diff --git a/.github/workflows/tests.yaml b/.github/workflows/tests.yaml
index 04c17f536..fddad0374 100644
--- a/.github/workflows/tests.yaml
+++ b/.github/workflows/tests.yaml
@@ -8,16 +8,18 @@ on:
       - 'tests/**'
       - 'pyproject.toml'
       - '.github/workflows/**'
+  # No paths filter here: lint and test are required checks, and a workflow that
+  # never triggers never reports, leaving such PRs permanently blocked. The filter
+  # lives in the `changes` job below, which skips the expensive steps instead.
   pull_request:
     types: [opened, synchronize, reopened, ready_for_review]
     branches: ["**"]
-    paths:
-      - 'flixopt/**'
-      - 'tests/**'
-      - 'pyproject.toml'
-      - '.github/workflows/**'
   workflow_dispatch:
 
+permissions:
+  contents: read
+  pull-requests: read  # `changes` reads the PR's file list via gh
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
@@ -30,6 +32,37 @@ env:
   FLIXOPT_CI: false
 
 jobs:
+  changes:
+    name: Detect source changes
+    runs-on: ubuntu-24.04
+    outputs:
+      source: ${{ steps.detect.outputs.source }}
+    steps:
+      - name: Detect source changes
+        id: detect
+        env:
+          GH_TOKEN: ${{ github.token }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          REPO: ${{ github.repository }}
+        run: |
+          if [[ "${{ github.event_name }}" != 'pull_request' ]]; then
+            echo 'source=true' >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          # Treat a failed, empty or truncated listing as a source change, so a
+          # detection failure costs a test run rather than skipping one. The guard
+          # is required: steps run under `bash -e`, where a failing command
+          # substitution aborts the step instead of falling through.
+          if ! paths=$(gh pr view "$PR_NUMBER" --repo "$REPO" --json files --jq '.files[].path'); then
+            echo 'source=true' >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          if [[ -z "$paths" ]] || grep -qE '^(flixopt/|tests/|pyproject\.toml$|\.github/workflows/)' <<< "$paths"; then
+            echo 'source=true' >> "$GITHUB_OUTPUT"
+          else
+            echo 'source=false' >> "$GITHUB_OUTPUT"
+          fi
+
   lint:
     runs-on: ubuntu-24.04
     steps:
@@ -52,7 +85,7 @@ jobs:
   test:
     runs-on: ubuntu-24.04
     timeout-minutes: 30
-    needs: lint
+    needs: [lint, changes]
     strategy:
       fail-fast: false
       matrix:
@@ -70,9 +103,11 @@ jobs:
           python-version: ${{ matrix.python-version }}
 
       - name: Install dependencies
+        if: needs.changes.outputs.source == 'true'
         run: uv pip install --system .[dev]
 
       - name: Run tests
+        if: needs.changes.outputs.source == 'true'
         run: |
           if [[ "${{ github.event.pull_request.draft }}" == "true" ]]; then
             # Draft PR: skip examples, slow, and deprecated_api
@@ -82,6 +117,10 @@ jobs:
             pytest -v --numprocesses=auto
           fi
 
+      - name: Report skipped tests
+        if: needs.changes.outputs.source != 'true'
+        run: echo 'No changes under flixopt/, tests/, pyproject.toml or .github/workflows/ - tests skipped.'
+
   test-examples:
     runs-on: ubuntu-24.04
     timeout-minutes: 45