diff --git a/src/interface/abi/Ephapaxiser/ABI/FfiSeam.idr b/src/interface/abi/Ephapaxiser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..93793f5
--- /dev/null
+++ b/src/interface/abi/Ephapaxiser/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: Seals the ABI<->FFI seam for ephapaxiser.
+|||
+||| The structural gate (scripts/abi-ffi-gate.py) checks that the Idris and Zig
+||| `Result` enums agree by name+value. This module supplies the PROOF-SIDE
+||| guarantee that the encoding `resultToInt : Result -> Bits32` is SOUND:
+|||
+|||   (a) it is injective  — distinct ABI outcomes never collide on the wire;
+|||   (b) it round-trips    — a decoder `intToResult` faithfully recovers the
+|||       original `Result` from the C integer, so the encoding is lossless.
+|||
+||| Injectivity is then DERIVED from the round-trip (the cleanest route), and a
+||| direct case-analysis injectivity proof is also given as cross-validation.
+|||
+||| Plus positive controls (concrete decodes = Refl) and a non-vacuity /
+||| negative control (two distinct codes have provably distinct ints).
+
+module Ephapaxiser.ABI.FfiSeam
+
+import Ephapaxiser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Decoder (faithful inverse of resultToInt)
+--------------------------------------------------------------------------------
+
+||| Decode a C integer back into a `Result`. Built with boolean `==` on
+||| concrete `Bits32` literals (which reduces definitionally on constants), so
+||| the round-trip `Refl`s below typecheck. Any out-of-range integer decodes to
+||| `Nothing`, modelling an unrecognised wire value.
+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 AlreadyConsumed
+  else if x == 6 then Just ResourceLeaked
+  else if x == 7 then Just DoubleFree
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- (b) Round-trip: the encoding is lossless / faithful
+--------------------------------------------------------------------------------
+
+||| `intToResult` is a left inverse of `resultToInt`: every `Result` survives a
+||| trip out to C and back unchanged. This is the core soundness theorem of the
+||| seam — the C integer carries exactly the ABI outcome and nothing is lost.
+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 AlreadyConsumed = Refl
+resultRoundTrip ResourceLeaked = Refl
+resultRoundTrip DoubleFree = Refl
+
+--------------------------------------------------------------------------------
+-- (a) Injectivity, DERIVED from the round-trip
+--------------------------------------------------------------------------------
+
+||| `Just` is injective. Proved directly by matching on the single inhabiting
+||| constructor — no library dependency.
+public export
+justInj : {0 x, y : a} -> Just x = Just y -> x = y
+justInj Refl = Refl
+
+||| Distinct ABI outcomes never collide on the wire. Derived cleanly from the
+||| round-trip: if `resultToInt a = resultToInt b` then decoding both sides
+||| gives the same `Just`, and `Just` is injective.
+|||
+|||   intToResult (resultToInt a) = intToResult (resultToInt b)   [cong]
+|||   Just a                      = Just b                          [round-trip, twice]
+|||   a                           = b                               [justInj]
+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 evaluate as expected)
+--------------------------------------------------------------------------------
+
+||| `0` on the wire decodes to `Ok`.
+public export
+decodeZeroIsOk : intToResult 0 = Just Ok
+decodeZeroIsOk = Refl
+
+||| `7` on the wire decodes to `DoubleFree` (the last code).
+public export
+decodeSevenIsDoubleFree : intToResult 7 = Just DoubleFree
+decodeSevenIsDoubleFree = Refl
+
+||| An out-of-range wire value (8) decodes to `Nothing`.
+public export
+decodeEightIsNothing : intToResult 8 = Nothing
+decodeEightIsNothing = Refl
+
+||| Concrete round-trip control: `AlreadyConsumed` survives encode/decode.
+public export
+roundTripAlreadyConsumed : intToResult (resultToInt AlreadyConsumed) = Just AlreadyConsumed
+roundTripAlreadyConsumed = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity control (the seam is not trivially collapsing)
+--------------------------------------------------------------------------------
+
+||| Two DISTINCT result codes have DISTINCT ints. Machine-checked proof that
+||| the encoding does not vacuously map everything to the same value — without
+||| this, injectivity would be content-free. `resultToInt Ok = 0` and
+||| `resultToInt Error = 1` reduce to distinct primitive `Bits32` literals,
+||| which Idris's coverage checker discharges via `Refl impossible`.
+public export
+okNotError : Not (resultToInt Ok = resultToInt Error)
+okNotError = \case Refl impossible
+
+||| A second non-vacuity witness across non-adjacent codes.
+public export
+okNotDoubleFree : Not (resultToInt Ok = resultToInt DoubleFree)
+okNotDoubleFree = \case Refl impossible
diff --git a/src/interface/abi/ephapaxiser-abi.ipkg b/src/interface/abi/ephapaxiser-abi.ipkg
index fd7563f..88e65b2 100644
--- a/src/interface/abi/ephapaxiser-abi.ipkg
+++ b/src/interface/abi/ephapaxiser-abi.ipkg
@@ -11,3 +11,4 @@ modules = Ephapaxiser.ABI.Types
         , Ephapaxiser.ABI.Proofs
         , Ephapaxiser.ABI.Semantics
         , Ephapaxiser.ABI.Invariants
+        , Ephapaxiser.ABI.FfiSeam