diff --git a/.github/workflows/app-build-verify.yml b/.github/workflows/app-build-verify.yml
index fcf0692e..8fb291be 100644
--- a/.github/workflows/app-build-verify.yml
+++ b/.github/workflows/app-build-verify.yml
@@ -24,8 +24,8 @@ on:
         default: ""
         type: string
       ios-simulator:
-        description: "The iOS simulator image to use"
-        default: "iPhone 16e::iOS 18.5"
+        description: "The iOS simulator image to use; leave blank to let Briefcase select an appropriate default device."
+        default: ""
         type: string
       framework:
         description: "Framework to use to build the App, e.g. toga."
@@ -511,9 +511,12 @@ jobs:
         INPUT_IOS_SIMULATOR: ${{ inputs.ios-simulator }}
         CREATE_OPTIONS: ${{ steps.output-format.outputs.template-override }} ${{ inputs.create-options }}
       run: |
+        IOS_SIMULATOR_OPTION=()
+        [ -n "${INPUT_IOS_SIMULATOR}" ] && IOS_SIMULATOR_OPTION=(-d "${INPUT_IOS_SIMULATOR}")
+
         briefcase create iOS xcode ${CREATE_OPTIONS} ${BRIEFCASE_VERBOSITY}
         briefcase build iOS xcode ${BRIEFCASE_VERBOSITY}
-        briefcase run iOS xcode -d "${INPUT_IOS_SIMULATOR}" ${BRIEFCASE_VERBOSITY}
+        briefcase run iOS xcode "${IOS_SIMULATOR_OPTION[@]}" ${BRIEFCASE_VERBOSITY}
         briefcase package iOS xcode --adhoc-sign ${BRIEFCASE_VERBOSITY}
 
         # Clean up built artefacts to preserve disk space
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c9ef4b1b..07c49b83 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -211,7 +211,7 @@ jobs:
       fail-fast: false
       matrix:
         framework: [ toga, pyside6 ]
-        runner-os: [ macos-15, windows-latest, ubuntu-24.04, ubuntu-24.04-arm ]
+        runner-os: [ macos-26, windows-2025, ubuntu-24.04, ubuntu-24.04-arm ]
 
   test-app-build-verify:
     name: App Build Verify (Default templates)
@@ -227,7 +227,7 @@ jobs:
       fail-fast: false
       matrix:
         framework: [ toga ]
-        runner-os: [ macos-15-intel, macos-15, windows-latest, ubuntu-24.04, ubuntu-24.04-arm ]
+        runner-os: [ macos-26-intel, macos-26, windows-2025, ubuntu-24.04, ubuntu-24.04-arm ]
 
   test-app-build-verify-template:
     name: App Build Verify (Briefcase template)
@@ -242,7 +242,7 @@ jobs:
       fail-fast: false
       matrix:
         framework: [ toga ]
-        runner-os: [ macos-15, windows-latest, ubuntu-24.04, ubuntu-24.04-arm ]
+        runner-os: [ macos-26, windows-2025, ubuntu-24.04, ubuntu-24.04-arm ]
 
   test-app-build-verify-android:
     name: App Build Verify (Android template)
@@ -258,7 +258,7 @@ jobs:
       fail-fast: false
       matrix:
         framework: [ toga ]  # toga only
-        runner-os: [ macos-15, windows-latest, ubuntu-24.04 ]
+        runner-os: [ macos-26, windows-2025, ubuntu-24.04 ]
 
   test-app-build-verify-iOS:
     name: App Build Verify (iOS template)
@@ -266,7 +266,7 @@ jobs:
     uses: ./.github/workflows/app-build-verify.yml
     with:
       repository: beeware/briefcase-iOS-xcode-template
-      runner-os: macos-15
+      runner-os: macos-26
       target-platform: iOS
       target-format: xcode
       framework: ${{ matrix.framework }}
@@ -338,7 +338,7 @@ jobs:
     uses: ./.github/workflows/app-build-verify.yml
     with:
       repository: beeware/briefcase-macos-${{ matrix.format }}-template
-      runner-os: macos-15
+      runner-os: macos-26
       target-platform: macOS
       target-format: ${{ matrix.format }}
       framework: ${{ matrix.framework }}
@@ -369,7 +369,7 @@ jobs:
     uses: ./.github/workflows/app-build-verify.yml
     with:
       repository: beeware/briefcase-windows-${{ matrix.format }}-template
-      runner-os: windows-latest
+      runner-os: windows-2025
       target-platform: windows
       target-format: ${{ matrix.format }}
       framework: ${{ matrix.framework }}