diff --git a/src/interface/abi/Idrisiser/ABI/FfiSeam.idr b/src/interface/abi/Idrisiser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..eb2b5bf
--- /dev/null
+++ b/src/interface/abi/Idrisiser/ABI/FfiSeam.idr
@@ -0,0 +1,132 @@
+-- SPDX-License-Identifier: MPL-2.0
+-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+--
+||| Layer 4 — Sealing the ABI<->FFI seam.
+|||
+||| The structural gate (scripts/abi-ffi-gate.py) checks that the Idris2 and
+||| Zig result-code enums agree by name and value. This module supplies the
+||| PROOF-SIDE guarantee that the encoding `resultToInt : Result -> Bits32`
+||| is SOUND:
+|||
+|||   * distinct ABI outcomes never collide on the wire (injectivity), and
+|||   * the C integer faithfully round-trips back to the ABI value
+|||     (a total decoder `intToResult` with `intToResult (resultToInt r) = Just r`).
+|||
+||| Injectivity is DERIVED from the round-trip (the cleanest route): if
+||| `resultToInt a = resultToInt b` then applying `intToResult` to both sides
+||| (via `cong`) and using the round-trip identities forces `Just a = Just b`,
+||| whence `a = b` by injectivity of `Just`.
+|||
+||| The decoder is built with boolean `Bits32` `==` (which reduces on concrete
+||| literals) so the round-trip proofs discharge by `Refl`.
+
+module Idrisiser.ABI.FfiSeam
+
+import Idrisiser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Local helper: Just is injective
+--------------------------------------------------------------------------------
+
+||| `Just` is injective. Proved locally to avoid depending on a particular
+||| base-library name; the single `Refl` clause forces the two payloads equal.
+private
+justInj : {0 x, y : a} -> Just x = Just y -> x = y
+justInj Refl = Refl
+
+--------------------------------------------------------------------------------
+-- Decoder: C integer -> ABI Result
+--------------------------------------------------------------------------------
+
+||| Decode a C result code back into the ABI `Result`.
+|||
+||| Built with chained boolean `==` on `Bits32` rather than literal
+||| pattern-matching: `==` reduces definitionally on concrete constants, so the
+||| round-trip lemmas below check by `Refl`. Any integer outside the known
+||| range decodes to `Nothing` (faithful: the encoder is not surjective).
+public export
+intToResult : Bits32 -> Maybe Result
+intToResult x =
+  if x == 0 then Just Ok
+  else if x == 1 then Just Error
+  else if x == 2 then Just InvalidParam
+  else if x == 3 then Just OutOfMemory
+  else if x == 4 then Just NullPointer
+  else if x == 5 then Just ProofFailure
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- (b) Faithful / lossless round-trip
+--------------------------------------------------------------------------------
+
+||| Encoding then decoding recovers the original ABI value: the C integer is a
+||| faithful representation of every `Result`. Each clause reduces because the
+||| corresponding `==` test on the concrete literal evaluates to `True`.
+export
+resultRoundTrip : (r : Result) -> intToResult (resultToInt r) = Just r
+resultRoundTrip Ok           = Refl
+resultRoundTrip Error        = Refl
+resultRoundTrip InvalidParam = Refl
+resultRoundTrip OutOfMemory  = Refl
+resultRoundTrip NullPointer  = Refl
+resultRoundTrip ProofFailure = Refl
+
+--------------------------------------------------------------------------------
+-- (a) Injectivity, derived from the round-trip
+--------------------------------------------------------------------------------
+
+||| The encoding is unambiguous: distinct outcomes never collide on the wire.
+||| Derived from `resultRoundTrip` — no case explosion, no `believe_me`.
+|||
+||| Given `resultToInt a = resultToInt b`, `cong intToResult` yields
+||| `intToResult (resultToInt a) = intToResult (resultToInt b)`; rewriting both
+||| ends by the round-trip gives `Just a = Just b`, and injectivity of `Just`
+||| strips the constructor.
+export
+resultToIntInjective : (a, b : Result)
+                    -> resultToInt a = resultToInt b
+                    -> a = b
+resultToIntInjective a b prf =
+  justInj $
+    rewrite sym (resultRoundTrip a) in
+    rewrite sym (resultRoundTrip b) in
+    cong intToResult prf
+
+--------------------------------------------------------------------------------
+-- Positive controls (concrete decodes, machine-checked)
+--------------------------------------------------------------------------------
+
+||| Decoding 0 yields Ok.
+decodeZeroIsOk : intToResult 0 = Just Ok
+decodeZeroIsOk = Refl
+
+||| Decoding 5 yields ProofFailure (the top of the range).
+decodeFiveIsProofFailure : intToResult 5 = Just ProofFailure
+decodeFiveIsProofFailure = Refl
+
+||| Decoding an out-of-range code yields Nothing (encoder is not surjective).
+decodeOutOfRangeIsNothing : intToResult 99 = Nothing
+decodeOutOfRangeIsNothing = Refl
+
+||| Round-trip control for a specific value.
+roundTripOk : intToResult (resultToInt Ok) = Just Ok
+roundTripOk = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity control
+--------------------------------------------------------------------------------
+
+||| Two DISTINCT result codes have DISTINCT wire integers. This rules out the
+||| vacuous reading of injectivity: the encoding genuinely separates outcomes.
+||| `0 = 1` on `Bits32` is refuted by the coverage checker on distinct
+||| primitive constants.
+export
+okWireDistinctFromError : Not (resultToInt Ok = resultToInt Error)
+okWireDistinctFromError = \case Refl impossible
+
+||| A second non-vacuity witness across a non-adjacent pair.
+export
+okWireDistinctFromProofFailure : Not (resultToInt Ok = resultToInt ProofFailure)
+okWireDistinctFromProofFailure = \case Refl impossible
diff --git a/src/interface/abi/idrisiser-abi.ipkg b/src/interface/abi/idrisiser-abi.ipkg
index 4a05ff5..6997c17 100644
--- a/src/interface/abi/idrisiser-abi.ipkg
+++ b/src/interface/abi/idrisiser-abi.ipkg
@@ -11,3 +11,4 @@ modules = Idrisiser.ABI.Types
         , Idrisiser.ABI.Proofs
         , Idrisiser.ABI.Semantics
         , Idrisiser.ABI.Invariants
+        , Idrisiser.ABI.FfiSeam