diff --git a/src/browser/tools.zig b/src/browser/tools.zig
index 464b28a6..cd6a3c1c 100644
--- a/src/browser/tools.zig
+++ b/src/browser/tools.zig
@@ -818,12 +818,7 @@ fn execTree(arena: std.mem.Allocator, session: *lp.Session, registry: *CDPNode.R
     const args = try parseArgsOrDefault(TreeParams, arena, arguments);
     const page = try ensurePage(session, registry, args.url, args.timeout, args.waitUntil);
 
-    var root_node = page.document.asNode();
-    if (args.backendNodeId) |node_id| {
-        if (registry.lookup_by_id.get(node_id)) |n| {
-            root_node = n.dom;
-        }
-    }
+    const root_node = (try resolveOptionalNode(registry, args.backendNodeId)) orelse page.document.asNode();
 
     const st = lp.SemanticTree{
         .dom_node = root_node,
diff --git a/src/mcp/tools.zig b/src/mcp/tools.zig
index 5f9fa096..3d3b0e04 100644
--- a/src/mcp/tools.zig
+++ b/src/mcp/tools.zig
@@ -627,6 +627,20 @@ test "MCP - scriptStep accepts comment line" {
     try testing.expect(std.mem.indexOf(u8, out.written(), "\"isError\":true") == null);
 }
 
+test "MCP - tree rejects stale backendNodeId instead of dumping whole document" {
+    defer testing.reset();
+    var out: std.io.Writer.Allocating = .init(testing.arena_allocator);
+    const server = try testLoadPage("about:blank", &out.writer);
+    defer server.deinit();
+
+    const msg =
+        \\{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"tree","arguments":{"backendNodeId":999999}}}
+    ;
+    try router.handleMessage(server, testing.arena_allocator, msg);
+    const written = out.written();
+    try testing.expect(std.mem.indexOf(u8, written, "NodeNotFound") != null);
+}
+
 test "MCP - PascalCase argument keys from LLMs are normalized to canonical" {
     defer testing.reset();
     var out: std.io.Writer.Allocating = .init(testing.arena_allocator);