feat(cdp): emit Target.targetDestroyed when target is closed

This improves protocol compliance and helps automation frameworks properly clean up their internal state when targets are closed or replaced.

src/Config.zig

@@ -24,6 +24,7 @@ const log = @import("log.zig");
 const dump = @import("browser/dump.zig");
 
 const WebBotAuthConfig = @import("network/WebBotAuth.zig").Config;
+const mcp = @import("mcp.zig");
 
 pub const RunMode = enum {
     help,
@@ -222,6 +223,7 @@ pub const Serve = struct {
 
 pub const Mcp = struct {
     common: Common = .{},
+    version: mcp.Version = .default,
 };
 
 pub const DumpFormat = enum {
@@ -453,6 +455,12 @@ pub fn printUsageAndExit(self: *const Config, success: bool) void {
         \\Starts an MCP (Model Context Protocol) server over stdio
         \\Example: {s} mcp
         \\
+        \\Options:
+        \\--version
+        \\                Override the reported MCP version.
+        \\                Valid: 2024-11-05, 2025-03-26, 2025-06-18, 2025-11-25.
+        \\                Defaults to "2024-11-05".
+        \\
     ++ common_options ++
         \\
         \\version command
@@ -640,10 +648,22 @@ fn parseMcpArgs(
     allocator: Allocator,
     args: *std.process.ArgIterator,
 ) !Mcp {
-    var mcp: Mcp = .{};
+    var result: Mcp = .{};
 
     while (args.next()) |opt| {
-        if (try parseCommonArg(allocator, opt, args, &mcp.common)) {
+        if (std.mem.eql(u8, "--version", opt)) {
+            const str = args.next() orelse {
+                log.fatal(.mcp, "missing argument value", .{ .arg = opt });
+                return error.InvalidArgument;
+            };
+            result.version = std.meta.stringToEnum(mcp.Version, str) orelse {
+                log.fatal(.mcp, "invalid protocol version", .{ .value = str });
+                return error.InvalidArgument;
+            };
+            continue;
+        }
+
+        if (try parseCommonArg(allocator, opt, args, &result.common)) {
             continue;
         }
 
@@ -651,7 +671,7 @@ fn parseMcpArgs(
         return error.UnkownOption;
     }
 
-    return mcp;
+    return result;
 }
 
 fn parseFetchArgs(

src/browser/HttpClient.zig

@@ -821,16 +821,16 @@ fn processOneMessage(self: *Client, msg: http.Handles.MultiMessage, transfer: *T
         break :blk std.ascii.eqlIgnoreCase(hdr.value, "close");
     };
 
-    if (msg.err != null and !is_conn_close_recv) {
-        transfer.requestFailed(transfer._callback_error orelse msg.err.?, true);
-        return true;
-    }
-
     // make sure the transfer can't be immediately aborted from a callback
     // since we still need it here.
     transfer._performing = true;
     defer transfer._performing = false;
 
+    if (msg.err != null and !is_conn_close_recv) {
+        transfer.requestFailed(transfer._callback_error orelse msg.err.?, true);
+        return true;
+    }
+
     if (!transfer._header_done_called) {
         // In case of request w/o data, we need to call the header done
         // callback now.
@@ -873,7 +873,6 @@ fn processMessages(self: *Client) !bool {
     var processed = false;
     while (self.handles.readMessage()) |msg| {
         const transfer = try Transfer.fromConnection(&msg.conn);
-
         const done = self.processOneMessage(msg, transfer) catch |err| blk: {
             log.err(.http, "process_messages", .{ .err = err, .req = transfer });
             transfer.requestFailed(err, true);
@@ -1068,6 +1067,24 @@ pub const Transfer = struct {
         if (self.req.shutdown_callback) |cb| {
             cb(self.ctx);
         }
+
+        if (self._performing or self.client.performing) {
+            // We're currently inside of a callback. This client, and libcurl
+            // generally don't expect a transfer to become deinitialized during
+            // a callback. We can flag the transfer as aborted (which is what
+            // we do when transfer.abort() is called in this condition) AND,
+            // since this "kill()"should prevent any future callbacks, the best
+            // we can do is null/noop them.
+            self.aborted = true;
+            self.req.start_callback = null;
+            self.req.shutdown_callback = null;
+            self.req.header_callback = Noop.headerCallback;
+            self.req.data_callback = Noop.dataCallback;
+            self.req.done_callback = Noop.doneCallback;
+            self.req.error_callback = Noop.errorCallback;
+            return;
+        }
+
         self.deinit();
     }
 
