diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -378,6 +378,7 @@
     'ArcanistNoParentScopeXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistNoParentScopeXHPASTLinterRule.php',
     'ArcanistNoParentScopeXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistNoParentScopeXHPASTLinterRuleTestCase.php',
     'ArcanistNoURIConduitException' => 'conduit/ArcanistNoURIConduitException.php',
+    'ArcanistNonblockingGuard' => 'utils/ArcanistNonblockingGuard.php',
     'ArcanistNoneLintRenderer' => 'lint/renderer/ArcanistNoneLintRenderer.php',
     'ArcanistObjectListHardpoint' => 'hardpoint/ArcanistObjectListHardpoint.php',
     'ArcanistObjectOperatorSpacingXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistObjectOperatorSpacingXHPASTLinterRule.php',
@@ -1434,6 +1435,7 @@
     'ArcanistNoParentScopeXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistNoParentScopeXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistNoURIConduitException' => 'ArcanistConduitException',
+    'ArcanistNonblockingGuard' => 'Phobject',
     'ArcanistNoneLintRenderer' => 'ArcanistLintRenderer',
     'ArcanistObjectListHardpoint' => 'ArcanistHardpoint',
     'ArcanistObjectOperatorSpacingXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
diff --git a/src/toolset/ArcanistPrompt.php b/src/toolset/ArcanistPrompt.php
--- a/src/toolset/ArcanistPrompt.php
+++ b/src/toolset/ArcanistPrompt.php
@@ -93,16 +93,9 @@
 
     // NOTE: We're making stdin nonblocking so that we can respond to signals
     // immediately. If we don't, and you ^C during a prompt, the program does
-    // not handle the signal until fgets() returns.
+    // not handle the signal until fgets() returns. See also T13649.
 
-    // On Windows, we skip this because stdin can not be made nonblocking.
-
-    if (!phutil_is_windows()) {
-      $ok = stream_set_blocking($stdin, false);
-      if (!$ok) {
-        throw new Exception(pht('Unable to set stdin nonblocking.'));
-      }
-    }
+    $guard = ArcanistNonblockingGuard::newForStream($stdin);
 
     echo "\n";
 
@@ -123,7 +116,7 @@
 
         $is_saved = false;
 
-        if (phutil_is_windows()) {
+        if (!$guard->getIsNonblocking()) {
           $response = fgets($stdin);
         } else {
           while (true) {
diff --git a/src/utils/ArcanistNonblockingGuard.php b/src/utils/ArcanistNonblockingGuard.php
new file mode 100644
--- /dev/null
+++ b/src/utils/ArcanistNonblockingGuard.php
@@ -0,0 +1,56 @@
+<?php
+
+final class ArcanistNonblockingGuard
+  extends Phobject {
+
+  private $stream;
+  private $didSetNonblocking;
+
+  public static function newForStream($stream) {
+    $guard = new self();
+    $guard->stream = $stream;
+
+    if (phutil_is_windows()) {
+
+      // On Windows, we skip this because stdin can not be made nonblocking.
+
+    } else if (!function_exists('pcntl_signal')) {
+
+      // If we can't handle signals, we: can't reset the flag if we're
+      // interrupted; but also don't benefit from setting it in the first
+      // place since it's only relevant for handling interrupts during
+      // prompts. So just skip this.
+
+    } else {
+
+      // See T13649. Note that the "blocked" key identifies whether the
+      // stream is blocking or nonblocking, not whether it will block when
+      // read or written.
+
+      $metadata = stream_get_meta_data($stream);
+      $is_blocking = idx($metadata, 'blocked');
+      if ($is_blocking) {
+        $ok = stream_set_blocking($stream, false);
+        if (!$ok) {
+          throw new Exception(pht('Unable to set stream nonblocking.'));
+        }
+        $guard->didSetNonblocking = true;
+      }
+    }
+
+    return $guard;
+  }
+
+  public function getIsNonblocking() {
+    return $this->didSetNonblocking;
+  }
+
+  public function __destruct() {
+    if ($this->stream && $this->didSetNonblocking) {
+      stream_set_blocking($this->stream, true);
+    }
+
+    $this->stream = null;
+  }
+
+}