diff --git a/acp-model/api/acp-model.api b/acp-model/api/acp-model.api
index 3dd270d..c0b3301 100644
--- a/acp-model/api/acp-model.api
+++ b/acp-model/api/acp-model.api
@@ -172,16 +172,18 @@ public abstract interface class com/agentclientprotocol/model/AcpWithSessionId {
 public final class com/agentclientprotocol/model/AgentCapabilities : com/agentclientprotocol/model/AcpCapabilities, com/agentclientprotocol/model/AcpWithMeta {
 	public static final field Companion Lcom/agentclientprotocol/model/AgentCapabilities$Companion;
 	public fun <init> ()V
-	public fun <init> (ZLcom/agentclientprotocol/model/PromptCapabilities;Lcom/agentclientprotocol/model/McpCapabilities;Lcom/agentclientprotocol/model/SessionCapabilities;Lkotlinx/serialization/json/JsonElement;)V
-	public synthetic fun <init> (ZLcom/agentclientprotocol/model/PromptCapabilities;Lcom/agentclientprotocol/model/McpCapabilities;Lcom/agentclientprotocol/model/SessionCapabilities;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public fun <init> (ZLcom/agentclientprotocol/model/PromptCapabilities;Lcom/agentclientprotocol/model/McpCapabilities;Lcom/agentclientprotocol/model/SessionCapabilities;Lcom/agentclientprotocol/model/AuthCapabilities;Lkotlinx/serialization/json/JsonElement;)V
+	public synthetic fun <init> (ZLcom/agentclientprotocol/model/PromptCapabilities;Lcom/agentclientprotocol/model/McpCapabilities;Lcom/agentclientprotocol/model/SessionCapabilities;Lcom/agentclientprotocol/model/AuthCapabilities;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Z
 	public final fun component2 ()Lcom/agentclientprotocol/model/PromptCapabilities;
 	public final fun component3 ()Lcom/agentclientprotocol/model/McpCapabilities;
 	public final fun component4 ()Lcom/agentclientprotocol/model/SessionCapabilities;
-	public final fun component5 ()Lkotlinx/serialization/json/JsonElement;
-	public final fun copy (ZLcom/agentclientprotocol/model/PromptCapabilities;Lcom/agentclientprotocol/model/McpCapabilities;Lcom/agentclientprotocol/model/SessionCapabilities;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/AgentCapabilities;
-	public static synthetic fun copy$default (Lcom/agentclientprotocol/model/AgentCapabilities;ZLcom/agentclientprotocol/model/PromptCapabilities;Lcom/agentclientprotocol/model/McpCapabilities;Lcom/agentclientprotocol/model/SessionCapabilities;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/AgentCapabilities;
+	public final fun component5 ()Lcom/agentclientprotocol/model/AuthCapabilities;
+	public final fun component6 ()Lkotlinx/serialization/json/JsonElement;
+	public final fun copy (ZLcom/agentclientprotocol/model/PromptCapabilities;Lcom/agentclientprotocol/model/McpCapabilities;Lcom/agentclientprotocol/model/SessionCapabilities;Lcom/agentclientprotocol/model/AuthCapabilities;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/AgentCapabilities;
+	public static synthetic fun copy$default (Lcom/agentclientprotocol/model/AgentCapabilities;ZLcom/agentclientprotocol/model/PromptCapabilities;Lcom/agentclientprotocol/model/McpCapabilities;Lcom/agentclientprotocol/model/SessionCapabilities;Lcom/agentclientprotocol/model/AuthCapabilities;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/AgentCapabilities;
 	public fun equals (Ljava/lang/Object;)Z
+	public final fun getAuth ()Lcom/agentclientprotocol/model/AuthCapabilities;
 	public final fun getLoadSession ()Z
 	public final fun getMcpCapabilities ()Lcom/agentclientprotocol/model/McpCapabilities;
 	public final fun getPromptCapabilities ()Lcom/agentclientprotocol/model/PromptCapabilities;
@@ -241,6 +243,35 @@ public final class com/agentclientprotocol/model/Annotations$Companion {
 	public final fun serializer ()Lkotlinx/serialization/KSerializer;
 }
 
