diff --git a/.github/workflows/swift.yml b/.github/workflows/swift.yml
index c5afdbb..d24aae1 100644
--- a/.github/workflows/swift.yml
+++ b/.github/workflows/swift.yml
@@ -5,10 +5,17 @@ on: [push, pull_request]
 jobs:
   build-test:
     name: build & test
-    runs-on: macos-latest
+    # SpecificationCore 1.0.0 currently does not compile with Swift 6.3+.
+    # Keep CI on a known-compatible compiler until the dependency is updated.
+    runs-on: macos-15
     steps:
       - uses: actions/checkout@v4
 
+      - name: Select Xcode 16.4 (Swift 6.1.2)
+        uses: maxim-lobanov/setup-xcode@v1
+        with:
+          xcode-version: '16.4'
+
       - name: Swift version
         run: swift --version
 
@@ -43,4 +50,4 @@ jobs:
         run: |
           python3 -m venv /tmp/ontology-venv
           /tmp/ontology-venv/bin/pip install --quiet pyyaml
-          /tmp/ontology-venv/bin/python Examples/ontology-backend/backend.py --check
\ No newline at end of file
+          /tmp/ontology-venv/bin/python Examples/ontology-backend/backend.py --check