MCP/CDP: unify node registration

This fixes a bug in MCP where interactive elements were not assigned
a backendNodeId, preventing agents from clicking or filling them. Also
extracts link collection to a shared browser module.

src/browser/interactive.zig

@@ -36,6 +36,7 @@ pub const InteractivityType = enum {
 };
 
 pub const InteractiveElement = struct {
+    backendNodeId: ?u32 = null,
     node: *Node,
     tag_name: []const u8,
     role: ?[]const u8,
@@ -55,6 +56,11 @@ pub const InteractiveElement = struct {
     pub fn jsonStringify(self: *const InteractiveElement, jw: anytype) !void {
         try jw.beginObject();
 
+        if (self.backendNodeId) |id| {
+            try jw.objectField("backendNodeId");
+            try jw.write(id);
+        }
+
         try jw.objectField("tagName");
         try jw.write(self.tag_name);
 
@@ -123,6 +129,15 @@ pub const InteractiveElement = struct {
     }
 };
 
+/// Populate backendNodeId on each interactive element by registering
+/// their nodes in the given registry. Works with both CDP and MCP registries.
+pub fn registerNodes(elements: []InteractiveElement, registry: anytype) !void {
+    for (elements) |*el| {
+        const registered = try registry.register(el.node);
+        el.backendNodeId = registered.id;
+    }
+}
+
 /// Collect all interactive elements under `root`.
 pub fn collectInteractiveElements(
     root: *Node,

src/browser/links.zig

@@ -0,0 +1,54 @@
+// Copyright (C) 2023-2026  Lightpanda (Selecy SAS)
+//
+// Francis Bouvier <francis@lightpanda.io>
+// Pierre Tachoire <pierre@lightpanda.io>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+const std = @import("std");
+
+const Element = @import("webapi/Element.zig");
+const Node = @import("webapi/Node.zig");
+const Page = @import("Page.zig");
+const Selector = @import("webapi/selector/Selector.zig");
+
+const Allocator = std.mem.Allocator;
+
+/// Collect all links (href attributes from anchor tags) under `root`.
+/// Returns a slice of strings allocated with `arena`.
+pub fn collectLinks(arena: Allocator, root: *Node, page: *Page) ![]const []const u8 {
+    var links: std.ArrayList([]const u8) = .empty;
+
+    if (Selector.querySelectorAll(root, "a[href]", page)) |list| {
+        defer list.deinit(page._session);
+
+        for (list._nodes) |node| {
+            if (node.is(Element.Html.Anchor)) |anchor| {
+                const href = anchor.getHref(page) catch |err| {
+                    @import("../lightpanda.zig").log.err(.app, "resolve href failed", .{ .err = err });
+                    continue;
+                };
+
+                if (href.len > 0) {
+                    try links.append(arena, href);
+                }
+            }
+        }
+    } else |err| {
+        @import("../lightpanda.zig").log.err(.app, "query links failed", .{ .err = err });
+        return err;
+    }
+
+    return links.items;
+}

src/cdp/domains/lp.zig

@@ -135,17 +135,10 @@ fn getInteractiveElements(cmd: anytype) !void {
         page.document.asNode();
 
     const elements = try interactive.collectInteractiveElements(root, cmd.arena, page);
-
+    try interactive.registerNodes(elements, &bc.node_registry);
-    // Register nodes so nodeIds are valid for subsequent CDP calls.
-    var node_ids: std.ArrayList(Node.Id) = try .initCapacity(cmd.arena, elements.len);
-    for (elements) |el| {
-        const registered = try bc.node_registry.register(el.node);
-        node_ids.appendAssumeCapacity(registered.id);
-    }
 
     return cmd.sendResult(.{
         .elements = elements,
-        .nodeIds = node_ids.items,
     }, .{});
 }
 
@@ -308,7 +301,6 @@ test "cdp.lp: getInteractiveElements" {
 
     const result = (try ctx.getSentMessage(0)).?.object.get("result").?.object;
     try testing.expect(result.get("elements") != null);
-    try testing.expect(result.get("nodeIds") != null);
 }
 
 test "cdp.lp: getStructuredData" {

src/lightpanda.zig

@@ -35,6 +35,7 @@ pub const markdown = @import("browser/markdown.zig");
 pub const SemanticTree = @import("SemanticTree.zig");
 pub const CDPNode = @import("cdp/Node.zig");
 pub const interactive = @import("browser/interactive.zig");
+pub const links = @import("browser/links.zig");
 pub const forms = @import("browser/forms.zig");
 pub const actions = @import("browser/actions.zig");
 pub const structured_data = @import("browser/structured_data.zig");

src/mcp/tools.zig

@@ -203,23 +203,12 @@ const ToolStreamingText = struct {
                 log.err(.mcp, "markdown dump failed", .{ .err = err });
             },
             .links => {
-                if (Selector.querySelectorAll(self.page.document.asNode(), "a[href]", self.page)) |list| {
+                if (lp.links.collectLinks(self.page.call_arena, self.page.document.asNode(), self.page)) |links| {
-                    defer list.deinit(self.page._session);
-
                     var first = true;
-                    for (list._nodes) |node| {
+                    for (links) |href| {
-                        if (node.is(Element.Html.Anchor)) |anchor| {
+                        if (!first) try w.writeByte('\n');
-                            const href = anchor.getHref(self.page) catch |err| {
+                        try w.writeAll(href);
-                                log.err(.mcp, "resolve href failed", .{ .err = err });
+                        first = false;
-                                continue;
-                            };
-
-                            if (href.len > 0) {
-                                if (!first) try w.writeByte('\n');
-                                try w.writeAll(href);
-                                first = false;
-                            }
-                        }
                     }
                 } else |err| {
                     log.err(.mcp, "query links failed", .{ .err = err });
@@ -417,6 +406,12 @@ fn handleInteractiveElements(server: *Server, arena: std.mem.Allocator, id: std.
         log.err(.mcp, "elements collection failed", .{ .err = err });
         return server.sendError(id, .InternalError, "Failed to collect interactive elements");
     };
+
+    lp.interactive.registerNodes(elements, &server.node_registry) catch |err| {
+        log.err(.mcp, "node registration failed", .{ .err = err });
+        return server.sendError(id, .InternalError, "Failed to register element nodes");
+    };
+
     var aw: std.Io.Writer.Allocating = .init(arena);
     try std.json.Stringify.value(elements, .{}, &aw.writer);