OpenAPI.NET 3.4 appears to emit `unevaluatedProperties: false` by default for generated schemas.

[mdaneri]

Describe the issue

OpenAPI.NET 3.4 appears to emit unevaluatedProperties: false by
default for generated schemas.

This changes the effective validation semantics of generated OpenAPI 3.1
documents by making objects strict unless explicitly modeled otherwise.

In JSON Schema 2020-12, unevaluatedProperties is optional. If omitted,
unevaluated properties are allowed. Emitting
unevaluatedProperties: false by default makes the generated schema
more restrictive than the default JSON Schema / OpenAPI behavior.

Breaking change

This behavior also represents a breaking change in generated OpenAPI
documents compared to previous OpenAPI.NET versions.

OpenAPI documents generated with version 3.4.x differ from those
generated by earlier versions, even when the input model has not
changed. The serializer now injects unevaluatedProperties: false into
schemas that previously did not contain this keyword.

This has several consequences:

• Generated OpenAPI documents change between versions without changes
  to the source model.
• Tools performing document comparison (CI validation, contract
  checks, snapshot tests) detect unexpected differences.
• Clients or validators using strict JSON Schema validation may now
  reject payloads that were previously valid.

Because OpenAPI specifications are often treated as stable API
contracts, such changes can impact downstream tooling and consumers.

Why this is a problem

Setting unevaluatedProperties: false by default means that unknown
properties are rejected even when the schema author did not explicitly
request strict object validation.

For example, a simple schema like this:

{
  "type": "object",
  "properties": {
    "name": {
      "type": "string"
    }
  }
}

normally allows:

{
  "name": "Max",
  "nickname": "Maxy"
}

But when OpenAPI.NET emits:

{
  "type": "object",
  "properties": {
    "name": {
      "type": "string"
    }
  },
  "unevaluatedProperties": false
}

the same instance becomes invalid.

This is not just cosmetic output; it materially changes the meaning of
the generated schema.

Expected behavior

OpenAPI.NET should not emit unevaluatedProperties: false unless strict
object validation was explicitly requested by the author or configured
by the generator.

The default generated schema should preserve the standard JSON Schema /
OpenAPI behavior, which is equivalent to allowing unevaluated properties
when the keyword is omitted.

Actual behavior

Generated schemas include unevaluatedProperties: false by default,
which makes the resulting schemas strict even when that was not
explicitly modeled.

Impact

This can cause several issues:

• generated OpenAPI documents become more restrictive than expected
• clients and validators may reject payloads that would otherwise be
  valid
• downstream users may assume strict object semantics were
  intentionally authored
• future-compatible or extensible request/response objects become
  harder to represent
• generated OpenAPI specifications change between OpenAPI.NET versions

Suggested fix

Do not emit unevaluatedProperties by default.

Instead:

• omit the keyword unless strict object validation is explicitly
  intended
• optionally provide a configuration switch for generators/users who
  want strict object output
• if emitted, only emit it where it is semantically meaningful

Additional note

There is a separate but related issue where unevaluatedProperties is
also being emitted for non-object schemas such as arrays and primitive
types. That appears to be serializer noise.

This issue is specifically about the default semantic choice of emitting
unevaluatedProperties: false for schemas without an explicit request
for strictness.

Version info

• OpenAPI.NET: 3.4.0
• OpenAPI document version: 3.1.x

[baywet]

Hi @mdaneri,

Thank you for using the SDK and for reaching out.

Can you please provide a code snippet to reproduce the issue?