Documentation/Extensions.md

@@ -2,7 +2,8 @@
 
 Foundation, value-type, string, collection, numeric, date, and crypto helpers, plus
 the `LCEssentials` namespace itself. UIKit extensions live in
-[UIKit.md](UIKit.md); SwiftUI helpers in [SwiftUI.md](SwiftUI.md).
+[UIKit.md](UIKit.md); SwiftUI helpers in [SwiftUI.md](SwiftUI.md); the
+`LCFeatureControl` sub-product in [FeatureControl.md](FeatureControl.md).
 
 Every section is a collapsible block — click a heading to expand it.
 

Documentation/FeatureControl.md

@@ -0,0 +1,175 @@
+# LCEssentials — Feature Control
+
+A typed Swift client for Atomenta's Feature Control API: flag evaluation with a
+TTL cache and safe-degrade fallback, an in-app notifications inbox, and batched
+exposure telemetry. Lives in its own SPM product, `LCFeatureControl`, not in
+`LCEssentials` itself — an optional sub-package that target-depends on
+`LCEssentials` (same shape as [`LCECryptoKit`](Extensions.md), see its section
+there for the sub-SPM rationale).
+
+Every section is a collapsible block — click a heading to expand it.
+
+## Contents
+
+- [Setup](#setup)
+- [Evaluating flags](#evaluating-flags)
+- [Notifications inbox](#notifications-inbox)
+- [Exposure telemetry](#exposure-telemetry)
+- [Errors](#errors)
+
+---
+
+## Setup
+
+Add the product explicitly alongside `LCEssentials` to a consumer's `Package.swift`:
+
+```swift
+.product(name: "LCEssentials", package: "LCEssentials"),
+.product(name: "LCFeatureControl", package: "LCEssentials"),
+```
+
+```swift
+import LCFeatureControl
+```
+
+<details>
+<summary><b>FeatureControlConfiguration</b> — wiring for one module deployment</summary>
+
+```swift
+let configuration = FeatureControlConfiguration(
+    baseURL: "https://api.example.com",
+    environment: "production",
+    auth: FeatureControlBearerAuth { await session.currentJWT() }
+)
+```
+
+| Property | Default | Notes |
+| --- | --- | --- |
+| `cacheTTL` | `45` seconds | In-memory client-side cache window |
+| `requestTimeout` | `3` seconds | Per-request timeout budget |
+| `evaluatePath` / `notificationsPath` / `telemetryPath` | `/api/feature-control/...` | Override only for a non-default BFF mount |
+
+</details>
+
+<details>
+<summary><b>Auth strategies</b> — <code>FeatureControlAuthorizing</code></summary>
+
+No `Atomenta-Token` (module token) type exists in this package on purpose — that
+token must never be embedded in a customer-facing app. Pick one:
+
+```swift
+// Internal/admin app hitting Atomenta directly with a panel-role JWT.
+let auth = FeatureControlBearerAuth { await session.currentJWT() }
+
+// Customer app calling its own BFF, which enforces its own auth —
+// adds exactly the given headers, never synthesizes Authorization.
+let auth = FeatureControlHeaderAuth(headers: ["X-BFF-Session": sessionToken])
+```
+
+</details>
+
+## Evaluating flags
+
+<details open>
+<summary><b>FeatureControlManager</b> — cache, safe-degrade, invalidation</summary>
+
+```swift
+let manager = FeatureControlManager(
+    configuration: configuration,
+    defaults: ["fc.checkout_v2": FeatureControlFlag(enabled: false, variant: nil,
+                                                     payload: nil, reason: "default")]
+)
+
+let context = FeatureControlContext(subjectType: .customer, subjectId: user.id,
