diff --git a/src/main/java/org/unlaxer/tinyexpression/instances/FileBaseTinyExpressionInstancesCache.java b/src/main/java/org/unlaxer/tinyexpression/instances/FileBaseTinyExpressionInstancesCache.java
index 9b4ecf22..449df10d 100644
--- a/src/main/java/org/unlaxer/tinyexpression/instances/FileBaseTinyExpressionInstancesCache.java
+++ b/src/main/java/org/unlaxer/tinyexpression/instances/FileBaseTinyExpressionInstancesCache.java
@@ -35,10 +35,22 @@ public class FileBaseTinyExpressionInstancesCache implements TinyExpressionInsta
   public FileBaseTinyExpressionInstancesCache(Path rootFolder ,
       FormulaInfoAdditionalFields formulaInfoAdditionalFields) {
     super();
-    this.rootFolder = rootFolder;
+    this.rootFolder = rootFolder != null ? rootFolder.toAbsolutePath().normalize() : null;
     this.formulaInfoAdditionalFields = formulaInfoAdditionalFields;
   }
 
+  static Path resolveUnderRoot(Path rootFolder, String tenantIdString) {
+    if (rootFolder == null) {
+      throw new IllegalArgumentException("rootFolder is not set");
+    }
+    Path resolved = rootFolder.resolve(tenantIdString).normalize();
+    if (!resolved.startsWith(rootFolder)) {
+      throw new IllegalArgumentException(
+          "tenantId escapes rootFolder: " + tenantIdString);
+    }
+    return resolved;
+  }
+
   @Override
   public boolean clearCache(TenantID tenantID) {
     calculatorsByTenantId.remove(tenantID);
@@ -51,7 +63,7 @@ public List<Calculator> cache(TenantID tenantID, Comparator<Calculator> comparat
     List<Calculator> cache =
         calculatorsByTenantId.computeIfAbsent(tenantID,
             tenantId->{
-              Path resolve = rootFolder.resolve(tenantId.asString()).resolve(FILENAME);
+              Path resolve = resolveUnderRoot(rootFolder, tenantId.asString()).resolve(FILENAME);
               try(InputStream inputStream = Files.newInputStream(resolve);){
                 Try<FormulaInfoList> parse =
                     FormulaInfoList.parse(inputStream, formulaInfoAdditionalFields, classLoader);
diff --git a/src/main/java/org/unlaxer/tinyexpression/mcp/McpServer.java b/src/main/java/org/unlaxer/tinyexpression/mcp/McpServer.java
index 609cde15..9340f5d5 100644
--- a/src/main/java/org/unlaxer/tinyexpression/mcp/McpServer.java
+++ b/src/main/java/org/unlaxer/tinyexpression/mcp/McpServer.java
@@ -41,6 +41,8 @@ public class McpServer {
 
     private static final ObjectMapper MAPPER = new ObjectMapper();
 
+    static final long MAX_REQUEST_BODY_BYTES = 16L * 1024 * 1024;
+
     private final HttpServer server;
     private final boolean allowJavaCode;
 
@@ -85,7 +87,13 @@ public void handle(HttpExchange ex) throws IOException {
                 sessionId = UUID.randomUUID().toString();
             }
 
-            String bodyStr = readBody(ex);
+            String bodyStr;
+            try {
+                bodyStr = readBody(ex);
+            } catch (IOException e) {
+                sendWithSession(ex, 413, MAPPER.writeValueAsString(errorResp(null, -32603, "Request body too large")), sessionId);
+                return;
+            }
             JsonNode req;
             try {
                 req = MAPPER.readTree(bodyStr);
@@ -859,8 +867,19 @@ private static class BatchFormula {
     // ─── HTTP helpers ──────────────────────────────────────────
 
