[Bug]: OpenAPI3 merges discriminated union envelopes when variants share a payload type

[tadfisher]

Describe the bug

With @typespec/openapi3 1.16.0, distinct variants of a discriminated union reuse the first envelope schema when their payload types match. The emitter repeats that schema in oneOf and points every discriminator mapping to it. The other envelope schemas are missing from the output.

For example:

@discriminated(#{
  discriminatorPropertyName: "tag",
  envelopePropertyName: "contents"
})
union Limit {
  daily: string,
  weekly: string,
  monthly: string,
}

This produces a oneOf schema that references LimitDaily three times, and rejects any payload with "tag":"weekly" or "tag":"monthly".

Reproduction

Playground

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[github-actions]

🎯 Issue Triage
The OpenAPI3 emitter collapses distinct discriminated-union envelope variants when their payload types are identical.
This produces repeated oneOf references and discriminator mappings to the first envelope, making valid variants fail schema validation.
Category: Bug
Area: emitter:openapi3

🔗 [Playground Reproduction]((typespec.io/redacted)

📋 Reproduction code

@discriminated(#{
  discriminatorPropertyName: "tag",
  envelopePropertyName: "contents"
})
union Limit {
  daily: string,
  weekly: string,
  monthly: string,
}

🔍 Analysis

The affected component is @typespec/openapi3 1.16.0's schema generation for enveloped discriminated unions. The output appears to deduplicate generated envelope schemas by the shared payload type without retaining the variant identity and discriminator literal, causing LimitWeekly and LimitMonthly to collapse into LimitDaily. Searches found related discriminator and schema-deduplication issues, but none describing this same root cause.

📝 Suggested next steps

• Add an OpenAPI3 emitter regression test with multiple variants sharing one payload type.
• Ensure generated envelope identity includes the union variant/discriminator value, then verify oneOf and discriminator mappings reference all envelopes.

Generated by Agentic Triage

[timotheeguerin]

Seems like this regressed in version 1.6.0 as it was workin in 1.5.0