The XSLT Debugging Problem for Logic Apps Developers

Azure Logic Apps uses XSLT for XML transformations. While the Azure Data Mapper allows basic testing, it lacks true debugging capabilities — no message capture, no variable inspection, no execution tracing.

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XSLT's Debug Tool: xsl:message

XSLT provides xsl:message for diagnostic logging:

<xsl:message>Processing started</xsl:message>
<xsl:message select="'Found ' || count(//book) || ' books'" />

Messages output to a separate stream without affecting transformation results. However, different processors handle it differently:

.NET (XslCompiledTransform)

transform.XsltMessageEncountered += (sender, e) => {
    Console.WriteLine($"[XSLT] {e.Message}");
};

• XSLT 1.0 only
• Event-based capture
• Silent if no handler registered

📘 Microsoft Docs

Saxon (XSLT 2.0/3.0)

transformer.setMessageListener((content, terminate, location) -> {
    System.out.println("[XSLT] " + content);
});

• XSLT 2.0/3.0 support
• Listener interface
• Includes location info

📘 Saxonica Docs

Feature	.NET	Saxon
XSLT Version	1.0	2.0 / 3.0
Location Info	❌	✅
select Attribute	❌	✅

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Existing Tools: Azure Data Mapper

Azure provides the Data Mapper for testing XSLT transformations:

📘 Logic Apps Data Mapper - Compiling and Testing

What it does:

• Run transformations locally
• View output results

What it doesn't do:

• No xsl:message capture — Can't view debug logs
• No variable inspection — Can't see intermediate values
• No execution tracing — Can't track template calls or logic flow
• Limited error context — Basic error messages without detailed diagnostics

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The Debugging Gap

What's still missing for effective XSLT debugging:

1. Message capture — Can't view xsl:message output
2. Variable inspection — Can't see values during execution
3. Execution tracing — Can't understand transformation flow
4. Rich error diagnostics — Need detailed context and line numbers

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What's Needed

A proper XSLT debugger for Logic Apps should:

• Run transformations locally in VS Code
• Capture and display xsl:message output
• Support both .NET and Saxon processors
• Provide fast edit-test cycles
• Show clear errors with line numbers

This is what the XSLT Debugger extension delivers — built on .NET with open-source Saxon to provide debugging capabilities for Logic Apps developers without additional licensing costs.