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
@@ -537,6 +537,7 @@
     'PhageAgentAction' => 'phage/action/PhageAgentAction.php',
     'PhageAgentBootloader' => 'phage/bootloader/PhageAgentBootloader.php',
     'PhageAgentTestCase' => 'phage/__tests__/PhageAgentTestCase.php',
+    'PhageExecWorkflow' => 'phage/workflow/PhageExecWorkflow.php',
     'PhageExecuteAction' => 'phage/action/PhageExecuteAction.php',
     'PhageLocalAction' => 'phage/action/PhageLocalAction.php',
     'PhagePHPAgent' => 'phage/agent/PhagePHPAgent.php',
@@ -1484,6 +1485,7 @@
     'PhageAgentAction' => 'PhageAction',
     'PhageAgentBootloader' => 'Phobject',
     'PhageAgentTestCase' => 'PhutilTestCase',
+    'PhageExecWorkflow' => 'PhageWorkflow',
     'PhageExecuteAction' => 'PhageAction',
     'PhageLocalAction' => 'PhageAgentAction',
     'PhagePHPAgent' => 'Phobject',
diff --git a/src/future/exec/ExecFuture.php b/src/future/exec/ExecFuture.php
--- a/src/future/exec/ExecFuture.php
+++ b/src/future/exec/ExecFuture.php
@@ -752,20 +752,29 @@
     }
 
     if ($is_done) {
+      $signal_info = null;
+
       // If the subprocess got nuked with `kill -9`, we get a -1 exitcode.
       // Upgrade this to a slightly more informative value by examining the
       // terminating signal code.
       $err = $status['exitcode'];
       if ($err == -1) {
         if ($status['signaled']) {
-          $err = 128 + $status['termsig'];
+          $signo = $status['termsig'];
+
+          $err = 128 + $signo;
+
+          $signal_info = pht(
+            "<Process was terminated by signal %s (%d).>\n\n",
+            phutil_get_signal_name($signo),
+            $signo);
         }
       }
 
       $result = array(
         $err,
         $this->stdout,
-        $this->stderr,
+        $signal_info.$this->stderr,
       );
 
       $this->setResult($result);
diff --git a/src/phage/action/PhageLocalAction.php b/src/phage/action/PhageLocalAction.php
--- a/src/phage/action/PhageLocalAction.php
+++ b/src/phage/action/PhageLocalAction.php
@@ -4,7 +4,17 @@
   extends PhageAgentAction {
 
   protected function newAgentFuture(PhutilCommandString $command) {
-    return new ExecFuture('sh -c %s', $command);
+    $arcanist_src = phutil_get_library_root('arcanist');
+    $bin_dir = Filesystem::concatenatePaths(
+      array(
+        dirname($arcanist_src),
+        'bin',
+      ));
+
+    $future = id(new ExecFuture('%s exec -- %C', './phage', $command))
+      ->setCWD($bin_dir);
+
+    return $future;
   }
 
 }
diff --git a/src/phage/workflow/PhageExecWorkflow.php b/src/phage/workflow/PhageExecWorkflow.php
new file mode 100644
--- /dev/null
+++ b/src/phage/workflow/PhageExecWorkflow.php
@@ -0,0 +1,69 @@
+<?php
+
+final class PhageExecWorkflow
+  extends PhageWorkflow {
+
+  public function getWorkflowName() {
+    return 'exec';
+  }
+
+  public function getWorkflowArguments() {
+    return array(
+      $this->newWorkflowArgument('argv')
+        ->setHelp(pht('Command to execute.'))
+        ->setWildcard(true),
+    );
+  }
+
+  public function getWorkflowInformation() {
+    return $this->newWorkflowInformation()
+      ->setSynopsis(pht('Execute a Phage subprocess.'));
+  }
+
+  protected function runWorkflow() {
+    $argv = $this->getArgument('argv');
+    if (!$argv) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Specify a command to execute using one or more arguments.'));
+    }
+
+    // This workflow is just a thin wrapper around running a subprocess as
+    // its own process group leader.
+    //
+    // If the Phage parent process executes a subprocess and that subprocess
+    // does not turn itself into a process group leader, sending "^C" to the
+    // parent process will also send the signal to the subprocess. Phage
+    // handles SIGINT as an input and we don't want it to propagate to children
+    // by default.
+    //
+    // Some versions of Linux have a binary named "setsid" which does the same
+    // thing, but this binary doesn't exist on macOS.
+
+    // NOTE: This calls is documented as ever being able to fail. For now,
+    // trust the documentation?
+
+    $pid = posix_getpid();
+
+    $pgid = posix_getpgid($pid);
+    if ($pgid === false) {
+      throw new Exception(pht('Call to "posix_getpgid(...)" failed!'));
+    }
+
+    // If this process was run directly from a shell with "phage exec ...",
+    // we'll already be the process group leader. In this case, we don't need
+    // to become the leader of a new session (and the call will fail if we
+    // try).
+
+    $is_leader = ($pid === $pgid);
+    if (!$is_leader) {
+      $sid = posix_setsid();
+      if ($sid === -1) {
+        throw new Exception(pht('Call to "posix_setsid()" failed!'));
+      }
+    }
+
+    return phutil_passthru('exec -- %Ls', $argv);
+  }
+
+}
diff --git a/src/workflow/ArcanistWorkflow.php b/src/workflow/ArcanistWorkflow.php
--- a/src/workflow/ArcanistWorkflow.php
+++ b/src/workflow/ArcanistWorkflow.php
@@ -126,6 +126,20 @@
       ->setArguments($specs);
 
     $information = $this->getWorkflowInformation();
+
+    if ($information !== null) {
+      if (!($information instanceof ArcanistWorkflowInformation)) {
+        throw new Exception(
+          pht(
+            'Expected workflow ("%s", of class "%s") to return an '.
+            '"ArcanistWorkflowInformation" object from call to '.
+            '"getWorkflowInformation()", got %s.',
+            $this->getWorkflowName(),
+            get_class($this),
+            phutil_describe_type($information)));
+      }
+    }
+
     if ($information) {
       $synopsis = $information->getSynopsis();
       if (strlen($synopsis)) {
@@ -2255,10 +2269,14 @@
     return $this->getConfigurationSourceList()->getConfig($key);
   }
 
-  final public function canHandleSignal($signo) {
+  public function canHandleSignal($signo) {
     return false;
   }
 
+  public function handleSignal($signo) {
+    return;
+  }
+
   final public function newCommand(PhutilExecutableFuture $future) {
     return id(new ArcanistCommand())
       ->setLogEngine($this->getLogEngine())