diff --git a/src/boot/system_test.c b/src/boot/system_test.c
index ba13571e..0b4c65b5 100644
--- a/src/boot/system_test.c
+++ b/src/boot/system_test.c
@@ -51,9 +51,11 @@ int system_test() {
     assert(janet_equals(janet_wrap_number(1.4), janet_wrap_number(1.4)));
     assert(janet_equals(janet_wrap_number(3.14159265), janet_wrap_number(3.14159265)));
 #ifdef NAN
-#ifndef JANET_PLAN9
-    assert(janet_checktype(janet_wrap_number(NAN), JANET_NUMBER));
+#ifdef JANET_PLAN9
+	// Plan 9 traps NaNs by default; disable that.
+	setfcr(0);
 #endif
+    assert(janet_checktype(janet_wrap_number(NAN), JANET_NUMBER));
 #else
     assert(janet_checktype(janet_wrap_number(0.0 / 0.0), JANET_NUMBER));
 #endif
diff --git a/src/core/os.c b/src/core/os.c
index 66140c41..e183bf43 100644
--- a/src/core/os.c
+++ b/src/core/os.c
@@ -29,6 +29,11 @@
 
 #include <stdlib.h>
 
+#ifdef JANET_PLAN9
+#include <unistd.h>
+#endif
+
+
 #ifndef JANET_REDUCED_OS
 
 #include <time.h>
@@ -288,7 +293,20 @@ JANET_CORE_FN(os_exit,
     return janet_wrap_nil();
 }
 
-#ifndef JANET_REDUCED_OS
+#ifdef JANET_PLAN9
+
+JANET_CORE_FN(os_isatty,
+              "(os/isatty &opt file)",
+              "Returns true if `file` is a terminal. If `file` is not specified, "
+              "it will default to standard output.") {
+    janet_arity(argc, 0, 1);
+    FILE *f = (argc == 1) ? janet_getfile(argv, 0, NULL) : stdout;
+    int fd = fileno(f);
+    if (fd == -1) janet_panic(janet_strerror(errno));
+    return janet_wrap_boolean(isatty(fd));
+}
+
+#elif !defined(JANET_REDUCED_OS)
 
 JANET_CORE_FN(os_cpu_count,
               "(os/cpu-count &opt dflt)",
@@ -2847,7 +2865,9 @@ void janet_lib_os(JanetTable *env) {
         JANET_CORE_REG("os/which", os_which),
         JANET_CORE_REG("os/arch", os_arch),
         JANET_CORE_REG("os/compiler", os_compiler),
-#ifndef JANET_REDUCED_OS
+#ifdef JANET_PLAN9
+        JANET_CORE_REG("os/isatty", os_isatty),
+#elif !defined(JANET_REDUCED_OS)
 
         /* misc (un-sandboxed) */
         JANET_CORE_REG("os/cpu-count", os_cpu_count),
diff --git a/src/include/janet.h b/src/include/janet.h
index 4b47a7ae..0401bf0e 100644
--- a/src/include/janet.h
+++ b/src/include/janet.h
@@ -305,10 +305,6 @@ extern "C" {
 #define JANET_STACK_MAX 0x7fffffff
 #endif
 
-#ifdef JANET_PLAN9
-#undef NAN
-#endif
-
 /* Use nanboxed values - uses 8 bytes per value instead of 12 or 16.
  * To turn of nanboxing, for debugging purposes or for certain
  * architectures (Nanboxing only tested on x86 and x64), comment out
diff --git a/src/mainclient/shell.c b/src/mainclient/shell.c
index 93546db2..beb938dd 100644
--- a/src/mainclient/shell.c
+++ b/src/mainclient/shell.c
@@ -309,7 +309,9 @@ static int curpos(void) {
     char buf[32];
     int cols, rows;
     unsigned int i = 0;
+#ifndef JANET_PLAN9
     if (write_console("\x1b[6n", 4) != 4) return -1;
+#endif
     while (i < sizeof(buf) - 1) {
         if (read_console(buf + i, 1) != 1) break;
         if (buf[i] == 'R') break;
@@ -327,6 +329,10 @@ static int getcols(void) {
     CONSOLE_SCREEN_BUFFER_INFO csbi;
     GetConsoleScreenBufferInfo(GetStdHandle(STD_OUTPUT_HANDLE), &csbi);
     return (int)(csbi.srWindow.Right - csbi.srWindow.Left + 1);
+#elif defined(JANET_PLAN9)
+	//FIXME(noam): this can probably read /dev/window to get window size, and
+	//divide by the active font width, since plan9 requires fixed-width glyphs.
+	return 80;
 #else
     struct winsize ws;
     if (ioctl(1, TIOCGWINSZ, &ws) == -1 || ws.ws_col == 0) {
@@ -1133,6 +1139,10 @@ int main(int argc, char **argv) {
     JanetArray *args;
     JanetTable *env;
 
+#ifdef JANET_PLAN9
+	setfcr(0);
+#endif
+
 #ifdef _WIN32
     setup_console_output();
 #endif