Fields starting with lowercase non ASCII characters are silently dropped when serializing

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I searched in the issues and found nothing similar.
I have confirmed that the same problem is not reproduced if I exclude the KotlinModule.
I searched in the issues of databind and other modules used and found nothing similar.
I have confirmed that the problem does not reproduce in Java and only occurs when using Kotlin and KotlinModule.

Describe the bug

When a field name starts with a (lowercase) non ASCII character, it is silently dropped when serialized. This behaviour did not exist in Jackson2.

Example:

data class Foo(
    val øx: String = "bar",
)

Results in an empty JSON: {}

I think this happens because the field ends up with a lowercase character in the getter in Kotlin Bytecode.
E.g. val foo results in getFoo, but val øx results in getøx.

To Reproduce

data class Foo(
    val øx: String = "disappears",
    val yø: String = "ok",
)

fun main() {
    val jackson2Mapper = com.fasterxml.jackson.module.kotlin.jacksonObjectMapper()
    val jackson2Json = jackson2Mapper.writeValueAsString(Foo())
    println("Jackson 2: $jackson2Json")

    val jackson3Mapper = tools.jackson.module.kotlin.jacksonMapperBuilder().build()
    val jackson3Json = jackson3Mapper.writeValueAsString(Foo())
    println("Jackson 3: $jackson3Json")

    // Output:
    // Jackson 2: {"øx":"disappears","yø":"ok"}
    // Jackson 3: {"yø":"ok"}
}

Expected behavior

I expected the field to be serialized like fields consisting of ASCII characters (like it did in Jackson2), or for an error to be thrown.

Versions

Kotlin: 2.3.20
Jackson-module-kotlin: 3.1.2

Additional context

@JsonProperty can be used as a workaround.

data class Workaround1(
    @JsonProperty("øx")
    val ox: String,
)

data class Workaround2(
    @get:JsonProperty("øx")
    val øx: String,
)

Likely due to FasterXML/jackson-databind#2882 which is mentioned in 3.0 Release notes (https://github.com/FasterXML/jackson/wiki/Jackson-Release-3.0#behavioral-changes).
So, intentional change on Java side at least; not sure about Kotlin module -- but I think implementation should actually consider non-ASCII letters too.

Could you file a jackson-databind issue for this one too? Apologies for extra work.

NOTE: plain @JsonProperty would probably work as work-around, without needing to duplicate name.

Thanks! Just want to make sure I understand the issue here before filing an issue in jackson-databind.
Consider this example:

data class Foo(
    // Kotlin Bytecode: public final getøx()Ljava/lang/String;
    val øx: String = "test",

    // Kotlin Bytecode: public final getØk()Ljava/lang/String;
    val Øk: String = "test",

    // Kotlin Bytecode: public final getName()Ljava/lang/String;
    val name: String = "test",
)

fun main() {
    val jackson2Mapper = com.fasterxml.jackson.module.kotlin.jacksonObjectMapper()
    val jackson2Json = jackson2Mapper.writeValueAsString(Foo())
    println("Jackson 2: $jackson2Json")

    val jackson3Mapper = tools.jackson.module.kotlin.jacksonMapperBuilder().build()
    val jackson3Json = jackson3Mapper.writeValueAsString(Foo())
    println("Jackson 3: $jackson3Json")

    /*
    Output:
    Jackson 2: {"øx":"test","name":"test","øk":"test"}
    Jackson 3: {"Øk":"test","name":"test"}
     */
}

When checking the Kotlin Bytecode, I see that fields starting with a non ASCII character does not get a uppercase letter in the getter. However, if the field starts with an uppercase non ASCII character, it gets a uppercase letter in the getter, and then the field is serialized with Jackson3.

This makes me think that the real issue is on the Kotlin side: Kotlin generates getøx() instead of getØx().
Should this be handled in jackson-module-kotlin instead, since the Kotlin compiler is the one generating the "non-standard" getter name?