     private static String readBody(HttpExchange ex) throws IOException {
+        String contentLength = ex.getRequestHeaders().getFirst("Content-Length");
+        if (contentLength != null) {
+            try {
+                long len = Long.parseLong(contentLength.trim());
+                if (len > MAX_REQUEST_BODY_BYTES) {
+                    throw new IOException("Request body too large: " + len + " > " + MAX_REQUEST_BODY_BYTES);
+                }
+            } catch (NumberFormatException ignored) {
+                // fall through to bounded read
+            }
+        }
         try (InputStream is = ex.getRequestBody()) {
-            return new String(is.readAllBytes(), StandardCharsets.UTF_8);
+            return new String(is.readNBytes((int) MAX_REQUEST_BODY_BYTES), StandardCharsets.UTF_8);
         }
     }
 
diff --git a/src/test/java/org/unlaxer/tinyexpression/instances/FileBaseTinyExpressionInstancesCacheTest.java b/src/test/java/org/unlaxer/tinyexpression/instances/FileBaseTinyExpressionInstancesCacheTest.java
new file mode 100644
index 00000000..52054f28
--- /dev/null
+++ b/src/test/java/org/unlaxer/tinyexpression/instances/FileBaseTinyExpressionInstancesCacheTest.java
@@ -0,0 +1,41 @@
+package org.unlaxer.tinyexpression.instances;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import org.junit.Test;
+
+public class FileBaseTinyExpressionInstancesCacheTest {
+
+  @Test
+  public void resolveUnderRoot_acceptsSimpleId() {
+    Path root = Paths.get("/tmp/formula-root");
+    Path resolved = FileBaseTinyExpressionInstancesCache.resolveUnderRoot(root, "69");
+    assertEquals(root.resolve("69"), resolved);
+    assertTrue(resolved.startsWith(root));
+  }
+
+  @Test
+  public void resolveUnderRoot_rejectsTraversal() {
+    Path root = Paths.get("/tmp/formula-root");
+    assertThrows(IllegalArgumentException.class,
+        () -> FileBaseTinyExpressionInstancesCache.resolveUnderRoot(root, "../../etc"));
+  }
+
+  @Test
+  public void resolveUnderRoot_rejectsAbsolutePath() {
+    Path root = Paths.get("/tmp/formula-root");
+    assertThrows(IllegalArgumentException.class,
+        () -> FileBaseTinyExpressionInstancesCache.resolveUnderRoot(root, "/etc"));
+  }
+
+  @Test
+  public void resolveUnderRoot_rejectsNullRoot() {
+    assertThrows(IllegalArgumentException.class,
+        () -> FileBaseTinyExpressionInstancesCache.resolveUnderRoot(null, "69"));
+  }
+}
diff --git a/src/test/java/org/unlaxer/tinyexpression/mcp/McpServerTest.java b/src/test/java/org/unlaxer/tinyexpression/mcp/McpServerTest.java
index 11c832e0..fb75a7d1 100644
--- a/src/test/java/org/unlaxer/tinyexpression/mcp/McpServerTest.java
+++ b/src/test/java/org/unlaxer/tinyexpression/mcp/McpServerTest.java
@@ -270,6 +270,22 @@ public void resourcesRead_guide_returnsMarkdown() throws Exception {
         assertTrue(text.contains("tinyexpression MCP"));
     }
 
+    @Test
+    public void oversizedBody_rejectedWith413() throws Exception {
+        long max = McpServer.MAX_REQUEST_BODY_BYTES;
+        byte[] padding = new byte[(int) Math.min(max + 1024, Integer.MAX_VALUE - 8)];
+        java.util.Arrays.fill(padding, (byte) ' ');
+        String body = new String(padding, java.nio.charset.StandardCharsets.UTF_8);
+
+        HttpRequest req = HttpRequest.newBuilder()
+                .uri(URI.create("http://127.0.0.1:" + port + "/mcp"))
+                .header("Content-Type", "application/json")
+                .POST(HttpRequest.BodyPublishers.ofString(body))
+                .build();
+        HttpResponse<String> resp = client.send(req, HttpResponse.BodyHandlers.ofString());
+        assertEquals(413, resp.statusCode());
+    }
+
     // ─── helpers ──────────────────────────────────────────────────
 
     private JsonNode rpc(String method, JsonNode params) throws Exception {