diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 76262d70..75c0dc71 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -109,10 +109,15 @@ jobs:
 
   test-with-coverage:
     runs-on: ubuntu-latest
+    outputs:
+      diff-coverage: ${{ steps.diff_cover.outputs.percent }}
 
     steps:
       - name: Checkout code
         uses: https://github.com/actions/checkout@v5
+        with:
+          # diff-cover needs the base branch present to diff against
+          fetch-depth: 0
 
       - name: Setup
         uses: ./.gitea/actions/install
@@ -129,14 +134,41 @@ jobs:
           source .venv/bin/activate
           pytest --cov=aare --cov-config=./pyproject.toml --cov-branch --cov-report=xml --no-cov-on-fail ./tests/unit
 
+      # No --fail-under here: the gate lives in coverage-analysis so that the PR
+      # comment is posted before the build is failed.
+      - name: Diff coverage
+        id: diff_cover
+        env:
+          BASE_BRANCH: ${{ github.base_ref || 'main' }}
+        run: |
+          source .venv/bin/activate
+          diff-cover coverage.xml \
+            --compare-branch="origin/${BASE_BRANCH}" \
+            --format "markdown:diff-cover.md,json:diff-cover.json"
+          percent=$(python -c "import json; print(json.load(open('diff-cover.json'))['total_percent_covered'])")
+          echo "percent=${percent}" >> "$GITHUB_OUTPUT"
+          echo "Diff coverage: ${percent}%"
+
       - name: Build coverage summary
+        env:
+          BASE_BRANCH: ${{ github.base_ref || 'main' }}
+          DIFF_COVERAGE: ${{ steps.diff_cover.outputs.percent }}
         run: |
           source .venv/bin/activate
           {
             echo "<!-- coverage-report -->"
             echo "### Coverage report"
             echo
-            echo "**Total line + branch coverage: $(coverage report --format=total)%**"
+            echo "| Metric | Value |"
+            echo "| --- | --- |"
+            echo "| Total line + branch coverage | $(coverage report --format=total)% |"
+            echo "| Diff coverage vs \`${BASE_BRANCH}\` | ${DIFF_COVERAGE}% (minimum 80%) |"
+            echo
+            echo "<details><summary>Diff coverage detail</summary>"
+            echo
+            cat diff-cover.md
+            echo
+            echo "</details>"
             echo
             echo "<details><summary>Per-file breakdown</summary>"
             echo
@@ -182,3 +214,19 @@ jobs:
           issue-number: ${{ github.event.pull_request.number }}
           body-path: coverage-summary.md
           edit-mode: replace
+
+      # Last, so the report is always visible on the PR even when this fails.
+      - name: Enforce diff coverage
+        env:
+          DIFF_COVERAGE: ${{ needs.test-with-coverage.outputs.diff-coverage }}
+          MINIMUM: "80"
+        run: |
+          if [ -z "${DIFF_COVERAGE}" ]; then
+            echo "::error::test-with-coverage produced no diff coverage value"
+            exit 1
+          fi
+          echo "Diff coverage: ${DIFF_COVERAGE}% (minimum ${MINIMUM}%)"
+          if awk -v p="${DIFF_COVERAGE}" -v m="${MINIMUM}" 'BEGIN { exit !(p + 0 < m + 0) }'; then
+            echo "::error::Diff coverage ${DIFF_COVERAGE}% is below the required ${MINIMUM}%"
+            exit 1
+          fi
diff --git a/pyproject.toml b/pyproject.toml
index 005125bc..3408b544 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -43,6 +43,7 @@ test = [
     "pytest-qt",
     "pytest-asyncio",
     "pytest-timeout",
+    "diff-cover",
     "ruff>=0.15"
 ]