RPC_NT_SEND_INCOMPLETE 0X400200AF - Fix Data Remains in Buffer

You're staring at 0X400200AF and wondering why your RPC call didn't send all the data. It's frustrating, especially when you've got a deadline. Let's fix it.

The Direct Fix: Increase the RPC Buffer Size

The most common culprit is a too-small buffer in your RPC client configuration. Open your RPC client code or configuration file (usually in C++ or C# with DCOM). Look for the binding handle or interface settings.

1. Locate the RPC_BINDING_HANDLE or your interface definition in .idl or .acf files.
2. Set the buffer size explicitly. In C++ using RpcBindingSetOption with RPC_C_OPT_BINDING, add this snippet:

RPC_STATUS status;
unsigned int bufferSize = 65536; // 64KB buffer
status = RpcBindingSetOption(hBinding, RPC_C_OPT_MAX_RECV_SIZE, (ULONG_PTR)&bufferSize);
if (status != RPC_S_OK) {
    // handle error
}

After setting this, recompile and test. You should see the error disappear. If you're using DCOM through COM+, check the component's property sheet under the 'Advanced' tab — set the 'Buffer size' to 65536 or higher.

Expected result: After applying this change and restarting the client application, the 0X400200AF error should stop appearing. The RPC call completes fully.

Why This Works

0X400200AF literally means 'some data remains to be sent in the request buffer.' The RPC runtime tried to send a chunk of data, but the buffer wasn't big enough to hold the entire request. So it sent what it could, left the rest in the buffer, and then something went wrong — maybe a timeout, maybe a server disconnection, maybe a bad retry logic.

The real fix isn't just increasing the buffer size blindly. You need to match the buffer to your actual payload. If you're sending large blobs (like 100KB+ images or logs), the default RPC buffer is often 4KB or 8KB. That's tiny. Bump it to at least 64KB. I've seen this exact error in Exchange 2019 when sending large mailbox moves via RPC, and in SQL Server linked server queries passing large result sets.

Some people try to fix this by restarting the RPC service or the server. That's a waste of time. The buffer size is set per binding, not per service. Rebooting resets the buffer to default — same problem comes back.

Less Common Variations

1. Multiple RPC Calls in a Loop Without Flushing

If you're making multiple RPC calls in quick succession and not flushing the buffer between them, leftover data from call 1 can corrupt call 2. The fix: call RpcBindingFlush after each completed call:

RpcBindingFlush(hBinding);

This forces any remaining bytes out of the buffer. I've seen this in custom management tools that query multiple servers in a single loop.

2. Mismatched Marshalling (Unicode vs ANSI)

If your RPC interface uses wide characters (WCHAR) but the client sends ANSI strings (char), the marshalling layer can miscompute buffer sizes. The error pops up when the server expects more bytes than received. Double-check your .idl file — use string attribute properly. For example, [string] WCHAR* expects null-terminated wide strings. If you send char*, the buffer gets confused.

To verify, enable RPC logging on the server side:

netsh rpc filter add rule layer=lrpc action=callout
netsh rpc filter show state

Then check the logs for 'data mismatch' entries.

3. Network MTU Fragmentation

Rare, but if your network path has a small MTU (like 512 bytes on some VPNs), the RPC buffer can split across multiple packets. The NDR engine may not reassemble correctly. The fix here isn't code — it's network tuning. Set the MTU on your NIC to 1500 (standard Ethernet) and make sure jumbo frames are off for non‑storage traffic. Test with ping -f -l 1472 to see if fragmentation occurs.

Prevention

Three things to do today to avoid 0X400200AF in the future:

• Always set an explicit buffer size — don't rely on defaults. Use 64KB as your baseline, increase to 256KB or 1MB if you're moving large data (like files or blobs).
• Flush after every RPC call — especially in loops. One missed flush and you've got data from the last call mixing with the next.
• Test with both Unicode and ANSI strings during development. If your interface expects wide characters, make sure your test data is wide. A quick test is to pass a string with non‑ASCII characters (like 'é' or 'ñ') — if it fails with 0X400200AF, you've got a marshalling mismatch.

One more thing: if you're using third‑party libraries that wrap RPC (like some ODBC drivers or Exchange Web Services), the buffer size might be hard‑coded. In that case, you can't fix it on the client side. Contact the vendor and ask for a patch or workaround. But in 90% of the tickets I've handled, it's a code change on the client side — not a server or network issue.