diff --git a/src/interface/abi/Atsiser/ABI/FfiSeam.idr b/src/interface/abi/Atsiser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..7cdbf1d
--- /dev/null
+++ b/src/interface/abi/Atsiser/ABI/FfiSeam.idr
@@ -0,0 +1,128 @@
+-- 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 for atsiser.
+|||
+||| The structural gate (scripts/abi-ffi-gate.py) checks that the Idris
+||| `Result` enum and the Zig FFI enum agree by name and value. This module
+||| supplies the PROOF-SIDE guarantee that the integer encoding `resultToInt`
+||| is SOUND:
+|||
+|||   * Faithfulness (round-trip): a decoder `intToResult` recovers exactly
+|||     the `Result` that `resultToInt` produced — the C integer faithfully
+|||     round-trips back to the ABI value, losing nothing.
+|||   * Injectivity: distinct ABI outcomes never collide on the wire. This is
+|||     DERIVED from the round-trip via `justInjective` + `cong`, the cleanest
+|||     route, since the round-trip `Refl`s reduce definitionally.
+|||
+||| Positive controls (concrete decodes = Refl) and a non-vacuity / negative
+||| control (two distinct codes have distinct ints, machine-checked) guard
+||| against a vacuously-true encoding.
+
+module Atsiser.ABI.FfiSeam
+
+import Atsiser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Decoder
+--------------------------------------------------------------------------------
+
+||| Decode a C integer back into a `Result`.
+|||
+||| Built with boolean `Bits32` `==` chained through `if ... then ... else`,
+||| because that form reduces definitionally on concrete literals — so the
+||| round-trip `Refl`s below check. Any integer outside the defined range
+||| decodes to `Nothing` (no spurious ABI value is fabricated).
+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 OwnershipViolation
+  else if x == 6 then Just BoundsViolation
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- Faithfulness: round-trip
+--------------------------------------------------------------------------------
+
+||| The encoding is lossless: decoding the encoding of any `Result` returns
+||| exactly that `Result`. Each case reduces by `Refl` because the `if`/`==`
+||| decoder evaluates on the concrete `Bits32` literal that `resultToInt`
+||| emits.
+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 OwnershipViolation = Refl
+resultRoundTrip BoundsViolation = Refl
+
+--------------------------------------------------------------------------------
+-- Injectivity (derived from the round-trip)
+--------------------------------------------------------------------------------
+
+||| The encoding is injective: distinct ABI outcomes never collide on the
+||| wire. If `resultToInt a = resultToInt b`, then decoding both sides yields
+||| the same `Just`, and `justInjective` extracts `a = b`.
+|||
+||| Derivation:
+|||   Just a = intToResult (resultToInt a)   -- sym (resultRoundTrip a)
+|||          = intToResult (resultToInt b)   -- cong intToResult prf
+|||          = Just b                        -- resultRoundTrip b
+||| then `Just a = Just b` is inverted to `a = b` (constructors are injective).
+public export
+resultToIntInjective : (a, b : Result)
+                    -> resultToInt a = resultToInt b
+                    -> a = b
+resultToIntInjective a b prf =
+  let justEq : (the (Maybe Result) (Just a) = Just b)
+             = trans (sym (resultRoundTrip a))
+                     (trans (cong intToResult prf) (resultRoundTrip b))
+  in case justEq of Refl => Refl
+
+--------------------------------------------------------------------------------
+-- Positive controls (concrete decodes)
+--------------------------------------------------------------------------------
+
+||| Concrete decode: integer 0 decodes to success.
+export
+decodeZeroIsOk : intToResult 0 = Just Ok
+decodeZeroIsOk = Refl
+
+||| Concrete decode: integer 6 decodes to the bounds-violation code.
+export
+decodeSixIsBounds : intToResult 6 = Just BoundsViolation
+decodeSixIsBounds = Refl
+
+||| Concrete decode: an out-of-range integer decodes to nothing — no ABI
+||| value is invented for codes the contract does not define.
+export
+decodeOutOfRangeIsNothing : intToResult 7 = Nothing
+decodeOutOfRangeIsNothing = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity control
+--------------------------------------------------------------------------------
+
+||| Non-vacuity: two DISTINCT result codes encode to DISTINCT integers, so
+||| the injectivity theorem above is not vacuously true. Discharged by the
+||| coverage checker — distinct primitive `Bits32` literals (0 and 1) are
+||| provably unequal.
+export
+okEncodesDistinctFromError : Not (resultToInt Ok = resultToInt Error)
+okEncodesDistinctFromError = \case Refl impossible
+
+||| A second distinct pair, machine-checked the same way (success vs. the
+||| highest code), to underline that the separation is not an artefact of one
+||| adjacent pair.
+export
+okEncodesDistinctFromBounds : Not (resultToInt Ok = resultToInt BoundsViolation)
+okEncodesDistinctFromBounds = \case Refl impossible
diff --git a/src/interface/abi/atsiser-abi.ipkg b/src/interface/abi/atsiser-abi.ipkg
index 7ae70d5..1881cad 100644
--- a/src/interface/abi/atsiser-abi.ipkg
+++ b/src/interface/abi/atsiser-abi.ipkg
@@ -9,3 +9,4 @@ modules = Atsiser.ABI.Types
         , Atsiser.ABI.Proofs
         , Atsiser.ABI.Semantics
         , Atsiser.ABI.Invariants
+        , Atsiser.ABI.FfiSeam