diff --git a/.github/workflows/ci-core.yml b/.github/workflows/ci-core.yml
index 9aef9380..f622b027 100644
--- a/.github/workflows/ci-core.yml
+++ b/.github/workflows/ci-core.yml
@@ -14,6 +14,27 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  # Detect whether this PR/push touches code that mutation testing covers.
+  # Infection is expensive, so the `infection` job below runs only when src/
+  # or test/ changed — a docs-only or config-only change skips it.
+  changes:
+    name: Detect code changes
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: read
+    outputs:
+      code: ${{ steps.filter.outputs.code }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dorny/paths-filter@v3
+        id: filter
+        with:
+          filters: |
+            code:
+              - 'src/**'
+              - 'test/**'
+
   phpunit:
     # 8.4 is the supported/default runtime and runs the full suite minus the
     # `php85` group; a dedicated 8.5 container runs only that group, which
@@ -116,7 +137,10 @@ jobs:
   infection:
     name: Mutation testing
     runs-on: ubuntu-latest
-    needs: phpunit
+    # Only after unit tests pass, and only when src/ or test/ changed
+    # (see the `changes` job) — mutation is skipped for docs/config-only work.
+    needs: [phpunit, changes]
+    if: needs.changes.outputs.code == 'true'
     steps:
       - uses: actions/checkout@v4