diff --git a/src/interface/abi/Eclexiaiser/ABI/FfiSeam.idr b/src/interface/abi/Eclexiaiser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..5a82b9e
--- /dev/null
+++ b/src/interface/abi/Eclexiaiser/ABI/FfiSeam.idr
@@ -0,0 +1,133 @@
+-- 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 Eclexiaiser.
+|||
+||| 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 is
+||| the proof-side counterpart: it shows the on-the-wire encoding
+||| `resultToInt : Result -> Bits32` is SOUND.
+|||
+||| Soundness here means two things:
+|||   1. FAITHFUL / LOSSLESS — there is a decoder `intToResult` such that
+||| every ABI outcome round-trips through the C integer back to itself
+||| (`resultRoundTrip`).
+|||   2. UNAMBIGUOUS / INJECTIVE — distinct ABI outcomes never collide on the
+||| wire (`resultToIntInjective`), DERIVED from the round-trip so the two
+||| facts cannot drift apart.
+|||
+||| Genuine proof only: no believe_me / idris_crash / assert_total / postulate /
+||| sorry. Every claim is machine-checked by the totality + coverage checker.
+|||
+||| @see Eclexiaiser.ABI.Types for the `Result` enum and `resultToInt`.
+
+module Eclexiaiser.ABI.FfiSeam
+
+import Eclexiaiser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Decoder: C integer -> ABI Result
+--------------------------------------------------------------------------------
+
+||| Decode a C integer (as produced by the Zig FFI) back into an ABI `Result`.
+||| Unknown codes decode to `Nothing` — the wire format is closed.
+|||
+||| Built with concrete `==` tests on primitive `Bits32` literals: these reduce
+||| definitionally on concrete constants, so the round-trip proofs below check
+||| by `Refl`.
+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 BudgetExceeded
+  else if x == 6 then Just CarbonLimitExceeded
+  else if x == 7 then Just CounterUnavailable
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- (b) Faithful / lossless encoding: the round-trip law
+--------------------------------------------------------------------------------
+
+||| FAITHFULNESS: encoding a `Result` to its C integer and decoding it back
+||| recovers exactly the original outcome. Nothing is lost across the seam.
+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 BudgetExceeded = Refl
+resultRoundTrip CarbonLimitExceeded = Refl
+resultRoundTrip CounterUnavailable = Refl
+
+--------------------------------------------------------------------------------
+-- (a) Unambiguous encoding: injectivity, DERIVED from the round-trip
+--------------------------------------------------------------------------------
+
+||| `Just` is injective — local lemma so injectivity is derived cleanly from the
+||| round-trip without depending on a particular base-library name.
+||| Both arguments are bound explicitly as erased implicits (no auto-bind warn).
+private
+justInj : {0 a, b : Result} -> Just a = Just b -> a = b
+justInj Refl = Refl
+
+||| UNAMBIGUITY: `resultToInt` is injective — distinct ABI outcomes can never
+||| collide on the wire. Derived from `resultRoundTrip`: if two results encode
+||| to the same integer, decoding that integer gives the same `Just`, and the
+||| round-trip law pins each side back to its own constructor.
+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)
+--------------------------------------------------------------------------------
+
+||| The success code 0 decodes to `Ok`.
+export
+decodeZeroIsOk : intToResult 0 = Just Ok
+decodeZeroIsOk = Refl
+
+||| The budget-exceeded code 5 decodes to `BudgetExceeded` (Zig FFI contract).
+export
+decodeFiveIsBudgetExceeded : intToResult 5 = Just BudgetExceeded
+decodeFiveIsBudgetExceeded = Refl
+
+||| The highest defined code 7 decodes to `CounterUnavailable`.
+export
+decodeSevenIsCounterUnavailable : intToResult 7 = Just CounterUnavailable
+decodeSevenIsCounterUnavailable = Refl
+
+||| An out-of-range code decodes to `Nothing` — the wire format is closed.
+export
+decodeUnknownIsNothing : intToResult 8 = Nothing
+decodeUnknownIsNothing = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity control
+--------------------------------------------------------------------------------
+
+||| NON-VACUITY: two DISTINCT result codes really do encode to DISTINCT
+||| integers. Without this, an encoding mapping everything to 0 would satisfy
+||| injectivity vacuously. Machine-checked: 0 = 1 is refuted by the coverage
+||| checker on primitive `Bits32` literals.
+export
+okEncodingDiffersFromError : Not (resultToInt Ok = resultToInt Error)
+okEncodingDiffersFromError = \case Refl impossible
+
+||| A second non-vacuity witness on a non-adjacent pair, to pin that the
+||| separation is not an artefact of the first two codes.
+export
+okEncodingDiffersFromCounterUnavailable :
+  Not (resultToInt Ok = resultToInt CounterUnavailable)
+okEncodingDiffersFromCounterUnavailable = \case Refl impossible
diff --git a/src/interface/abi/eclexiaiser-abi.ipkg b/src/interface/abi/eclexiaiser-abi.ipkg
index 8b44efe..301373e 100644
--- a/src/interface/abi/eclexiaiser-abi.ipkg
+++ b/src/interface/abi/eclexiaiser-abi.ipkg
@@ -9,3 +9,4 @@ modules = Eclexiaiser.ABI.Types
         , Eclexiaiser.ABI.Proofs
         , Eclexiaiser.ABI.Semantics
         , Eclexiaiser.ABI.Invariants
+        , Eclexiaiser.ABI.FfiSeam