diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index f9e6b40..32e5e58 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -186,4 +186,4 @@ jobs:
 
       - name: "verify used"
         run: |
-          go test -v ./testing/ci '-verify-used'
+          go test -v ./testing/ci '-verify-used' '-relative-open'
diff --git a/testing/ci/ci_test.go b/testing/ci/ci_test.go
index d86adfd..cf1b321 100644
--- a/testing/ci/ci_test.go
+++ b/testing/ci/ci_test.go
@@ -4,15 +4,48 @@
 package ci
 
 import (
+	"bytes"
 	"encoding/json"
 	"flag"
 	"net/http"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/tailscale/gomodfs"
 )
 
-var runVerifyUsed = flag.Bool("verify-used", false, "if set, runs TestVerifyUsed")
+var (
+	runVerifyUsed   = flag.Bool("verify-used", false, "if set, runs TestVerifyUsed")
+	runRelativeOpen = flag.Bool("relative-open", false, "if set, runs TestRelativeOpen against the mounted $GOMODCACHE")
+)
+
+// TestRelativeOpen opens files in the mounted module cache by paths relative
+// to a working directory on the mount, as cmd/asm does with #include files.
+// On Windows, WinFsp used to pass such opens to gomodfs with the working
+// directory's path upcased, because gomodfs's volume claimed to be
+// case-insensitive and not case-preserving.
+func TestRelativeOpen(t *testing.T) {
+	if !*runRelativeOpen {
+		t.Skip("only runs in CI with --relative-open set")
+	}
+	modCache := os.Getenv("GOMODCACHE")
+	if modCache == "" {
+		t.Fatal("GOMODCACHE not set")
+	}
+	t.Chdir(filepath.Join(modCache, "golang.org", "x", "sys@v0.16.0"))
+
+	b, err := os.ReadFile("go.mod")
+	if err != nil {
+		t.Fatalf("reading go.mod relative to the module dir: %v", err)
+	}
+	if !bytes.Contains(b, []byte("module golang.org/x/sys")) {
+		t.Errorf("go.mod contents = %q; want the golang.org/x/sys module", b)
+	}
+	if _, err := os.Stat("no-such-file"); !os.IsNotExist(err) {
+		t.Errorf("Stat of a missing relative path: err = %v; want not exist", err)
+	}
+}
 
 func TestVerifyUsed(t *testing.T) {
 	if !*runVerifyUsed {
diff --git a/winfsp.go b/winfsp.go
index fc35918..e2efe75 100644
--- a/winfsp.go
+++ b/winfsp.go
@@ -36,7 +36,13 @@ func (mfs *FS) MountWinFSP(mntPoint string) (MountRunner, error) {
 		},
 		mntPoint, // e.g. "M:"
 		winfsp.FileSystemName("gomodfs"),
-		winfsp.Attributes(winfsp.FspFSAttributeReadOnlyVolume),
+		// gomodfs paths are case-sensitive, like the GOMODCACHE layout it
+		// emulates (which escapes capitals as "!x"). Say so: on a volume
+		// that claims to be case-insensitive and not case-preserving,
+		// Windows passes relative opens to us with the working directory's
+		// path upcased, which we don't recognize.
+		winfsp.CaseSensitive(true),
+		winfsp.Attributes(winfsp.FspFSAttributeReadOnlyVolume|winfsp.FspFSAttributeCasePreservedNames),
 	)
 	if err != nil {
 		return nil, fmt.Errorf("winfsp.Mount: %w", err)