@@ -1492,3 +1509,12 @@ pub const Transfer = struct {
         return null;
     }
 };
+
+const Noop = struct {
+    fn headerCallback(_: *Transfer) !bool {
+        return true;
+    }
+    fn dataCallback(_: *Transfer, _: []const u8) !void {}
+    fn doneCallback(_: *anyopaque) !void {}
+    fn errorCallback(_: *anyopaque, _: anyerror) void {}
+};

src/browser/URL.zig

@@ -77,41 +77,6 @@ pub fn resolve(allocator: Allocator, base: [:0]const u8, path: anytype, comptime
         return processResolved(allocator, result, opts);
     }
 
-    if (path.len >= 4) { // Minimum: "ws:x"
-        if (std.mem.indexOfScalar(u8, path[0..@min(path.len, 6)], ':')) |pos| {
-            // we know this isn't a complete URL, else the very first check in
-            // this function would have handled it.
-            const possible_special_protocol = path[0..pos];
-            const special_schemes = [_][]const u8{ "https", "http", "ws", "wss", "file", "ftp" };
-            for (special_schemes) |special_scheme| {
-                if (std.ascii.eqlIgnoreCase(possible_special_protocol, special_scheme)) {
-                    const rest = path[pos + 1 ..];
-
-                    // Check if base has the same scheme
-                    const base_scheme_end = std.mem.indexOf(u8, base, "://") orelse 0;
-                    if (base_scheme_end > 0 and std.ascii.eqlIgnoreCase(base[0..base_scheme_end], special_scheme)) {
-                        // Same scheme - strip it and resolve rest as relative
-                        return resolve(allocator, base, rest, opts);
-                    }
-
-                    // Different scheme - construct absolute URL
-                    // Skip any leading slashes in rest
-                    var rest_start: usize = 0;
-                    while (rest_start < rest.len and (rest[rest_start] == '/' or rest[rest_start] == '\\')) {
-                        rest_start += 1;
-                    }
-                    const rest_trimmed = rest[rest_start..];
-
-                    // file: scheme needs empty host (triple slash)
-                    const separator = if (std.mem.eql(u8, special_scheme, "file")) ":///" else "://";
-                    const normalized = try std.mem.joinZ(allocator, "", &.{ special_scheme, separator, rest_trimmed });
-                    return resolve(allocator, "", normalized, opts);
-                }
-            }
-            // Don't know what this is, just try to resolve it through our normal logic
-        }
-    }
-
     const scheme_end = std.mem.indexOf(u8, base, "://");
     const authority_start = if (scheme_end) |end| end + 3 else 0;
     const path_start = std.mem.indexOfScalarPos(u8, base, authority_start, '/') orelse base.len;
@@ -1605,84 +1570,3 @@ test "URL: getOrigin" {
         }
     }
 }
-
-test "URL: resolve path scheme" {
-    const Case = struct {
-        base: [:0]const u8,
-        path: [:0]const u8,
-        expected: [:0]const u8,
-    };
-
-    const cases = [_]Case{
-        .{
-            .base = "https://www.example.com/example",
-            .path = "https:/about",
-            .expected = "https://www.example.com/about",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "https:about",
-            .expected = "https://www.example.com/about",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "https://about",
-            .expected = "https://about",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "http:about",
-            .expected = "http://about",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "http:/about",
-            .expected = "http://about",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "http://about",
-            .expected = "http://about",
-        },
-        .{
-            .base = "https://site/",
-            .path = "https://path",
-            .expected = "https://path",
-        },
-        .{
-            .base = "http://localhost/",
-            .path = "data:test",
-            .expected = "data:test",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "ws://about",
-            .expected = "ws://about",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "wss://about",
-            .expected = "wss://about",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "ftp://about",
-            .expected = "ftp://about",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "file://path/to/file",
-            .expected = "file://path/to/file",
-        },
-        .{
-            .base = "https://www.example.com/example",
-            .path = "file:/path/to/file",
-            .expected = "file:///path/to/file",
-        },
-    };
-
-    for (cases) |case| {
-        const result = try resolve(testing.arena_allocator, case.base, case.path, .{});
-        try testing.expectString(case.expected, result);
-    }
-}

