diff --git a/src/cli/cli.cpp b/src/cli/cli.cpp
index e41d3a798..1bf96d7a8 100644
--- a/src/cli/cli.cpp
+++ b/src/cli/cli.cpp
@@ -76,8 +76,6 @@ const struct Command Interpreter::sCommands[] =
     { "whitelist", &ProcessWhitelist },
 };
 
-ResponseBuffer Interpreter::sResponse;
-
 otNetifAddress Interpreter::sAddress;
 
 Ip6::IcmpEcho Interpreter::sIcmpEcho(&HandleEchoResponse, NULL);
@@ -139,11 +137,11 @@ void Interpreter::AppendResult(ThreadError error)
 {
     if (error == kThreadError_None)
     {
-        sResponse.Append("Done\r\n");
+        sServer->OutputFormat("Done\r\n");
     }
     else
     {
-        sResponse.Append("Error %d\r\n", error);
+        sServer->OutputFormat("Error %d\r\n", error);
     }
 }
 
@@ -158,7 +156,7 @@ void Interpreter::ProcessHelp(int argc, char *argv[])
 {
     for (unsigned int i = 0; i < sizeof(sCommands) / sizeof(sCommands[0]); i++)
     {
-        sResponse.Append("%s\r\n", sCommands[i].mName);
+        sServer->OutputFormat("%s\r\n", sCommands[i].mName);
     }
 }
 
@@ -169,7 +167,7 @@ void Interpreter::ProcessChannel(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%d\r\n", otGetChannel());
+        sServer->OutputFormat("%d\r\n", otGetChannel());
     }
     else
     {
@@ -188,7 +186,7 @@ void Interpreter::ProcessChildTimeout(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%d\r\n", otGetChildTimeout());
+        sServer->OutputFormat("%d\r\n", otGetChildTimeout());
     }
     else
     {
@@ -207,7 +205,7 @@ void Interpreter::ProcessContextIdReuseDelay(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%d\r\n", otGetContextIdReuseDelay());
+        sServer->OutputFormat("%d\r\n", otGetContextIdReuseDelay());
     }
     else
     {
@@ -222,9 +220,9 @@ exit:
 void Interpreter::ProcessExtAddress(int argc, char *argv[])
 {
     const uint8_t *extAddress = otGetExtendedAddress();
-    sResponse.Append("%02x%02x%02x%02x%02x%02x%02x%02x\r\n",
-                     extAddress[0], extAddress[1], extAddress[2], extAddress[3],
-                     extAddress[4], extAddress[5], extAddress[6], extAddress[7]);
+    sServer->OutputFormat("%02x%02x%02x%02x%02x%02x%02x%02x\r\n",
+                          extAddress[0], extAddress[1], extAddress[2], extAddress[3],
+                          extAddress[4], extAddress[5], extAddress[6], extAddress[7]);
     AppendResult(kThreadError_None);
 }
 
