diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index f9553ab2..46e38aa1 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -108,6 +108,32 @@ jobs:
             exit 1
           fi
 
+      - name: Assert deploy-obs writes obs-secrets.env via an unquoted heredoc (#603)
+        shell: bash
+        run: |
+          # Inside a composite action, secrets arrive as $VAR from env: (secrets.*
+          # is unavailable there), so the obs-secrets.env heredoc MUST use an
+          # unquoted delimiter (<<EOF) for $VAR to expand. A quoted delimiter
+          # (<<'EOF') would write the literal string "$GRAFANA_ADMIN_PASSWORD",
+          # and the action's five-key non-empty guard would STILL pass (the line
+          # is present, just wrong). This guard enforces the invariant in CI so a
+          # future re-quote cannot ship broken obs auth green. See ADR-029 / #603.
+          action='.gitea/actions/deploy-obs/action.yml'
+          quoted='obs-secrets\.env\s*<<-?\s*[\x27\x22]'
+          # Self-test: the regex must catch a quoted delimiter and ignore the unquoted one.
+          printf "obs-secrets.env <<'EOF'\n" | grep -qP "$quoted" \
+            || { echo "FAIL: guard self-test — regex missed the quoted <<'EOF' form"; exit 1; }
+          printf 'obs-secrets.env <<EOF\n' | grep -qvP "$quoted" \
+            || { echo "FAIL: guard self-test — regex wrongly flagged the unquoted <<EOF form"; exit 1; }
+          # Positive: the unquoted heredoc must be present at all.
+          grep -qP 'obs-secrets\.env\s*<<-?EOF\b' "$action" \
+            || { echo "::error::$action no longer writes obs-secrets.env via an unquoted <<EOF heredoc (ADR-029 / #603)"; exit 1; }
+          # Negative: never a quoted delimiter on the obs-secrets.env heredoc.
+          if grep -nP "$quoted" "$action"; then
+            echo "::error::$action writes obs-secrets.env with a quoted heredoc delimiter — secrets would be written as literal \$VAR strings. Use unquoted <<EOF (ADR-029 / #603)."
+            exit 1
+          fi
+
       - name: Run unit and component tests with coverage
         shell: bash
         run: |