diff --git a/.github/workflows/ios-sim.yml b/.github/workflows/ios-sim.yml
index 22628fa600..4bf096cc68 100644
--- a/.github/workflows/ios-sim.yml
+++ b/.github/workflows/ios-sim.yml
@@ -76,6 +76,9 @@ jobs:
           xcrun --show-sdk-path
           ls /Library/Developer/CommandLineTools/SDKs || true
 
+      - name: Use the macOS 26 SDK
+        run: scripts/ci/ios-sim/use-macos26-sdk.sh
+
       # without ~/.cabal, cabal 3.10 keeps its store in XDG directories, not where setup-haskell reports it
       - name: Use ~/.cabal for cabal
         run: echo "CABAL_DIR=$HOME/.cabal" >> "$GITHUB_ENV"
@@ -144,6 +147,9 @@ jobs:
           xcrun --show-sdk-path
           ls /Library/Developer/CommandLineTools/SDKs || true
 
+      - name: Use the macOS 26 SDK
+        run: scripts/ci/ios-sim/use-macos26-sdk.sh
+
       - name: Use ~/.cabal for cabal
         run: echo "CABAL_DIR=$HOME/.cabal" >> "$GITHUB_ENV"
 
@@ -212,6 +218,9 @@ jobs:
           ls /Library/Developer/CommandLineTools/SDKs || true
           xcrun simctl list runtimes
 
+      - name: Use the macOS 26 SDK
+        run: scripts/ci/ios-sim/use-macos26-sdk.sh
+
       - name: Download core library
         uses: actions/download-artifact@v4
         with:
diff --git a/scripts/ci/ios-sim/use-macos26-sdk.sh b/scripts/ci/ios-sim/use-macos26-sdk.sh
new file mode 100755
index 0000000000..d69a6a6af8
--- /dev/null
+++ b/scripts/ci/ios-sim/use-macos26-sdk.sh
@@ -0,0 +1,12 @@
+#!/bin/bash
+
+# Runner images from 20261013 add Command Line Tools whose default SDK is macOS 27, which Xcode 26.6's linker cannot read
+# ("tapi error: malformed file"); GHC's links reach it despite DEVELOPER_DIR, so the default goes back to macOS 26.5.
+
+set -euo pipefail
+
+sdks=/Library/Developer/CommandLineTools/SDKs
+if [ -L "$sdks/MacOSX.sdk" ] && [ -d "$sdks/MacOSX26.5.sdk" ]; then
+  sudo ln -sfn MacOSX26.5.sdk "$sdks/MacOSX.sdk"
+fi
+ls -l "$sdks"