diff --git a/src/interface/abi/Typedqliser/ABI/FfiSeam.idr b/src/interface/abi/Typedqliser/ABI/FfiSeam.idr
new file mode 100644
index 0000000..86aaa34
--- /dev/null
+++ b/src/interface/abi/Typedqliser/ABI/FfiSeam.idr
@@ -0,0 +1,150 @@
+-- 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 TypedQLiser.
+|||
+||| The Idris2 ABI (`Typedqliser.ABI.Types`) defines the FFI result-code enum
+||| `Result` together with `resultToInt : Result -> Bits32` (the integer the Zig
+||| FFI hands back to C), and likewise `ProofStatus` / `statusToInt`. The estate
+||| has a STRUCTURAL gate (`scripts/abi-ffi-gate.py`) checking the Idris and Zig
+||| enums agree by name+value. THIS module is the PROOF-SIDE guarantee: the
+||| encodings are SOUND — distinct ABI outcomes never collide on the wire, and
+||| the C integer faithfully round-trips back to the ABI value.
+|||
+||| Strategy: build total decoders with `if x == n` over boolean `Bits32` `(==)`
+||| (which reduces on concrete literals), prove the round-trip lossless by `Refl`,
+||| and DERIVE injectivity from the round-trip via `justInj` + `cong`.
+
+module Typedqliser.ABI.FfiSeam
+
+import Typedqliser.ABI.Types
+
+%default total
+
+--------------------------------------------------------------------------------
+-- Local lemma
+--------------------------------------------------------------------------------
+
+||| `Just` is injective. Proved locally (no library dependency) so the seam
+||| module stands on its own.
+private
+justInj : {0 x, y : a} -> Just x = Just y -> x = y
+justInj Refl = Refl
+
+--------------------------------------------------------------------------------
+-- Result: decoder
+--------------------------------------------------------------------------------
+
+||| Decode a C integer back into a `Result`. Total; unrecognised codes -> Nothing.
+||| Written with `if … == …` so that the boolean `Bits32` equality reduces on
+||| concrete literals and the round-trip `Refl`s below check definitionally.
+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 InvalidQuery
+  else if x == 3 then Just SchemaError
+  else if x == 4 then Just NullPointer
+  else Nothing
+
+--------------------------------------------------------------------------------
+-- Result: round-trip (faithful / lossless encoding)
+--------------------------------------------------------------------------------
+
+||| The C integer round-trips back to the originating ABI value, for every
+||| `Result`. This is the faithfulness of the wire encoding.
+public export
+resultRoundTrip : (r : Result) -> intToResult (resultToInt r) = Just r
+resultRoundTrip Ok           = Refl
+resultRoundTrip Error        = Refl
+resultRoundTrip InvalidQuery = Refl
+resultRoundTrip SchemaError  = Refl
+resultRoundTrip NullPointer  = Refl
+
+--------------------------------------------------------------------------------
+-- Result: injectivity (no two outcomes collide on the wire)
+--------------------------------------------------------------------------------
+
+||| The encoding is unambiguous: distinct `Result`s map to distinct integers.
+||| DERIVED from the round-trip — if `resultToInt a = resultToInt b` then
+||| `intToResult` of both sides are equal, i.e. `Just a = Just b`, hence `a = b`.
+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))
+
+--------------------------------------------------------------------------------
+-- ProofStatus: decoder, round-trip, injectivity
+--------------------------------------------------------------------------------
+
+||| Decode a C integer back into a `ProofStatus`. Total; unknown -> Nothing.
+public export
+intToStatus : Bits32 -> Maybe ProofStatus
+intToStatus x =
+  if x == 0 then Just Proven
+  else if x == 1 then Just Refuted
+  else if x == 2 then Just Skipped
+  else if x == 3 then Just Timeout
+  else Nothing
+
+||| The proof-status integer round-trips back to its `ProofStatus`.
+public export
+statusRoundTrip : (s : ProofStatus) -> intToStatus (statusToInt s) = Just s
+statusRoundTrip Proven  = Refl
+statusRoundTrip Refuted = Refl
+statusRoundTrip Skipped = Refl
+statusRoundTrip Timeout = Refl
+
+||| `statusToInt` is injective — distinct proof statuses never collide on the wire.
+public export
+statusToIntInjective : (a, b : ProofStatus) -> statusToInt a = statusToInt b -> a = b
+statusToIntInjective a b prf =
+  justInj $
+    trans (sym (statusRoundTrip a)) (trans (cong intToStatus prf) (statusRoundTrip b))
+
+--------------------------------------------------------------------------------
+-- Positive controls (concrete decodes reduce to Refl)
+--------------------------------------------------------------------------------
+
+||| Decoding the wire value `0` yields `Ok`.
+public export
+decodeOk : intToResult 0 = Just Ok
+decodeOk = Refl
+
+||| Decoding the wire value `4` yields `NullPointer`.
+public export
+decodeNullPointer : intToResult 4 = Just NullPointer
+decodeNullPointer = Refl
+
+||| Decoding an out-of-range wire value yields `Nothing`.
+public export
+decodeUnknown : intToResult 99 = Nothing
+decodeUnknown = Refl
+
+||| Decoding the wire value `0` yields `Proven`.
+public export
+decodeProven : intToStatus 0 = Just Proven
+decodeProven = Refl
+
+--------------------------------------------------------------------------------
+-- Negative / non-vacuity controls (machine-checked dis-equalities)
+--------------------------------------------------------------------------------
+
+||| NON-VACUITY: two DISTINCT result codes encode to DISTINCT integers.
+||| `Ok` -> 0 and `Error` -> 1, and the primitive `Bits32` literals `0` and `1`
+||| are provably unequal, so the coverage checker discharges `Refl impossible`.
+public export
+okNotError : Not (resultToInt Ok = resultToInt Error)
+okNotError Refl impossible
+
+||| NON-VACUITY: `SchemaError` (3) and `NullPointer` (4) differ on the wire.
+public export
+schemaNotNull : Not (resultToInt SchemaError = resultToInt NullPointer)
+schemaNotNull Refl impossible
+
+||| NON-VACUITY for `ProofStatus`: `Proven` (0) and `Refuted` (1) differ.
+public export
+provenNotRefuted : Not (statusToInt Proven = statusToInt Refuted)
+provenNotRefuted Refl impossible
diff --git a/src/interface/abi/typedqliser-abi.ipkg b/src/interface/abi/typedqliser-abi.ipkg
index acb74a8..c0ff7cd 100644
--- a/src/interface/abi/typedqliser-abi.ipkg
+++ b/src/interface/abi/typedqliser-abi.ipkg
@@ -13,3 +13,4 @@ modules = Typedqliser.ABI.Types
         , Typedqliser.ABI.SchemaBound
         , Typedqliser.ABI.TypeCompat
         , Typedqliser.ABI.Invariants
+        , Typedqliser.ABI.FfiSeam