diff --git a/src/parser/argument/PhutilArgumentSpellingCorrector.php b/src/parser/argument/PhutilArgumentSpellingCorrector.php
index 8652598..dd99912 100644
--- a/src/parser/argument/PhutilArgumentSpellingCorrector.php
+++ b/src/parser/argument/PhutilArgumentSpellingCorrector.php
@@ -1,132 +1,155 @@
 <?php
 
 final class PhutilArgumentSpellingCorrector extends Phobject {
 
   private $editDistanceMatrix;
   private $maximumDistance;
+  private $mode;
+
+  const MODE_COMMANDS = 'commands';
+  const MODE_FLAGS = 'flags';
 
   /**
    * Build a new corrector with parameters for correcting commands, like
    * fixing "dfif" into "diff" in "arc diff".
    *
    * @return PhutilArgumentSpellingCorrector Configured corrector.
    */
   public static function newCommandCorrector() {
     // When correcting argument spelling, we're relatively liberal about
     // selecting alternatives and allow multiple mistakes.
     $max_distance = 2;
 
     // Adjust to the scaled edit costs we use below, so "2" roughly means
     // "2 edits".
     $max_distance = $max_distance * 3;
 
     // These costs are somewhat made up, but the theory is that it is far more
     // likely you will mis-strike a key ("lans" for "land") or press two keys
     // out of order ("alnd" for "land") than omit keys or press extra keys.
     $matrix = id(new PhutilEditDistanceMatrix())
       ->setInsertCost(4)
       ->setDeleteCost(4)
       ->setReplaceCost(3)
       ->setTransposeCost(2);
 
     return id(new self())
       ->setEditDistanceMatrix($matrix)
+      ->setMode(self::MODE_COMMANDS)
       ->setMaximumDistance($max_distance);
   }
 
 
   /**
    * Build a new corrector with parameters for correcting flags, like
    * fixing "--nolint" into "--no-lint".
    *
    * @return PhutilArgumentSpellingCorrector Configured corrector.
    */
   public static function newFlagCorrector() {
     // When correcting flag spelling, we're stricter than we are when
     // correcting command spelling: we allow only one inserted or deleted
     // character. It is mainly to handle cases like "--no-lint" versus
     // "--nolint" or "--reviewer" versus "--reviewers".
     $max_distance = 1;
 
     $matrix = id(new PhutilEditDistanceMatrix())
       ->setInsertCost(1)
       ->setDeleteCost(1)
       ->setReplaceCost(10);
 
     return id(new self())
       ->setEditDistanceMatrix($matrix)
+      ->setMode(self::MODE_FLAGS)
       ->setMaximumDistance($max_distance);
   }
 
+  public function setMode($mode) {
+    $this->mode = $mode;
+    return $this;
+  }
+
+  public function getMode() {
+    return $this->mode;
+  }
+
   public function setEditDistanceMatrix(PhutilEditDistanceMatrix $matrix) {
     $this->editDistanceMatrix = $matrix;
     return $this;
   }
 
   public function getEditDistanceMatrix() {
     return $this->editDistanceMatrix;
   }
 
   public function setMaximumDistance($maximum_distance) {
     $this->maximumDistance = $maximum_distance;
     return $this;
   }
 
   public function getMaximumDistance() {
     return $this->maximumDistance;
   }
 
   public function correctSpelling($input, array $options) {
     $matrix = $this->getEditDistanceMatrix();
     if (!$matrix) {
       throw new PhutilInvalidStateException('setEditDistanceMatrix');
     }
 
     $max_distance = $this->getMaximumDistance();
     if (!$max_distance) {
       throw new PhutilInvalidStateException('setMaximumDistance');
     }
 
+    // If we're correcting commands, never correct an input which begins
+    // with "-", since this is almost certainly intended to be a flag.
+    if ($this->getMode() === self::MODE_COMMANDS) {
+      if (preg_match('/^-/', $input)) {
+        return array();
+      }
+    }
+
     $input = $this->normalizeString($input);
     foreach ($options as $key => $option) {
       $options[$key] = $this->normalizeString($option);
     }
 
     $distances = array();
     $inputv = phutil_utf8v($input);
     foreach ($options as $option) {
       $optionv = phutil_utf8v($option);
       $matrix->setSequences($optionv, $inputv);
       $distances[$option] = $matrix->getEditDistance();
     }
 
     asort($distances);
     $best = min($max_distance, head($distances));
     foreach ($distances as $option => $distance) {
       if ($distance > $best) {
         unset($distances[$option]);
       }
     }
 
     // Before filtering, check if we have multiple equidistant matches and
     // return them if we do. This prevents us from, e.g., matching "alnd" with
     // both "land" and "amend", then dropping "land" for being too short, and
     // incorrectly completing to "amend".
     if (count($distances) > 1) {
       return array_keys($distances);
     }
 
     foreach ($distances as $option => $distance) {
       if (phutil_utf8_strlen($option) < $distance) {
         unset($distances[$option]);
       }
     }
 
     return array_keys($distances);
   }
 
   private function normalizeString($string) {
     return phutil_utf8_strtolower($string);
   }
 
 }