A workaround you can use is to move additionalProperties to the extending schema and redeclare the properties from the extended schema.

Because additionalProperties only recognizes properties declared in the same subschema, it considers anything other than “street_address”, “city”, and “state” to be additional. Combining the schemas with allOf doesn’t change that.

It’s important to note that additionalProperties only recognizes properties declared in the same subschema as itself. So, additionalProperties can restrict you from “extending” a schema using Schema Composition keywords such as allOf. In the following example, we can see how the additionalProperties can cause attempts to extend the address schema example to fail.

In your scenario, which many, many people encounter, you expect that properties defined in schema1 will be known to schema2; but this is not the case and will never be.

When you do: "allOf": [ { "schema1": "here" }, { "schema2": "here" } ] schema1 and schema2 have no knowledge of one another; they are evaluated in their own context.

I'm not sure if there's a reason why additionalProperties only looks at the sibling-level when checking allowed properties but IMHO this should be changed.

It's unfortunate that additionalProperties only takes the immediate, sibling-level properties into account

additionalProperties applies to all properties that are not accounted-for by properties or patternProperties in the immediate schema.

additionalProperties here applies to all properties, because there is no sibling-level properties entry - the one inside allOf does not count.

unevaluatedProperties is like additionalProperties, except that it can "see through" $ref and "see inside" allOf, anyOf, oneOf, if, then, else

What I want is to use "additionalProperties: false" to validate a union of schemas, but it seems it isn't possible. I already tried with sevaral different combination, but I didn't make it works.

additionalProperties: false works on it, but not along with allOf, because only validate one schema or another.