diff --git a/lib/std/Build/Step.zig b/lib/std/Build/Step.zig
index 37fc2ca023..0dd4b93280 100644
--- a/lib/std/Build/Step.zig
+++ b/lib/std/Build/Step.zig
@@ -539,6 +539,7 @@ fn zigProcessUpdate(s: *Step, zp: *ZigProcess, watch: bool, web_server: ?*Build.
     if (!watch) try sendMessage(io, zp.child.stdin.?, .exit);
 
     var result: ?Path = null;
+    var eos_err: error{EndOfStream}!void = {};
 
     const stdout = zp.multi_reader.fileReader(0);
 
@@ -549,7 +550,13 @@ fn zigProcessUpdate(s: *Step, zp: *ZigProcess, watch: bool, web_server: ?*Build.
             error.ReadFailed => return stdout.err.?,
         };
         const body = stdout.interface.take(header.bytes_len) catch |err| switch (err) {
-            error.EndOfStream => |e| return e,
+            error.EndOfStream => |e| {
+                // Better to report the crash with stderr below, but we set
+                // this in case the child exits successfully while violating
+                // this protocol.
+                eos_err = e;
+                break;
+            },
             error.ReadFailed => return stdout.err.?,
         };
         switch (header.tag) {
@@ -647,6 +654,8 @@ fn zigProcessUpdate(s: *Step, zp: *ZigProcess, watch: bool, web_server: ?*Build.
         try s.result_error_msgs.append(arena, try arena.dupe(u8, stderr_contents));
     }
 
+    try eos_err;
+
     return result;
 }
 
diff --git a/lib/std/Io/File/MultiReader.zig b/lib/std/Io/File/MultiReader.zig
index a1ea42a7d8..0cfa777e96 100644
--- a/lib/std/Io/File/MultiReader.zig
+++ b/lib/std/Io/File/MultiReader.zig
@@ -246,6 +246,14 @@ pub fn fill(mr: *MultiReader, unused_capacity: usize, timeout: Io.Timeout) FillE
     if (!any_completed) return error.EndOfStream;
 }
 
+/// Wait until all streams fail or reach the end.
+pub fn fillRemaining(mr: *MultiReader, timeout: Io.Timeout) Io.Batch.WaitError!void {
+    while (fill(mr, 1, timeout)) |_| {} else |err| switch (err) {
+        error.EndOfStream => return,
+        else => |e| return e,
+    }
+}
+
 fn rebaseGrowing(mr: *MultiReader, context: *Context, capacity: usize) Allocator.Error!void {
     const gpa = mr.gpa;
     const r = &context.fr.interface;
diff --git a/lib/std/process.zig b/lib/std/process.zig
index b5de41f5d8..6f3c155f6d 100644
--- a/lib/std/process.zig
+++ b/lib/std/process.zig
@@ -488,6 +488,7 @@ pub const RunOptions = struct {
     create_no_window: bool = true,
     /// Darwin-only. Disable ASLR for the child process.
     disable_aslr: bool = false,
+    timeout: Io.Timeout = .none,
 };
 
 pub const RunResult = struct {
@@ -529,6 +530,7 @@ pub fn run(gpa: Allocator, io: Io, options: RunOptions) RunError!RunResult {
         .stderr = &stderr,
         .stdout_limit = options.stdout_limit,
         .stderr_limit = options.stderr_limit,
+        .timeout = options.timeout,
     });
 
     const term = try child.wait(io);
diff --git a/lib/std/process/Child.zig b/lib/std/process/Child.zig
index 19c974ff9f..fe6dfa389d 100644
--- a/lib/std/process/Child.zig
+++ b/lib/std/process/Child.zig
@@ -137,6 +137,7 @@ pub const CollectOutputOptions = struct {
     allocator: ?Allocator = null,
     stdout_limit: Io.Limit = .unlimited,
     stderr_limit: Io.Limit = .unlimited,
+    timeout: Io.Timeout = .none,
 };
 
 /// Collect the output from the process's stdout and stderr. Will return once
@@ -173,7 +174,7 @@ pub fn collectOutput(child: *const Child, io: Io, options: CollectOutputOptions)
         remaining += 1;
     }
     while (remaining > 0) {
-        try batch.wait(io, .none);
+        try batch.wait(io, options.timeout);
         while (batch.next()) |op| {
             const n = try reads[op].file_read_streaming.status.result;
             if (n == 0) {
diff --git a/lib/std/zig/LibCInstallation.zig b/lib/std/zig/LibCInstallation.zig
index 02b3df54dc..6a3f4b4813 100644
--- a/lib/std/zig/LibCInstallation.zig
+++ b/lib/std/zig/LibCInstallation.zig
@@ -268,7 +268,8 @@ fn findNativeIncludeDirPosix(self: *LibCInstallation, gpa: Allocator, io: Io, ar
     });
 
     const run_res = std.process.run(gpa, io, .{
-        .max_output_bytes = 1024 * 1024,
+        .stdout_limit = .limited(1024 * 1024),
+        .stderr_limit = .limited(1024 * 1024),
         .argv = argv.items,
         .environ_map = &environ_map,
         // Some C compilers, such as Clang, are known to rely on argv[0] to find the path
@@ -584,7 +585,8 @@ fn ccPrintFileName(gpa: Allocator, io: Io, args: CCPrintFileNameOptions) ![]u8 {
     try argv.append(arg1);
 
