diff --git a/src/interface/abi/Nimiser/ABI/FfiSeam.idr b/src/interface/abi/Nimiser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..40cef1e
--- /dev/null
+++ b/src/interface/abi/Nimiser/ABI/FfiSeam.idr
@@ -0,0 +1,125 @@
+-- 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 Nimiser.
+|||
+||| The structural gate (scripts/abi-ffi-gate.py) checks that the Idris
+||| `Result` enum and the Zig FFI enum agree by name+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 (`resultToIntInjective`).
+|||   (b) there is a total decoder `intToResult` and a round-trip law
+|||       `resultRoundTrip` showing the C integer faithfully reconstructs
+|||       the ABI value (lossless / faithful encoding).
+|||
+||| Injectivity is then DERIVED from the round-trip via `justInjective . cong`,
+||| which is the cleanest route and depends only on the round-trip Refls
+||| reducing. The decoder is written with boolean `==` on Bits32 so that
+||| `intToResult (resultToInt r)` reduces definitionally for each concrete
+||| constructor, making every round-trip clause `Refl`.
+|||
+||| There is no `ProofStatus`/`statusToInt` (or any other FFI enum encoder)
+||| in this repo's ABI, so `Result` is the sole seam to seal here.
+
+module Nimiser.ABI.FfiSeam
+
+import Nimiser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Local helper
+--------------------------------------------------------------------------------
+
+||| `Just` is injective: equal wrapped values come from equal payloads.
+||| (Defined locally; the base library in this toolchain does not export it.)
+private
+justInjective : {0 x, y : a} -> Just x = Just y -> x = y
+justInjective Refl = Refl
+
+--------------------------------------------------------------------------------
+-- Decoder: C integer -> ABI Result
+--------------------------------------------------------------------------------
+
+||| Decode a C integer result code back into the ABI `Result`.
+||| Uses boolean `==` on Bits32 (which reduces on concrete literals) so the
+||| round-trip law below holds definitionally. 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 CompilationFailed
+  else if x == 6 then Just TemplateError
+  else if x == 7 then Just MacroError
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- Round-trip: faithful / lossless encoding
+--------------------------------------------------------------------------------
+
+||| Every ABI result survives a round trip through its C integer encoding:
+||| decoding the encoded value reconstructs exactly the original `Result`.
+||| Each clause reduces to `Refl` because the boolean `==` comparisons on the
+||| concrete literal produced by `resultToInt` evaluate at type-check time.
+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 CompilationFailed = Refl
+resultRoundTrip TemplateError     = Refl
+resultRoundTrip MacroError        = Refl
+
+--------------------------------------------------------------------------------
+-- Injectivity, derived from the round trip
+--------------------------------------------------------------------------------
+
+||| The encoding is unambiguous: distinct ABI outcomes never collide on the
+||| wire. Derived cleanly from the round-trip law — if two results encode to
+||| the same integer, decoding that integer gives `Just a` and `Just b`, so
+||| `Just a = Just b`, hence `a = b`.
+export
+resultToIntInjective : (a, b : Result) -> resultToInt a = resultToInt b -> a = b
+resultToIntInjective a b prf =
+  justInjective $
+    rewrite sym (resultRoundTrip a) in
+    rewrite sym (resultRoundTrip b) in
+    cong intToResult prf
+
+--------------------------------------------------------------------------------
+-- Positive controls (concrete decodes, machine-checked)
+--------------------------------------------------------------------------------
+
+||| Positive control: the integer 0 decodes to `Ok`.
+export
+decodeZeroIsOk : intToResult 0 = Just Ok
+decodeZeroIsOk = Refl
+
+||| Positive control: the integer 7 (the largest code) decodes to `MacroError`.
+export
+decodeSevenIsMacroError : intToResult 7 = Just MacroError
+decodeSevenIsMacroError = Refl
+
+||| Positive control: an out-of-range code (8) decodes to `Nothing`.
+export
+decodeEightIsNothing : intToResult 8 = Nothing
+decodeEightIsNothing = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity control (machine-checked)
+--------------------------------------------------------------------------------
+
+||| Non-vacuity control: two DISTINCT result codes have DISTINCT integers.
+||| If the encoding were trivial/collapsing, this proof would be impossible.
+||| Distinct primitive Bits32 literals are provably unequal, discharged by the
+||| coverage checker via `Refl impossible`.
+export
+okIntDistinctFromErrorInt : Not (resultToInt Ok = resultToInt Error)
+okIntDistinctFromErrorInt = \case Refl impossible
diff --git a/src/interface/abi/nimiser-abi.ipkg b/src/interface/abi/nimiser-abi.ipkg
index c080159..6bedff8 100644
--- a/src/interface/abi/nimiser-abi.ipkg
+++ b/src/interface/abi/nimiser-abi.ipkg
@@ -1,4 +1,4 @@
 -- SPDX-License-Identifier: MPL-2.0
 package nimiser-abi
 sourcedir = "."
-modules = Nimiser.ABI.Types, Nimiser.ABI.Layout, Nimiser.ABI.Foreign, Nimiser.ABI.Proofs, Nimiser.ABI.Semantics, Nimiser.ABI.Invariants
+modules = Nimiser.ABI.Types, Nimiser.ABI.Layout, Nimiser.ABI.Foreign, Nimiser.ABI.Proofs, Nimiser.ABI.Semantics, Nimiser.ABI.Invariants, Nimiser.ABI.FfiSeam