+                                    storeId: store.id, platform: "ios",
+                                    appVersion: appVersion)
+
+let snapshot = await manager.evaluate(keys: ["fc.checkout_v2"], context: context)
+snapshot.flags["fc.checkout_v2"]?.enabled   // Bool
+```
+
+`evaluate(keys:context:)` **never throws** — fresh fetch → stale cache → the
+`defaults` passed at init, in that order. Use it everywhere a flag gates real
+product behaviour. `evaluateOrThrow(keys:context:)` surfaces the real failure
+instead, for admin/debug tooling only.
+
+Concurrent calls for the same key on a cold cache are coalesced onto a single
+in-flight request — three callers evaluating the same key at once produce one
+network call, not three.
+
+```swift
+await manager.invalidateCache()   // call on user/store change
+```
+
+The cache key is `environment + subjectId + sortedKeys + platform + appVersion`
+— it does **not** include `storeId` or `attributes`. If a subject's flags can
+legitimately differ by store, call `invalidateCache()` on store switch.
+
+</details>
+
+## Notifications inbox
+
+<details>
+<summary><b>FeatureControlNotificationsClient</b> — JWT-only</summary>
+
+The OpenAPI fragment declares `security: [bearerAuth]` for every notifications
+route, no module-token alternative — configure with `FeatureControlBearerAuth`.
+
+```swift
+let client = FeatureControlNotificationsClient(configuration: configuration)
+
+let (items, nextCursor) = try await client.list(status: .unread, limit: 20)
+let (more, _) = try await client.list(status: .unread, limit: 20, cursor: nextCursor)
+
+try await client.markRead(id: notification.id)
+try await client.markAllRead()
+```
+
+</details>
+
+## Exposure telemetry
+
+<details>
+<summary><b>recordExposure(_:)</b> / <b>flushExposures()</b> — best-effort, batched</summary>
+
+```swift
+await manager.recordExposure(FeatureControlExposureEvent(
+    featureKey: "fc.checkout_v2", variant: "on",
+    subjectType: .customer, storeId: store.id
+))
+// ... later, e.g. on app background or a periodic timer:
+await manager.flushExposures()
+```
+
+Buffers locally — nothing is sent until `flushExposures()` is called. Splits
+into batches of ≤100 (server `maxItems: 100`). Not a critical-path operation: a
+failed batch is dropped, never retried indefinitely.
+
+</details>
+
+## Errors
+
+<details>
+<summary><b>FeatureControlError</b> — from <code>evaluateOrThrow</code> and the notifications client</summary>
+
+```swift
+public enum FeatureControlError: Error, Sendable, Equatable {
+    case invalidContext(code: String)   // 400
+    case unauthorized(code: String)     // 401
+    case forbidden(code: String)        // 403
+    case rateLimited(code: String)      // 429
+    case server(code: String, status: Int)
+    case transport(message: String)     // decode failure / non-HTTP error
+}
+```
+
+The safe `evaluate(...)` path never surfaces this — it degrades to cache/defaults
+instead. `evaluateOrThrow` also throws `.server(code:status: 200)` if the server
+responds `200` with an `{"error": true}` envelope (a degraded-but-200 response).
+
+</details>
+
+---
+
+See [Extensions.md](Extensions.md) for the rest of `LCEssentials`' Foundation
+helpers, including the sibling `LCECryptoKit` sub-product.

