Setting some attributes to multivalued causes 400 errors

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Area

admin/api

Describe the bug

In trying to get Keycloak to send verification and account recovery emails to multiple addresses I turned on multivalued for the email attribute.

After doing this, adding emails to an account causes a 400 error:

{"error":"unknown_error","error_description":"Cannot parse the JSON"}

In the debug logs I also see the following:

kc.sh[99219]: 2025-09-18 18:43:28,738 DEBUG [org.jboss.resteasy.reactive.server.handlers.RequestDeserializeHandler] (executor-thread-4) Error occurred during deserialization of input: com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.lang.String` from Array value (token `JsonToken.START_ARRAY`)
kc.sh[99219]:  at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 1, column: 170] (through reference chain: org.keycloak.representations.idm.UserRepresentation["email"])

I assume this error means that somewhere the value for email is hardcoded as the type String.

Version

26.3.4

Regression

• The issue is a regression

Expected behavior

Either the multivalued toggle is not present for attributes where it isn't supported, and/or functionality is expended to support multiple email addresses.

Actual behavior

Enabled multivalued for certain attributes causes an error.

How to Reproduce?

Enabled multivalued for email, or username, and try updating those fields in user info.

Anything else?

No response