diff --git a/src/interface/abi/Iseriser/ABI/FfiSeam.idr b/src/interface/abi/Iseriser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..d7a6a87
--- /dev/null
+++ b/src/interface/abi/Iseriser/ABI/FfiSeam.idr
@@ -0,0 +1,117 @@
+-- SPDX-License-Identifier: MPL-2.0
+-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+--
+||| Layer-4 proof: SEALING THE ABI<->FFI SEAM for iseriser.
+|||
+||| The ABI defines `resultToInt : Result -> Bits32`, the integer the Zig FFI
+||| returns to C. The estate's structural gate (`scripts/abi-ffi-gate.py`)
+||| checks the Idris and Zig enums agree by name+value. THIS module supplies
+||| the PROOF-SIDE guarantee that the encoding is SOUND:
+|||
+|||   * `intToResult`         — a decoder Bits32 -> Maybe Result
+|||   * `resultRoundTrip`     — the encoding is faithful/lossless: decoding an
+|||                             encoded result recovers exactly that result
+|||   * `resultToIntInjective`— distinct ABI outcomes never collide on the wire,
+|||                             DERIVED from the round-trip via cong + justInjective
+|||
+||| Plus positive controls (concrete decodes by Refl) and a machine-checked
+||| NON-VACUITY control (two distinct codes have distinct ints).
+
+module Iseriser.ABI.FfiSeam
+
+import Iseriser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Decoder: the inverse of resultToInt
+--------------------------------------------------------------------------------
+
+||| Decode a C integer back to a `Result`. Built with boolean `==` on Bits32
+||| literals (which reduces on concrete constants) so the round-trip `Refl`s
+||| check definitionally. Any value outside 0..5 is not a valid ABI code.
+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 InvalidLanguage
+  else if x == 3 then Just TemplateError
+  else if x == 4 then Just OutputError
+  else if x == 5 then Just NullPointer
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- Faithfulness: the encoding is lossless (round-trips through the wire)
+--------------------------------------------------------------------------------
+
+||| Decoding the encoding of any `Result` recovers exactly that `Result`.
+||| Each clause reduces because `resultToInt` produces a concrete Bits32
+||| literal and the decoder's boolean `==` chain evaluates on that literal.
+public export
+resultRoundTrip : (r : Result) -> intToResult (resultToInt r) = Just r
+resultRoundTrip Ok = Refl
+resultRoundTrip Error = Refl
+resultRoundTrip InvalidLanguage = Refl
+resultRoundTrip TemplateError = Refl
+resultRoundTrip OutputError = Refl
+resultRoundTrip NullPointer = Refl
+
+--------------------------------------------------------------------------------
+-- Injectivity: distinct ABI outcomes never collide on the wire
+--------------------------------------------------------------------------------
+
+||| `Just` is injective: recover the underlying equality from wrapped values.
+||| (Local helper to avoid depending on a Prelude name that may not be in
+||| scope; the off-diagonal `Just x = Nothing` cannot arise here.)
+justEq : {0 x, y : Result} -> Just x = Just y -> x = y
+justEq Refl = Refl
+
+||| The encoding is unambiguous: if two results encode to the same integer,
+||| they ARE the same result. Derived from the round-trip — applying the
+||| decoder to both sides of `resultToInt a = resultToInt b` and chaining the
+||| two round-trip facts forces `Just a = Just b`, hence `a = b`.
+public export
+resultToIntInjective : (a, b : Result) ->
+                       resultToInt a = resultToInt b -> a = b
+resultToIntInjective a b prf =
+  justEq $
+    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 5 yields NullPointer (the highest valid code).
+decodeFiveIsNullPointer : intToResult 5 = Just NullPointer
+decodeFiveIsNullPointer = Refl
+
+||| An out-of-range code decodes to Nothing.
+decodeSixIsNothing : intToResult 6 = Nothing
+decodeSixIsNothing = Refl
+
+||| Concrete round-trip control through the middle of the range.
+roundTripTemplateError : intToResult (resultToInt TemplateError) = Just TemplateError
+roundTripTemplateError = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity control (machine-checked)
+--------------------------------------------------------------------------------
+
+||| Distinct primitive Bits32 literals are provably unequal: the coverage
+||| checker discharges `Refl impossible` for `0 = 1`.
+okIntNotErrorInt : Not (the Bits32 0 = the Bits32 1)
+okIntNotErrorInt = \case Refl impossible
+
+||| NON-VACUITY: two DISTINCT result codes have DISTINCT wire integers.
+||| `resultToInt Ok = 0` and `resultToInt Error = 1` reduce, so any proof
+||| that they are equal would prove `0 = 1`, which is refuted above. This
+||| guarantees `resultToIntInjective` is not vacuously true.
+public export
+okEncodingNotErrorEncoding : Not (resultToInt Ok = resultToInt Error)
+okEncodingNotErrorEncoding prf = okIntNotErrorInt prf
diff --git a/src/interface/abi/iseriser-abi.ipkg b/src/interface/abi/iseriser-abi.ipkg
index b48942d..1dc8105 100644
--- a/src/interface/abi/iseriser-abi.ipkg
+++ b/src/interface/abi/iseriser-abi.ipkg
@@ -11,3 +11,4 @@ modules = Iseriser.ABI.Types
         , Iseriser.ABI.Proofs
         , Iseriser.ABI.Semantics
         , Iseriser.ABI.Invariants
+        , Iseriser.ABI.FfiSeam