Package.swift

@@ -3,18 +3,19 @@ import PackageDescription
 
 // LCECryptoKit ships as a prebuilt .xcframework (`Frameworks/LCECryptoKit.xcframework`) —
 // vendored locally in this repo, no remote package dependency. iOS device + simulator slices
-// only; consumers who need macOS/tvOS/watchOS simply don't link the `LCECryptoKit` product.
+// only.
 //
 // Sub-SPM rule: `LCEssentials` installs standalone (no sub required). Every sub
-// (`LCECryptoKit` here) target-depends on `LCEssentials`, so linking the sub's product always
+// (`LCECryptoKit`, `LCFeatureControl`) target-depends on `LCEssentials`, so linking the sub's
-// pulls `LCEssentials` in too — a consumer never has to declare it separately.
+// product always pulls `LCEssentials` in too — a consumer never has to declare it separately.
+//
+// Platforms are iOS + watchOS only: `LCEssentials.API` (used by `LCFeatureControl`) is
+// `#if os(iOS) || os(watchOS)`, and the vendored xcframework has no macOS/tvOS slice.
 
 let package = Package(
     name: "LCEssentials",
     platforms: [
         .iOS(.v15),
-        .macOS(.v10_15),
-        .tvOS(.v13),
         .watchOS(.v8)
     ],
     products: [

README.md

@@ -35,6 +35,7 @@ import LCEssentials
 | **[Extensions.md](Documentation/Extensions.md)** | Foundation / value-type / string / collection / numeric / date / crypto extensions and the `LCEssentials` namespace |
 | **[SwiftUI.md](Documentation/SwiftUI.md)** | SwiftUI components (`LCENavigationView`) and `View` helpers |
 | **[UIKit.md](Documentation/UIKit.md)** | Programmatic layout & constraints, view/control extensions, navigation, tables, and drop-in components (`LCSnackBarView`, image picker/zoom, GIF loading) |
+| **[FeatureControl.md](Documentation/FeatureControl.md)** | `LCFeatureControl` — Atomenta Feature Control client: flag evaluation, cache/safe-degrade, notifications inbox, exposure telemetry |
 
 ---
 

Sources/LCFeatureControl/FeatureControlManager.swift

@@ -21,6 +21,9 @@ public actor FeatureControlManager: FeatureControlEvaluating {
     private let defaults: [String: FeatureControlFlag]
     private let cache: FeatureControlCache
     private var exposureBuffer: [FeatureControlExposureEvent] = []
+    /// Coalesces concurrent cold-cache callers onto one in-flight request per key,
+    /// instead of firing one POST per caller (which was hammering the 429 limit).
+    private var inFlight: [FeatureControlCacheKey: Task<FeatureControlSnapshot, Error>] = [:]
 
     /// `configVersion` sentinel returned when no network response and no cache exist —
     /// distinguishes "never evaluated" from any real server value (server versions are ≥ 0).
@@ -52,25 +55,40 @@ public actor FeatureControlManager: FeatureControlEvaluating {
         let key = cacheKey(keys: keys, context: context)
         if let fresh = await cache.get(key, allowStale: false) { return fresh }
 
-        var headers: [String: String] = [:]
+        if let inFlightTask = inFlight[key] {
-        await configuration.auth.authorize(&headers)
+            return try await inFlightTask.value
-
-        let body = jsonBody(FeatureControlEvaluateRequestBody(
-            environment: configuration.environment, keys: keys, context: context))
-
-        do {
-            let envelope: FeatureControlEvaluateEnvelope = try await api.request(
-                url: configuration.baseURL + configuration.evaluatePath,
-                method: .post,
-                body: body,
-                headers: headers,
-                timeoutInterval: configuration.requestTimeout
-            )
-            await cache.set(key, snapshot: envelope.result, ttl: configuration.cacheTTL)
-            return envelope.result
-        } catch {
-            throw FeatureControlErrorMapper.map(error)
         }
+
+        let task = Task<FeatureControlSnapshot, Error> {
+            var headers: [String: String] = [:]
+            await self.configuration.auth.authorize(&headers)
+
+            let body = jsonBody(FeatureControlEvaluateRequestBody(
+                environment: self.configuration.environment, keys: keys, context: context))
+
+            do {
+                let envelope: FeatureControlEvaluateEnvelope = try await self.api.request(
+                    url: self.configuration.baseURL + self.configuration.evaluatePath,
+                    method: .post,
+                    body: body,
+                    headers: headers,
+                    debug: false,
+                    timeoutInterval: self.configuration.requestTimeout
+                )
+                guard !envelope.error else {
+                    throw FeatureControlError.server(code: envelope.code ?? "UNKNOWN", status: 200)
+                }
+                await self.cache.set(key, snapshot: envelope.result, ttl: self.configuration.cacheTTL)
+                return envelope.result
+            } catch let error as FeatureControlError {
+                throw error
+            } catch {
+                throw FeatureControlErrorMapper.map(error)
+            }
+        }
+        inFlight[key] = task
+        defer { inFlight[key] = nil }
+        return try await task.value
     }
 
     public func invalidateCache() async {
@@ -110,6 +128,7 @@ extension FeatureControlManager {
                 method: .post,
                 body: body,
                 headers: headers,
+                debug: false,
                 timeoutInterval: configuration.requestTimeout
             ) as FeatureControlExposureBatchEnvelope
         }

