diff --git a/libc/calls/fcntl-nt.c b/libc/calls/fcntl-nt.c
index 05f34bd7..e67b855d 100644
--- a/libc/calls/fcntl-nt.c
+++ b/libc/calls/fcntl-nt.c
@@ -16,6 +16,7 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
 #include "libc/calls/internal.h"
 #include "libc/nt/enum/accessmask.h"
 #include "libc/nt/enum/fileflagandattributes.h"
@@ -31,24 +32,15 @@ textwindows int fcntl$nt(int fd, int cmd, unsigned arg) {
   if (__isfdkind(fd, kFdFile) || __isfdkind(fd, kFdSocket)) {
     switch (cmd) {
       case F_GETFL:
-        return g_fds.p[fd].flags;
+        return g_fds.p[fd].flags & (O_ACCMODE | O_APPEND | O_ASYNC | O_DIRECT |
+                                    O_NOATIME | O_NONBLOCK);
       case F_SETFL:
-        if (ReOpenFile(
-                g_fds.p[fd].handle,
-                (arg & O_APPEND)
-                    ? kNtFileAppendData
-                    : (arg & O_ACCMODE) == O_RDONLY
-                          ? kNtGenericExecute | kNtFileGenericRead
-                          : kNtGenericExecute | kNtFileGenericRead |
-                                kNtFileGenericWrite,
-                (arg & O_EXCL) == O_EXCL
-                    ? kNtFileShareExclusive
-                    : kNtFileShareRead | kNtFileShareWrite | kNtFileShareDelete,
-                kNtFileAttributeNotContentIndexed | kNtFileAttributeNormal)) {
-          return (g_fds.p[fd].flags = arg);
-        } else {
-          return __winerr();
-        }
+        /*
+         * - O_APPEND doesn't appear to be tunable at cursory glance
+         * - O_NONBLOCK might require we start doing all i/o in threads
+         * - O_DSYNC / O_RSYNC / O_SYNC maybe if we fsync() everything
+         */
+        return einval();
       case F_GETFD:
         if (g_fds.p[fd].flags & O_CLOEXEC) {
           return FD_CLOEXEC;
@@ -64,7 +56,7 @@ textwindows int fcntl$nt(int fd, int cmd, unsigned arg) {
           return 0;
         }
       default:
-        return 0; /* TODO(jart): Implement me. */
+        return einval();
     }
   } else {
     return ebadf();
diff --git a/test/libc/calls/fcntl_test.c b/test/libc/calls/fcntl_test.c
new file mode 100644
index 00000000..11f00e12
--- /dev/null
+++ b/test/libc/calls/fcntl_test.c
@@ -0,0 +1,68 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2021 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "libc/fmt/fmt.h"
+#include "libc/log/check.h"
+#include "libc/macros.h"
+#include "libc/runtime/runtime.h"
+#include "libc/sysv/consts/f.h"
+#include "libc/sysv/consts/o.h"
+#include "libc/testlib/testlib.h"
+#include "libc/x/x.h"
+
+char basedir[PATH_MAX];
+char testdir[PATH_MAX];
+
+void SetUp(void) {
+  getcwd(basedir, ARRAYLEN(basedir));
+  sprintf(testdir, "o/tmp/%s.%d", program_invocation_short_name, getpid());
+  makedirs(testdir, 0755);
+  CHECK_NE(-1, chdir(testdir));
+}
+
+void TearDown(void) {
+  CHECK_NE(-1, chdir(basedir));
+  CHECK_NE(-1, rmrf(testdir));
+}
+
+TEST(fcntl_getfl, testRemembersAccessMode) {
+  int fd;
+  ASSERT_NE(-1, (fd = open("foo", O_CREAT | O_RDWR, 0644)));
+  EXPECT_EQ(O_RDWR, fcntl(fd, F_GETFL) & O_ACCMODE);
+  EXPECT_NE(-1, close(fd));
+  ASSERT_NE(-1, (fd = open("foo", O_WRONLY, 0644)));
+  EXPECT_EQ(O_WRONLY, fcntl(fd, F_GETFL) & O_ACCMODE);
+  EXPECT_NE(-1, close(fd));
+}
+
+TEST(fcntl_setfl, testChangeAppendStatus) {
+  int fd;
+  char buf[8] = {0};
+  if (IsWindows()) return; /* doesn't appear possible on windows */
+  ASSERT_NE(-1, (fd = open("foo", O_CREAT | O_RDWR, 0644)));
+  EXPECT_EQ(3, write(fd, "foo", 3));
+  EXPECT_NE(-1, lseek(fd, 0, SEEK_SET));
+  EXPECT_NE(-1, fcntl(fd, F_SETFL, O_APPEND));
+  EXPECT_EQ(3, write(fd, "bar", 3));
+  EXPECT_NE(-1, lseek(fd, 0, SEEK_SET));
+  EXPECT_NE(-1, fcntl(fd, F_SETFL, 0));
+  EXPECT_EQ(6, read(fd, buf, 6));
+  EXPECT_STREQ("foobar", buf);
+  EXPECT_NE(-1, close(fd));
+}