diff --git a/lib/std/process.zig b/lib/std/process.zig
index 4a021879a5..b203838e3f 100644
--- a/lib/std/process.zig
+++ b/lib/std/process.zig
@@ -455,6 +455,7 @@ pub fn spawnPath(io: Io, dir: Io.Dir, options: SpawnOptions) SpawnError!Child {
 
 pub const RunError = CurrentPathError || posix.ReadError || SpawnError || posix.PollError || error{
     StreamTooLong,
+    ConcurrencyUnavailable,
 };
 
 pub const RunOptions = struct {
diff --git a/lib/std/process/Child.zig b/lib/std/process/Child.zig
index fc31014520..e64f6106fa 100644
--- a/lib/std/process/Child.zig
+++ b/lib/std/process/Child.zig
@@ -125,14 +125,15 @@ pub fn wait(child: *Child, io: Io) WaitError!Term {
     return io.vtable.childWait(io.userdata, child);
 }
 
-pub const CollectOutputError = error{StreamTooLong} || Allocator.Error || Io.File.Reader.Error;
+pub const CollectOutputError = error{
+    StreamTooLong,
+    ConcurrencyUnavailable,
+} || Allocator.Error || Io.File.Reader.Error;
 
 pub const CollectOutputOptions = struct {
     stdout: *std.ArrayList(u8),
     stderr: *std.ArrayList(u8),
-    /// Used for `stdout` and `stderr`. If not provided, only the existing
-    /// capacity will be used.
-    allocator: ?Allocator = null,
+    allocator: Allocator,
     stdout_limit: Io.Limit = .unlimited,
     stderr_limit: Io.Limit = .unlimited,
 };
@@ -144,56 +145,24 @@ pub const CollectOutputOptions = struct {
 /// The process must have been started with stdout and stderr set to
 /// `process.SpawnOptions.StdIo.pipe`.
 pub fn collectOutput(child: *const Child, io: Io, options: CollectOutputOptions) CollectOutputError!void {
-    const files: [2]Io.File = .{ child.stdout.?, child.stderr.? };
-    const lists: [2]*std.ArrayList(u8) = .{ options.stdout, options.stderr };
-    const limits: [2]Io.Limit = .{ options.stdout_limit, options.stderr_limit };
-    var dones: [2]bool = .{ false, false };
-    var reads: [2]Io.Operation = undefined;
-    var vecs: [2][1][]u8 = undefined;
-    while (true) {
-        for (&reads, &lists, &files, dones, &vecs) |*read, list, file, done, *vec| {
-            if (done) {
-                read.* = .noop;
-                continue;
-            }
-            if (options.allocator) |gpa| try list.ensureUnusedCapacity(gpa, 1);
-            const cap = list.unusedCapacitySlice();
-            if (cap.len == 0) return error.StreamTooLong;
-            vec[0] = cap;
-            read.* = .{ .file_read_streaming = .{
-                .file = file,
-                .data = vec,
-                .nonblocking = true,
-                .result = undefined,
-            } };
-        }
-        var all_done = true;
-        var any_canceled = false;
-        var other_err: (error{StreamTooLong} || Io.File.Reader.Error)!void = {};
-        io.vtable.operate(io.userdata, &reads);
-        for (&reads, &lists, &limits, &dones) |*read, list, limit, *done| {
-            if (done.*) continue;
-            const n = read.file_read_streaming.result catch |err| switch (err) {
-                error.Canceled => {
-                    any_canceled = true;
-                    continue;
-                },
-                error.WouldBlock => continue,
-                else => |e| {
-                    other_err = e;
-                    continue;
-                },
-            };
-            if (n == 0) {
-                done.* = true;
-            } else {
-                all_done = false;
-            }
-            list.items.len += n;
-            if (list.items.len > @intFromEnum(limit)) other_err = error.StreamTooLong;
-        }
-        if (any_canceled) return error.Canceled;
-        try other_err;
-        if (all_done) return;
-    }
+    var stdout = try io.concurrent(collectStream, .{
+        io, options.allocator, child.stdout.?, options.stdout, options.stdout_limit,
+    });
+    defer stdout.cancel(io) catch {};
+
+    var stderr = try io.concurrent(collectStream, .{
+        io, options.allocator, child.stderr.?, options.stderr, options.stderr_limit,
+    });
+    defer stderr.cancel(io) catch {};
+
+    try stdout.await(io);
+    try stderr.await(io);
+}
+
+fn collectStream(io: Io, gpa: Allocator, file: File, list: *std.ArrayList(u8), limit: Io.Limit) CollectOutputError!void {
+    var fr = file.readerStreaming(io, &.{});
+    fr.interface.appendRemaining(gpa, list, limit) catch |err| switch (err) {
+        error.ReadFailed => return fr.err.?,
+        else => |e| return e,
+    };
 }