If you're using a custom business process to handle the message, you could perform the truncate in an OnRequest() callback.

If you're using a BPL, you would add an action near the beginning of the process to perform the truncate. The request object will be a single batch object containing a collection of records; the next step would be to iterate over the collection and perform the inserts.

You could do essentially the same thing in a DTL; use an SQL rule at the beginning to truncate, then add a foreach to iterate over the batch and populate the DB.

The key is the fact that your BP is getting all of the records in a single object. So when that object arrives, it's time to truncate the table and repopulate it. You will have to skip the first record if it contains headers, of course.

With IRIS 2025.1.1 on Windows, I cannot reproduce your issue. Both sets of instructions execute normally. 

VS Code Version: 1.105.1 (user setup)
Commit: 7d842fb85a0275a4a8e4d7e040d2625abbf7f084
Date: 2025-10-14T22:33:36.618Z
Electron: 37.6.0
ElectronBuildId: 12502201
Chromium: 138.0.7204.251
Node.js: 22.19.0
V8: 13.8.258.32-electron.0
OS: Windows_NT x64 10.0.19045

ISC ObjectScript Extension 3.2.1-beta.1

ISC Server Manager 3.10.6-beta.3

ISC Language Server 2.8.0

You are correct in that ReplyCodeAction E* does not evaluate the contents of the ACK message MSA segment; it only looks at the IRIS error code and text.

For your specific example, though, I'm wondering why you're not simply setting a condition to exclude invalid messages in the Routing Rule?