diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8a9de6be..5b1552e9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -103,7 +103,7 @@ jobs:
           - macos-latest
           - ubuntu-latest
 
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ vars.CI_5070X_ENABLED == 'true' && (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && contains(fromJSON('["ubuntu-latest","ubuntu-24.04"]'), matrix.os) && 'ci-5070x-linux' || matrix.os }}
     timeout-minutes: 30
     env:
       SMOKE_TAG: v0.0.0-smoketest
@@ -248,7 +248,7 @@ jobs:
     # #418: exercise release.yml's archive download contract without its
     # write token, provenance action, tag trigger, or release publication.
     needs: cross-platform-smoke
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.CI_5070X_ENABLED == 'true' && (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && 'ci-5070x-linux' || 'ubuntu-latest' }}
     timeout-minutes: 10
     permissions:
       contents: read
diff --git a/.github/workflows/codegraph-conformance.yml b/.github/workflows/codegraph-conformance.yml
index 57d747d4..6bc1eddc 100644
--- a/.github/workflows/codegraph-conformance.yml
+++ b/.github/workflows/codegraph-conformance.yml
@@ -26,7 +26,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ vars.CI_5070X_ENABLED == 'true' && (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && contains(fromJSON('["ubuntu-latest","ubuntu-24.04"]'), matrix.os) && 'ci-5070x-linux' || matrix.os }}
     timeout-minutes: 30
     env:
       DO_NOT_TRACK: '1'
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 70c9beac..a1073c24 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -80,7 +80,7 @@ jobs:
           - macos-latest
           - ubuntu-latest
 
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ vars.CI_5070X_ENABLED == 'true' && (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && contains(fromJSON('["ubuntu-latest","ubuntu-24.04"]'), matrix.os) && 'ci-5070x-linux' || matrix.os }}
 
     steps:
       - name: Checkout
diff --git a/.github/workflows/skill-check.yml b/.github/workflows/skill-check.yml
index a0a3b10a..63634bb7 100644
--- a/.github/workflows/skill-check.yml
+++ b/.github/workflows/skill-check.yml
@@ -22,7 +22,7 @@ jobs:
   # Run darwin-skill 8-dim scoring on every new/modified SKILL.md
   score-skills:
     name: Darwin score (8-dim)
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.CI_5070X_ENABLED == 'true' && (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && 'ci-5070x-linux' || 'ubuntu-latest' }}
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
@@ -140,7 +140,7 @@ jobs:
   # Validate SKILL.md frontmatter (YAML)
   frontmatter-check:
     name: Frontmatter YAML validation
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.CI_5070X_ENABLED == 'true' && (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && 'ci-5070x-linux' || 'ubuntu-latest' }}
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1