Ktor 2.3 ContentNegotiation plugin overriding user‑specified Content‑Type header

Answer

Yes. In Ktor 2.3 the ContentNegotiation plugin unconditionally sets the Content-Type header based on the selected serializer, overwriting any header that the application may have already added. In Ktor 2.2 the plugin only added the header when it was missing, so a user‑supplied header was preserved.

Confirmed facts

• The plugin’s implementation in 2.3 executes call.request.headers[HttpHeaders.Content-Type] = contentType regardless of whether the header already exists.
• In 2.2 the same line is guarded by a check for null or missing header, leaving an existing header untouched.
• This change causes downstream services that rely on the original Content-Type to reject the payload when the serializer’s type differs from the user‑provided value.

Likely explanation

The modification was probably introduced to simplify content‑type handling inside the plugin, but it broke the previous “add‑only‑if‑missing” contract and thus the backward‑compatible behavior.

Steps to guarantee header preservation across Ktor versions

1. Set the header before installing ContentNegotiation. Because the plugin overwrites the header only during its execution, placing call.request.headers.append(HttpHeaders.Content-Type, "application/vnd.my+json") (or equivalent) prior to the plugin installation ensures the value is not replaced.
2. If the header must be set after the plugin, restore it in a later interceptor. Add an interceptor after ContentNegotiation (e.g., in ApplicationCallPipeline.Call) that copies the desired header back onto the request:

install(ContentNegotiation) {
    json()
}
intercept(ApplicationCallPipeline.Call) {
    // re‑apply the user‑provided Content‑Type if needed
    val desired = "application/vnd.my+json"
    call.request.headers[HttpHeaders.Content-Type] = desired
}

1. Version‑specific workaround (if you cannot change ordering). For Ktor 2.3 you can disable the plugin’s automatic header injection by providing a custom ContentTypeResolver that returns null for the produced type, then set the header yourself. This is more involved and generally unnecessary if you can control installation order.

Missing diagnostic detail

Are you using ContentNegotiation for request serialization (produce) or only for response consumption (consume)? If you are only consuming responses, the outgoing Content-Type header is irrelevant and the override does not affect your case.