diff --git a/.github/workflows/ios.yml b/.github/workflows/ios.yml
index 843c3b42..142defe4 100644
--- a/.github/workflows/ios.yml
+++ b/.github/workflows/ios.yml
@@ -19,8 +19,9 @@ concurrency:
     cancel-in-progress: true
 
 jobs:
+    # Xcode 27.1+ is required: the app uses iOS 27.1 SDK APIs (e.g. UIHingeInteraction).
     package-tests:
-        runs-on: macos-15
+        runs-on: xcode-27
         steps:
             - uses: actions/checkout@v4
             - name: Show toolchain versions
@@ -31,7 +32,7 @@ jobs:
               run: swift test --package-path ios/Packages/HapiKit
 
     app-build:
-        runs-on: macos-15
+        runs-on: xcode-27
         steps:
             - uses: actions/checkout@v4
             - name: Show toolchain versions
diff --git a/docs/guide/native-apps.md b/docs/guide/native-apps.md
index 893e7828..712bd51f 100644
--- a/docs/guide/native-apps.md
+++ b/docs/guide/native-apps.md
@@ -22,7 +22,7 @@ or Apple developer setup is needed on your hub. See [Notifications](#notificatio
 ### Build from source (developers)
 
 The app source is in the repository. Development builds follow the
-[iOS README](https://github.com/tiann/hapi/blob/main/ios/README.md) (Xcode 16
+[iOS README](https://github.com/tiann/hapi/blob/main/ios/README.md) (Xcode 27.1
 or newer, shared `Hapi` scheme; device builds require signing) or the
 [Android README](https://github.com/tiann/hapi/blob/main/android/README.md)
 (JDK 17+, Android SDK, Gradle wrapper). Self-built apps need their own push
diff --git a/ios/Packages/HapiKit/Tests/HapiClientTests/MultipartEncoderTests.swift b/ios/Packages/HapiKit/Tests/HapiClientTests/MultipartEncoderTests.swift
index e4fb4b44..18ada527 100644
--- a/ios/Packages/HapiKit/Tests/HapiClientTests/MultipartEncoderTests.swift
+++ b/ios/Packages/HapiKit/Tests/HapiClientTests/MultipartEncoderTests.swift
@@ -15,18 +15,19 @@ struct MultipartEncoderTests {
             data: Data([0x01, 0x02])
         )
 
+        // Joined rather than `+`-chained: Swift 6.4 times out type-checking the chain.
         var expected = Data(
-            (
-                "--B\r\n"
-                    + "Content-Disposition: form-data; name=\"provider\"\r\n\r\n"
-                    + "openai\r\n"
-                    + "--B\r\n"
-                    + "Content-Disposition: form-data; name=\"mode\"\r\n\r\n"
-                    + "standard\r\n"
-                    + "--B\r\n"
-                    + "Content-Disposition: form-data; name=\"file\"; filename=\"clip.m4a\"\r\n"
-                    + "Content-Type: audio/mp4\r\n\r\n"
-            ).utf8
+            [
+                "--B\r\n",
+                "Content-Disposition: form-data; name=\"provider\"\r\n\r\n",
+                "openai\r\n",
+                "--B\r\n",
+                "Content-Disposition: form-data; name=\"mode\"\r\n\r\n",
+                "standard\r\n",
+                "--B\r\n",
+                "Content-Disposition: form-data; name=\"file\"; filename=\"clip.m4a\"\r\n",
+                "Content-Type: audio/mp4\r\n\r\n",
+            ].joined().utf8
         )
         expected.append(Data([0x01, 0x02]))
         expected.append(Data("\r\n--B--\r\n".utf8))
diff --git a/ios/README.md b/ios/README.md
index 362f3027..2cce8f20 100644
--- a/ios/README.md
+++ b/ios/README.md
@@ -26,7 +26,8 @@ for platform differences, web-only features and everyday use.
 
 ## Requirements
 
-- Xcode 16 or newer with Swift 6 support.
+- Xcode 27.1 or newer (the app uses iOS 27.1 SDK APIs such as
+  `UIHingeInteraction`).
 - Deployment target: iOS 17.0.
 - Runtime dependencies (SPM, declared in `Packages/HapiKit/Package.swift`,
   used only by the `HapiUI` target): `swiftlang/swift-markdown` (GFM parsing
@@ -52,8 +53,8 @@ swift test --package-path ios/Packages/HapiKit
 ```
 
 CI runs package tests, the simulator build, and app-hosted transcript tests
-on `macos-15` via `.github/workflows/ios.yml` (triggered by changes under
-`ios/**` and `shared/fixtures/**`).
+on the `xcode-27` runner image via `.github/workflows/ios.yml` (triggered by
+changes under `ios/**` and `shared/fixtures/**`).
 
 ### Linux verification (no Mac needed)