diff --git a/migration/app/proven/ProvenError.affine b/migration/app/proven/ProvenError.affine
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+++ b/migration/app/proven/ProvenError.affine
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+// SPDX-License-Identifier: AGPL-3.0-or-later
+// Copyright (c) 2026 Joshua B. Jewell and Jonathan D.A. Jewell
+//
+// ProvenError.affine — error types for proven-safe modules.
+//
+// Translation of src/app/proven/ProvenError.res (32 LoC). Self-contained
+// pure-data + pure-constructors module — no FFI, no cross-module deps,
+// no Tea_/Pixi/Gossamer surface. Cleanest possible Mode-A AffineScript
+// port in idaptik's src/app/ tree.
+//
+// Translation choices:
+//   `type x = { field: T, ... }` → unchanged (no trailing comma).
+//   `let f = (~name: T, ...): U => body` → `fn f(name: T, ...) -> U { body }`
+//     (AS uses positional args, not labelled — the ReScript ~prefix
+//     drops on the call site for `notANumber(operation, message)` etc.).
+//   `{operation, message, errorType}`     → `#{operation: operation, ...}`
+//     punning shorthand drops; AS record literals are explicit.
+//   `` `[${a}] ${b}: ${c}` `` template-literal → `"[" ++ a ++ "] " ++ b ++ ": " ++ c`
+//     AS uses `++` concatenation; template literals are JS-specific.
+//
+// What this PR is NOT
+//   Does NOT replace src/app/proven/ProvenError.res in the build. The .res
+//   still owns the runtime contract — SafeFloat / SafeJson / SafeAngle
+//   reference it directly. Replacement requires the same .affine→.mjs
+//   codegen story tracked under standards#279 STEP 8.
+//
+// Refs: standards#279 (megaport step), standards#252 (campaign umbrella).
+
+module ProvenError;
+
+type ProvenError = {
+  operation: String,
+  message: String,
+  errorType: String,
+}
+
+fn notANumber(operation: String, message: String) -> ProvenError {
+  #{ operation: operation, message: message, errorType: "NaN" }
+}
+
+fn infinity(operation: String, message: String) -> ProvenError {
+  #{ operation: operation, message: message, errorType: "Infinity" }
+}
+
+fn parseFailure(operation: String, message: String) -> ProvenError {
+  #{ operation: operation, message: message, errorType: "ParseFailure" }
+}
+
+fn divisionByZero(operation: String, message: String) -> ProvenError {
+  #{ operation: operation, message: message, errorType: "DivisionByZero" }
+}
+
+fn invalidArgument(operation: String, message: String) -> ProvenError {
+  #{ operation: operation, message: message, errorType: "InvalidArgument" }
+}
+
+fn toString(err: ProvenError) -> String {
+  "[" ++ err.errorType ++ "] " ++ err.operation ++ ": " ++ err.message
+}