existing exported witnesses/theorems — no new domain
+||| theorem is proved here. If ANY prior layer were unsound (its witness
+||| missing or its theorem ill-typed), this single value would fail to
+||| typecheck. Thus the capstone is an end-to-end statement: manifest -> ABI
+||| proofs (flagship semantic equivalence + deeper tiling invariant) -> FFI
+||| seam injectivity, all discharged simultaneously.
+|||
+||| Genuine composition only: no believe_me / idris_crash / assert_total /
+||| postulate / sorry / %hint hacks. Every field is a real exported value.
+module Halideiser.ABI.Capstone
+
+import Halideiser.ABI.Types
+import Halideiser.ABI.Semantics
+import Halideiser.ABI.Invariants
+import Halideiser.ABI.FfiSeam
+
+%default total
+
+--------------------------------------------------------------------------------
+-- The capstone certificate
+--------------------------------------------------------------------------------
+
+||| `ABISound` bundles the key proven facts of the halideiser ABI, one per
+||| layer. To inhabit it you must supply, simultaneously:
+|||
+|||   * `flagship` — a Layer-2 `SplitEquivalent` witness on the canonical
+|||     positive-control instance (the `double` stage, split 3x4). This is the
+|||     exact type of `Semantics.doubleSplit3x4Equivalent`.
+|||
+|||   * `invariant` — a Layer-3 `ScheduleEquiv` witness that the tiled run of
+|||     the same stage equals the flat run. This is the exact type of
+|||     `Invariants.doubleTile2x3x2Equivalent`.
+|||
+|||   * `ffiInjective` — the Layer-4 seam theorem that `resultToInt` is
+|||     injective, stored as the proven function value itself. This is the
+|||     exact type of `FfiSeam.resultToIntInjective`.
+|||
+||| None of these can be faked: each field's type is precisely a prior layer's
+||| exported proof obligation.
+public export
+record ABISound where
+  constructor MkABISound
+  flagship     : SplitEquivalent Semantics.double 3 4
+  invariant    : ScheduleEquiv (runTile Semantics.double 2 3 2)
+                               (runFlat Semantics.double ((2 * 3) * 2))
+  ffiInjective : (a, b : Result) -> resultToInt a = resultToInt b -> a = b
+
+--------------------------------------------------------------------------------
+-- The single inhabited capstone value
+--------------------------------------------------------------------------------
+
+||| THE CAPSTONE. One inhabited certificate assembled entirely from the
+||| already-exported witnesses of Layers 2, 3 and 4. Its very existence is the
+||| end-to-end soundness statement: the full ABI contract — flagship semantic
+||| equivalence, deeper tiling invariant, and FFI-seam injectivity — is
+||| discharged together. If any prior layer regressed, this value would not
+||| typecheck.
+public export
+abiContractDischarged : ABISound
+abiContractDischarged =
+  MkABISound
+    doubleSplit3x4Equivalent     -- Layer 2: flagship positive control
+    doubleTile2x3x2Equivalent    -- Layer 3: deeper tiling invariant
+    resultToIntInjective         -- Layer 4: FFI seam injectivity
+
+--------------------------------------------------------------------------------
+-- Field projections (the certificate really exposes each proven fact)
+--------------------------------------------------------------------------------
+
+||| Project the Layer-2 flagship witness out of the discharged certificate.
+public export
+capstoneFlagship : SplitEquivalent Semantics.double 3 4
+capstoneFlagship = abiContractDischarged.flagship
+
+||| Project the Layer-3 invariant witness out of the discharged certificate.
+public export
+capstoneInvariant : ScheduleEquiv (runTile Semantics.double 2 3 2)
+                                  (runFlat Semantics.double ((2 * 3) * 2))
+capstoneInvariant = abiContractDischarged.invariant
+
+||| Use the certificate's FFI-seam field at a concrete pair: equal codes force
+||| equal results. Here the trivial `Refl` premise yields `Ok = Ok`, confirming
+||| the stored injectivity proof really reduces.
+public export
+capstoneFfiOkOk : Ok = Ok
+capstoneFfiOkOk = abiContractDischarged.ffiInjective Ok Ok Refl
diff --git a/src/interface/abi/halideiser-abi.ipkg b/src/interface/abi/halideiser-abi.ipkg
index 7a050bb..7b9e52d 100644
--- a/src/interface/abi/halideiser-abi.ipkg
+++ b/src/interface/abi/halideiser-abi.ipkg
@@ -12,3 +12,4 @@ modules = Halideiser.ABI.Types
         , Halideiser.ABI.Semantics
         , Halideiser.ABI.Invariants
         , Halideiser.ABI.FfiSeam
+        , Halideiser.ABI.Capstone