diff --git a/.github/workflows/__multi-language-autodetect.yml b/.github/workflows/__multi-language-autodetect.yml
index ea9704165..4c75f5e5d 100644
--- a/.github/workflows/__multi-language-autodetect.yml
+++ b/.github/workflows/__multi-language-autodetect.yml
@@ -77,7 +77,7 @@ jobs:
             version: stable-v2.22.4
           - os: ubuntu-latest
             version: stable-v2.23.9
-          - os: macos-15-xlarge
+          - os: macos-latest-xlarge
             version: stable-v2.23.9
           - os: ubuntu-latest
             version: stable-v2.24.3
diff --git a/pr-checks/checks/multi-language-autodetect.yml b/pr-checks/checks/multi-language-autodetect.yml
index 263dd9bc0..1608fcc6b 100644
--- a/pr-checks/checks/multi-language-autodetect.yml
+++ b/pr-checks/checks/multi-language-autodetect.yml
@@ -2,16 +2,8 @@ name: "Multi-language repository"
 description: "An end-to-end integration test of a multi-language repository using automatic language detection"
 operatingSystems:
   - ubuntu
-  # Newer CodeQL CLI versions support Swift 6.2, so analyse Swift on the latest macOS runner
-  # (currently macOS 26) with its default Xcode. This exercises the common combination of a
-  # recent CLI on a recent runner.
   - os: macos
     runner-image: macos-latest-xlarge
-    codeql-versions:
-      - stable-v2.24.3
-      - default
-      - linked
-      - nightly-latest
   # Older CodeQL CLI versions only support Swift up to 6.1, which requires Xcode 16. That is
   # not available on macOS 26, so run these versions on macOS 15 where we select Xcode 16
   # below. See https://github.com/actions/runner-images/issues/14167.
@@ -22,7 +14,6 @@ operatingSystems:
       - stable-v2.20.7
       - stable-v2.21.4
       - stable-v2.22.4
-      - stable-v2.23.9
 env:
   CODEQL_ACTION_RESOLVE_SUPPORTED_LANGUAGES_USING_CLI: true
 installGo: true
diff --git a/pr-checks/sync.ts b/pr-checks/sync.ts
index 0ee33e4f2..035f5b34d 100755
--- a/pr-checks/sync.ts
+++ b/pr-checks/sync.ts
@@ -55,9 +55,10 @@ type OperatingSystem =
       /** Optional runner image label. */
       "runner-image"?: string;
       /**
-       * Optional CodeQL versions to run on this entry. If specified, only these versions are
-       * tested on this runner image. This allows running different runner images for different
-       * CodeQL versions of the same OS.
+       * Optional CodeQL versions to run on this entry. If specified, this entry runs only these
+       * versions. A sibling entry for the same OS that omits `codeql-versions` runs all versions
+       * not claimed by any sibling entry. This allows pinning specific CodeQL versions to a
+       * particular runner image while letting the remaining versions default to another.
        */
       "codeql-versions"?: string[];
     };
@@ -358,6 +359,28 @@ function generateJobMatrix(
 ): Array<Record<string, any>> {
   let matrix: Array<Record<string, any>> = [];
 
+  const operatingSystems = checkSpecification.operatingSystems ?? ["ubuntu"];
+
+  // For each OS, collect the CodeQL versions explicitly claimed by entries that specify
+  // `codeql-versions`. A sibling entry for the same OS that omits `codeql-versions` runs all
+  // versions not in this set.
+  const claimedVersionsByOs = new Map<string, Set<string>>();
+  for (const operatingSystemConfig of operatingSystems) {
+    if (typeof operatingSystemConfig === "string") {
+      continue;
+    }
+    const entryVersions = operatingSystemConfig["codeql-versions"];
+    if (!entryVersions) {
+      continue;
+    }
+    const claimed =
+      claimedVersionsByOs.get(operatingSystemConfig.os) ?? new Set<string>();
+    for (const entryVersion of entryVersions) {
+      claimed.add(entryVersion);
+    }
+    claimedVersionsByOs.set(operatingSystemConfig.os, claimed);
+  }
+
   for (const version of checkSpecification.versions ?? defaultTestVersions) {
     if (version === "latest") {
       throw new Error(
@@ -370,7 +393,6 @@ function generateJobMatrix(
       "macos-latest",
       "windows-latest",
     ];
-    const operatingSystems = checkSpecification.operatingSystems ?? ["ubuntu"];
 
     for (const operatingSystemConfig of operatingSystems) {
       const operatingSystem =
@@ -385,12 +407,16 @@ function generateJobMatrix(
         continue;
       }
 
-      // If this OS entry restricts itself to specific CodeQL versions, skip other versions.
+      // An entry that specifies `codeql-versions` runs only those versions. A sibling entry for
+      // the same OS that omits `codeql-versions` runs all versions not claimed by its siblings.
       const entryVersions =
         typeof operatingSystemConfig === "string"
           ? undefined
           : operatingSystemConfig["codeql-versions"];
-      if (entryVersions && !entryVersions.includes(version)) {
+      const runsThisVersion = entryVersions
+        ? entryVersions.includes(version)
+        : !claimedVersionsByOs.get(operatingSystem)?.has(version);
+      if (!runsThisVersion) {
         continue;
       }