diff --git a/lsp/cache/fs_memoized_test.go b/lsp/cache/fs_memoized_test.go
new file mode 100644
index 0000000..7497940
--- /dev/null
+++ b/lsp/cache/fs_memoized_test.go
@@ -0,0 +1,95 @@
+package cache
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.lsp.dev/uri"
+)
+
+// writeWithMtime creates path with content and pins its mtime more than
+// the 2s freshness window in the past, so memoization decisions do not
+// depend on test timing.
+func writeWithMtime(t *testing.T, path, content string) {
+	t.Helper()
+
+	require.NoError(t, os.WriteFile(path, []byte(content), 0o600))
+
+	mtime := time.Now().Add(-time.Hour)
+	require.NoError(t, os.Chtimes(path, mtime, mtime))
+}
+
+// TestMemoizedFS_MemoHit pins the core contract: two reads of an
+// unchanged file return the same cached handle.
+func TestMemoizedFS_MemoHit(t *testing.T) {
+	fs := &memoizedFS{filesByID: map[FileID][]*DiskFile{}}
+	ctx := context.Background()
+
+	path := filepath.Join(t.TempDir(), "a.thrift")
+	writeWithMtime(t, path, "struct A {}")
+
+	u := uri.File(path)
+
+	first, err := fs.ReadFile(ctx, u)
+	require.NoError(t, err)
+
+	content, err := first.Content()
+	require.NoError(t, err)
+	assert.Equal(t, "struct A {}", string(content))
+
+	second, err := fs.ReadFile(ctx, u)
+	require.NoError(t, err)
+
+	assert.Same(t, first.(*DiskFile), second.(*DiskFile), "unchanged file must reuse the cached handle")
+}
+
+// TestMemoizedFS_MtimeInvalidation pins the behavior the freshness
+// heuristic exists for: when the mtime changes, the stale handle is
+// dropped even though the inode is the same.
+func TestMemoizedFS_MtimeInvalidation(t *testing.T) {
+	fs := &memoizedFS{filesByID: map[FileID][]*DiskFile{}}
+	ctx := context.Background()
+
+	path := filepath.Join(t.TempDir(), "a.thrift")
+	u := uri.File(path)
+
+	writeWithMtime(t, path, "before")
+
+	before, err := fs.ReadFile(ctx, u)
+	require.NoError(t, err)
+
+	content, err := before.Content()
+	require.NoError(t, err)
+	assert.Equal(t, "before", string(content))
+
+	writeWithMtime(t, path, "after")
+
+	after, err := fs.ReadFile(ctx, u)
+	require.NoError(t, err)
+
+	content, err = after.Content()
+	require.NoError(t, err)
+	assert.Equal(t, "after", string(content))
+
+	assert.NotSame(t, before.(*DiskFile), after.(*DiskFile))
+}
+
+// TestMemoizedFS_MissingFile pins that reads of nonexistent files return
+// a handle carrying the error instead of failing the call: one bad path
+// must not fail a workspace walk.
+func TestMemoizedFS_MissingFile(t *testing.T) {
+	fs := &memoizedFS{filesByID: map[FileID][]*DiskFile{}}
+
+	u := uri.File(filepath.Join(t.TempDir(), "ghost.thrift"))
+
+	fh, err := fs.ReadFile(context.Background(), u)
+	require.NoError(t, err)
+
+	_, cerr := fh.Content()
+	require.Error(t, cerr)
+}