fix: Implement polymorphic serialization for EmbeddedResourceResource and add unit tests

acp-model/api/acp-model.api

@@ -790,8 +790,6 @@ public final class com/agentclientprotocol/model/EmbeddedResource$Companion {
 
 public abstract class com/agentclientprotocol/model/EmbeddedResourceResource : com/agentclientprotocol/model/AcpWithMeta {
 	public static final field Companion Lcom/agentclientprotocol/model/EmbeddedResourceResource$Companion;
-	public synthetic fun <init> (ILkotlinx/serialization/internal/SerializationConstructorMarker;)V
-	public static final synthetic fun write$Self (Lcom/agentclientprotocol/model/EmbeddedResourceResource;Lkotlinx/serialization/encoding/CompositeEncoder;Lkotlinx/serialization/descriptors/SerialDescriptor;)V
 }
 
 public final class com/agentclientprotocol/model/EmbeddedResourceResource$BlobResourceContents : com/agentclientprotocol/model/EmbeddedResourceResource {

acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Content.kt

@@ -3,11 +3,21 @@
 
 package com.agentclientprotocol.model
 
+import kotlinx.serialization.DeserializationStrategy
 import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.SerializationException
 import kotlinx.serialization.json.JsonClassDiscriminator
 import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonContentPolymorphicSerializer
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+
+/**
+ * Common discriminator key used for polymorphic serialization in ACP.
+ */
+internal const val TYPE_DISCRIMINATOR = "type"
 
 /**
  * Content blocks represent displayable information in the Agent Client Protocol.
@@ -18,10 +28,10 @@ import kotlinx.serialization.json.JsonElement
  * See protocol docs: [Content](https://agentclientprotocol.com/protocol/content)
  */
 @Serializable
-@JsonClassDiscriminator("type")
+@JsonClassDiscriminator(TYPE_DISCRIMINATOR)
 public sealed class ContentBlock : AcpWithMeta {
     public abstract val annotations: Annotations?
-    
+
     /**
      * Plain text content
      *
@@ -101,7 +111,7 @@ public sealed class ContentBlock : AcpWithMeta {
 /**
  * Resource content that can be embedded in a message.
  */
-@Serializable
+@Serializable(with = EmbeddedResourceResourceSerializer::class)
 public sealed class EmbeddedResourceResource : AcpWithMeta {
     /**
      * Text-based resource contents.
@@ -128,6 +138,28 @@ public sealed class EmbeddedResourceResource : AcpWithMeta {
     ) : EmbeddedResourceResource()
 }
 
+/**
+ * Embedded resources are discriminator-less in the protocol; choose subtype by fields if
+ * discriminator is absent, but still honor an explicit discriminator when provided.
+ */
+internal object EmbeddedResourceResourceSerializer :
+    JsonContentPolymorphicSerializer<EmbeddedResourceResource>(EmbeddedResourceResource::class) {
+    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<EmbeddedResourceResource> {
+        val obj = element.jsonObject
+
+        val explicitType = obj[TYPE_DISCRIMINATOR]?.jsonPrimitive?.content
+        when (explicitType) {
+            EmbeddedResourceResource.TextResourceContents::class.simpleName -> return EmbeddedResourceResource.TextResourceContents.serializer()
+            EmbeddedResourceResource.BlobResourceContents::class.simpleName -> return EmbeddedResourceResource.BlobResourceContents.serializer()
+        }
+
+        if (EmbeddedResourceResource.TextResourceContents::text.name in obj) return EmbeddedResourceResource.TextResourceContents.serializer()
+        if (EmbeddedResourceResource.BlobResourceContents::blob.name in obj) return EmbeddedResourceResource.BlobResourceContents.serializer()
+
+        throw SerializationException("Cannot determine EmbeddedResourceResource type; expected '${EmbeddedResourceResource.TextResourceContents::text.name}' or '${EmbeddedResourceResource.BlobResourceContents::blob.name}'")
+    }
+}
+
 /**
  * The contents of a resource, embedded into a prompt or tool call result.
  */
@@ -151,4 +183,4 @@ public data class ResourceLink(
     val title: String? = null,
     val annotations: Annotations? = null,
     override val _meta: JsonElement? = null
-) : AcpWithMeta
+) : AcpWithMeta

acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/SessionUpdate.kt

@@ -15,7 +15,7 @@ import kotlinx.serialization.json.JsonElement
  * Specifies how the agent should collect input for this command.
  */
 @Serializable
-@JsonClassDiscriminator("type")
+@JsonClassDiscriminator(TYPE_DISCRIMINATOR)
 public sealed class AvailableCommandInput {
     /**
      * All text typed after the command name is provided as unstructured input.

acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/EmbeddedResourceResourceTest.kt

@@ -0,0 +1,53 @@
+package com.agentclientprotocol.model
+
+import com.agentclientprotocol.rpc.ACPJson
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class EmbeddedResourceResourceTest {
+
+    @Test
+    fun `decodes resource content without discriminator`() {
+        val payload = """
+            {
+              "type": "resource",
+              "resource": {
+                "text": "hello",
+                "uri": "file:///tmp/example.txt"
+              }
+            }
+        """.trimIndent()
+
+        val block = ACPJson.decodeFromString(ContentBlock.serializer(), payload)
+
+        assertTrue(block is ContentBlock.Resource)
+        val resource = block.resource
+        assertTrue(resource is EmbeddedResourceResource.TextResourceContents)
+        assertEquals("hello", resource.text)
+        assertEquals("file:///tmp/example.txt", resource.uri)
+    }
+
+    @Test
+    fun `decodes blob resource content without discriminator`() {
+        val payload = """
+            {
+              "type": "resource",
+              "resource": {
+                "blob": "ZGF0YQ==",
+                "mimeType": "application/octet-stream",
+                "uri": "file:///tmp/data.bin"
+              }
+            }
+        """.trimIndent()
+
+        val block = ACPJson.decodeFromString(ContentBlock.serializer(), payload)
+
+        assertTrue(block is ContentBlock.Resource)
+        val resource = block.resource
+        assertTrue(resource is EmbeddedResourceResource.BlobResourceContents)
+        assertEquals("ZGF0YQ==", resource.blob)
+        assertEquals("application/octet-stream", resource.mimeType)
+        assertEquals("file:///tmp/data.bin", resource.uri)
+    }
+}

build.gradle.kts

@@ -7,7 +7,7 @@ plugins {
 private val buildNumber: String? = System.getenv("GITHUB_RUN_NUMBER")
 private val isReleasePublication = System.getenv("RELEASE_PUBLICATION")?.toBoolean() ?: false
 
-private val baseVersion = "0.9.1"
+private val baseVersion = "0.9.2"
 
 allprojects {
     group = "com.agentclientprotocol"