diff --git a/src/interface/abi/Betlangiser/ABI/FfiSeam.idr b/src/interface/abi/Betlangiser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..10867bc
--- /dev/null
+++ b/src/interface/abi/Betlangiser/ABI/FfiSeam.idr
@@ -0,0 +1,154 @@
+-- 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 Betlangiser.
+|||
+||| 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 C-integer encoding is SOUND:
+|||
+|||   (a) `resultToIntInjective` — distinct ABI outcomes never collide on the
+|||       wire (the encoding is unambiguous);
+|||   (b) `intToResult` + `resultRoundTrip` — the C integer faithfully and
+|||       losslessly round-trips back to the ABI value, and injectivity is
+|||       DERIVED from the round-trip via `justInjective`;
+|||   (c) the SAME guarantees for the `TernaryBool` FFI enum encoder
+|||       (`ternaryToInt` / `intToTernary`), which is the other C-integer
+|||       result-style encoder this ABI exposes across the seam.
+|||
+||| Method: the decoders are built with boolean `==` on `Bits32` so that the
+||| round-trip `Refl`s reduce definitionally on the concrete primitive
+||| literals. Injectivity is then derived from the round-trip. Positive
+||| controls (concrete `decode = Refl`) and a machine-checked negative /
+||| non-vacuity control (two distinct codes have distinct ints) are included.
+|||
+||| No `believe_me`, `idris_crash`, `assert_total`, `postulate`, or `sorry`:
+||| this is a genuine, total proof.
+
+module Betlangiser.ABI.FfiSeam
+
+import Betlangiser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Generic helper: injectivity of the `Just` constructor
+--------------------------------------------------------------------------------
+
+||| `Just` is injective. (Idris2 0.7.0 base does not export `justInjective`,
+||| so we prove it locally by pattern matching on the equality witness.)
+private
+justInj : {0 x, y : a} -> Just x = Just y -> x = y
+justInj Refl = Refl
+
+--------------------------------------------------------------------------------
+-- (b) Decoder + round-trip for Result
+--------------------------------------------------------------------------------
+
+||| Decode a C integer back into a `Result`. Total: every `Bits32` not in the
+||| valid range maps to `Nothing`. Uses boolean `==` so the comparisons reduce
+||| on concrete primitive literals (enabling the round-trip `Refl`s to 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 InvalidDistribution
+  else if x == 6 then Just SamplingFailed
+  else Nothing
+
+||| Faithful / lossless encoding: decoding the encoding of any `Result`
+||| recovers exactly that `Result`. This is the core seam-soundness theorem —
+||| no ABI outcome is lost or corrupted when it crosses to C and back.
+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 InvalidDistribution = Refl
+resultRoundTrip SamplingFailed = Refl
+
+--------------------------------------------------------------------------------
+-- (a) Injectivity of resultToInt, DERIVED from the round-trip
+--------------------------------------------------------------------------------
+
+||| The encoding is unambiguous: distinct `Result` outcomes never collide on
+||| the wire. Derived cleanly from `resultRoundTrip` via `justInj`:
+||| if `resultToInt a = resultToInt b`, then applying `intToResult` to both
+||| sides gives `Just a = Just b`, whence `a = b`.
+public export
+resultToIntInjective : (a, b : Result) ->
+                       resultToInt a = resultToInt b -> a = b
+resultToIntInjective a b prf =
+  justInj (rewrite sym (resultRoundTrip a) in
+           rewrite prf in
+           resultRoundTrip b)
+
+--------------------------------------------------------------------------------
+-- (c) Same guarantees for the TernaryBool FFI enum encoder
+--------------------------------------------------------------------------------
+-- Types.idr already defines `ternaryToInt` (0=False,1=True,2=Unknown) and the
+-- decoder `intToTernary`. We prove the same round-trip and injectivity here so
+-- the second C-integer encoder crossing the seam is equally sealed.
+
+||| Faithful / lossless encoding for the ternary-logic FFI enum.
+public export
+ternaryRoundTrip : (x : TernaryBool) -> intToTernary (ternaryToInt x) = Just x
+ternaryRoundTrip TFalse = Refl
+ternaryRoundTrip TTrue = Refl
+ternaryRoundTrip TUnknown = Refl
+
+||| The ternary encoding is unambiguous, derived from its round-trip.
+public export
+ternaryToIntInjective : (x, y : TernaryBool) ->
+                        ternaryToInt x = ternaryToInt y -> x = y
+ternaryToIntInjective x y prf =
+  justInj (rewrite sym (ternaryRoundTrip x) in
+           rewrite prf in
+           ternaryRoundTrip y)
+
+--------------------------------------------------------------------------------
+-- Positive controls (concrete decodes, machine-checked by Refl)
+--------------------------------------------------------------------------------
+
+||| Positive control: the wire value 0 decodes to `Ok`.
+public export
+decodeOkControl : intToResult 0 = Just Ok
+decodeOkControl = Refl
+
+||| Positive control: the wire value 6 decodes to `SamplingFailed`.
+public export
+decodeSamplingFailedControl : intToResult 6 = Just SamplingFailed
+decodeSamplingFailedControl = Refl
+
+||| Positive control: the wire value 2 decodes to `TUnknown` (ternary enum).
+public export
+decodeTUnknownControl : intToTernary 2 = Just TUnknown
+decodeTUnknownControl = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity controls (distinct codes => distinct ints)
+--------------------------------------------------------------------------------
+-- These guarantee the injectivity theorems are not vacuously true: there
+-- genuinely EXIST distinct outcomes whose encodings differ. The refutations
+-- are discharged by the coverage checker on distinct primitive Bits32 literals.
+
+||| Non-vacuity: `Ok` and `Error` do NOT collide on the wire.
+public export
+okErrorDistinct : Not (resultToInt Ok = resultToInt Error)
+okErrorDistinct = \case Refl impossible
+
+||| Non-vacuity: `OutOfMemory` and `SamplingFailed` do NOT collide on the wire.
+public export
+oomSamplingDistinct : Not (resultToInt OutOfMemory = resultToInt SamplingFailed)
+oomSamplingDistinct = \case Refl impossible
+
+||| Non-vacuity for the ternary enum: `TTrue` and `TUnknown` do NOT collide.
+public export
+ternaryTrueUnknownDistinct : Not (ternaryToInt TTrue = ternaryToInt TUnknown)
+ternaryTrueUnknownDistinct = \case Refl impossible
diff --git a/src/interface/abi/betlangiser-abi.ipkg b/src/interface/abi/betlangiser-abi.ipkg
index 9c1ed84..aef5256 100644
--- a/src/interface/abi/betlangiser-abi.ipkg
+++ b/src/interface/abi/betlangiser-abi.ipkg
@@ -9,3 +9,4 @@ modules = Betlangiser.ABI.Types
         , Betlangiser.ABI.Proofs
         , Betlangiser.ABI.Semantics
         , Betlangiser.ABI.Invariants
+        , Betlangiser.ABI.FfiSeam