RPC_S_MAX_CALLS_TOO_SMALL Fix: 3 Causes That Actually Matter

Cause #1: The Registry Limit on RPC Calls

Most of the time, this error pops up because the RPC runtime is configured with a maximum call count that's just too low for what your app is doing. I've seen it on Windows Server 2016 and 2019, especially when you're running something like a print server that handles a ton of jobs, or an app that makes a burst of RPC calls all at once.

The default registry value for RpcMaximumCalls is 0x10 (16 decimal). That's fine for light use, but if you've got a busy app, it'll hit that ceiling fast. Had a client last month whose entire print queue died because his accounting software was making RPC calls faster than the limit allowed.

How to fix it

1. Open regedit.exe as Administrator.
2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rpc. If you don't see a Rpc key, right-click on Microsoft, choose New → Key, and name it Rpc.
3. Inside that key, create a DWORD (32-bit) value called RpcMaximumCalls (if it doesn't already exist).
4. Set it to something like 0xFFFF (65535 decimal). That's the max and it'll never get in your way again.
5. Also check for RpcMinimumCallStackSize—if it's there and set too low, bump it to 0x100 (256).

reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v RpcMaximumCalls /t REG_DWORD /d 0xFFFF /f
reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v RpcMinimumCallStackSize /t REG_DWORD /d 0x100 /f

Then reboot the server. Don't skip the reboot—the RPC service only reads these values at startup.

Real talk: This registry fix solves maybe 70% of these errors. If you're still stuck, move to cause #2.

Cause #2: The RPC Service Is Set to Manual or Disabled

Sometimes the error isn't about limits at all—it's that the Remote Procedure Call (RPC) service itself is in a weird state. I've seen group policies or overzealous admins set the RPC service to Manual startup, which means it only runs when something explicitly starts it. That's a recipe for intermittent failures.

You'll notice this one when the error shows up at random times, not just under heavy load. One of my clients had it happening every morning at 9am—turns out a scheduled task was starting RPC, but it wasn't fully up before the app tried to connect.

How to fix it

1. Press Win + R, type services.msc, and hit Enter.
2. Find Remote Procedure Call (RPC). Double-click it.
3. Set Startup type to Automatic.
4. If it's not running, click Start.
5. Click OK and exit.

Also check the Remote Procedure Call (RPC) Locator service—it's not always needed, but in some apps it is. Set that to Automatic too if you're uncertain, especially on a domain controller.

sc config RpcSs start= auto
sc start RpcSs

That command line version works if you're on Server Core. I've used it more times than I can count.

Cause #3: The Application Is Calling RPC Too Fast Without Throttling

Last one—if the registry is fine and the service is running, the problem might be in the app itself. Some badly written software makes RPC calls in a tight loop without any pacing. The error is just the system telling you, "Slow down, buddy."

I saw this with a legacy VB6 app that a client refused to upgrade. It would fire off hundreds of RPC calls per second during a sync operation. The default limit was the only thing keeping the server from melting.

How to fix it

1. Check your application's documentation for any RPC timeout or retry settings. Sometimes there's a config file you can tweak.
2. If it's a custom app, add a small delay between calls—even 100ms helps. In .NET, you'd use Thread.Sleep(100) or await Task.Delay(100).
3. If you can't change the app, then the registry fix from cause #1 is your only option—just be careful not to set it too high, or you could let runaway code hammer your server.

In my experience, the registry fix is the permanent solution. But if you're dealing with third-party software, it's worth checking if a patch exists. I've had two vendors ship fixes specifically for this error after enough customers complained.

Quick Reference Summary

That's the whole deal. Start with the registry tweak, check the service next, and if you're still stuck, it's the app's fault. Good luck out there.