From dd1dbf7f3b0a76d07cc4d659d98d847b161bb72b Mon Sep 17 00:00:00 2001
From: Guthrie McAfee Armstrong <git@gmarmstrong.dev>
Date: Wed, 26 Aug 2026 16:34:42 -0400
Subject: [PATCH] forwarder: preserve platform in NewContainer

Fixes #7081.

Signed-off-by: Guthrie McAfee Armstrong <git@gmarmstrong.dev>
---
 frontend/gateway/forwarder/forward.go      |  1 +
 frontend/gateway/forwarder/forward_test.go | 64 ++++++++++++++++++++++
 2 files changed, 65 insertions(+)
 create mode 100644 frontend/gateway/forwarder/forward_test.go

diff --git a/frontend/gateway/forwarder/forward.go b/frontend/gateway/forwarder/forward.go
index 4b8fa2612744..9294fed7699b 100644
--- a/frontend/gateway/forwarder/forward.go
+++ b/frontend/gateway/forwarder/forward.go
@@ -264,6 +264,7 @@ func (c *BridgeClient) NewContainer(ctx context.Context, req client.NewContainer
 		NetMode:     req.NetMode,
 		Hostname:    req.Hostname,
 		Mounts:      make([]container.Mount, len(req.Mounts)),
+		Platform:    req.Platform,
 	}
 
 	eg, ctx := errgroup.WithContext(ctx)
diff --git a/frontend/gateway/forwarder/forward_test.go b/frontend/gateway/forwarder/forward_test.go
new file mode 100644
index 000000000000..d8e27cfe015b
--- /dev/null
+++ b/frontend/gateway/forwarder/forward_test.go
@@ -0,0 +1,64 @@
+package forwarder
+
+import (
+	"context"
+	"runtime"
+	"testing"
+
+	"github.com/moby/buildkit/cache"
+	buildkitclient "github.com/moby/buildkit/client"
+	"github.com/moby/buildkit/executor"
+	resourcestypes "github.com/moby/buildkit/executor/resources/types"
+	gwclient "github.com/moby/buildkit/frontend/gateway/client"
+	"github.com/moby/buildkit/solver/pb"
+	"github.com/moby/buildkit/util/system"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewContainerPreservesPlatform(t *testing.T) {
+	exec := &recordingExecutor{process: make(chan executor.ProcessInfo, 1)}
+	c := &BridgeClient{
+		workers:  emptyWorkerInfos{},
+		executor: exec,
+	}
+	targetOS := "windows"
+	if runtime.GOOS == targetOS {
+		targetOS = "linux"
+	}
+	ctx := t.Context()
+
+	ctr, err := c.NewContainer(ctx, gwclient.NewContainerRequest{
+		Platform: &pb.Platform{OS: targetOS, Architecture: "amd64"},
+	})
+	require.NoError(t, err)
+
+	proc, err := ctr.Start(ctx, gwclient.StartRequest{})
+	require.NoError(t, err)
+	require.Contains(t, (<-exec.process).Meta.Env, "PATH="+system.DefaultPathEnv(targetOS))
+	require.NoError(t, proc.Wait())
+	require.NoError(t, ctr.Release(ctx))
+}
+
+type emptyWorkerInfos struct{}
+
+func (emptyWorkerInfos) DefaultCacheManager() (cache.Manager, error) {
+	return nil, nil
+}
+
+func (emptyWorkerInfos) WorkerInfos() []buildkitclient.WorkerInfo {
+	return nil
+}
+
+type recordingExecutor struct {
+	process chan executor.ProcessInfo
+}
+
+func (e *recordingExecutor) Run(_ context.Context, _ string, _ executor.Mount, _ []executor.Mount, process executor.ProcessInfo, started chan<- struct{}) (resourcestypes.Recorder, error) {
+	e.process <- process
+	close(started)
+	return nil, nil
+}
+
+func (e *recordingExecutor) Exec(context.Context, string, executor.ProcessInfo) error {
+	return nil
+}