diff --git a/contrib/chg/chg.c b/contrib/chg/chg.c
--- a/contrib/chg/chg.c
+++ b/contrib/chg/chg.c
@@ -373,8 +373,15 @@
 }
 
 /*
- * Test whether the command is unsupported or not. This is not designed to
- * cover all cases. But it's fast, does not depend on the server.
+ * Test whether the command and the environment is unsupported or not.
+ *
+ * If any of the stdio file descriptors are not present (rare, but some tools
+ * might spawn new processes without stdio instead of redirecting them to the
+ * null device), then mark it as not supported because attachio won't work
+ * correctly.
+ *
+ * The command list is not designed to cover all cases. But it's fast, and does
+ * not depend on the server.
  */
 static int isunsupported(int argc, const char *argv[])
 {
@@ -384,6 +391,13 @@
 	};
 	unsigned int state = 0;
 	int i;
+	/* use fcntl to test missing stdio fds */
+	if (fcntl(STDIN_FILENO, F_GETFD) == -1 ||
+	    fcntl(STDOUT_FILENO, F_GETFD) == -1 ||
+	    fcntl(STDERR_FILENO, F_GETFD) == -1) {
+		debugmsg("stdio fds are missing");
+		return 1;
+	}
 	for (i = 0; i < argc; ++i) {
 		if (strcmp(argv[i], "--") == 0)
 			break;
diff --git a/tests/test-chg.t b/tests/test-chg.t
--- a/tests/test-chg.t
+++ b/tests/test-chg.t
@@ -197,6 +197,14 @@
 
   $ cd ..
 
+missing stdio
+-------------
+
+  $ CHGDEBUG=1 chg version -q 0<&-
+  chg: debug: * stdio fds are missing (glob)
+  chg: debug: * execute original hg (glob)
+  Mercurial Distributed SCM * (glob)
+
 server lifecycle
 ----------------