src/browser/tests/canvas/canvas_rendering_context_2d.html

@@ -148,3 +148,13 @@
 
 }
 </script>
+
+<script id=identity>
+{
+  const element = document.createElement('canvas');
+  const ctx = element.getContext('2d');
+
+  testing.expectTrue(ctx === element.getContext('2d'));
+  testing.expectEqual(null, element.getContext('webgl'));
+}
+</script>

src/browser/tests/canvas/webgl_rendering_context.html

@@ -85,3 +85,13 @@
   loseContext.restoreContext();
 }
 </script>
+
+<script id=identity>
+{
+  const element = document.createElement('canvas');
+  const ctx = element.getContext('webgl');
+
+  testing.expectTrue(ctx === element.getContext('webgl'));
+  testing.expectEqual(null, element.getContext('2d'));
+}
+</script>

src/browser/tests/navigator/navigator.html

@@ -15,8 +15,9 @@
   testing.expectEqual(true, validPlatforms.includes(navigator.platform));
   testing.expectEqual('en-US', navigator.language);
   testing.expectEqual(true, Array.isArray(navigator.languages));
-  testing.expectEqual(1, navigator.languages.length);
+  testing.expectEqual(2, navigator.languages.length);
   testing.expectEqual('en-US', navigator.languages[0]);
+  testing.expectEqual('en', navigator.languages[1]);
   testing.expectEqual(true, navigator.onLine);
   testing.expectEqual(true, navigator.cookieEnabled);
   testing.expectEqual(true, navigator.hardwareConcurrency > 0);

src/browser/webapi/DOMException.zig

@@ -59,6 +59,7 @@ pub fn fromError(err: anyerror) ?DOMException {
         error.TimeoutError => .{ ._code = .timeout_error },
         error.InvalidNodeType => .{ ._code = .invalid_node_type_error },
         error.DataClone => .{ ._code = .data_clone_error },
+        error.InvalidAccessError => .{ ._code = .invalid_access_error },
         else => null,
     };
 }

src/browser/webapi/Navigator.zig

@@ -40,8 +40,8 @@ pub fn getUserAgent(_: *const Navigator, page: *Page) []const u8 {
     return page._session.browser.app.config.http_headers.user_agent;
 }
 
-pub fn getLanguages(_: *const Navigator) [1][]const u8 {
+pub fn getLanguages(_: *const Navigator) [2][]const u8 {
-    return .{"en-US"};
+    return .{ "en-US", "en" };
 }
 
 pub fn getPlatform(_: *const Navigator) []const u8 {

src/browser/webapi/element/html/Canvas.zig

@@ -29,6 +29,9 @@ const OffscreenCanvas = @import("../../canvas/OffscreenCanvas.zig");
 
 const Canvas = @This();
 _proto: *HtmlElement,
+_cached: ?DrawingContext = null,
+
+const ContextType = enum { none, @"2d", webgl };
 
 pub fn asElement(self: *Canvas) *Element {
     return self._proto._proto;
@@ -68,17 +71,28 @@ const DrawingContext = union(enum) {
 };
 
 pub fn getContext(self: *Canvas, context_type: []const u8, page: *Page) !?DrawingContext {
-    if (std.mem.eql(u8, context_type, "2d")) {
+    if (self._cached) |cached| {
-        const ctx = try page._factory.create(CanvasRenderingContext2D{ ._canvas = self });
+        const matches = switch (cached) {
-        return .{ .@"2d" = ctx };
+            .@"2d" => std.mem.eql(u8, context_type, "2d"),
+            .webgl => std.mem.eql(u8, context_type, "webgl") or std.mem.eql(u8, context_type, "experimental-webgl"),
+        };
+        return if (matches) cached else null;
     }
 
-    if (std.mem.eql(u8, context_type, "webgl") or std.mem.eql(u8, context_type, "experimental-webgl")) {
+    const drawing_context: DrawingContext = blk: {
-        const ctx = try page._factory.create(WebGLRenderingContext{});
+        if (std.mem.eql(u8, context_type, "2d")) {
-        return .{ .webgl = ctx };
+            const ctx = try page._factory.create(CanvasRenderingContext2D{ ._canvas = self });
-    }
+            break :blk .{ .@"2d" = ctx };
+        }
 