+public final class com/agentclientprotocol/model/AuthCapabilities : com/agentclientprotocol/model/AcpWithMeta {
+	public static final field Companion Lcom/agentclientprotocol/model/AuthCapabilities$Companion;
+	public fun <init> ()V
+	public fun <init> (Lkotlinx/serialization/json/JsonElement;)V
+	public synthetic fun <init> (Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public final fun component1 ()Lkotlinx/serialization/json/JsonElement;
+	public final fun copy (Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/AuthCapabilities;
+	public static synthetic fun copy$default (Lcom/agentclientprotocol/model/AuthCapabilities;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/AuthCapabilities;
+	public fun equals (Ljava/lang/Object;)Z
+	public fun get_meta ()Lkotlinx/serialization/json/JsonElement;
+	public fun hashCode ()I
+	public fun toString ()Ljava/lang/String;
+}
+
+public final synthetic class com/agentclientprotocol/model/AuthCapabilities$$serializer : kotlinx/serialization/internal/GeneratedSerializer {
+	public static final field INSTANCE Lcom/agentclientprotocol/model/AuthCapabilities$$serializer;
+	public final fun childSerializers ()[Lkotlinx/serialization/KSerializer;
+	public final fun deserialize (Lkotlinx/serialization/encoding/Decoder;)Lcom/agentclientprotocol/model/AuthCapabilities;
+	public synthetic fun deserialize (Lkotlinx/serialization/encoding/Decoder;)Ljava/lang/Object;
+	public final fun getDescriptor ()Lkotlinx/serialization/descriptors/SerialDescriptor;
+	public final fun serialize (Lkotlinx/serialization/encoding/Encoder;Lcom/agentclientprotocol/model/AuthCapabilities;)V
+	public synthetic fun serialize (Lkotlinx/serialization/encoding/Encoder;Ljava/lang/Object;)V
+	public fun typeParametersSerializers ()[Lkotlinx/serialization/KSerializer;
+}
+
+public final class com/agentclientprotocol/model/AuthCapabilities$Companion {
+	public final fun serializer ()Lkotlinx/serialization/KSerializer;
+}
+
 public abstract class com/agentclientprotocol/model/AuthMethod : com/agentclientprotocol/model/AcpWithMeta {
 	public static final field Companion Lcom/agentclientprotocol/model/AuthMethod$Companion;
 	public abstract fun getDescription ()Ljava/lang/String;
diff --git a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Capabilities.kt b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Capabilities.kt
index 03e6d57..a9473ac 100644
--- a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Capabilities.kt
+++ b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Capabilities.kt
@@ -37,6 +37,23 @@ public data class PromptCapabilities(
     override val _meta: JsonElement? = null
 ) : AcpWithMeta
 
+/**
+ * **UNSTABLE**
+ *
+ * This capability is not part of the spec yet, and may be removed or changed at any point.
+ *
+ * Authentication capabilities supported by the client.
+ *
+ * Advertised during initialization to inform the agent which authentication
+ * method types the client can handle. This governs opt-in types that require
+ * additional client-side support.
+ */
+@UnstableApi
+@Serializable
+public data class AuthCapabilities(
+    override val _meta: JsonElement? = null
+): AcpWithMeta
+
 /**
  * Capabilities supported by the client.
  *
@@ -128,5 +145,7 @@ public data class AgentCapabilities(
     @EncodeDefault val promptCapabilities: PromptCapabilities = PromptCapabilities(),
     @EncodeDefault val mcpCapabilities: McpCapabilities = McpCapabilities(),
     @EncodeDefault val sessionCapabilities: SessionCapabilities = SessionCapabilities(),
+    @property:UnstableApi
+    @EncodeDefault val auth: AuthCapabilities? = null,
     override val _meta: JsonElement? = null
 ) : AcpCapabilities, AcpWithMeta
\ No newline at end of file
diff --git a/build.gradle.kts b/build.gradle.kts
index 76c9d89..d05042f 100644
--- a/build.gradle.kts
+++ b/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 = "0.16.3"
+private val baseVersion = "0.16.4"
 
 allprojects {
     group = "com.agentclientprotocol"