fix: import OpenAPI 3.1/3.2 contentEncoding base64 as bytes with @encode decorator

[Copilot]

• Analyze the issue: OpenAPI 3.1/3.2 schemas with contentEncoding: base64 should be imported as bytes type with @encode("base64", string) decorator
• Understand the codebase: The import/convert functionality is in packages/openapi3/src/cli/actions/convert/
• Modify generate-types.ts (getStringType function) to handle contentEncoding: base64 and return bytes type
• Modify decorators.ts (getStringSchemaDecorators function) to add @encode("base64", string) decorator when contentEncoding: base64
• Add tests to verify the fix
• Run format and lint commands
• Add changelog entry
• Run code review and address feedback
• Manually verify the generated TypeSpec output is correct
• Fix TypeScript build error in data-types.test.ts

Summary

This PR fixes the import of OpenAPI 3.1/3.2 schemas with contentEncoding: base64.

Before this fix:

# OpenAPI 3.1/3.2
properties:
  b64_json:
    type: string
    contentEncoding: base64

Was incorrectly imported as:

model Foo {
  b64_json?: string;  // Wrong!
}

After this fix:

model Foo {
  @encode("base64", string) b64_json?: bytes;  // Correct!
}

Security Summary

The changes are minimal and focused on type detection during import. No security vulnerabilities were introduced.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>import - base64 content encoding should be imported as bytes</issue_title>
<issue_description>### Clear and concise description of the problem

The following TypeSpec definition

model Foo {
  @encode("base64", string)
  b64_json?: bytes;
}

Leads to the follow OpenAPI 3.1/3.2 descriptions

openapi: 3.2.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths: {}
components:
  schemas:
    Foo:
      type: object
      properties:
        b64_json:
          type: string
          contentEncoding: base64

And the following OpenAPI 3.0 description

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths: {}
components:
  schemas:
    Foo:
      type: object
      properties:
        b64_json:
          type: string
          format: base64

But importing those to TypeSpec leads to the following result in 3.1/3.2

model Foo {
  b64_json?: string;
}

And this result in 3.0

model Foo {
  @format("base64") b64_json?: string;
}

The import result looks correct for OpenAPI 3.0 due to the lack of support for contentEncoding. However the import in 3.1/3.2 should lead to a bytes type and an encode decorator.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.</issue_description>

<agent_instructions>make sure you run the format and lint commands before each commit.
use conventional commits.
run npx @chronus/chronus add to add a bugfix changelog entry when done with the changes.</agent_instructions>

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

• Fixes import - base64 content encoding should be imported as bytes #9150

---

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[baywet]

@copilot I'm getting the following error when building, fix it.

test/tsp-openapi3/data-types.test.ts(83,11): error TS2353: Object literal may only specify known properties, and 'contentEncoding' does not exist in type 'Refable<OpenAPI3Schema>'

[Copilot]

@copilot I'm getting the following error when building, fix it.

...

Fixed by casting the test schema to any in commit 1ac932b. The contentEncoding property is only available in OpenAPI 3.1+ schemas (OpenAPISchema3_1), but the test utility uses OpenAPI3Schema type which is for OpenAPI 3.0. The cast allows the test to compile while still correctly testing the functionality.

Warning

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• telemetry.astro.build
  • Triggering command: /usr/local/bin/node node /home/REDACTED/work/typespec/typespec/website/node_modules/.bin/../astro/astro.js build sh sion�� ebsite/src/content/docs/docs/emitters/openapi3/reference node .0/bin/sh /library-linter node . rsioning/referengenerate-scenarios-summary node tool�� lmstxt --output-dir ../../website/src/content/docs/docs/libraries/http/reference node _modules/.bin/node /library-linter sh . st/node-gyp-bin/tsx ./.scripts/regen-compiler-docs.ts sh (dns block)

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[pkg-pr-new]

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commit: 15b3a9b

[github-actions]

All changed packages have been documented.

• ✅ @typespec/openapi3

Show changes

@typespec/openapi3 - fix ✏️

Import OpenAPI 3.1/3.2 schemas with contentEncoding: base64 as bytes type with @encode("base64", string) decorator