Sources/LCFeatureControl/FeatureControlNotificationsClient.swift

@@ -3,7 +3,7 @@ import LCEssentials
 
 /// Protocol seam for DI, mirroring `FeatureControlEvaluating`.
 public protocol FeatureControlNotifying: Sendable {
-    func list(status: FeatureControlNotificationStatus, limit: Int) async throws
+    func list(status: FeatureControlNotificationStatus, limit: Int, cursor: String?) async throws
         -> (items: [FeatureControlNotification], nextCursor: String?)
     func markRead(id: String) async throws
     func markAllRead() async throws
@@ -22,20 +22,29 @@ public actor FeatureControlNotificationsClient: FeatureControlNotifying {
         self.api = api
     }
 
-    public func list(status: FeatureControlNotificationStatus = .all, limit: Int = 20) async throws
+    public func list(status: FeatureControlNotificationStatus = .all, limit: Int = 20, cursor: String? = nil) async throws
         -> (items: [FeatureControlNotification], nextCursor: String?) {
         var headers: [String: String] = [:]
         await configuration.auth.authorize(&headers)
 
-        let query = "?status=\(status.rawValue)&limit=\(limit)"
+        var query = "?status=\(status.rawValue)&limit=\(limit)"
+        if let cursor, let encodedCursor = cursor.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed) {
+            query += "&cursor=\(encodedCursor)"
+        }
         do {
             let envelope: FeatureControlNotificationListEnvelope = try await api.request(
                 url: configuration.baseURL + configuration.notificationsPath + query,
                 method: .get,
                 headers: headers,
+                debug: false,
                 timeoutInterval: configuration.requestTimeout
             )
+            guard !envelope.error else {
+                throw FeatureControlError.server(code: "UNKNOWN", status: 200)
+            }
             return (envelope.result.items, envelope.result.nextCursor)
+        } catch let error as FeatureControlError {
+            throw error
         } catch {
             throw FeatureControlErrorMapper.map(error)
         }
@@ -51,6 +60,7 @@ public actor FeatureControlNotificationsClient: FeatureControlNotifying {
                 url: configuration.baseURL + configuration.notificationsPath + "/\(encodedId)/read",
                 method: .post,
                 headers: headers,
+                debug: false,
                 timeoutInterval: configuration.requestTimeout
             )
         } catch {
@@ -67,6 +77,7 @@ public actor FeatureControlNotificationsClient: FeatureControlNotifying {
                 url: configuration.baseURL + configuration.notificationsPath + "/read-all",
                 method: .post,
                 headers: headers,
+                debug: false,
                 timeoutInterval: configuration.requestTimeout
             )
         } catch {

Tests/LCFeatureControlTests/FeatureControlErrorTests.swift

@@ -4,18 +4,10 @@ import XCTest
 
 final class FeatureControlErrorTests: XCTestCase {
 
-    private var api: API!
+    private var api = API.lce_featureControlTestInstance()
-
-    override func setUp() {
-        super.setUp()
-        let cfg = URLSessionConfiguration.ephemeral
-        cfg.protocolClasses = [StubURLProtocol.self]
-        api = API(testConfiguration: cfg)
-    }
 
     override func tearDown() {
         StubURLProtocol.reset()
-        api = nil
         super.tearDown()
     }
 

Tests/LCFeatureControlTests/FeatureControlExposureTests.swift

@@ -4,18 +4,10 @@ import XCTest
 
 final class FeatureControlExposureTests: XCTestCase {
 
-    private var api: API!
+    private var api = API.lce_featureControlTestInstance()
-
-    override func setUp() {
-        super.setUp()
-        let cfg = URLSessionConfiguration.ephemeral
-        cfg.protocolClasses = [StubURLProtocol.self]
-        api = API(testConfiguration: cfg)
-    }
 
     override func tearDown() {
         StubURLProtocol.reset()
-        api = nil
         super.tearDown()
     }
 
@@ -51,6 +43,20 @@ final class FeatureControlExposureTests: XCTestCase {
         XCTAssertEqual(StubURLProtocol.requestCount, 2)
     }
 
+    func testFlushSplitsExactlyAtTheServerBatchLimit() async {
+        StubURLProtocol.setStub(.init(statusCode: 200, body: Data(#"{"error":false,"code":"OK"}"#.utf8)))
+        let manager = makeManager()
+        for i in 0..<101 { await manager.recordExposure(event(i)) } // maxItems: 100, plus 1
+
+        await manager.flushExposures()
+
+        XCTAssertEqual(StubURLProtocol.requestCount, 2)
+        let eventCountsPerBatch = StubURLProtocol.capturedBodies.map { body in
+            String(decoding: body, as: UTF8.self).components(separatedBy: "\"featureKey\"").count - 1
+        }
+        XCTAssertEqual(eventCountsPerBatch, [100, 1])
+    }
+
     func testFailedBatchIsDroppedNotRetriedIndefinitely() async {
         StubURLProtocol.setStub(.init(statusCode: 500, body: Data()))
         let manager = makeManager()

Tests/LCFeatureControlTests/FeatureControlManagerTests.swift

@@ -9,18 +9,10 @@ private final class MutableClock: @unchecked Sendable {
 
 final class FeatureControlManagerTests: XCTestCase {
 
-    private var api: API!
+    private var api = API.lce_featureControlTestInstance()
-
-    override func setUp() {
-        super.setUp()
-        let cfg = URLSessionConfiguration.ephemeral
-        cfg.protocolClasses = [StubURLProtocol.self]
-        api = API(testConfiguration: cfg)
-    }
 
     override func tearDown() {
         StubURLProtocol.reset()
-        api = nil
         super.tearDown()
     }
 
@@ -157,4 +149,39 @@ final class FeatureControlManagerTests: XCTestCase {
         // No `try` above this line compiles only because `evaluate` truly never throws.
         _ = await manager.evaluate(keys: ["x"], context: context())
     }
+
+    // MARK: - Concurrency & envelope validation
+
+    func testConcurrentEvaluateOnColdCacheCoalescesIntoSingleRequest() async {
+        StubURLProtocol.setStub(.init(statusCode: 200, body: evaluateResponseBody(configVersion: 7)))
+        let manager = makeManager()
+        let ctx = context()
+
+        async let first = manager.evaluate(keys: ["fc.checkout_v2"], context: ctx)
+        async let second = manager.evaluate(keys: ["fc.checkout_v2"], context: ctx)
+        async let third = manager.evaluate(keys: ["fc.checkout_v2"], context: ctx)
+        let results = await [first, second, third]
+
+        XCTAssertEqual(Set(results.map(\.configVersion)), [7])
+        XCTAssertEqual(StubURLProtocol.requestCount, 1)
+    }
+
+    func testEvaluateOrThrowThrowsWhenEnvelopeErrorIsTrueDespite200() async {
+        let body = """
+        {"error": true, "code": "FEATURE_CONTROL_DEGRADED", "result": {
+            "evaluatedAt": "2026-04-16T12:00:00.000Z", "configVersion": 1, "flags": {}
+        }}
+        """
+        StubURLProtocol.setStub(.init(statusCode: 200, body: Data(body.utf8)))
+        let manager = makeManager()
+
+        do {
+            _ = try await manager.evaluateOrThrow(keys: ["x"], context: context())
+            XCTFail("expected throw")
+        } catch let error as FeatureControlError {
+            XCTAssertEqual(error, .server(code: "FEATURE_CONTROL_DEGRADED", status: 200))
+        } catch {
+            XCTFail("expected FeatureControlError, got \(error)")
+        }
+    }
 }

Tests/LCFeatureControlTests/FeatureControlModelsTests.swift

@@ -51,7 +51,7 @@ final class FeatureControlModelsTests: XCTestCase {
         let envelope = try JSONDecoder().decode(FeatureControlEvaluateEnvelope.self, from: data)
 
         var calendar = Calendar(identifier: .gregorian)
-        calendar.timeZone = TimeZone(identifier: "UTC")!
+        calendar.timeZone = try XCTUnwrap(TimeZone(identifier: "UTC"))
         let components = calendar.dateComponents([.year, .month, .day, .hour, .minute, .second],
                                                   from: envelope.result.evaluatedAt)
         XCTAssertEqual(components.year, 2026)
@@ -60,6 +60,34 @@ final class FeatureControlModelsTests: XCTestCase {
         XCTAssertEqual(components.hour, 12)
     }
 
