diff --git a/src/interface/abi/Lustreiser/ABI/FfiSeam.idr b/src/interface/abi/Lustreiser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..07b3db0
--- /dev/null
+++ b/src/interface/abi/Lustreiser/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 — ABI<->FFI Seam Soundness Proof for Lustreiser
+|||
+||| The structural gate (scripts/abi-ffi-gate.py) checks that the Idris2
+||| `Result` enum and the Zig FFI enum agree by name and value. This module
+||| supplies the *proof-side* guarantee that the encoding itself is SOUND:
+|||
+|||   (a) `resultToInt` is INJECTIVE — distinct ABI outcomes never collide on
+|||       the wire (no two `Result` values map to the same C integer).
+|||   (b) A decoder `intToResult : Bits32 -> Maybe Result` round-trips every
+|||       `Result` faithfully (`resultRoundTrip`), proving the encoding is
+|||       lossless; injectivity is then DERIVED from the round-trip.
+|||
+||| Together these seal the seam: the C integer returned by the Zig FFI
+||| faithfully and unambiguously reconstructs the ABI `Result` it came from.
+|||
+||| Lustreiser defines exactly one FFI result-code encoder (`resultToInt`);
+||| there is no `ProofStatus`/`statusToInt` (or other FFI enum encoder), so
+||| clause (c) is vacuous here.
+
+module Lustreiser.ABI.FfiSeam
+
+import Lustreiser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Local lemma: Just is injective
+--------------------------------------------------------------------------------
+
+||| `Just` is injective. Defined locally (Idris2 0.7.0 has no `justInjective`
+||| in scope here). The implicit equands are erased to avoid auto-binding a
+||| free lowercase name with a warning.
+justInj : {0 x, y : t} -> Just x = Just y -> x = y
+justInj Refl = Refl
+
+--------------------------------------------------------------------------------
+-- (b) Faithful decoder and round-trip
+--------------------------------------------------------------------------------
+
+||| Decode a C integer back into a `Result`. Built with boolean `==` on
+||| concrete `Bits32` literals, which reduces definitionally on each constant,
+||| so the round-trip `Refl`s below check.
+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 DeadlineViolation
+  else if x == 6 then Just ClockError
+  else Nothing
+
+||| Faithful (lossless) encoding: decoding the encoding of any `Result`
+||| recovers exactly that `Result`. Each case reduces because `intToResult`
+||| branches on concrete `Bits32` literals via boolean `==`.
+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 DeadlineViolation = Refl
+resultRoundTrip ClockError = Refl
+
+--------------------------------------------------------------------------------
+-- (a) Injectivity of the encoding (derived from the round-trip)
+--------------------------------------------------------------------------------
+
+||| The encoding is unambiguous: if two `Result`s encode to the same C
+||| integer, they are the same `Result`. Derived cleanly from the round-trip:
+||| `intToResult` applied to both sides yields `Just a = Just b`, and
+||| `justInj` strips the constructor.
+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, machine-checked = Refl)
+--------------------------------------------------------------------------------
+
+||| Decoding 0 yields `Ok`.
+decodeOk : intToResult 0 = Just Ok
+decodeOk = Refl
+
+||| Decoding 6 yields `ClockError` (the largest code).
+decodeClockError : intToResult 6 = Just ClockError
+decodeClockError = Refl
+
+||| An out-of-range code decodes to `Nothing`.
+decodeUnknown : intToResult 7 = Nothing
+decodeUnknown = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity control (distinct codes are distinct on the wire)
+--------------------------------------------------------------------------------
+
+||| Two DISTINCT result codes map to DISTINCT C integers. This is the
+||| non-vacuity witness: injectivity would be trivially true if every
+||| `Result` collapsed to one integer. `resultToInt Ok` reduces to the
+||| primitive `Bits32` literal 0 and `resultToInt Error` to 1; the coverage
+||| checker discharges `Refl impossible` for the two distinct constants.
+okNotError : Not (resultToInt Ok = resultToInt Error)
+okNotError Refl impossible
+
+||| A second distinct-code witness, away from the 0/1 boundary (5 vs 6).
+deadlineNotClock : Not (resultToInt DeadlineViolation = resultToInt ClockError)
+deadlineNotClock Refl impossible
diff --git a/src/interface/abi/lustreiser-abi.ipkg b/src/interface/abi/lustreiser-abi.ipkg
index 03b8966..4b76945 100644
--- a/src/interface/abi/lustreiser-abi.ipkg
+++ b/src/interface/abi/lustreiser-abi.ipkg
@@ -11,3 +11,4 @@ modules = Lustreiser.ABI.Types
         , Lustreiser.ABI.Proofs
         , Lustreiser.ABI.Semantics
         , Lustreiser.ABI.Invariants
+        , Lustreiser.ABI.FfiSeam