fix(cdp): auto-close existing target on createTarget

When creating a new target via `Target.createTarget`, any existing target in the browser context is now automatically closed and detached. This resolves an issue where automation frameworks (like Stagehand) would fail with a TargetAlreadyLoaded error when attempting to open multiple tabs sequentially.

Closes #1962

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/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/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/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")) {
-        const ctx = try page._factory.create(CanvasRenderingContext2D{ ._canvas = self });
-        return .{ .@"2d" = ctx };
+    if (self._cached) |cached| {
+        const matches = switch (cached) {
+            .@"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 ctx = try page._factory.create(WebGLRenderingContext{});
-        return .{ .webgl = ctx };
-    }
+    const drawing_context: DrawingContext = blk: {
+        if (std.mem.eql(u8, context_type, "2d")) {
+            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/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
-    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;
+    try doCloseTarget(cmd, bc);
 }
 
 fn getTargetInfo(cmd: anytype) !void {
@@ -468,6 +448,37 @@ 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;
+    }
+
+    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();
@@ -771,6 +782,51 @@ 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 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." }, .{});
+        // 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/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{
-        .protocolVersion = "2025-11-25",
+    const version: protocol.Version = switch (server.app.config.mode) {
+        .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;