-    return null;
+        if (std.mem.eql(u8, context_type, "webgl") or std.mem.eql(u8, context_type, "experimental-webgl")) {
+            const ctx = try page._factory.create(WebGLRenderingContext{});
+            break :blk .{ .webgl = ctx };
+        }
+        return null;
+    };
+    self._cached = drawing_context;
+    return drawing_context;
 }
 
 /// Transfers control of the canvas to an OffscreenCanvas.

src/cdp/CDP.zig

@@ -363,6 +363,11 @@ pub fn BrowserContext(comptime CDP_T: type) type {
         inspector_session: *js.Inspector.Session,
         isolated_worlds: std.ArrayList(*IsolatedWorld),
 
+        // Scripts registered via Page.addScriptToEvaluateOnNewDocument.
+        // Evaluated in each new document after navigation completes.
+        scripts_on_new_document: std.ArrayList(ScriptOnNewDocument) = .empty,
+        next_script_id: u32 = 1,
+
         http_proxy_changed: bool = false,
 
         // Extra headers to add to all requests.
@@ -762,6 +767,11 @@ pub fn BrowserContext(comptime CDP_T: type) type {
 /// Clients create this to be able to create variables and run code without interfering with the
 /// normal namespace and values of the webpage. Similar to the main context we need to pretend to recreate it after
 /// a executionContextsCleared event which happens when navigating to a new page. A client can have a command be executed
+const ScriptOnNewDocument = struct {
+    identifier: u32,
+    source: []const u8,
+};
+
 /// in the isolated world by using its Context ID or the worldName.
 /// grantUniveralAccess Indecated whether the isolated world can reference objects like the DOM or other JS Objects.
 /// An isolated world has it's own instance of globals like Window.

src/cdp/domains/page.zig

@@ -37,6 +37,7 @@ pub fn processMessage(cmd: anytype) !void {
         getFrameTree,
         setLifecycleEventsEnabled,
         addScriptToEvaluateOnNewDocument,
+        removeScriptToEvaluateOnNewDocument,
         createIsolatedWorld,
         navigate,
         reload,
@@ -51,6 +52,7 @@ pub fn processMessage(cmd: anytype) !void {
         .getFrameTree => return getFrameTree(cmd),
         .setLifecycleEventsEnabled => return setLifecycleEventsEnabled(cmd),
         .addScriptToEvaluateOnNewDocument => return addScriptToEvaluateOnNewDocument(cmd),
+        .removeScriptToEvaluateOnNewDocument => return removeScriptToEvaluateOnNewDocument(cmd),
         .createIsolatedWorld => return createIsolatedWorld(cmd),
         .navigate => return navigate(cmd),
         .reload => return doReload(cmd),
@@ -147,22 +149,55 @@ fn setLifecycleEventsEnabled(cmd: anytype) !void {
     return cmd.sendResult(null, .{});
 }
 
-// TODO: hard coded method
-// With the command we receive a script we need to store and run for each new document.
-// Note that the worldName refers to the name given to the isolated world.
 fn addScriptToEvaluateOnNewDocument(cmd: anytype) !void {
-    // const params = (try cmd.params(struct {
+    const params = (try cmd.params(struct {
-    //     source: []const u8,
+        source: []const u8,
-    //     worldName: ?[]const u8 = null,
+        worldName: ?[]const u8 = null,
-    //     includeCommandLineAPI: bool = false,
+        includeCommandLineAPI: bool = false,
-    //     runImmediately: bool = false,
+        runImmediately: bool = false,
-    // })) orelse return error.InvalidParams;
+    })) orelse return error.InvalidParams;
 
+    const bc = cmd.browser_context orelse return error.BrowserContextNotLoaded;
+
+    if (params.runImmediately) {
+        log.warn(.not_implemented, "addScriptOnNewDocument", .{ .param = "runImmediately" });
+    }
+
+    const script_id = bc.next_script_id;
+    bc.next_script_id += 1;
+
+    const source_dupe = try bc.arena.dupe(u8, params.source);
+    try bc.scripts_on_new_document.append(bc.arena, .{
+        .identifier = script_id,
+        .source = source_dupe,
+    });
+
+    var id_buf: [16]u8 = undefined;
+    const id_str = std.fmt.bufPrint(&id_buf, "{d}", .{script_id}) catch "1";
     return cmd.sendResult(.{
-        .identifier = "1",
+        .identifier = id_str,
     }, .{});
 }
 
+fn removeScriptToEvaluateOnNewDocument(cmd: anytype) !void {
+    const params = (try cmd.params(struct {
+        identifier: []const u8,
+    })) orelse return error.InvalidParams;
+
+    const bc = cmd.browser_context orelse return error.BrowserContextNotLoaded;
+
+    const target_id = std.fmt.parseInt(u32, params.identifier, 10) catch
+        return cmd.sendResult(null, .{});
+
+    for (bc.scripts_on_new_document.items, 0..) |script, i| {
+        if (script.identifier == target_id) {
+            _ = bc.scripts_on_new_document.orderedRemove(i);
+            break;
+        }
+    }
+    return cmd.sendResult(null, .{});
+}
+
 fn close(cmd: anytype) !void {
     const bc = cmd.browser_context orelse return error.BrowserContextNotLoaded;
 
@@ -482,6 +517,27 @@ pub fn pageNavigated(arena: Allocator, bc: anytype, event: *const Notification.P
         );
     }
 
+    // Evaluate scripts registered via Page.addScriptToEvaluateOnNewDocument.
+    // Must run after the execution context is created but before the client
+    // receives frameNavigated/loadEventFired so polyfills are available for
+    // subsequent CDP commands.
+    if (bc.scripts_on_new_document.items.len > 0) {
+        var ls: js.Local.Scope = undefined;
+        page.js.localScope(&ls);
+        defer ls.deinit();
+
+        for (bc.scripts_on_new_document.items) |script| {
+            var try_catch: lp.js.TryCatch = undefined;
+            try_catch.init(&ls.local);
+            defer try_catch.deinit();
+
+            ls.local.eval(script.source, null) catch |err| {
+                const caught = try_catch.caughtOrError(arena, err);
+                log.warn(.cdp, "script on new doc", .{ .caught = caught });
+            };
+        }
+    }
+
     // frameNavigated event
     try cdp.sendEvent("Page.frameNavigated", .{
         .type = "Navigation",
@@ -840,3 +896,55 @@ test "cdp.page: reload" {
         try ctx.processMessage(.{ .id = 32, .method = "Page.reload", .params = .{ .ignoreCache = true } });
     }
 }
+
+test "cdp.page: addScriptToEvaluateOnNewDocument" {
+    var ctx = try testing.context();
+    defer ctx.deinit();
+
+    var bc = try ctx.loadBrowserContext(.{ .id = "BID-9", .url = "hi.html", .target_id = "FID-000000000X".* });
+
+    {
+        // Register a script — should return unique identifier "1"
+        try ctx.processMessage(.{ .id = 20, .method = "Page.addScriptToEvaluateOnNewDocument", .params = .{ .source = "window.__test = 1" } });
+        try ctx.expectSentResult(.{
+            .identifier = "1",
+        }, .{ .id = 20 });
+    }
+
+    {
+        // Register another script — should return identifier "2"
+        try ctx.processMessage(.{ .id = 21, .method = "Page.addScriptToEvaluateOnNewDocument", .params = .{ .source = "window.__test2 = 2" } });
+        try ctx.expectSentResult(.{
+            .identifier = "2",
+        }, .{ .id = 21 });
+    }
+
+    {
+        // Remove the first script — should succeed
+        try ctx.processMessage(.{ .id = 22, .method = "Page.removeScriptToEvaluateOnNewDocument", .params = .{ .identifier = "1" } });
+        try ctx.expectSentResult(null, .{ .id = 22 });
+    }
+
+    {
+        // Remove a non-existent identifier — should succeed silently
+        try ctx.processMessage(.{ .id = 23, .method = "Page.removeScriptToEvaluateOnNewDocument", .params = .{ .identifier = "999" } });
+        try ctx.expectSentResult(null, .{ .id = 23 });
+    }
+
+    {
+        try ctx.processMessage(.{ .id = 34, .method = "Page.reload" });
+        // wait for this event, which is sent after we've run the registered scripts
+        try ctx.expectSentEvent("Page.frameNavigated", .{
+            .frame = .{ .loaderId = "LID-0000000002" },
+        }, .{});
+
+        const page = bc.session.currentPage() orelse unreachable;
+
+        var ls: js.Local.Scope = undefined;
+        page.js.localScope(&ls);
+        defer ls.deinit();
+
+        const test_val = try ls.local.exec("window.__test2", null);
+        try testing.expectEqual(2, try test_val.toI32());
+    }
+}

src/cdp/domains/target.zig

@@ -158,15 +158,20 @@ fn createTarget(cmd: anytype) !void {
         else => return err,
     };
 
