diff --git a/src/interface/abi/Julianiser/ABI/FfiSeam.idr b/src/interface/abi/Julianiser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..57e2819
--- /dev/null
+++ b/src/interface/abi/Julianiser/ABI/FfiSeam.idr
@@ -0,0 +1,131 @@
+-- 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 proofs for Julianiser.
+|||
+||| The structural gate (scripts/abi-ffi-gate.py) checks that the Idris2 `Result`
+||| enum and the Zig FFI enum agree by name+value. THIS module is the proof-side
+||| guarantee that the wire encoding is itself SOUND:
+|||
+|||   (a) `resultToIntInjective` — distinct ABI `Result` outcomes never collide
+|||       on the wire (the encoding is unambiguous).
+|||   (b) `intToResult` + `resultRoundTrip` — the C integer faithfully and
+|||       losslessly round-trips back to the originating ABI `Result`.
+|||
+||| Injectivity (a) is DERIVED from the round-trip (b) via `cong` + `justInj`,
+||| which is the cleanest argument: if two results encode to the same int, then
+||| decoding that int yields the same `Just`, hence the results are equal.
+|||
+||| Julianiser has exactly one FFI enum encoder (`Result`/`resultToInt`); there
+||| is no `ProofStatus`/`statusToInt` or other encoder to mirror, so clause (c)
+||| of the seam obligation is vacuous here.
+|||
+||| Genuine proof only: no `believe_me`, `idris_crash`, `assert_total`,
+||| `postulate`, `sorry`. Decidable primitive Bits32 equality discharges the
+||| negative (non-vacuity) control.
+
+module Julianiser.ABI.FfiSeam
+
+import Julianiser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Local lemma
+--------------------------------------------------------------------------------
+
+||| `Just` is injective. Proved by pattern matching on the single inhabitant
+||| of the equality (the `Just x = Just y` shape forces `x = y`).
+private
+justInj : {0 x, y : a} -> Just x = Just y -> x = y
+justInj Refl = Refl
+
+--------------------------------------------------------------------------------
+-- Decoder: the inverse of resultToInt
+--------------------------------------------------------------------------------
+
+||| Decode a C integer result code back to the ABI `Result`. Built with nested
+||| boolean `if`/`==` on concrete Bits32 literals so the round-trip equalities
+||| reduce definitionally to `Refl`. Unknown codes decode to `Nothing`.
+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 ParseError
+  else if x == 6 then Just CodegenError
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- (b) Faithful / lossless round-trip
+--------------------------------------------------------------------------------
+
+||| The wire encoding is lossless: decoding the encoding of any `Result`
+||| recovers exactly that `Result`. Each clause reduces through the boolean
+||| `==` comparisons on concrete literals.
+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 ParseError   = Refl
+resultRoundTrip CodegenError = Refl
+
+--------------------------------------------------------------------------------
+-- (a) Injectivity, DERIVED from the round-trip
+--------------------------------------------------------------------------------
+
+||| The encoding is unambiguous: distinct `Result` values never map to the same
+||| C integer. Derived from the round-trip — if `resultToInt a = resultToInt b`,
+||| then applying `intToResult` to both sides (via `cong`) gives
+||| `Just a = Just b` (using both round-trips), and `justInj` yields
+||| `a = b`. No case analysis on the 49-cell matrix is needed.
+export
+resultToIntInjective : (a, b : Result) ->
+                       resultToInt a = resultToInt b -> a = b
+resultToIntInjective a b prf =
+  justInj $
+    rewrite sym (resultRoundTrip a) in
+    rewrite sym (resultRoundTrip b) in
+    cong intToResult prf
+
+--------------------------------------------------------------------------------
+-- Positive controls (concrete decodes machine-checked = Refl)
+--------------------------------------------------------------------------------
+
+||| Positive control: code 0 decodes to Ok.
+export
+decodeZeroIsOk : intToResult 0 = Just Ok
+decodeZeroIsOk = Refl
+
+||| Positive control: code 6 decodes to CodegenError (the largest code).
+export
+decodeSixIsCodegenError : intToResult 6 = Just CodegenError
+decodeSixIsCodegenError = Refl
+
+||| Positive control: an out-of-range code decodes to Nothing.
+export
+decodeSevenIsNothing : intToResult 7 = Nothing
+decodeSevenIsNothing = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity control (machine-checked)
+--------------------------------------------------------------------------------
+
+||| Non-vacuity: two DISTINCT result codes have DISTINCT wire integers. If this
+||| were not machine-checkable the injectivity theorem above would be vacuously
+||| true. Discharged by Idris's coverage checker on distinct primitive Bits32
+||| literals (0 vs 1).
+export
+okIntNotErrorInt : Not (resultToInt Ok = resultToInt Error)
+okIntNotErrorInt = \case Refl impossible
+
+||| A second distinct-pair witness across non-adjacent codes (0 vs 6).
+export
+okIntNotCodegenInt : Not (resultToInt Ok = resultToInt CodegenError)
+okIntNotCodegenInt = \case Refl impossible
diff --git a/src/interface/abi/julianiser-abi.ipkg b/src/interface/abi/julianiser-abi.ipkg
index d844c3c..406e92e 100644
--- a/src/interface/abi/julianiser-abi.ipkg
+++ b/src/interface/abi/julianiser-abi.ipkg
@@ -11,3 +11,4 @@ modules = Julianiser.ABI.Types
         , Julianiser.ABI.Proofs
         , Julianiser.ABI.Semantics
         , Julianiser.ABI.Invariants
+        , Julianiser.ABI.FfiSeam