@@ -235,9 +233,9 @@ void Interpreter::ProcessExtPanId(int argc, char *argv[])
     if (argc == 0)
     {
         const uint8_t *extPanId = otGetExtendedPanId();
-        sResponse.Append("%02x%02x%02x%02x%02x%02x%02x%02x\r\n",
-                         extPanId[0], extPanId[1], extPanId[2], extPanId[3],
-                         extPanId[4], extPanId[5], extPanId[6], extPanId[7]);
+        sServer->OutputFormat("%02x%02x%02x%02x%02x%02x%02x%02x\r\n",
+                              extPanId[0], extPanId[1], extPanId[2], extPanId[3],
+                              extPanId[4], extPanId[5], extPanId[6], extPanId[7]);
     }
     else
     {
@@ -291,11 +289,11 @@ void Interpreter::ProcessIpAddr(int argc, char *argv[])
     {
         for (const otNetifAddress *addr = otGetUnicastAddresses(); addr; addr = addr->mNext)
         {
-            sResponse.Append("%x:%x:%x:%x:%x:%x:%x:%x\r\n",
-                             HostSwap16(addr->mAddress.m16[0]), HostSwap16(addr->mAddress.m16[1]),
-                             HostSwap16(addr->mAddress.m16[2]), HostSwap16(addr->mAddress.m16[3]),
-                             HostSwap16(addr->mAddress.m16[4]), HostSwap16(addr->mAddress.m16[5]),
-                             HostSwap16(addr->mAddress.m16[6]), HostSwap16(addr->mAddress.m16[7]));
+            sServer->OutputFormat("%x:%x:%x:%x:%x:%x:%x:%x\r\n",
+                                  HostSwap16(addr->mAddress.m16[0]), HostSwap16(addr->mAddress.m16[1]),
+                                  HostSwap16(addr->mAddress.m16[2]), HostSwap16(addr->mAddress.m16[3]),
+                                  HostSwap16(addr->mAddress.m16[4]), HostSwap16(addr->mAddress.m16[5]),
+                                  HostSwap16(addr->mAddress.m16[6]), HostSwap16(addr->mAddress.m16[7]));
         }
     }
     else
@@ -321,7 +319,7 @@ void Interpreter::ProcessKeySequence(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%d\r\n", otGetKeySequenceCounter());
+        sServer->OutputFormat("%d\r\n", otGetKeySequenceCounter());
     }
     else
     {
@@ -340,7 +338,7 @@ void Interpreter::ProcessLeaderWeight(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%d\r\n", otGetLocalLeaderWeight());
+        sServer->OutputFormat("%d\r\n", otGetLocalLeaderWeight());
     }
     else
     {
@@ -363,10 +361,10 @@ void Interpreter::ProcessMasterKey(int argc, char *argv[])
 
         for (int i = 0; i < keyLength; i++)
         {
-            sResponse.Append("%02x", key[i]);
+            sServer->OutputFormat("%02x", key[i]);
         }
 
-        sResponse.Append("\r\n");
+        sServer->OutputFormat("\r\n");
     }
     else
     {
@@ -391,25 +389,25 @@ void Interpreter::ProcessMode(int argc, char *argv[])
 
         if (linkMode.mRxOnWhenIdle)
         {
-            sResponse.Append("r");
+            sServer->OutputFormat("r");
         }
 
         if (linkMode.mSecureDataRequests)
         {
-            sResponse.Append("s");
+            sServer->OutputFormat("s");
         }
 
         if (linkMode.mDeviceType)
         {
-            sResponse.Append("d");
+            sServer->OutputFormat("d");
         }
 
         if (linkMode.mNetworkData)
         {
-            sResponse.Append("n");
+            sServer->OutputFormat("n");
         }
 
-        sResponse.Append("\r\n");
+        sServer->OutputFormat("\r\n");
     }
     else
     {
@@ -461,7 +459,7 @@ void Interpreter::ProcessNetworkIdTimeout(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%d\r\n", otGetNetworkIdTimeout());
+        sServer->OutputFormat("%d\r\n", otGetNetworkIdTimeout());
     }
     else
     {
@@ -479,7 +477,7 @@ void Interpreter::ProcessNetworkName(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%.*s\r\n", OT_NETWORK_NAME_SIZE, otGetNetworkName());
+        sServer->OutputFormat("%.*s\r\n", OT_NETWORK_NAME_SIZE, otGetNetworkName());
     }
     else
     {
@@ -497,7 +495,7 @@ void Interpreter::ProcessPanId(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%d\r\n", otGetPanId());
+        sServer->OutputFormat("%d\r\n", otGetPanId());
     }
     else
     {
@@ -515,16 +513,13 @@ void Interpreter::HandleEchoResponse(void *aContext, Message &aMessage, const Ip
 
     aMessage.Read(aMessage.GetOffset(), sizeof(icmp6Header), &icmp6Header);
 
-    sResponse.Init();
-    sResponse.Append("%d bytes from ", aMessage.GetLength() - aMessage.GetOffset());
-    sResponse.Append("%x:%x:%x:%x:%x:%x:%x:%x",
-                     HostSwap16(aMessageInfo.GetPeerAddr().m16[0]), HostSwap16(aMessageInfo.GetPeerAddr().m16[1]),
-                     HostSwap16(aMessageInfo.GetPeerAddr().m16[2]), HostSwap16(aMessageInfo.GetPeerAddr().m16[3]),
-                     HostSwap16(aMessageInfo.GetPeerAddr().m16[4]), HostSwap16(aMessageInfo.GetPeerAddr().m16[5]),
-                     HostSwap16(aMessageInfo.GetPeerAddr().m16[6]), HostSwap16(aMessageInfo.GetPeerAddr().m16[7]));
-    sResponse.Append(": icmp_seq=%d hlim=%d\r\n", icmp6Header.GetSequence(), aMessageInfo.mHopLimit);
-
-    sServer->Output(sResponse.GetResponse(), sResponse.GetResponseLength());
+    sServer->OutputFormat("%d bytes from ", aMessage.GetLength() - aMessage.GetOffset());
+    sServer->OutputFormat("%x:%x:%x:%x:%x:%x:%x:%x",
+                          HostSwap16(aMessageInfo.GetPeerAddr().m16[0]), HostSwap16(aMessageInfo.GetPeerAddr().m16[1]),
+                          HostSwap16(aMessageInfo.GetPeerAddr().m16[2]), HostSwap16(aMessageInfo.GetPeerAddr().m16[3]),
+                          HostSwap16(aMessageInfo.GetPeerAddr().m16[4]), HostSwap16(aMessageInfo.GetPeerAddr().m16[5]),
+                          HostSwap16(aMessageInfo.GetPeerAddr().m16[6]), HostSwap16(aMessageInfo.GetPeerAddr().m16[7]));
+    sServer->OutputFormat(": icmp_seq=%d hlim=%d\r\n", icmp6Header.GetSequence(), aMessageInfo.mHopLimit);
 }
 
 void Interpreter::ProcessPing(int argc, char *argv[])
@@ -545,8 +540,6 @@ void Interpreter::ProcessPing(int argc, char *argv[])
 
     sIcmpEcho.SendEchoRequest(sSockAddr, sEchoRequest, length);
 
-    sResponse.Init();
-
     return;
 
 exit:
@@ -710,8 +703,8 @@ exit:
 
 void Interpreter::ProcessRloc16(int argc, char *argv[])
 {
-    sResponse.Append("%04x\r\n", otGetRloc16());
-    sResponse.Append("Done\r\n");
+    sServer->OutputFormat("%04x\r\n", otGetRloc16());
+    sServer->OutputFormat("Done\r\n");
 }
 
 ThreadError Interpreter::ProcessRouteAdd(int argc, char *argv[])
@@ -834,7 +827,7 @@ void Interpreter::ProcessRouterUpgradeThreshold(int argc, char *argv[])
 
     if (argc == 0)
     {
-        sResponse.Append("%d\r\n", otGetRouterUpgradeThreshold());
+        sServer->OutputFormat("%d\r\n", otGetRouterUpgradeThreshold());
     }
     else
     {
@@ -859,8 +852,8 @@ void Interpreter::ProcessScan(int argc, char *argv[])
     }
 
     SuccessOrExit(error = otActiveScan(scanChannels, 0, &HandleActiveScanResult));
-    sResponse.Append("| J | Network Name     | Extended PAN     | PAN  | MAC Address      | Ch | dBm |\r\n");
-    sResponse.Append("+---+------------------+------------------+------+------------------+----+-----+\r\n");
+    sServer->OutputFormat("| J | Network Name     | Extended PAN     | PAN  | MAC Address      | Ch | dBm |\r\n");
+    sServer->OutputFormat("+---+------------------+------------------+------+------------------+----+-----+\r\n");
 
     return;
 
@@ -872,46 +865,44 @@ void Interpreter::HandleActiveScanResult(otActiveScanResult *aResult)
 {
     const uint8_t *bytes;
 
-    sResponse.Init();
-
     if (aResult == NULL)
     {
-        sResponse.Append("Done\r\n");
+        sServer->OutputFormat("Done\r\n");
         ExitNow();
     }
 
-    sResponse.Append("| %d ", aResult->mIsJoinable);
+    sServer->OutputFormat("| %d ", aResult->mIsJoinable);
 
     if (aResult->mNetworkName != NULL)
     {
-        sResponse.Append("| %-16s ", aResult->mNetworkName);
+        sServer->OutputFormat("| %-16s ", aResult->mNetworkName);
     }
     else
     {
-        sResponse.Append("| ---------------- ");
+        sServer->OutputFormat("| ---------------- ");
     }
 
     if (aResult->mExtPanId != NULL)
     {
         bytes = aResult->mExtPanId;
-        sResponse.Append("| %02x%02x%02x%02x%02x%02x%02x%02x ",
-                         bytes[0], bytes[1], bytes[2], bytes[3], bytes[4], bytes[5], bytes[6], bytes[7]);
+        sServer->OutputFormat("| %02x%02x%02x%02x%02x%02x%02x%02x ",
+                              bytes[0], bytes[1], bytes[2], bytes[3], bytes[4], bytes[5], bytes[6], bytes[7]);
     }
     else
     {
-        sResponse.Append("| ---------------- ");
+        sServer->OutputFormat("| ---------------- ");
     }
 
-    sResponse.Append("| %04x ", aResult->mPanId);
+    sServer->OutputFormat("| %04x ", aResult->mPanId);
 
     bytes = aResult->mExtAddress.m8;
-    sResponse.Append("| %02x%02x%02x%02x%02x%02x%02x%02x ",
-                     bytes[0], bytes[1], bytes[2], bytes[3], bytes[4], bytes[5], bytes[6], bytes[7]);
-    sResponse.Append("| %02d ", aResult->mChannel);
-    sResponse.Append("| %03d |\r\n", aResult->mRssi);
+    sServer->OutputFormat("| %02x%02x%02x%02x%02x%02x%02x%02x ",
+                          bytes[0], bytes[1], bytes[2], bytes[3], bytes[4], bytes[5], bytes[6], bytes[7]);
+    sServer->OutputFormat("| %02d ", aResult->mChannel);
+    sServer->OutputFormat("| %03d |\r\n", aResult->mRssi);
 
 exit:
-    sServer->Output(sResponse.GetResponse(), sResponse.GetResponseLength());
+    return;
 }
 
 void Interpreter::ProcessStart(int argc, char *argv[])
@@ -933,23 +924,23 @@ void Interpreter::ProcessState(int argc, char *argv[])
         switch (otGetDeviceRole())
         {
         case kDeviceRoleDisabled:
-            sResponse.Append("disabled\r\n");
+            sServer->OutputFormat("disabled\r\n");
             break;
 
         case kDeviceRoleDetached:
-            sResponse.Append("detached\r\n");
+            sServer->OutputFormat("detached\r\n");
             break;
 
         case kDeviceRoleChild:
-            sResponse.Append("child\r\n");
+            sServer->OutputFormat("child\r\n");
             break;
 
         case kDeviceRoleRouter:
-            sResponse.Append("router\r\n");
+            sServer->OutputFormat("router\r\n");
             break;
 
         case kDeviceRoleLeader:
-            sResponse.Append("leader\r\n");
+            sServer->OutputFormat("leader\r\n");
             break;
         }
     }
@@ -1060,8 +1051,6 @@ void Interpreter::ProcessLine(char *aBuf, uint16_t aBufLength, Server &aServer)
         }
     }
 
-    sResponse.Init();
-
     for (unsigned int i = 0; i < sizeof(sCommands) / sizeof(sCommands[0]); i++)
     {
         if (strcmp(cmd, sCommands[i].mName) == 0)
@@ -1071,11 +1060,6 @@ void Interpreter::ProcessLine(char *aBuf, uint16_t aBufLength, Server &aServer)
         }
     }
 
-    if (sResponse.GetResponseLength() > 0)
-    {
-        aServer.Output(sResponse.GetResponse(), sResponse.GetResponseLength());
-    }
-
 exit:
     return;
 }
diff --git a/src/cli/cli.hpp b/src/cli/cli.hpp
index 0ff3bcd09..e27c806ca 100644
--- a/src/cli/cli.hpp
+++ b/src/cli/cli.hpp
@@ -50,55 +50,6 @@ namespace Thread {
  */
 namespace Cli {
 
-/**
- * This class implements the response buffer for CLI command results.
- *
- */
-class ResponseBuffer
-{
-public:
-    /**
-     * This method initializes the response buffer.
-     *
-     */
-    void Init(void) { mEnd = mBuffer; }
-
-    /**
-     * This method appends output according to the format string.
-     *
-     * @param[in]  fmt  A pointer to the NULL-terminated format string.
-     * @param[in]  ...  Arguments for the format specification.
-     *
-     */
-    void Append(const char *fmt, ...) {
-        va_list ap;
-        va_start(ap, fmt);
-        vsnprintf(mEnd, sizeof(mBuffer) - (mEnd - mBuffer), fmt, ap);
-        va_end(ap);
-        mEnd += strlen(mEnd);
-    }
-
-    /**
-     * This method returns a pointer to the response string.
-     *
-     * @returns A pointer to the response string.
-     *
-     */
-    const char *GetResponse(void) { return mBuffer; }
-
-    /**
-     * This method returns the length of the response string.
-     *
-     * @returns The length to the response string.
-     *
-     */
-    uint16_t GetResponseLength(void) { return mEnd - mBuffer; }
-
-private:
-    char mBuffer[512];
-    char *mEnd;
-};
-
 /**
  * This structure represents a CLI command.
  *
@@ -173,8 +124,6 @@ private:
     static ThreadError ParseLong(char *argv, long &value);
 
     static const struct Command sCommands[];
-    static ResponseBuffer sResponse;
-
     static otNetifAddress sAddress;
 
     static Ip6::SockAddr sSockAddr;
diff --git a/src/cli/cli_serial.cpp b/src/cli/cli_serial.cpp
index ff1dcf2ab..9ecb65bc8 100644
--- a/src/cli/cli_serial.cpp
+++ b/src/cli/cli_serial.cpp
@@ -31,6 +31,7 @@
  *   This file implements the CLI server on the serial service.
  */
 
+#include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -45,11 +46,12 @@
 namespace Thread {
 namespace Cli {
 
-static const uint8_t sEraseString[] = {'\b', ' ', '\b'};
-static const uint8_t CRNL[] = {'\r', '\n'};
+static const char sEraseString[] = {'\b', ' ', '\b'};
+static const char CRNL[] = {'\r', '\n'};
 static Serial *sServer;
 
 Tasklet Serial::sReceiveTask(&ReceiveTask, NULL);
+Tasklet Serial::sSendDoneTask(&SendDoneTask, NULL);
 
 Serial::Serial(void)
 {
@@ -59,14 +61,13 @@ Serial::Serial(void)
 ThreadError Serial::Start(void)
 {
     mRxLength = 0;
+    mTxHead = 0;
+    mTxLength = 0;
+    mSendLength = 0;
     otPlatSerialEnable();
     return kThreadError_None;
 }
 
-extern "C" void otPlatSerialSignalSendDone(void)
-{
-}
-
 extern "C" void otPlatSerialSignalReceive(void)
 {
     Serial::sReceiveTask.Post();
@@ -92,7 +93,7 @@ void Serial::ReceiveTask(void)
         {
         case '\r':
         case '\n':
-            otPlatSerialSend(CRNL, sizeof(CRNL));
+            Output(CRNL, sizeof(CRNL));
 
             if (mRxLength > 0)
             {
@@ -104,7 +105,7 @@ void Serial::ReceiveTask(void)
 
         case '\b':
         case 127:
-            otPlatSerialSend(sEraseString, sizeof(sEraseString));
+            Output(sEraseString, sizeof(sEraseString));
 
             if (mRxLength > 0)
             {
@@ -114,7 +115,7 @@ void Serial::ReceiveTask(void)
             break;
 
         default:
-            otPlatSerialSend(buf, 1);
+            Output(reinterpret_cast<const char *>(buf), 1);
             mRxBuffer[mRxLength++] = *buf;
             break;
         }
@@ -144,9 +145,79 @@ ThreadError Serial::ProcessCommand(void)
     return error;
 }
 
-ThreadError Serial::Output(const char *aBuf, uint16_t aBufLength)
+int Serial::Output(const char *aBuf, uint16_t aBufLength)
 {
-    return otPlatSerialSend(reinterpret_cast<const uint8_t *>(aBuf), aBufLength);
+    uint16_t remaining = kTxBufferSize - mTxLength;
+    uint16_t tail = (mTxHead + mTxLength) % kTxBufferSize;
+
+    if (aBufLength > remaining)
+    {
+        aBufLength = remaining;
+    }
+
+    for (int i = 0; i < aBufLength; i++)
+    {
+        tail = (mTxHead + mTxLength) % kTxBufferSize;
+        mTxBuffer[tail] = *aBuf++;
+        mTxLength++;
+    }
+
+    Send();
+
+    return aBufLength;
+}
+
+int Serial::OutputFormat(const char *fmt, ...)
+{
+    char buf[kMaxLineLength];
+    va_list ap;
+
+    va_start(ap, fmt);
+    vsnprintf(buf, sizeof(buf), fmt, ap);
+    va_end(ap);
+
+    return Output(buf, strlen(buf));
+}
+
+void Serial::Send(void)
+{
+    VerifyOrExit(mSendLength == 0, ;);
+
+    if (mTxHead + mTxLength > kTxBufferSize)
+    {
+        mSendLength = kTxBufferSize - mTxHead;
+    }
+    else
+    {
+        mSendLength = mTxLength;
+    }
+
+    if (mSendLength > 0)
+    {
+        otPlatSerialSend(reinterpret_cast<uint8_t *>(mTxBuffer + mTxHead), mSendLength);
+    }
+
+exit:
+    return;
+}
+
+extern "C" void otPlatSerialSignalSendDone(void)
+{
+    Serial::sSendDoneTask.Post();
+}
+
+void Serial::SendDoneTask(void *aContext)
+{
+    sServer->SendDoneTask();
+}
+
+void Serial::SendDoneTask(void)
+{
+    mTxHead = (mTxHead + mSendLength) % kTxBufferSize;
+    mTxLength -= mSendLength;
+    mSendLength = 0;
+
+    Send();
 }
 
 }  // namespace Cli
diff --git a/src/cli/cli_serial.hpp b/src/cli/cli_serial.hpp
index f31ca105e..3a76753e0 100644
--- a/src/cli/cli_serial.hpp
+++ b/src/cli/cli_serial.hpp
@@ -59,31 +59,54 @@ public:
     ThreadError Start(void);
 
     /**
-     * This method delivers output to the client.
+     * This method delivers raw characters to the client.
      *
      * @param[in]  aBuf        A pointer to a buffer.
      * @param[in]  aBufLength  Number of bytes in the buffer.
      *
-     * @retval kThreadError_None  Successfully delivered output the client.
+     * @returns The number of bytes placed in the output queue.
      *
      */
-    ThreadError Output(const char *aBuf, uint16_t aBufLength);
+    int Output(const char *aBuf, uint16_t aBufLength);
+
+    /**
+     * This method delivers formatted output to the client.
+     *
+     * @param[in]  aFmt  A pointer to the format string.
+     * @param[in]  ...   A variable list of arguments to format.
+     *
+     * @returns The number of bytes placed in the output queue.
+     *
+     */
+    int OutputFormat(const char *fmt, ...);
 
     static Tasklet sReceiveTask;
+    static Tasklet sSendDoneTask;
 
 private:
     enum
     {
         kRxBufferSize = 128,
+        kTxBufferSize = 512,
+        kMaxLineLength = 80,
     };
 
     static void ReceiveTask(void *aContext);
+    static void SendDoneTask(void *aContext);
 
     ThreadError ProcessCommand(void);
     void ReceiveTask(void);
+    void SendDoneTask(void);
+    void Send(void);
 
     char mRxBuffer[kRxBufferSize];
     uint16_t mRxLength;
+
+    char mTxBuffer[kTxBufferSize];
+    uint16_t mTxHead;
+    uint16_t mTxLength;
+
+    uint16_t mSendLength;
 };
 
 }  // namespace Cli
diff --git a/src/cli/cli_server.hpp b/src/cli/cli_server.hpp
index e0b5d6f01..b6e61322d 100644
--- a/src/cli/cli_server.hpp
+++ b/src/cli/cli_server.hpp
@@ -55,15 +55,26 @@ public:
     virtual ThreadError Start() = 0;
 
     /**
-     * This method delivers output to the client.
+     * This method delivers raw characters to the client.
      *
      * @param[in]  aBuf        A pointer to a buffer.
      * @param[in]  aBufLength  Number of bytes in the buffer.
      *
-     * @retval kThreadError_None  Successfully delivered output the client.
+     * @returns The number of bytes placed in the output queue.
      *
      */
-    virtual ThreadError Output(const char *aBuf, uint16_t aBufLength) = 0;
+    virtual int Output(const char *aBuf, uint16_t aBufLength) = 0;
+
+    /**
+     * This method delivers formatted output to the client.
+     *
+     * @param[in]  aFmt  A pointer to the format string.
+     * @param[in]  ...   A variable list of arguments to format.
+     *
+     * @returns The number of bytes placed in the output queue.
+     *
+     */
+    virtual int OutputFormat(const char *fmt, ...) = 0;
 };
 
 }  // namespace Cli
diff --git a/src/cli/cli_udp.cpp b/src/cli/cli_udp.cpp
index c7708734e..1cf32314c 100644
--- a/src/cli/cli_udp.cpp
+++ b/src/cli/cli_udp.cpp
@@ -31,6 +31,7 @@
  *   This file implements the CLI server on a UDP socket.
  */
 
+#include <stdarg.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -87,7 +88,7 @@ exit:
     {}
 }
 
-ThreadError Udp::Output(const char *aBuf, uint16_t aBufLength)
+int Udp::Output(const char *aBuf, uint16_t aBufLength)
 {
     ThreadError error = kThreadError_None;
     otMessage message;
@@ -102,9 +103,22 @@ exit:
     if (error != kThreadError_None && message != NULL)
     {
         otFreeMessage(message);
+        aBufLength = 0;
     }
 
-    return error;
+    return aBufLength;
+}
+
+int Udp::OutputFormat(const char *fmt, ...)
+{
+    char buf[kMaxLineLength];
+    va_list ap;
+
+    va_start(ap, fmt);
+    vsnprintf(buf, sizeof(buf), fmt, ap);
+    va_end(ap);
+
+    return Output(buf, strlen(buf));
 }
 
 }  // namespace Cli
diff --git a/src/cli/cli_udp.hpp b/src/cli/cli_udp.hpp
index 4e205dd5f..4dd994203 100644
--- a/src/cli/cli_udp.hpp
+++ b/src/cli/cli_udp.hpp
@@ -56,17 +56,33 @@ public:
     ThreadError Start(void);
 
