diff --git a/WorkoutsTests/SplitDocumentCodableTests.swift b/WorkoutsTests/SplitDocumentCodableTests.swift
new file mode 100644
index 0000000..4a5846e
--- /dev/null
+++ b/WorkoutsTests/SplitDocumentCodableTests.swift
@@ -0,0 +1,45 @@
+import Foundation
+import Testing
+import IndieSync
+@testable import Workouts
+
+/// Round-trips `SplitDocument` through `DocumentCoder` — the exact JSON codec
+/// (ISO-8601 dates, sorted keys, pretty-printed) `DocumentFileStore` uses to
+/// write and read every document in this app's iCloud container.
+struct SplitDocumentCodableTests {
+    /// A whole-second epoch so ISO-8601 round-trips (no fractional-second loss).
+    private static let stableTimestamp = Date(timeIntervalSince1970: 1_700_000_000)
+
+    @Test func encodeDecodeRoundTrip() throws {
+        let exercise = ExerciseDocument(
+            id: ULID.make(),
+            name: "Bench Press",
+            order: 0,
+            sets: 4,
+            reps: 10,
+            weight: 135,
+            loadType: 0,
+            durationSeconds: 0,
+            weightLastUpdated: Self.stableTimestamp,
+            weightReminderWeeks: 4
+        )
+        let original = SplitDocument(
+            schemaVersion: SplitDocument.currentSchemaVersion,
+            id: ULID.make(),
+            name: "Push Day",
+            color: "blue",
+            systemImage: "dumbbell.fill",
+            order: 0,
+            createdAt: Self.stableTimestamp,
+            updatedAt: Self.stableTimestamp,
+            exercises: [exercise],
+            activityType: nil
+        )
+
+        let data = try DocumentCoder.encode(original)
+        let decoded = try DocumentCoder.decode(SplitDocument.self, from: data)
+
+        #expect(decoded == original)
+        #expect(decoded.relativePath == "Splits/\(original.id).json")
+    }
+}
diff --git a/project.yml b/project.yml
index 86a8de4..fcc88da 100644
--- a/project.yml
+++ b/project.yml
@@ -72,7 +72,23 @@ targets:
         TARGETED_DEVICE_FAMILY: "1"
         DEVELOPMENT_ASSET_PATHS: "\"Workouts/Preview Content\""
     scheme:
-      testTargets: []
+      testTargets:
+        - WorkoutsTests
+
+  # ---- unit tests (domain model / codec, against the iOS app target) --------
+  WorkoutsTests:
+    type: bundle.unit-test
+    platform: iOS
+    sources:
+      - WorkoutsTests
+    dependencies:
+      - target: Workouts
+      - package: IndieSync
+    settings:
+      base:
+        PRODUCT_BUNDLE_IDENTIFIER: dev.rzen.indie.WorkoutsTests
+        GENERATE_INFOPLIST_FILE: true
+        SWIFT_STRICT_CONCURRENCY: complete
 
   # ---- watchOS app (no iCloud; syncs through the phone via WatchConnectivity) -
   Workouts Watch App: