diff --git a/core/PlayerManager.cpp b/core/PlayerManager.cpp
index c48a38ff3..67a51b5a8 100644
--- a/core/PlayerManager.cpp
+++ b/core/PlayerManager.cpp
@@ -251,6 +251,8 @@ void PlayerManager::OnServerActivate(edict_t *pEdictList, int edictCount, int cl
 		memset(m_AuthQueue, 0, sizeof(unsigned int) * (ABSOLUTE_PLAYER_LIMIT + 1));
 
 		g_NumPlayersToAuth = &m_AuthQueue[0];
+
+		g_PluginSys.SyncMaxClients(clientMax);
 	}
 	g_Extensions.CallOnCoreMapStart(pEdictList, edictCount, clientMax);
 	m_onActivate->Execute(NULL);
diff --git a/core/smn_core.cpp b/core/smn_core.cpp
index 84692bca7..e75ae368e 100644
--- a/core/smn_core.cpp
+++ b/core/smn_core.cpp
@@ -622,6 +622,11 @@ static cell_t FindPluginByNumber(IPluginContext *pContext, const cell_t *params)
 	return pPlugin->GetMyHandle();
 }
 
+static cell_t VerifyCoreVersion(IPluginContext *pContext, const cell_t *params)
+{
+	return 4;
+}
+
 REGISTER_NATIVES(coreNatives)
 {
 	{"AutoExecConfig",			AutoExecConfig},
@@ -645,6 +650,7 @@ REGISTER_NATIVES(coreNatives)
 	{"LogToFileEx",				LogToFileEx},
 	{"GetExtensionFileStatus",	GetExtensionFileStatus},
 	{"FindPluginByNumber",		FindPluginByNumber},
+	{"VerifyCoreVersion",		VerifyCoreVersion},
 	{NULL,						NULL},
 };
 
diff --git a/core/systems/PluginSys.cpp b/core/systems/PluginSys.cpp
index 6fb2ca907..091a71c8f 100644
--- a/core/systems/PluginSys.cpp
+++ b/core/systems/PluginSys.cpp
@@ -68,6 +68,7 @@ CPlugin::CPlugin(const char *file)
 	m_LibraryMissing = false;
 	m_bGotAllLoaded = false;
 	m_pPhrases = g_Translator.CreatePhraseCollection();
+	m_MaxClientsVar = NULL;
 }
 
 CPlugin::~CPlugin()
@@ -218,7 +219,7 @@ void CPlugin::SetErrorState(PluginStatus status, const char *error_fmt, ...)
 	}
 }
 
-void CPlugin::UpdateInfo()
+bool CPlugin::UpdateInfo()
 {
 	/* Now grab the info */
 	uint32_t idx;
@@ -265,7 +266,7 @@ void CPlugin::UpdateInfo()
 		};
 		__version_info *info;
 		cell_t local_addr;
-		const char *pDate, *pTime;
+		const char *pDate, *pTime, *pFileVers;
 
 		pDate = "";
 		pTime = "";
@@ -278,11 +279,34 @@ void CPlugin::UpdateInfo()
 			base->LocalToString(info->time, (char **)&pTime);
 			UTIL_Format(m_DateTime, sizeof(m_DateTime), "%s %s", pDate, pTime);
 		}
+		if (m_FileVersion > 4)
+		{
+			base->LocalToString(info->filevers, (char **)&pFileVers);
+			SetErrorState(Plugin_Failed, "Newer SourceMod required (%s or higher)", pFileVers);
+			return false;
+		}
 	}
 	else
 	{
 		m_FileVersion = 0;
 	}
+
+	if ((err = base->FindPubvarByName("MaxClients", &idx)) == SP_ERROR_NONE)
+	{
+		base->GetPubvarByIndex(idx, &m_MaxClientsVar);
+	}
+
+	return true;
+}
+
+void CPlugin::SyncMaxClients(int max_clients)
+{
+	if (m_MaxClientsVar == NULL)
+	{
+		return;
+	}
+
+	*m_MaxClientsVar->offs = max_clients;
 }
 
 void CPlugin::Call_OnPluginStart()
@@ -294,6 +318,8 @@ void CPlugin::Call_OnPluginStart()
 
 	m_status = Plugin_Running;
 
+	SyncMaxClients(g_Players.MaxClients());
+
 	cell_t result;
 	IPluginFunction *pFunction = m_pRuntime->GetFunctionByName("OnPluginStart");
 	if (!pFunction)
@@ -427,6 +453,10 @@ PluginType CPlugin::GetType()
 
 const sm_plugininfo_t *CPlugin::GetPublicInfo()
 {
+	if (GetStatus() >= Plugin_Created)
+	{
+		return NULL;
+	}
 	return &m_info;
 }
 
@@ -959,11 +989,18 @@ LoadRes CPluginManager::_LoadPlugin(CPlugin **_plugin, const char *path, bool de
 				"Unable to load plugin (error %d: %s)", 
 				err, 
 				g_pSourcePawn2->GetErrorString(err));
+			pPlugin->m_status = Plugin_BadLoad;
 		}
 		else
 		{
-			pPlugin->UpdateInfo();
-			pPlugin->m_status = Plugin_Created;
+			if (pPlugin->UpdateInfo())
+			{
+				pPlugin->m_status = Plugin_Created;
+			}
+			else
+			{
+				UTIL_Format(error, maxlength, "%s", pPlugin->m_errormsg);
+			}
 		}
 	}
 
@@ -1052,10 +1089,13 @@ void CPluginManager::LoadAutoPlugin(const char *plugin)
 	if ((res=_LoadPlugin(&pl, plugin, false, PluginType_MapUpdated, error, sizeof(error))) == LoadRes_Failure)
 	{
 		g_Logger.LogError("[SM] Failed to load plugin \"%s\": %s", plugin, error);
-		pl->SetErrorState(Plugin_Failed, "%s", error);
+		pl->SetErrorState(
+			pl->GetStatus() == Plugin_BadLoad ? Plugin_BadLoad : Plugin_Failed, 
+			"%s",
+			error);
 	}
 
-	if (res == LoadRes_Successful)
+	if (res == LoadRes_Successful || res == LoadRes_Failure)
 	{
 		AddPlugin(pl);
 	}
@@ -1458,7 +1498,7 @@ bool CPluginManager::UnloadPlugin(IPlugin *plugin)
 	}
 
 	IPluginContext *pContext = plugin->GetBaseContext();
-	if (pContext->IsInExec())
+	if (pContext != NULL && pContext->IsInExec())
 	{
 		char buffer[255];
 		UTIL_Format(buffer, sizeof(buffer), "sm plugins unload %s\n", plugin->GetFilename());
@@ -1925,14 +1965,21 @@ void CPluginManager::OnRootConsoleCommand(const char *cmdname, const CCommand &c
 				{
 					len += UTIL_Format(buffer, sizeof(buffer), "  %02d", id);
 				}
-				len += UTIL_Format(&buffer[len], sizeof(buffer)-len, " \"%s\"", (IS_STR_FILLED(info->name)) ? info->name : pl->GetFilename());
-				if (IS_STR_FILLED(info->version))
+				if (pl->GetStatus() < Plugin_Created)
 				{
-					len += UTIL_Format(&buffer[len], sizeof(buffer)-len, " (%s)", info->version);
+					len += UTIL_Format(&buffer[len], sizeof(buffer)-len, " \"%s\"", (IS_STR_FILLED(info->name)) ? info->name : pl->GetFilename());
+					if (IS_STR_FILLED(info->version))
+					{
+						len += UTIL_Format(&buffer[len], sizeof(buffer)-len, " (%s)", info->version);
+					}
+					if (IS_STR_FILLED(info->author))
+					{
+						UTIL_Format(&buffer[len], sizeof(buffer)-len, " by %s", info->author);
+					}
 				}
-				if (IS_STR_FILLED(info->author))
+				else
 				{
-					UTIL_Format(&buffer[len], sizeof(buffer)-len, " by %s", info->author);
+					UTIL_Format(&buffer[len], sizeof(buffer)-len, " %s", pl->m_filename);
 				}
 				g_RootMenu.ConsolePrint("%s", buffer);
 			}
@@ -2025,8 +2072,16 @@ void CPluginManager::OnRootConsoleCommand(const char *cmdname, const CCommand &c
 			}
 
 			char name[PLATFORM_MAX_PATH];
-			const sm_plugininfo_t *info = pl->GetPublicInfo();
-			strcpy(name, (IS_STR_FILLED(info->name)) ? info->name : pl->GetFilename());
+
+			if (pl->GetStatus() < Plugin_Created)
+			{
+				const sm_plugininfo_t *info = pl->GetPublicInfo();
+				UTIL_Format(name, sizeof(name), (IS_STR_FILLED(info->name)) ? info->name : pl->GetFilename());
+			}
+			else
+			{
+				UTIL_Format(name, sizeof(name), "%s", pl->GetFilename());
+			}
 
 			if (UnloadPlugin(pl))
 			{
@@ -2172,12 +2227,15 @@ void CPluginManager::OnRootConsoleCommand(const char *cmdname, const CCommand &c
 			else
 			{
 				g_RootMenu.ConsolePrint("  Load error: %s", pl->m_errormsg);
-				g_RootMenu.ConsolePrint("  File info: (title \"%s\") (version \"%s\")", 
-										info->name ? info->name : "<none>",
-										info->version ? info->version : "<none>");
-				if (IS_STR_FILLED(info->url))
+				if (pl->GetStatus() < Plugin_Created)
 				{
-					g_RootMenu.ConsolePrint("  File URL: %s", info->url);
+					g_RootMenu.ConsolePrint("  File info: (title \"%s\") (version \"%s\")", 
+											info->name ? info->name : "<none>",
+											info->version ? info->version : "<none>");
+					if (IS_STR_FILLED(info->url))
+					{
+						g_RootMenu.ConsolePrint("  File URL: %s", info->url);
+					}
 				}
 			}
 
@@ -2551,3 +2609,18 @@ CPlugin *CPluginManager::FindPluginByConsoleArg(const char *arg)
 
 	return pl;
 }
+
+void CPluginManager::OnSourceModMaxPlayersChanged(int newvalue)
+{
+	SyncMaxClients(newvalue);
+}
+
+void CPluginManager::SyncMaxClients(int max_clients)
+{
+	List<CPlugin *>::iterator iter;
+
+	for (iter = m_plugins.begin(); iter != m_plugins.end(); iter++)
+	{
+		(*iter)->SyncMaxClients(max_clients);
+	}
+}
diff --git a/core/systems/PluginSys.h b/core/systems/PluginSys.h
index ea662b393..aa29ffaae 100644
--- a/core/systems/PluginSys.h
+++ b/core/systems/PluginSys.h
@@ -234,8 +234,9 @@ public:
 		m_Libraries.push_back(name);
 	}
 	void LibraryActions(bool dropping);
+	void SyncMaxClients(int max_clients);
 protected:
-	void UpdateInfo();
+	bool UpdateInfo();
 	void SetTimeStamp(time_t t);
 	void DependencyDropped(CPlugin *pOwner);
 private:
@@ -261,6 +262,7 @@ private:
 	char m_DateTime[256];
 	IPluginRuntime *m_pRuntime;
 	IPluginContext *m_pContext;
+	sp_pubvar_t *m_MaxClientsVar;
 };
 
 class CPluginManager : 
@@ -307,6 +309,7 @@ public: //IPluginManager
 public: //SMGlobalClass
 	void OnSourceModAllInitialized();
 	void OnSourceModShutdown();
+	void OnSourceModMaxPlayersChanged(int newvalue);
 public: //IHandleTypeDispatch
 	void OnHandleDestroy(HandleType_t type, void *object);
 	bool GetHandleApproxSize(HandleType_t type, void *object, unsigned int *pSize);
@@ -390,6 +393,8 @@ public:
 	void UnloadAll();
 
 	CPlugin *FindPluginByConsoleArg(const char *arg);
+
+	void SyncMaxClients(int max_clients);
 private:
 	LoadRes _LoadPlugin(CPlugin **pPlugin, const char *path, bool debug, PluginType type, char error[], size_t maxlength);
 
diff --git a/plugins/basechat.sp b/plugins/basechat.sp
index 3d968352b..307e1f3a2 100644
--- a/plugins/basechat.sp
+++ b/plugins/basechat.sp
@@ -237,8 +237,7 @@ public Action:Command_SmHsay(client, args)
  
 	decl String:nameBuf[MAX_NAME_LENGTH];
 	
-	new maxClients = GetMaxClients();
-	for (new i = 1; i <= maxClients; i++)
+	for (new i = 1; i <= MaxClients; i++)
 	{
 		if (!IsClientConnected(i) || IsFakeClient(i))
 		{
@@ -270,7 +269,6 @@ public Action:Command_SmTsay(client, args)
 	GetClientName(client, name, sizeof(name));
 		
 	new color = FindColor(colorStr);
-	new maxClients = GetMaxClients();
 	new String:nameBuf[MAX_NAME_LENGTH];
 	
 	if (color == -1)
@@ -279,7 +277,7 @@ public Action:Command_SmTsay(client, args)
 		len = 0;
 	}
 	
-	for (new i = 1; i <= maxClients; i++)
+	for (new i = 1; i <= MaxClients; i++)
 	{
 		if (!IsClientConnected(i) || IsFakeClient(i))
 		{
@@ -407,11 +405,9 @@ FindColor(String:color[])
 
 SendChatToAll(client, String:message[])
 {
-	new maxClients;
 	new String:nameBuf[MAX_NAME_LENGTH];
 	
-	maxClients = GetMaxClients();
-	for (new i = 1; i <= maxClients; i++)
+	for (new i = 1; i <= MaxClients; i++)
 	{
 		if (!IsClientConnected(i) || IsFakeClient(i))
 		{
@@ -433,9 +429,8 @@ SendChatToAll(client, String:message[])
 DisplayCenterTextToAll(client, String:message[])
 {
 	new String:nameBuf[MAX_NAME_LENGTH];
-	new maxClients = GetMaxClients();
 	
-	for (new i = 1; i < maxClients; i++)
+	for (new i = 1; i < MaxClients; i++)
 	{
 		if (!IsClientConnected(i) || IsFakeClient(i))
 		{
@@ -448,9 +443,7 @@ DisplayCenterTextToAll(client, String:message[])
 
 SendChatToAdmins(String:name[], String:message[])
 {
-	new iMaxClients = GetMaxClients();
-	
-	for (new i = 1; i <= iMaxClients; i++)
+	for (new i = 1; i <= MaxClients; i++)
 	{
 		if (IsClientInGame(i))
 		{
@@ -500,8 +493,7 @@ SendPanelToAll(String:name[], String:message[])
 	SetPanelCurrentKey(mSayPanel, 10);
 	DrawPanelItem(mSayPanel, "Exit", ITEMDRAW_CONTROL);
 
-	new MaxClients = GetMaxClients();
-	for(new i = 1; i < MaxClients; i++)
+	for(new i = 1; i <= MaxClients; i++)
 	{
 		if(IsClientInGame(i) && !IsFakeClient(i))
 		{
diff --git a/plugins/include/clients.inc b/plugins/include/clients.inc
index 01ba4b7b9..880eb0b0b 100644
--- a/plugins/include/clients.inc
+++ b/plugins/include/clients.inc
@@ -45,9 +45,19 @@ enum NetFlow
 	NetFlow_Both,			/**< Both values added together */
 };
 
-#define MAXPLAYERS		64	/**< Maximum number of players that can be in server */
+/**
+ * MAXPLAYERS is not the same as MaxClients.
+ * MAXPLAYERS is a hardcoded value as an upper limit.  MaxClients changes based on the server.
+ *
+ * Both GetMaxClients() and MaxClients are only available once the map is loaded, and should 
+ * not be used in OnPluginStart().
+ */
+
+#define MAXPLAYERS		64	/**< Maximum number of players SourceMod supports */
 #define MAX_NAME_LENGTH 32	/**< Maximum buffer required to store a client name */
 
+public const MaxClients;	/**< Maximum number of players the server supports (dynamic) */
+
 /**
  * Called on client connection.
  *
@@ -169,10 +179,16 @@ forward OnClientPostAdminFilter(client);
 forward OnClientPostAdminCheck(client);
 
 /**
+ * This function will be deprecated in a future release.  Use the MaxClients variable instead.
+ *
  * Returns the maximum number of clients allowed on the server.  This may 
  * return 0 if called before OnMapStart(), and thus should not be called 
  * in OnPluginStart().  
  *
+ * You should not globally cache the value to GetMaxClients() because it can change from 
+ * SourceTV or TF2's arena mode.  Use the "MaxClients" dynamic variable documented at the 
+ * top of this file.
+ *
  * @return				Maximum number of clients allowed.
  */
 native GetMaxClients();
diff --git a/plugins/include/core.inc b/plugins/include/core.inc
index 5ad7bcb5e..787cddb20 100644
--- a/plugins/include/core.inc
+++ b/plugins/include/core.inc
@@ -37,7 +37,9 @@
 
 #include <version>
 
-#define SOURCEMOD_PLUGINAPI_VERSION		3
+/** If this gets changed, you need to update Core's check. */
+#define SOURCEMOD_PLUGINAPI_VERSION		4
+
 struct PlVers
 {
 	version,
@@ -136,6 +138,25 @@ struct SharedPlugin
 public Float:NULL_VECTOR[3];		/**< Pass this into certain functions to act as a C++ NULL */
 public const String:NULL_STRING[1];	/**< pass this into certain functions to act as a C++ NULL */
 
+/**
+ * Horrible compatibility shim.
+ */
+public Extension:__ext_core = 
+{
+	name = "Core",
+	file = "core",
+	autoload = 0,
+	required = 0,
+};
+
+native VerifyCoreVersion();
+
+public __ext_core_SetNTVOptional()
+{
+	VerifyCoreVersion();
+}
+
+
 #define AUTOLOAD_EXTENSIONS
 #define REQUIRE_EXTENSIONS
 #define REQUIRE_PLUGIN