     /**
-     * This method delivers output to the client.
+     * This method delivers raw characters to the client.
      *
      * @param[in]  aBuf        A pointer to a buffer.
      * @param[in]  aBufLength  Number of bytes in the buffer.
      *
-     * @retval kThreadError_None  Successfully delivered output the client.
+     * @returns The number of bytes placed in the output queue.
      *
      */
-    ThreadError Output(const char *aBuf, uint16_t aBufLength);
+    int Output(const char *aBuf, uint16_t aBufLength);
+
+    /**
+     * This method delivers formatted output to the client.
+     *
+     * @param[in]  aFmt  A pointer to the format string.
+     * @param[in]  ...   A variable list of arguments to format.
+     *
+     * @returns The number of bytes placed in the output queue.
+     *
+     */
+    int OutputFormat(const char *fmt, ...);
 
 private:
+    enum
+    {
+        kMaxLineLength = 80,
+    };
+
     static void HandleUdpReceive(void *aContext, otMessage aMessage, const otMessageInfo *aMessageInfo);
     void HandleUdpReceive(otMessage aMessage, const otMessageInfo *aMessageInfo);
 
diff --git a/tests/scripts/thread-cert/node.py b/tests/scripts/thread-cert/node.py
index 3a5b80c71..ae587d666 100755
--- a/tests/scripts/thread-cert/node.py
+++ b/tests/scripts/thread-cert/node.py
@@ -148,7 +148,7 @@ class Node:
 
     def get_key_sequence(self):
         self.send_command('keysequence')
-        i = self.pexpect.expect('(\d+)')
+        i = self.pexpect.expect('(\d+)\r\n')
         if i == 0:
             key_sequence = int(self.pexpect.match.groups()[0])
         self.pexpect.expect('Done')
@@ -205,7 +205,7 @@ class Node:
 
     def get_timeout(self):
         self.send_command('childtimeout')
-        i = self.pexpect.expect('(\d+)')
+        i = self.pexpect.expect('(\d+)\r\n')
         if i == 0:
             timeout = self.pexpect.match.groups()[0]
         self.pexpect.expect('Done')
@@ -218,7 +218,7 @@ class Node:
 
     def get_weight(self):
         self.send_command('leaderweight')
-        i = self.pexpect.expect('(\d+)')
+        i = self.pexpect.expect('(\d+)\r\n')
         if i == 0:
             weight = self.pexpect.match.groups()[0]
         self.pexpect.expect('Done')
@@ -239,7 +239,7 @@ class Node:
         self.send_command('ipaddr')
 
         while True:
-            i = self.pexpect.expect(['(\S+:\S+)', 'Done'])
+            i = self.pexpect.expect(['(\S+:\S+)\r\n', 'Done'])
             if i == 0:
                 addrs.append(self.pexpect.match.groups()[0])
             elif i == 1:
@@ -249,7 +249,7 @@ class Node:
 
     def get_context_reuse_delay(self):
         self.send_command('contextreusedelay')
-        i = self.pexpect.expect('(\d+)')
+        i = self.pexpect.expect('(\d+)\r\n')
         if i == 0:
             timeout = self.pexpect.match.groups()[0]
         self.pexpect.expect('Done')
@@ -289,7 +289,7 @@ class Node:
 
         results = []
         while True:
-            i = self.pexpect.expect(['\|\s(\S+)\s+\|\s(\S+)\s+\|\s([0-9a-fA-F]{4})\s\|\s([0-9a-fA-F]{16})\s\|\s(\d+)',
+            i = self.pexpect.expect(['\|\s(\S+)\s+\|\s(\S+)\s+\|\s([0-9a-fA-F]{4})\s\|\s([0-9a-fA-F]{16})\s\|\s(\d+)\r\n',
                                      'Done'])
             if i == 0:
                 results.append(self.pexpect.match.groups())