diff --git a/src/interface/abi/Oblibeniser/ABI/FfiSeam.idr b/src/interface/abi/Oblibeniser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..bb4ddc0
--- /dev/null
+++ b/src/interface/abi/Oblibeniser/ABI/FfiSeam.idr
@@ -0,0 +1,115 @@
+-- SPDX-License-Identifier: MPL-2.0
+-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+--
+||| Layer 4: ABI<->FFI seam soundness for Oblibeniser.
+|||
+||| The structural gate (scripts/abi-ffi-gate.py) checks that the Idris2 `Result`
+||| enum and the Zig FFI enum agree by name+value. This module supplies the
+||| PROOF-SIDE guarantee that the encoding `resultToInt : Result -> Bits32` is
+||| SOUND: distinct ABI outcomes never collide on the wire, and the C integer
+||| faithfully round-trips back to the ABI value.
+|||
+||| Theorems:
+|||   * intToResult       — a total decoder Bits32 -> Maybe Result.
+|||   * resultRoundTrip   — intToResult (resultToInt r) = Just r  (lossless).
+|||   * resultToIntInjective — distinct Results never share an int code,
+|||     DERIVED from the round-trip via justInjective + cong.
+|||
+||| Plus positive controls (concrete decodes by Refl) and a non-vacuity /
+||| negative control (Ok and Error provably differ on the wire).
+
+module Oblibeniser.ABI.FfiSeam
+
+import Oblibeniser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Local lemma
+--------------------------------------------------------------------------------
+
+||| `Just` is injective: peel it off both sides. Genuine proof by matching the
+||| single inhabiting `Refl`.
+private
+justInj : {0 a, b : ty} -> Just a = Just b -> a = b
+justInj Refl = Refl
+
+--------------------------------------------------------------------------------
+-- Decoder
+--------------------------------------------------------------------------------
+
+||| Decode a C integer back to a Result. Built with boolean `==` on Bits32
+||| literals (which reduces definitionally on concrete constants), so the
+||| round-trip Refls below check. Any value outside 0..7 is unrepresentable
+||| and decodes to Nothing.
+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 NotReversible
+  else if x == 6 then Just AuditViolation
+  else if x == 7 then Just InverseProofFailed
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- Round-trip: the encoding is lossless
+--------------------------------------------------------------------------------
+
+||| Encoding then decoding recovers the original Result exactly.
+||| Each clause reduces through the boolean `==` ladder above.
+public 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 NotReversible      = Refl
+resultRoundTrip AuditViolation     = Refl
+resultRoundTrip InverseProofFailed = Refl
+
+--------------------------------------------------------------------------------
+-- Injectivity, derived from the round-trip
+--------------------------------------------------------------------------------
+
+||| The encoding is unambiguous: distinct ABI outcomes never collide on the
+||| wire. Derived purely from `resultRoundTrip`: if two Results encode to the
+||| same int, then decoding that int gives `Just a` and `Just b` for the same
+||| value, so `Just a = Just b`, hence `a = b`.
+public export
+resultToIntInjective : (a, b : Result) -> resultToInt a = resultToInt b -> a = b
+resultToIntInjective a b prf =
+  justInj $
+    trans (sym (resultRoundTrip a)) (trans (cong intToResult prf) (resultRoundTrip b))
+
+--------------------------------------------------------------------------------
+-- Positive controls (concrete decodes by Refl)
+--------------------------------------------------------------------------------
+
+||| Decoding 0 yields Ok.
+decodeZeroIsOk : intToResult 0 = Just Ok
+decodeZeroIsOk = Refl
+
+||| Decoding 7 yields InverseProofFailed (the last code).
+decodeSevenIsInverseProofFailed : intToResult 7 = Just InverseProofFailed
+decodeSevenIsInverseProofFailed = Refl
+
+||| Out-of-range codes decode to Nothing.
+decodeEightIsNothing : intToResult 8 = Nothing
+decodeEightIsNothing = Refl
+
+--------------------------------------------------------------------------------
+-- Non-vacuity / negative control
+--------------------------------------------------------------------------------
+
+||| Machine-checked proof that two DISTINCT result codes have DISTINCT ints:
+||| Ok (0) and Error (1) cannot share a wire value. This rules out a vacuous
+||| injectivity statement (which would hold trivially if the encoding were
+||| constant). The witness is `\case Refl impossible`: the coverage checker
+||| discharges it because the primitive Bits32 literals 0 and 1 differ.
+okErrorDistinctOnWire : Not (resultToInt Ok = resultToInt Error)
+okErrorDistinctOnWire = \case Refl impossible
diff --git a/src/interface/abi/oblibeniser-abi.ipkg b/src/interface/abi/oblibeniser-abi.ipkg
index 742d484..98e3684 100644
--- a/src/interface/abi/oblibeniser-abi.ipkg
+++ b/src/interface/abi/oblibeniser-abi.ipkg
@@ -11,3 +11,4 @@ modules = Oblibeniser.ABI.Types
         , Oblibeniser.ABI.Proofs
         , Oblibeniser.ABI.Semantics
         , Oblibeniser.ABI.Invariants
+        , Oblibeniser.ABI.FfiSeam