[spec-api] XML validation treats the declaration as body content

[live1206]

Describe the bug

validateXmlBodyEquals in @typespec/spec-api@0.1.0-alpha.15 rejects semantically equivalent XML when the expected body has an XML declaration and the actual request body does not.

The validator always prepends <?xml version='1.0' encoding='UTF-8'?> to a plain expected string. Since the migration from xml2js to fast-xml-parser in #10239, XMLParser preserves that declaration as a ?xml node by default. The parsed expected value therefore differs from an otherwise identical actual request without a declaration.

This causes 25 XML request-body Spector tests to fail in Azure/azure-sdk-for-net#61046. @typespec/spec-api@0.1.0-alpha.14 does not have this regression because xml2js ignored the declaration.

Reproduction

import { validateXmlBodyEquals } from "@typespec/spec-api";

validateXmlBodyEquals(
  { rawBody: "<Root><a>1</a></Root>" },
  "<Root><a>1</a></Root>",
);

With @typespec/spec-api@0.1.0-alpha.15, this throws:

ValidationError: Body provided doesn't match expected body

The parsed values differ as follows:

const parser = new XMLParser({ parseTagValue: false });
parser.parse("<Root><a>1</a></Root>");
// { Root: { a: "1" } }

parser.parse('<?xml version="1.0" encoding="UTF-8"?><Root><a>1</a></Root>');
// { "?xml": "", Root: { a: "1" } }

Expected behavior

XML declarations should not affect semantic request-body equality. The reproduction should not throw.

Using the following parser option restores the prior behavior:

new XMLParser({ parseTagValue: false, ignoreDeclaration: true });

Suggested regression coverage

The current test covers a declaration in the actual request and no declaration in the expected string. Please also cover the inverse path that occurs in Spector: the actual request omits the declaration while validateXmlBodyEquals adds one to the expected value.

Related

• Parser migration: Dependency audit #10239
• Related but distinct entity-decoding issue: [spec-api]: XMLParser does not decode 
 by default #10350

[github-actions]

🎯 Issue Triage

validateXmlBodyEquals in @typespec/spec-api@0.1.0-alpha.15 incorrectly fails when the actual request body omits the XML declaration that the validator prepends to the expected string, because fast-xml-parser (introduced in #10239) preserves the declaration as a ?xml node unlike the previous xml2js. This causes 25 Spector XML body tests to fail in Azure SDK for .NET.

Category: Bug
Area: spector

🔍 Analysis

• Root cause: validateXmlBodyEquals always prepends <?xml version='1.0' encoding='UTF-8'?> to the expected string. After migration to fast-xml-parser, the parser preserves this declaration as a ?xml key in the parsed object, whereas xml2js silently ignored it. When the actual request body has no declaration, the parsed objects differ ({ "?xml": "", Root: {...} } vs { Root: {...} }).
• Fix: Pass ignoreDeclaration: true to XMLParser so declarations are stripped before comparison, restoring prior behavior.
• Impact: Regression introduced in @typespec/spec-api@0.1.0-alpha.15, affecting 25 XML request-body Spector tests.
• Related: Parser migration Dependency audit #10239, entity-decoding issue [spec-api]: XMLParser does not decode 
 by default #10350.

📝 Suggested next steps

• Add ignoreDeclaration: true to the XMLParser options in validateXmlBodyEquals
• Add regression test: actual body without declaration, expected string without declaration (the current inverse of the existing test)

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