diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index 016d864fcc..2d46ff373e 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -25,7 +25,8 @@ run_queue: std.SinglyLinkedList = .{},
 join_requested: bool = false,
 threads: std.ArrayList(std.Thread),
 stack_size: usize,
-cpu_count: std.Thread.CpuCountError!usize,
+thread_capacity: std.atomic.Value(ThreadCapacity),
+thread_capacity_error: ?std.Thread.CpuCountError,
 concurrent_count: usize,
 
 wsa: if (is_windows) Wsa else struct {} = .{},
@@ -34,6 +35,21 @@ have_signal_handler: bool,
 old_sig_io: if (have_sig_io) posix.Sigaction else void,
 old_sig_pipe: if (have_sig_pipe) posix.Sigaction else void,
 
+pub const ThreadCapacity = enum(usize) {
+    unknown = 0,
+    _,
+
+    pub fn init(n: usize) ThreadCapacity {
+        assert(n != 0);
+        return @enumFromInt(n);
+    }
+
+    pub fn get(tc: ThreadCapacity) ?usize {
+        if (tc == .unknown) return null;
+        return @intFromEnum(tc);
+    }
+};
+
 threadlocal var current_closure: ?*Closure = null;
 
 const max_iovecs_len = 8;
@@ -104,18 +120,21 @@ pub fn init(
     /// here.
     gpa: Allocator,
 ) Threaded {
+    const cpu_count = std.Thread.getCpuCount();
+
     var t: Threaded = .{
         .allocator = gpa,
         .threads = .empty,
         .stack_size = std.Thread.SpawnConfig.default_stack_size,
-        .cpu_count = std.Thread.getCpuCount(),
+        .thread_capacity = .init(if (cpu_count) |n| .init(n) else |_| .unknown),
+        .thread_capacity_error = if (cpu_count) |_| null else |e| e,
         .concurrent_count = 0,
         .old_sig_io = undefined,
         .old_sig_pipe = undefined,
         .have_signal_handler = false,
     };
 
-    if (t.cpu_count) |n| {
+    if (cpu_count) |n| {
         t.threads.ensureTotalCapacityPrecise(gpa, n - 1) catch {};
     } else |_| {}
 
@@ -145,7 +164,8 @@ pub const init_single_threaded: Threaded = .{
     .allocator = .failing,
     .threads = .empty,
     .stack_size = std.Thread.SpawnConfig.default_stack_size,
-    .cpu_count = 1,
+    .thread_capacity = .init(.init(1)),
+    .thread_capacity_error = null,
     .concurrent_count = 0,
     .old_sig_io = undefined,
     .old_sig_pipe = undefined,
@@ -166,6 +186,18 @@ pub fn deinit(t: *Threaded) void {
     t.* = undefined;
 }
 
+pub fn setThreadCapacity(t: *Threaded, n: usize) void {
+    t.thread_capacity.store(.init(n), .monotonic);
+}
+
+pub fn getThreadCapacity(t: *Threaded) ?usize {
+    return t.thread_capacity.load(.monotonic).get();
+}
+
+pub fn getCurrentThreadId() usize {
+    @panic("TODO");
+}
+
 fn join(t: *Threaded) void {
     if (builtin.single_threaded) return;
     {
@@ -497,7 +529,7 @@ fn async(
     }
 
     const t: *Threaded = @ptrCast(@alignCast(userdata));
-    const cpu_count = t.cpu_count catch {
+    const cpu_count = t.getThreadCapacity() orelse {
         return concurrent(userdata, result.len, result_alignment, context, context_alignment, start) catch {
             start(context.ptr, result.ptr);
             return null;
@@ -556,7 +588,7 @@ fn concurrent(
     if (builtin.single_threaded) return error.ConcurrencyUnavailable;
 
     const t: *Threaded = @ptrCast(@alignCast(userdata));
-    const cpu_count = t.cpu_count catch 1;
+    const cpu_count = t.getThreadCapacity() orelse 1;
 
     const gpa = t.allocator;
     const ac = AsyncClosure.init(gpa, .concurrent, result_len, result_alignment, context, context_alignment, start) catch {
@@ -685,7 +717,7 @@ fn groupAsync(
     if (builtin.single_threaded) return start(group, context.ptr);
 
     const t: *Threaded = @ptrCast(@alignCast(userdata));
-    const cpu_count = t.cpu_count catch 1;
+    const cpu_count = t.getThreadCapacity() orelse 1;
 
     const gpa = t.allocator;
     const gc = GroupClosure.init(gpa, t, group, context, context_alignment, start) catch {
diff --git a/lib/std/Io/Threaded/test.zig b/lib/std/Io/Threaded/test.zig
index 7e6e687cf2..03582d4d95 100644
--- a/lib/std/Io/Threaded/test.zig
+++ b/lib/std/Io/Threaded/test.zig
@@ -10,7 +10,7 @@ test "concurrent vs main prevents deadlock via oversubscription" {
     defer threaded.deinit();
     const io = threaded.io();
 
-    threaded.cpu_count = 1;
+    threaded.setThreadCapacity(1);
 
     var queue: Io.Queue(u8) = .init(&.{});
 
@@ -38,7 +38,7 @@ test "concurrent vs concurrent prevents deadlock via oversubscription" {
     defer threaded.deinit();
     const io = threaded.io();
 
-    threaded.cpu_count = 1;
+    threaded.setThreadCapacity(1);
 
     var queue: Io.Queue(u8) = .init(&.{});