Using accessorNaming (from your link https://github.com/FasterXML/jackson/wiki/Jackson-Release-3.0#behavioral-changes) is also a possible workaround.

.accessorNaming(
    DefaultAccessorNamingStrategy.Provider()
        .withFirstCharAcceptance(true, false),
)

@olesls

@olesls Good point. Perhaps issue is indeed with Kotlin-side logic; and at least needs to be traced here first.

Looking at jackson-databind DefaultAccessorNamingStrategy.java I think it is checking for non-ASCII characters correctly, not limited to just ASCII.

So it does indeed sound like Kotlin is not recognizing non-ASCII leading character.
And if so, suggested work-around (allow leading lower-case letter) should work.
But ideally somehow Kotlin handling could be improved (but generation is not under Jackson's control so...).

I don't believe this is a bug, at least not in my opinion.
Both Kotlin and Jackson are functioning as designed.
If I had to guess, it might be considered a bug in Kotlin, but since it involves a change that affects the bytecode, it's highly unlikely to be fixed.

It seems that this is rarely an issue, and there are manual workarounds, so I don't currently intend to actively fix it as part of the kotlin-module.

Besides using JsonProperty or withFirstCharAcceptance, adding the JvmField annotation to the property also seems to work as a workaround.

I agree. It is more of a question about backwards compatibility. Given the workarounds, and the breaking changes mentioned here: https://github.com/FasterXML/jackson/wiki/Jackson-Release-3.0#behavioral-changes, I think it makes sense to not fix this issue. But maybe it is worth mentioning it in the 'behavioral changes'-section, in case any more unlucky souls stumble upon it.

@cowtowncoder

Since this is a Kotlin-specific issue, would it be better to add it to Changes, JVM Languages?
https://github.com/FasterXML/jackson/wiki/Jackson-Release-3.0#kotlin

@cowtowncoder
I'm thinking of adding the following; what do you think?

#952: ...
#1154: Due to changes in databind#2882 and Kotlin's bytecode generation specification, fields starting with lowercase non ASCII characters are silently dropped when serializing now.

@k163377

@k163377 Good idea! Maybe "dropped" -> "ignored" or "excluded"?

@cowtowncoder

@cowtowncoder
Thank you.
I've adopted "ignored" and updated the wiki.
https://github.com/FasterXML/jackson/wiki/Jackson-Release-3.0#kotlin

With the above update, this issue will be closed.

@k163377

@k163377 It would make sense to also mention this earlier in the Wiki:

https://github.com/FasterXML/jackson/wiki/Jackson-Release-3.0#behavioral-changes

@cowtowncoder

@cowtowncoder
Is this what you mean?

Behavioral changes

jackson-databind

...

Kotlin

#1154: Due to changes in databind#2882 and Kotlin's bytecode generation specification, fields starting with lowercase non ASCII characters are silently ignored when serializing now.

Yes. Although "Kotlin" -> "jackson-module-kotlin"

@cowtowncoder

@cowtowncoder
Reflected.
https://github.com/FasterXML/jackson/wiki/Jackson-Release-3.0#jackson-module-kotlin

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Fields starting with lowercase non ASCII characters are silently dropped when serializing #1154

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Describe the bug

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Fields starting with lowercase non ASCII characters are silently dropped when serializing #1154

Description

Search before asking

Describe the bug

To Reproduce

Expected behavior

Versions

Additional context

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cowtowncoder commented on Apr 27, 2026

olesls commented on Apr 28, 2026

cowtowncoder commented on Apr 28, 2026

k163377 commented on May 2, 2026

olesls commented on May 4, 2026

k163377 commented on May 7, 2026

cowtowncoder commented on May 7, 2026

k163377 commented on May 8, 2026

cowtowncoder commented on May 8, 2026

k163377 commented on May 8, 2026

Behavioral changes

jackson-databind

Kotlin

cowtowncoder commented on May 8, 2026

k163377 commented on May 9, 2026

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