+    func testEvaluatedAtDecodeFailsOnUnrecognizedDateFormat() {
+        let json = #"{"evaluatedAt": "not-a-date", "configVersion": 1, "flags": {}}"#
+        XCTAssertThrowsError(try JSONDecoder().decode(FeatureControlSnapshot.self, from: Data(json.utf8))) { error in
+            guard case DecodingError.dataCorrupted = error else {
+                return XCTFail("expected .dataCorrupted, got \(error)")
+            }
+        }
+    }
+
+    func testFlagDecodesObjectPayloadThroughEvaluateEnvelope() throws {
+        let json = """
+        {"error": false, "code": "FEATURE_CONTROL_EVALUATED", "result": {
+            "evaluatedAt": "2026-04-16T12:00:00.000Z",
+            "configVersion": 3,
+            "flags": {"fc.promo_banner": {"enabled": true, "variant": "on",
+                "payload": {"title": "Sale", "limit": 5, "tags": ["a", "b"]}, "reason": "rollout"}}
+        }}
+        """
+        let envelope = try JSONDecoder().decode(FeatureControlEvaluateEnvelope.self, from: Data(json.utf8))
+        let flag = try XCTUnwrap(envelope.result.flags["fc.promo_banner"])
+        guard case let .object(payload) = flag.payload else {
+            return XCTFail("expected .object payload, got \(String(describing: flag.payload))")
+        }
+        XCTAssertEqual(payload["title"], .string("Sale"))
+        XCTAssertEqual(payload["limit"], .number(5))
+        XCTAssertEqual(payload["tags"], .array([.string("a"), .string("b")]))
+    }
+
     func testContextEncodesExactOpenAPIShape() throws {
         let context = FeatureControlContext(subjectType: .customer, subjectId: "cust_123",
                                             storeId: "store_001", platform: "ios",
@@ -113,6 +141,18 @@ final class FeatureControlModelsTests: XCTestCase {
         XCTAssertNil(notification.ctaUrl)
     }
 
+    func testNotificationCreatedAtDecodeFailsOnUnrecognizedDateFormat() {
+        let json = """
+        {"id": "n3", "title": "T", "body": "B", "severity": "info",
+         "createdAt": "not-a-date", "read": false}
+        """
+        XCTAssertThrowsError(try JSONDecoder().decode(FeatureControlNotification.self, from: Data(json.utf8))) { error in
+            guard case DecodingError.dataCorrupted = error else {
+                return XCTFail("expected .dataCorrupted, got \(error)")
+            }
+        }
+    }
+
     func testExposureEventEncodesExpectedShape() throws {
         let event = FeatureControlExposureEvent(featureKey: "fc.checkout_v2", variant: "on",
                                                 subjectType: .customer, storeId: "store_001")

Tests/LCFeatureControlTests/FeatureControlNotificationsClientTests.swift

@@ -4,18 +4,10 @@ import XCTest
 
 final class FeatureControlNotificationsClientTests: XCTestCase {
 
-    private var api: API!
+    private var api = API.lce_featureControlTestInstance()
-
-    override func setUp() {
-        super.setUp()
-        let cfg = URLSessionConfiguration.ephemeral
-        cfg.protocolClasses = [StubURLProtocol.self]
-        api = API(testConfiguration: cfg)
-    }
 
     override func tearDown() {
         StubURLProtocol.reset()
-        api = nil
         super.tearDown()
     }
 
@@ -77,6 +69,43 @@ final class FeatureControlNotificationsClientTests: XCTestCase {
                        "https://api.example.com/api/feature-control/notifications/read-all")
     }
 