     const run_res = std.process.run(gpa, io, .{
-        .max_output_bytes = 1024 * 1024,
+        .stdout_limit = .limited(1024 * 1024),
+        .stderr_limit = .limited(1024 * 1024),
         .argv = argv.items,
         .environ_map = &environ_map,
         // Some C compilers, such as Clang, are known to rely on argv[0] to find the path
diff --git a/src/Compilation.zig b/src/Compilation.zig
index 98c1b56e38..6b6021ab3c 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -6873,6 +6873,7 @@ fn spawnZigRc(
     child_progress_node: std.Progress.Node,
 ) !void {
     const io = comp.io;
+    const gpa = comp.gpa;
     var node_name: std.ArrayList(u8) = .empty;
     defer node_name.deinit(arena);
 
@@ -6887,55 +6888,69 @@ fn spawnZigRc(
     });
     defer child.kill(io);
 
-    var poller = std.Io.poll(comp.gpa, enum { stdout, stderr }, .{
-        .stdout = child.stdout.?,
-        .stderr = child.stderr.?,
-    });
-    defer poller.deinit();
+    var multi_reader_buffer: Io.File.MultiReader.Buffer(2) = undefined;
+    var multi_reader: Io.File.MultiReader = undefined;
+    multi_reader.init(gpa, io, multi_reader_buffer.toStreams(), &.{ child.stdout.?, child.stderr.? });
+    defer multi_reader.deinit();
 
-    const stdout = poller.reader(.stdout);
+    const stdout = multi_reader.fileReader(0);
+    const MessageHeader = std.zig.Server.Message.Header;
 
-    poll: while (true) {
-        const MessageHeader = std.zig.Server.Message.Header;
-        while (stdout.buffered().len < @sizeOf(MessageHeader)) if (!try poller.poll()) break :poll;
-        const header = stdout.takeStruct(MessageHeader, .little) catch unreachable;
-        while (stdout.buffered().len < header.bytes_len) if (!try poller.poll()) break :poll;
-        const body = stdout.take(header.bytes_len) catch unreachable;
+    var eos_err: error{EndOfStream}!void = {};
 
+    while (true) {
+        const header = stdout.interface.takeStruct(MessageHeader, .little) catch |err| switch (err) {
+            error.EndOfStream => break,
+            error.ReadFailed => return stdout.err.?,
+        };
+        const body = stdout.interface.take(header.bytes_len) catch |err| switch (err) {
+            error.EndOfStream => |e| {
+                // Better to report the crash with stderr below, but we set
+                // this in case the child exits successfully while violating
+                // this protocol.
+                eos_err = e;
+                break;
+            },
+            error.ReadFailed => return stdout.err.?,
+        };
         switch (header.tag) {
             // We expect exactly one ErrorBundle, and if any error_bundle header is
             // sent then it's a fatal error.
             .error_bundle => {
-                const error_bundle = try std.zig.Server.allocErrorBundle(comp.gpa, body);
+                const error_bundle = try std.zig.Server.allocErrorBundle(gpa, body);
                 return comp.failWin32ResourceWithOwnedBundle(win32_resource, error_bundle);
             },
             else => {}, // ignore other messages
         }
     }
 
-    // Just in case there's a failure that didn't send an ErrorBundle (e.g. an error return trace)
-    const stderr = poller.reader(.stderr);
+    try multi_reader.fillRemaining(.none);
 
+    // Just in case there's a failure that didn't send an ErrorBundle (e.g. an error return trace)
     const term = child.wait(io) catch |err| {
         return comp.failWin32Resource(win32_resource, "unable to wait for {s} rc: {t}", .{ argv[0], err });
     };
 
+    const stderr = multi_reader.reader(1).buffered();
+
     switch (term) {
         .exited => |code| {
             if (code != 0) {
-                log.err("zig rc failed with stderr:\n{s}", .{stderr.buffered()});
+                log.err("zig rc failed with stderr:\n{s}", .{stderr});
                 return comp.failWin32Resource(win32_resource, "zig rc exited with code {d}", .{code});
             }
         },
         .signal => |sig| {
-            log.err("zig rc signaled {t} with stderr:\n{s}", .{ sig, stderr.buffered() });
+            log.err("zig rc signaled {t} with stderr:\n{s}", .{ sig, stderr });
             return comp.failWin32Resource(win32_resource, "zig rc terminated unexpectedly", .{});
         },
         else => {
-            log.err("zig rc terminated with stderr:\n{s}", .{stderr.buffered()});
+            log.err("zig rc terminated with stderr:\n{s}", .{stderr});
             return comp.failWin32Resource(win32_resource, "zig rc terminated unexpectedly", .{});
         },
     }
+
+    try eos_err;
 }
 
 pub fn tmpFilePath(comp: Compilation, ally: Allocator, suffix: []const u8) error{OutOfMemory}![]const u8 {