diff --git a/src/test/java/com/demcha/documentation/EnginePoiIsolationGuardTest.java b/src/test/java/com/demcha/documentation/EnginePoiIsolationGuardTest.java
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+++ b/src/test/java/com/demcha/documentation/EnginePoiIsolationGuardTest.java
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+package com.demcha.documentation;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.TreeSet;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Architectural guard that keeps Apache POI out of the lean engine module.
+ *
+ * <p>The semantic DOCX/PPTX backend — the only code that ever touched POI —
+ * lives in the separate {@code graph-compose-render-docx} module. The engine
+ * (root module) must stay free of every {@code org.apache.poi.*} import or
+ * fully-qualified reference so it never drags {@code poi-ooxml} back onto its
+ * classpath.</p>
+ *
+ * <p>This freezes that invariant at the source level: the retired {@code no-poi}
+ * CI job only ran the suite <em>without</em> POI on the classpath, it never
+ * asserted the sources were POI-free. Mirrors
+ * {@link PdfBackendIsolationGuardTest}'s grep-style walk/assert.</p>
+ */
+class EnginePoiIsolationGuardTest {
+
+    private static final Path PROJECT_ROOT = Path.of("").toAbsolutePath().normalize();
+    private static final String POI_IMPORT_PREFIX = "import org.apache.poi.";
+    private static final String POI_REFERENCE = "org.apache.poi.";
+
+    private static final List<Path> ENGINE_ROOTS = List.of(
+            PROJECT_ROOT.resolve("src/main/java/com/demcha/compose"),
+            PROJECT_ROOT.resolve("src/test/java/com/demcha/compose"));
+
+    @Test
+    void poiShouldStayOutOfTheEngineModule() throws IOException {
+        Map<String, Set<String>> violations = new LinkedHashMap<>();
+        for (Path file : engineJavaFiles()) {
+            Set<String> references = poiReferencesIn(file);
+            if (!references.isEmpty()) {
+                violations.put(relative(file), references);
+            }
+        }
+
+        assertThat(violations)
+                .describedAs("Apache POI must stay out of the lean engine module. The semantic "
+                        + "DOCX/PPTX backend lives in graph-compose-render-docx — keep every "
+                        + "org.apache.poi.* import and reference there, not under com.demcha.compose.")
+                .isEmpty();
+    }
+
+    private List<Path> engineJavaFiles() throws IOException {
+        List<Path> files = new ArrayList<>();
+        for (Path root : ENGINE_ROOTS) {
+            if (Files.notExists(root)) {
+                continue;
+            }
+            try (var stream = Files.walk(root)) {
+                stream.filter(Files::isRegularFile)
+                        .filter(p -> p.toString().endsWith(".java"))
+                        .forEach(files::add);
+            }
+        }
+        return files;
+    }
+
+    private Set<String> poiReferencesIn(Path file) throws IOException {
+        Set<String> references = new TreeSet<>();
+        for (String line : Files.readAllLines(file)) {
+            String trimmed = line.trim();
+            if (trimmed.startsWith(POI_IMPORT_PREFIX)) {
+                int semicolon = trimmed.indexOf(';');
+                if (semicolon >= 0) {
+                    references.add(trimmed.substring("import ".length(), semicolon).trim());
+                }
+            } else if (trimmed.contains(POI_REFERENCE)) {
+                references.add(trimmed);
+            }
+        }
+        return references;
+    }
+
+    private String relative(Path path) {
+        return PROJECT_ROOT.relativize(path).toString().replace('\\', '/');
+    }
+}