Fix RPC_NT_BYTE_COUNT_TOO_SMALL (0XC003000B)

You're staring at 0XC003000B and the service that was fine yesterday is throwing RPC_NT_BYTE_COUNT_TOO_SMALL every time it tries to talk to the server. That's annoying, but the fix is usually fast once you know what the RPC runtime is actually complaining about.

What To Do First

Stop the failing client service, then walk through this. Don't skip.

Open an elevated Command Prompt on the client that throws the error.
Run sc query rpcss. You should see STATE: 4 RUNNING. If it's stopped, run net start rpcss and try your app again.
Check the RPC event log. Open Event Viewer, go to Applications and Services Logs → Microsoft → Windows → RPC → Operational. Look for the last event before the failure. You'll usually see the interface UUID and the byte count the client sent vs. what the server expected.
Compare the interface UUID and version between client and server. Mismatched stub versions are the number one cause. On the server, run dcomcnfg, expand Component Services → Computers → My Computer → DCOM Config, find your app, right-click → Properties → Endpoints. Note the endpoints.
On the client, run reg query HKLM\SOFTWARE\Microsoft\Rpc /v MaxRpcSize. If MaxRpcSize is set below 5840, raise it. A common working value is 65536.
If you're on Windows Server 2016 or 2019 and you use RPC over HTTP, check HKLM\SOFTWARE\Microsoft\Rpc\Internet. The value MaxRpcSize here controls the HTTP transport buffer. Set it to 65536 and reboot.
Restart the client app. Watch the RPC Operational log. You should see event ID 5 (RPC call started) followed by event ID 6 (RPC call completed). If event 3 (RPC call failed) shows up again with the same byte count, the client is still sending old stub data.

If step 4 shows mismatched versions, recompile the client with the exact IDL/stub the server is running. There's no registry trick that fixes a version mismatch.

Real-world trigger: this bites hardest after a mid-week patch. Server patches Tuesday, client patches Thursday. Between those two days, a client with the old stub hits a server with the new stub, and the new stub expects a larger payload. Byte count too small, every call.

Why That Worked

RPC marshals data into a buffer before it goes on the wire. The NDR (Network Data Representation) engine writes the byte count into the header. When the receiving side unmarshals, it reads that header, checks the declared size against what the interface expects, and if the declared size is smaller, it bails with STATUS_RPC_NT_BYTE_COUNT_TOO_SMALL. The RPC runtime won't guess. It won't pad. It rejects the call outright because accepting a short buffer is how you get buffer overreads and remote code execution.

Raising MaxRpcSize clears the limit, but the real fix is almost always matching the stub. The MaxRpcSize value exists because older Windows builds defaulted to 5840 bytes to keep UDP RPC fragments small. Modern apps that pass larger structures over TCP or HTTP still hit that ceiling if someone set it manually. Bumping it to 65536 gives the marshaller room. It doesn't change the protocol, just the cap.

Less Common Causes

Custom MIDL stubs with a fixed `size_is`

If you wrote your own IDL, check the size_is and length_is attributes. A common bug is declaring [size_is(100)] byte buffer but the caller passes a 16-byte array. The marshaller sends 16 bytes. The unmarshaller expects 100. You get 0XC003000B. Fix the IDL and recompile both sides.

Third-party firewall doing RPC inspection

Some deep-packet inspection firewalls rewrite the RPC PDU header. Check the firewall log for the specific client IP around the failure timestamp. If you see the packet get truncated, disable RPC inspection on that rule. Palo Alto and Fortinet both have KB articles on this. The tell is that it fails only when the payload crosses a certain size, usually around 4 KB or 8 KB.

RPC over HTTP through a reverse proxy

IIS with the RPC Proxy role has its own request size limits. If you're tunneling RPC over HTTP through IIS, edit applicationHost.config and raise maxRequestBytes and maxAllowedContentLength on the RPC site. Default is 30000000 for content length, but some templates set it lower.

Corrupted RPC runtime registration

Rare, but it happens after a botched in-place upgrade. Run sfc /scannow. If that finds nothing, run dism /online /cleanup-image /restorehealth. Then net stop rpcss && net start rpcss. Reboot and retest. This one's a last resort. Try it only after the stub and size checks come back clean.

Prevention

Deploy client and server RPC stubs together. Same build, same patch level, same day. Split rollouts are how this error gets born.
Version your interfaces. Bump the major version in the IDL whenever you change the wire format. The RPC runtime will reject mismatches with a cleaner error than 0XC003000B.
Monitor the RPC Operational log with a simple alert on event ID 3. You'll catch version drift the same hour, not the same week.
Document your MaxRpcSize values per server. If someone sets it to 1024 during a troubleshooting session and forgets to change it back, the next app that sends a 2 KB struct will fall over.
Test your RPC calls with the largest payload you expect in production. A 16-byte test call will pass while a 4 KB call fails. Size matters here.

Once you've matched the stubs and cleared the size caps, 0XC003000B stops showing up. If it comes back, the RPC Operational log will tell you exactly which interface and which byte count, and that's the only clue you need.