-    if (bc.target_id != null) {
-        return error.TargetAlreadyLoaded;
-    }
     if (params.browserContextId) |param_browser_context_id| {
         if (std.mem.eql(u8, param_browser_context_id, bc.id) == false) {
             return error.UnknownBrowserContextId;
         }
     }
 
+    // If a target already exists, close it first. Lightpanda only supports
+    // one page at a time, so we replace the existing target rather than
+    // rejecting the request. This unblocks automation frameworks (e.g.
+    // Stagehand) that call createTarget multiple times.
+    if (bc.target_id != null) {
+        try doCloseTarget(cmd, bc);
+    }
+
     // if target_id is null, we should never have a page
     lp.assert(bc.session.page == null, "CDP.target.createTarget not null page", .{});
 
@@ -280,34 +285,9 @@ fn closeTarget(cmd: anytype) !void {
         return error.UnknownTargetId;
     }
 
-    // can't be null if we have a target_id
-    lp.assert(bc.session.page != null, "CDP.target.closeTarget null page", .{});
-
     try cmd.sendResult(.{ .success = true }, .{ .include_session_id = false });
 
-    // could be null, created but never attached
+    try doCloseTarget(cmd, bc);
-    if (bc.session_id) |session_id| {
-        // Inspector.detached event
-        try cmd.sendEvent("Inspector.detached", .{
-            .reason = "Render process gone.",
-        }, .{ .session_id = session_id });
-
-        // detachedFromTarget event
-        try cmd.sendEvent("Target.detachedFromTarget", .{
-            .targetId = target_id,
-            .sessionId = session_id,
-            .reason = "Render process gone.",
-        }, .{});
-
-        bc.session_id = null;
-    }
-
-    bc.session.removePage();
-    for (bc.isolated_worlds.items) |world| {
-        world.deinit();
-    }
-    bc.isolated_worlds.clearRetainingCapacity();
-    bc.target_id = null;
 }
 
 fn getTargetInfo(cmd: anytype) !void {
@@ -468,6 +448,41 @@ fn setAutoAttach(cmd: anytype) !void {
     try cmd.sendResult(null, .{});
 }
 
+/// Close the current target in a browser context: send detach events,
+/// remove the page, clean up isolated worlds, and clear the target_id.
+/// Shared by closeTarget and createTarget (which auto-closes before
+/// creating a replacement).
+fn doCloseTarget(cmd: anytype, bc: anytype) !void {
+    // can't be null if we have a target_id
+    lp.assert(bc.session.page != null, "CDP.target.doCloseTarget null page", .{});
+
+    // could be null, created but never attached
+    if (bc.session_id) |session_id| {
+        try cmd.sendEvent("Inspector.detached", .{
+            .reason = "Render process gone.",
+        }, .{ .session_id = session_id });
+
+        try cmd.sendEvent("Target.detachedFromTarget", .{
+            .targetId = &bc.target_id.?,
+            .sessionId = session_id,
+            .reason = "Render process gone.",
+        }, .{});
+
+        bc.session_id = null;
+    }
+
+    try cmd.sendEvent("Target.targetDestroyed", .{
+        .targetId = &bc.target_id.?,
+    }, .{});
+
+    bc.session.removePage();
+    for (bc.isolated_worlds.items) |world| {
+        world.deinit();
+    }
+    bc.isolated_worlds.clearRetainingCapacity();
+    bc.target_id = null;
+}
+
 fn doAttachtoTarget(cmd: anytype, target_id: []const u8) !void {
     const bc = cmd.browser_context.?;
     const session_id = bc.session_id orelse cmd.cdp.session_id_gen.next();
@@ -646,6 +661,7 @@ test "cdp.target: closeTarget" {
     {
         try ctx.processMessage(.{ .id = 11, .method = "Target.closeTarget", .params = .{ .targetId = "TID-000000000A" } });
         try ctx.expectSentResult(.{ .success = true }, .{ .id = 11 });
+        try ctx.expectSentEvent("Target.targetDestroyed", .{ .targetId = "TID-000000000A" }, .{});
         try testing.expectEqual(null, bc.session.page);
         try testing.expectEqual(null, bc.target_id);
     }
@@ -771,6 +787,53 @@ test "cdp.target: detachFromTarget" {
     }
 }
 
