diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7970a3b8d..9015f064f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,8 +3,9 @@
 This list is not intended to be all-encompassing - it will document major and breaking API 
 changes with their rationale when appropriate. Given version `A.B.C.D`, breaking changes are to be expected in version number increments where changes in the `A` or `B` sections:
 
-### v5.20.1.0 (uncut)
+### v5.21.0.0 (uncut)
 - **http4k-connect-*** - Upgrade dependencies including Kotlin to 2.0.10
+  - **http4k-connect-ai-openai-**** - [Breaking] Model ResponseFormat as a sealed class hierarchy. Removed ResponseFormatType as now inherent in the JSON marshalling. Alpha support for `json_schema` response format, but it's just a map right now with no class structure.
 
 ### v5.20.0.0
 - **http4k-connect-*** - Upgrade dependencies, including Kotshi to 3.0.0. Version bump to highlight that this could be a breaking change if you are using the Kotshi annotation processor.
diff --git a/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/OpenAIMoshi.kt b/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/OpenAIMoshi.kt
index 51dda78e7..f30bdc51b 100644
--- a/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/OpenAIMoshi.kt
+++ b/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/OpenAIMoshi.kt
@@ -29,7 +29,6 @@ object OpenAIMoshi : ConfigurableMoshi(
         .value(Timestamp)
         .value(TokenId)
         .value(User)
-        .value(ResponseFormatType)
         .done()
 )
 
diff --git a/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/action/ChatCompletion.kt b/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/action/ChatCompletion.kt
index e571824da..dded1654e 100644
--- a/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/action/ChatCompletion.kt
+++ b/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/action/ChatCompletion.kt
@@ -8,12 +8,14 @@ import org.http4k.connect.Http4kConnectAction
 import org.http4k.connect.asRemoteFailure
 import org.http4k.connect.model.ModelName
 import org.http4k.connect.openai.CompletionId
+import org.http4k.connect.openai.GPT3_5
 import org.http4k.connect.openai.ObjectType
 import org.http4k.connect.openai.OpenAIAction
 import org.http4k.connect.openai.OpenAIMoshi
 import org.http4k.connect.openai.OpenAIMoshi.autoBody
-import org.http4k.connect.openai.ResponseFormatType
 import org.http4k.connect.openai.Role
+import org.http4k.connect.openai.Role.Companion.System
+import org.http4k.connect.openai.Role.Companion.User
 import org.http4k.connect.openai.Timestamp
 import org.http4k.connect.openai.TokenId
 import org.http4k.connect.openai.User
@@ -28,6 +30,8 @@ import org.http4k.lens.Header.CONTENT_TYPE
 import se.ansman.kotshi.ExperimentalKotshiApi
 import se.ansman.kotshi.JsonProperty
 import se.ansman.kotshi.JsonSerializable
+import se.ansman.kotshi.Polymorphic
+import se.ansman.kotshi.PolymorphicLabel
 import java.io.BufferedReader
 import java.io.InputStreamReader
 import kotlin.text.Charsets.UTF_8
@@ -103,10 +107,23 @@ data class ChatCompletion(
     }
 }
 
+
 @JsonSerializable
-data class ResponseFormat(
-    val type: ResponseFormatType
-)
+@Polymorphic("type")
+sealed class ResponseFormat {
+    @JsonSerializable
+    @PolymorphicLabel("json_object")
+    data object Json : ResponseFormat()
+
+    @JsonSerializable
+    @PolymorphicLabel("url")
+    data object Url : ResponseFormat()
+
+    @JsonSerializable
+    @PolymorphicLabel("json_schema")
+    data class JsonSchema(val strict: Boolean, val json_schema: Map<String, Any>) : ResponseFormat()
+}
+
 
 @JsonSerializable
 data class Message(
diff --git a/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/model.kt b/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/model.kt
index 9966ff5c1..e9c2bfa55 100644
--- a/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/model.kt
+++ b/ai/openai/client/src/main/kotlin/org/http4k/connect/openai/model.kt
@@ -57,13 +57,6 @@ val ModelName.Companion.GPT4_TURBO_PREVIEW get() = ModelName.of("gpt-4-turbo-pre
 val ModelName.Companion.GPT3_5 get() = ModelName.of("gpt-3.5-turbo")
 val ModelName.Companion.TEXT_EMBEDDING_ADA_002 get() = ModelName.of("text-embedding-ada-002")
 
-class ResponseFormatType private constructor(value: String) : StringValue(value) {
-    companion object : NonBlankStringValueFactory<ResponseFormatType>(::ResponseFormatType) {
-        val JsonObject = ResponseFormatType.of("json_object")
-        val url = ResponseFormatType.of("url")
-    }
-}
-
 class Role private constructor(value: String) : StringValue(value) {
     companion object : NonBlankStringValueFactory<Role>(::Role) {
         val System = Role.of("system")
diff --git a/ai/openai/client/src/test/kotlin/org/http4k/connect/openai/RealOpenAITest.kt b/ai/openai/client/src/test/kotlin/org/http4k/connect/openai/RealOpenAITest.kt
index 182535348..a23ed5a35 100644
--- a/ai/openai/client/src/test/kotlin/org/http4k/connect/openai/RealOpenAITest.kt
+++ b/ai/openai/client/src/test/kotlin/org/http4k/connect/openai/RealOpenAITest.kt
@@ -16,7 +16,7 @@ class RealOpenAITest : OpenAIContract {
 
     override val openAi = OpenAI.Http(
         apiKey(ENV)!!,
-        JavaHttpClient().debug(debugStream = true),
+        JavaHttpClient().debug(),
         OpenAIOrg.of("org-Ydjc9eGanqJtCP70yPUwZsvs")
     )
 }