Add warning when bytes body is used with XML content type

[Copilot]

When bytes is used as a body type with an XML content type (e.g. application/xml), the payload is silently treated as raw binary data rather than structured XML. This adds a warning diagnostic at the @typespec/http level so all downstream emitters (OpenAPI3, AutoRest, etc.) surface the issue.

// Now emits @typespec/http/bytes-xml-body warning
@post op upload(@header contentType: "application/xml", @body body: bytes): void;

• packages/http/src/lib.ts — New bytes-xml-body warning diagnostic
• packages/http/src/payload.ts — Detection in resolveHttpPayload() after body resolution; matches XML content types via regex (application/xml, text/xml, application/*+xml); walks scalar inheritance chain to catch types extending bytes
• packages/http/test/parameters.test.ts — Tests for application/xml, text/xml, application/soap+xml, response bodies, and negative case (application/octet-stream)

Original prompt

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This section details on the original issue you should resolve

<issue_title>application/xml bytes treated as binary payload</issue_title>
<issue_description>Following the rule those bytes should be serialized as xml. Which on a different note not sure what it means

Playground Link

bug in typespec-autorest too</issue_description>

Comments on the Issue (you are @copilot in this section)

@markcowl - add a warning when a spec uses this pattern

• Fixes application/xml bytes treated as binary payload #9503

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npm i https://pkg.pr.new/@typespec/http@9972

commit: 7625812

[github-actions]

All changed packages have been documented.

• ✅ @typespec/http

Show changes

@typespec/http - feature ✏️

Add warning diagnostic when bytes is used as a body type with an XML content type (e.g. application/xml). The payload will be treated as raw binary data; use a model type for structured XML serialization.

[timotheeguerin]

I do wonder if this is a little to specific to be both in http library and not just a linting rule. Feel like the same should be true if you try to do that with json or any other structured media no?

[azure-sdk]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension

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