+test "cdp.target: createTarget closes existing target (issue #1962)" {
+    var ctx = try testing.context();
+    defer ctx.deinit();
+    const bc = try ctx.loadBrowserContext(.{ .id = "BID-9" });
+    {
+        // Create first target
+        try ctx.processMessage(.{ .id = 10, .method = "Target.createTarget", .params = .{ .browserContextId = "BID-9" } });
+        try testing.expectEqual(true, bc.target_id != null);
+        try ctx.expectSentResult(.{ .targetId = bc.target_id.? }, .{ .id = 10 });
+
+        // Create second target — should succeed by auto-closing the first
+        try ctx.processMessage(.{ .id = 11, .method = "Target.createTarget", .params = .{ .browserContextId = "BID-9" } });
+        try ctx.expectSentEvent("Target.targetDestroyed", .{ .targetId = "FID-0000000001" }, .{});
+        try testing.expectEqual(true, bc.target_id != null);
+        try ctx.expectSentResult(.{ .targetId = bc.target_id.? }, .{ .id = 11 });
+
+        // Page should exist (new target is active)
+        try testing.expectEqual(true, bc.session.page != null);
+    }
+}
+
+test "cdp.target: createTarget closes existing attached target (issue #1962)" {
+    var ctx = try testing.context();
+    defer ctx.deinit();
+    const bc = try ctx.loadBrowserContext(.{ .id = "BID-9" });
+    {
+        // Create and attach first target
+        try ctx.processMessage(.{ .id = 10, .method = "Target.createTarget", .params = .{ .browserContextId = "BID-9" } });
+        try testing.expectEqual(true, bc.target_id != null);
+        try ctx.expectSentResult(.{ .targetId = bc.target_id.? }, .{ .id = 10 });
+
+        try ctx.processMessage(.{ .id = 11, .method = "Target.attachToTarget", .params = .{ .targetId = bc.target_id.? } });
+        const session_id = bc.session_id.?;
+        try ctx.expectSentResult(.{ .sessionId = session_id }, .{ .id = 11 });
+
+        // Create second target — should close and detach the first
+        try ctx.processMessage(.{ .id = 12, .method = "Target.createTarget", .params = .{ .browserContextId = "BID-9" } });
+        // Should have sent detach events for the old session
+        try ctx.expectSentEvent("Inspector.detached", .{ .reason = "Render process gone." }, .{ .session_id = session_id });
+        try ctx.expectSentEvent("Target.detachedFromTarget", .{ .sessionId = session_id, .reason = "Render process gone." }, .{});
+        try ctx.expectSentEvent("Target.targetDestroyed", .{ .targetId = "FID-0000000001" }, .{});
+        // New target should be created
+        try testing.expectEqual(true, bc.target_id != null);
+        try ctx.expectSentResult(.{ .targetId = bc.target_id.? }, .{ .id = 12 });
+    }
+}
+
 test "cdp.target: detachFromTarget without session" {
     var ctx = try testing.context();
     defer ctx.deinit();

src/cdp/testing.zig

@@ -168,13 +168,26 @@ const TestContext = struct {
         index: ?usize = null,
     };
     pub fn expectSent(self: *TestContext, expected: anytype, opts: SentOpts) !void {
-        const serialized = try json.Stringify.valueAlloc(base.arena_allocator, expected, .{
+        const expected_json = blk: {
-            .whitespace = .indent_2,
+            // Zig makes this hard. When sendJSON is called, we're sending an anytype.
-            .emit_null_optional_fields = false,
+            // We can't record that in an ArrayList(???), so we serialize it to JSON.
-        });
+            // Now, ideally, we could just take our expected structure, serialize it to
+            // json and check if the two are equal.
+            // Except serializing to JSON isn't deterministic.
+            // So we serialize the JSON then we deserialize to json.Value. And then we can
+            // compare our anytype expectation with the json.Value that we captured
+
+            const serialized = try json.Stringify.valueAlloc(base.arena_allocator, expected, .{
+                .whitespace = .indent_2,
+                .emit_null_optional_fields = false,
+            });
+
+            break :blk try std.json.parseFromSliceLeaky(json.Value, base.arena_allocator, serialized, .{});
+        };
+
         for (0..5) |_| {
             for (self.received.items, 0..) |received, i| {
-                if (try compareExpectedToSent(serialized, received) == false) {
+                if (try base.isEqualJson(expected_json, received) == false) {
                     continue;
                 }
 
