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
@@ -866,6 +866,7 @@
     'phutil_decode_mime_header' => 'utils/utils.php',
     'phutil_deprecated' => 'init/lib/moduleutils.php',
     'phutil_describe_type' => 'utils/utils.php',
+    'phutil_encode_log' => 'utils/utils.php',
     'phutil_error_listener_example' => 'error/phlog.php',
     'phutil_escape_uri' => 'utils/utils.php',
     'phutil_escape_uri_path_component' => 'utils/utils.php',
diff --git a/src/channel/PhutilJSONProtocolChannel.php b/src/channel/PhutilJSONProtocolChannel.php
--- a/src/channel/PhutilJSONProtocolChannel.php
+++ b/src/channel/PhutilJSONProtocolChannel.php
@@ -43,7 +43,16 @@
    * @task protocol
    */
   protected function encodeMessage($message) {
-    $message = json_encode($message);
+    if (!is_array($message)) {
+      throw new Exception(
+        pht(
+          'JSON protocol message must be an array, got some other '.
+          'type ("%s").',
+          phutil_describe_type($message)));
+    }
+
+    $message = phutil_json_encode($message);
+
     $len = sprintf(
       '%0'.self::SIZE_LENGTH.'.'.self::SIZE_LENGTH.'d',
       strlen($message));
@@ -67,6 +76,21 @@
           $len = substr($this->buf, 0, self::SIZE_LENGTH);
           $this->buf = substr($this->buf, self::SIZE_LENGTH);
 
+          if (!preg_match('/^\d+\z/', $len)) {
+            $full_buffer = $len.$this->buf;
+            $full_length = strlen($full_buffer);
+
+            throw new Exception(
+              pht(
+                'Protocol channel expected %s-character, zero-padded '.
+                'numeric frame length, got something else ("%s"). Full '.
+                'buffer (of length %s) begins: %s',
+                new PhutilNumber(self::SIZE_LENGTH),
+                phutil_encode_log($len),
+                new PhutilNumber($full_length),
+                phutil_encode_log(substr($len.$this->buf, 0, 128))));
+          }
+
           $this->mode = self::MODE_OBJECT;
           $this->byteLengthOfNextChunk = (int)$len;
           break;
diff --git a/src/filesystem/PhutilDeferredLog.php b/src/filesystem/PhutilDeferredLog.php
--- a/src/filesystem/PhutilDeferredLog.php
+++ b/src/filesystem/PhutilDeferredLog.php
@@ -229,7 +229,7 @@
       if ($saw_percent) {
         $saw_percent = false;
         if (array_key_exists($c, $map)) {
-          $result .= addcslashes($map[$c], "\0..\37\\\177..\377");
+          $result .= phutil_encode_log($map[$c]);
         } else {
           $result .= '-';
         }
diff --git a/src/future/FuturePool.php b/src/future/FuturePool.php
--- a/src/future/FuturePool.php
+++ b/src/future/FuturePool.php
@@ -34,6 +34,10 @@
     return $this;
   }
 
+  public function getFutures() {
+    return $this->futures;
+  }
+
   public function hasFutures() {
     return (bool)$this->futures;
   }
diff --git a/src/phage/agent/PhagePHPAgent.php b/src/phage/agent/PhagePHPAgent.php
--- a/src/phage/agent/PhagePHPAgent.php
+++ b/src/phage/agent/PhagePHPAgent.php
@@ -4,27 +4,28 @@
 
   private $stdin;
   private $master;
-  private $exec = array();
+  private $futurePool;
 
   public function __construct($stdin) {
     $this->stdin = $stdin;
   }
 
   public function execute() {
+    $future_pool = $this->getFuturePool();
+
     while (true) {
-      if ($this->exec) {
-        $iterator = new FutureIterator($this->exec);
-        $iterator->setUpdateInterval(0.050);
-        foreach ($iterator as $key => $future) {
+      if ($future_pool->hasFutures()) {
+        while ($future_pool->hasFutures()) {
+          $future = $future_pool->resolve();
+
           if ($future === null) {
-            foreach ($this->exec as $read_key => $read_future) {
-              $this->readFuture($read_key, $read_future);
+            foreach ($future_pool->getFutures() as $read_future) {
+              $this->readFuture($read_future);
             }
-
             break;
-          } else {
-            $this->resolveFuture($key, $future);
           }
+
+          $this->resolveFuture($future);
         }
       } else {
         PhutilChannel::waitForAny(array($this->getMaster()));
@@ -34,6 +35,22 @@
     }
   }
 
+  private function getFuturePool() {
+    if (!$this->futurePool) {
+      $this->futurePool = $this->newFuturePool();
+    }
+    return $this->futurePool;
+  }
+
+  private function newFuturePool() {
+    $future_pool = new FuturePool();
+
+    $future_pool->getIteratorTemplate()
+      ->setUpdateInterval(0.050);
+
+    return $future_pool;
+  }
+
   private function getMaster() {
     if (!$this->master) {
       $raw_channel = new PhutilSocketChannel(
@@ -77,9 +94,10 @@
           $future->setTimeout(ceil($timeout));
         }
 
-        $future->isReady();
+        $future->setFutureKey($key);
 
-        $this->exec[$key] = $future;
+        $this->getFuturePool()
+          ->addFuture($future);
         break;
       case 'EXIT':
         $this->terminateAgent();
@@ -87,8 +105,9 @@
     }
   }
 
-  private function readFuture($key, ExecFuture $future) {
+  private function readFuture(ExecFuture $future) {
     $master = $this->getMaster();
+    $key = $future->getFutureKey();
 
     list($stdout, $stderr) = $future->read();
     $future->discardBuffers();
@@ -114,7 +133,8 @@
     }
   }
 
-  private function resolveFuture($key, ExecFuture $future) {
+  private function resolveFuture(ExecFuture $future) {
+    $key = $future->getFutureKey();
     $result = $future->resolve();
     $master = $this->getMaster();
 
@@ -127,8 +147,6 @@
         'stderr'  => $result[2],
         'timeout' => (bool)$future->getWasKilledByTimeout(),
       ));
-
-    unset($this->exec[$key]);
   }
 
   public function __destruct() {
@@ -136,9 +154,12 @@
   }
 
   private function terminateAgent() {
-    foreach ($this->exec as $key => $future) {
+    $pool = $this->getFuturePool();
+
+    foreach ($pool->getFutures() as $future) {
       $future->resolveKill();
     }
+
     exit(0);
   }
 
diff --git a/src/phage/bootloader/PhagePHPAgentBootloader.php b/src/phage/bootloader/PhagePHPAgentBootloader.php
--- a/src/phage/bootloader/PhagePHPAgentBootloader.php
+++ b/src/phage/bootloader/PhagePHPAgentBootloader.php
@@ -47,6 +47,7 @@
         'xsprintf/PhutilCommandString.php',
         'future/Future.php',
         'future/FutureIterator.php',
+        'future/FuturePool.php',
         'future/exec/PhutilExecutableFuture.php',
         'future/exec/ExecFuture.php',
         'future/exec/CommandException.php',
diff --git a/src/utils/utils.php b/src/utils/utils.php
--- a/src/utils/utils.php
+++ b/src/utils/utils.php
@@ -1926,3 +1926,7 @@
 
   return false;
 }
+
+function phutil_encode_log($message) {
+  return addcslashes($message, "\0..\37\\\177..\377");
+}