+    func testListIncludesEncodedCursorWhenProvided() async throws {
+        let body = #"{"error": false, "result": {"items": [], "nextCursor": null}}"#
+        StubURLProtocol.setStub(.init(statusCode: 200, body: Data(body.utf8)))
+        let client = makeClient()
+
+        _ = try await client.list(status: .all, limit: 20, cursor: "cursor abc")
+
+        let url = StubURLProtocol.capturedRequests.first?.url?.absoluteString
+        XCTAssertEqual(url, "https://api.example.com/api/feature-control/notifications?status=all&limit=20&cursor=cursor%20abc")
+    }
+
+    func testListOmitsCursorWhenNil() async throws {
+        let body = #"{"error": false, "result": {"items": [], "nextCursor": null}}"#
+        StubURLProtocol.setStub(.init(statusCode: 200, body: Data(body.utf8)))
+        let client = makeClient()
+
+        _ = try await client.list(status: .all, limit: 20)
+
+        let url = StubURLProtocol.capturedRequests.first?.url?.absoluteString
+        XCTAssertEqual(url, "https://api.example.com/api/feature-control/notifications?status=all&limit=20")
+    }
+
+    func testListThrowsWhenEnvelopeErrorIsTrueDespite200() async {
+        let body = #"{"error": true, "result": {"items": [], "nextCursor": null}}"#
+        StubURLProtocol.setStub(.init(statusCode: 200, body: Data(body.utf8)))
+        let client = makeClient()
+
+        do {
+            _ = try await client.list(status: .all, limit: 20)
+            XCTFail("expected throw")
+        } catch let error as FeatureControlError {
+            XCTAssertEqual(error, .server(code: "UNKNOWN", status: 200))
+        } catch {
+            XCTFail("expected FeatureControlError, got \(error)")
+        }
+    }
+
     func testUnauthorizedMapsToFeatureControlErrorNotCrash() async {
         StubURLProtocol.setStub(.init(statusCode: 401, body: Data(#"{"code":"FEATURE_CONTROL_MISSING_AUTH"}"#.utf8)))
         let client = makeClient()

Tests/LCFeatureControlTests/Support/StubURLProtocol.swift

@@ -1,4 +1,15 @@
 import Foundation
+@testable import LCEssentials
+
+/// Shared factory so test classes don't each declare a force-unwrapped `API!`
+/// var for the XCTest setUp/tearDown lifecycle.
+extension API {
+    static func lce_featureControlTestInstance() -> API {
+        let cfg = URLSessionConfiguration.ephemeral
+        cfg.protocolClasses = [StubURLProtocol.self]
+        return API(testConfiguration: cfg)
+    }
+}
 
 /// Test double for `URLProtocol`. Intercepts every request on a `URLSession`
 /// configured with it, records the outgoing `URLRequest`, and replays a canned
@@ -56,6 +67,13 @@ final class StubURLProtocol: URLProtocol, @unchecked Sendable {
         return _capturedRequests.count
     }
 
+    /// Bodies of every intercepted request, in order — for tests asserting batch
+    /// boundaries across multiple requests (`lastCapturedBody` only sees the last one).
+    static var capturedBodies: [Data] {
+        lock.lock(); defer { lock.unlock() }
+        return _capturedBodies
+    }
+
     private static func currentStub() -> Stub {
         lock.lock(); defer { lock.unlock() }
         return _stub
@@ -84,11 +102,14 @@ final class StubURLProtocol: URLProtocol, @unchecked Sendable {
             return
         }
 
-        let url = request.url ?? URL(string: "https://stub.invalid")!
+        guard let url = request.url ?? URL(string: "https://stub.invalid"),
-        let response = HTTPURLResponse(url: url,
+              let response = HTTPURLResponse(url: url,
-                                       statusCode: stub.statusCode,
+                                             statusCode: stub.statusCode,
-                                       httpVersion: "HTTP/1.1",
+                                             httpVersion: "HTTP/1.1",
-                                       headerFields: stub.headers)!
+                                             headerFields: stub.headers) else {
+            client.urlProtocol(self, didFailWithError: URLError(.badServerResponse))
+            return
+        }
         client.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
         client.urlProtocol(self, didLoad: stub.body)
         client.urlProtocolDidFinishLoading(self)