@@ -187,6 +200,15 @@ const TestContext = struct {
                 }
                 return;
             }
+
+            if (self.cdp_) |*cdp__| {
+                if (cdp__.browser_context) |*bc| {
+                    if (bc.session.page != null) {
+                        var runner = try bc.session.runner(.{});
+                        _ = try runner.tick(.{ .ms = 1000 });
+                    }
+                }
+            }
             std.Thread.sleep(5 * std.time.ns_per_ms);
             try self.read();
         }
@@ -299,17 +321,3 @@ pub fn context() !TestContext {
         .socket = pair[0],
     };
 }
-
-// Zig makes this hard. When sendJSON is called, we're sending an anytype.
-// We can't record that in an ArrayList(???), so we serialize it to JSON.
-// Now, ideally, we could just take our expected structure, serialize it to
-// json and check if the two are equal.
-// Except serializing to JSON isn't deterministic.
-// So we serialize the JSON then we deserialize to json.Value. And then we can
-// compare our anytype expectation with the json.Value that we captured
-
-fn compareExpectedToSent(expected: []const u8, actual: json.Value) !bool {
-    const expected_value = try std.json.parseFromSlice(json.Value, std.testing.allocator, expected, .{});
-    defer expected_value.deinit();
-    return base.isEqualJson(expected_value.value, actual);
-}

src/mcp.zig

@@ -1,6 +1,7 @@
 const std = @import("std");
 
 pub const protocol = @import("mcp/protocol.zig");
+pub const Version = protocol.Version;
 pub const router = @import("mcp/router.zig");
 pub const Server = @import("mcp/Server.zig");
 

src/mcp/Server.zig

@@ -114,7 +114,7 @@ test "MCP.Server - Integration: synchronous smoke test" {
 
     try router.processRequests(server, &in_reader);
 
-    try testing.expectJson(.{ .jsonrpc = "2.0", .id = 1 }, out_alloc.writer.buffered());
+    try testing.expectJson(.{ .jsonrpc = "2.0", .id = 1, .result = .{ .protocolVersion = "2024-11-05" } }, out_alloc.writer.buffered());
 }
 
 test "MCP.Server - Integration: ping request returns an empty result" {

src/mcp/protocol.zig

@@ -1,5 +1,14 @@
 const std = @import("std");
 
+pub const Version = enum {
+    @"2024-11-05",
+    @"2025-03-26",
+    @"2025-06-18",
+    @"2025-11-25",
+
+    pub const default: Version = .@"2024-11-05";
+};
+
 pub const Request = struct {
     jsonrpc: []const u8 = "2.0",
     id: ?std.json.Value = null,

src/mcp/router.zig

@@ -81,8 +81,12 @@ pub fn handleMessage(server: *Server, arena: std.mem.Allocator, msg: []const u8)
 
 fn handleInitialize(server: *Server, req: protocol.Request) !void {
     const id = req.id orelse return;
-    const result = protocol.InitializeResult{
+    const version: protocol.Version = switch (server.app.config.mode) {
-        .protocolVersion = "2025-11-25",
+        .mcp => |opts| opts.version,
+        else => .default,
+    };
+    const result: protocol.InitializeResult = .{
+        .protocolVersion = @tagName(version),
         .capabilities = .{
             .resources = .{},
             .tools = .{},
@@ -121,7 +125,7 @@ test "MCP.router - handleMessage - synchronous unit tests" {
         \\{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test-client","version":"1.0.0"}}}
     );
     try testing.expectJson(
-        \\{ "jsonrpc": "2.0", "id": 1, "result": { "capabilities": { "tools": {} } } }
+        \\{ "jsonrpc": "2.0", "id": 1, "result": { "protocolVersion": "2024-11-05", "capabilities": { "tools": {} } } }
     , out_alloc.writer.buffered());
     out_alloc.writer.end = 0;