diff --git a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Requests.kt b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Requests.kt
index 8d2650a..5339ac8 100644
--- a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Requests.kt
+++ b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Requests.kt
@@ -6,6 +6,7 @@ package com.agentclientprotocol.model
 import com.agentclientprotocol.annotations.UnstableApi
 import com.agentclientprotocol.rpc.RequestId
 import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.Polymorphic
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.JsonClassDiscriminator
@@ -53,7 +54,7 @@ public data class HttpHeader(
 @Serializable
 public sealed class McpServer {
     public abstract val name: String
-    
+
     /**
      * Stdio transport configuration
      *
@@ -67,7 +68,7 @@ public sealed class McpServer {
         val args: List<String>,
         val env: List<EnvVariable>
     ) : McpServer()
-    
+
     /**
      * HTTP transport configuration
      *
@@ -80,7 +81,7 @@ public sealed class McpServer {
         val url: String,
         val headers: List<HttpHeader>
     ) : McpServer()
-    
+
     /**
      * SSE transport configuration
      *
@@ -192,7 +193,7 @@ public data class AuthenticateRequest(
 @Serializable
 public data class NewSessionRequest(
     val cwd: String,
-    val mcpServers: List<McpServer>,
+    val mcpServers: List<@Polymorphic McpServer>,
     override val _meta: JsonElement? = null
 ) : AcpRequest
 
@@ -207,7 +208,7 @@ public data class NewSessionRequest(
 public data class LoadSessionRequest(
     override val sessionId: SessionId,
     val cwd: String,
-    val mcpServers: List<McpServer>,
+    val mcpServers: List<@Polymorphic McpServer>,
     override val _meta: JsonElement? = null
 ) : AcpRequest, AcpWithSessionId
 
@@ -534,4 +535,4 @@ public class CancelRequestNotification(
     public val requestId: RequestId,
     public val message: String?,
     override val _meta: JsonElement? = null,
-) : AcpNotification
\ No newline at end of file
+) : AcpNotification
diff --git a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/rpc/JsonRpc.kt b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/rpc/JsonRpc.kt
index 24d0755..527c105 100644
--- a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/rpc/JsonRpc.kt
+++ b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/rpc/JsonRpc.kt
@@ -3,6 +3,7 @@
 package com.agentclientprotocol.rpc
 
 import com.agentclientprotocol.model.AvailableCommandInput
+import com.agentclientprotocol.model.McpServer
 import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.KSerializer
 import kotlinx.serialization.Serializable
@@ -178,6 +179,12 @@ private val acpSerializersModule = SerializersModule {
         subclass(AvailableCommandInput.Unstructured::class, AvailableCommandInput.Unstructured.serializer())
         defaultDeserializer { AvailableCommandInput.Unstructured.serializer() }
     }
+    polymorphic(McpServer::class) {
+        subclass(McpServer.Stdio::class, McpServer.Stdio.serializer())
+        subclass(McpServer.Http::class, McpServer.Http.serializer())
+        subclass(McpServer.Sse::class, McpServer.Sse.serializer())
+        defaultDeserializer { McpServer.Stdio.serializer() }
+    }
 }
 
 @OptIn(ExperimentalSerializationApi::class)
@@ -223,4 +230,4 @@ public fun decodeJsonRpcMessage(jsonString: String): JsonRpcMessage {
         hasMethod -> ACPJson.decodeFromJsonElement(JsonRpcNotification.serializer(), element)
         else -> error("Unable to determine JsonRpcMessage type from JSON structure")
     }
-}
\ No newline at end of file
+}
diff --git a/acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/McpServerSerializerTest.kt b/acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/McpServerSerializerTest.kt
new file mode 100644
index 0000000..71fca11
--- /dev/null
+++ b/acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/McpServerSerializerTest.kt
@@ -0,0 +1,91 @@
+package com.agentclientprotocol.model
+
+import com.agentclientprotocol.rpc.ACPJson
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class McpServerSerializerTest {
+    @Test
+    fun `decodes stdio transport without discriminator`() {
+        val payload = """
+            {
+                "name": "filesystem",
+                "command": "/path/to/mcp-server",
+                "args": ["--stdio"],
+                "env": [{"name": "TOKEN", "value": "secret"}]
+            }
+        """.trimIndent()
+
+        val server = ACPJson.decodeFromString(McpServer.serializer(), payload)
+        assertTrue(server is McpServer.Stdio)
+        assertEquals("filesystem", server.name)
+        assertEquals("/path/to/mcp-server", server.command)
+        assertEquals(listOf("--stdio"), server.args)
+        assertEquals("TOKEN", server.env.first().name)
+        assertEquals("secret", server.env.first().value)
+    }
+
+    @Test
+    fun `decodes http transport with type discriminator`() {
+        val payload = """
+            {
+                "type": "http",
+                "name": "api-server",
+                "url": "https://api.example.com/mcp",
+                "headers": [{"name": "Authorization", "value": "Bearer token123"}]
+            }
+        """.trimIndent()
+
+        val server = ACPJson.decodeFromString(McpServer.serializer(), payload)
+        assertTrue(server is McpServer.Http)
+        assertEquals("api-server", server.name)
+        assertEquals("https://api.example.com/mcp", server.url)
+        assertEquals("Authorization", server.headers.first().name)
+        assertEquals("Bearer token123", server.headers.first().value)
+    }
+
+    @Test
+    fun `decodes sse transport when type is sse`() {
+        val payload = """
+            {
+                "type": "sse",
+                "name": "event-stream",
+                "url": "https://events.example.com/mcp",
+                "headers": [{"name": "X-API-Key", "value": "apikey456"}]
+            }
+        """.trimIndent()
+
+        val server = ACPJson.decodeFromString(McpServer.serializer(), payload)
+        assertTrue(server is McpServer.Sse)
+        assertEquals("event-stream", server.name)
+        assertEquals("https://events.example.com/mcp", server.url)
+        assertEquals("X-API-Key", server.headers.first().name)
+        assertEquals("apikey456", server.headers.first().value)
+    }
+
+    @Test
+    fun `encodes discriminator when serializing`() {
+        val server = McpServer.Sse(
+            name = "event-stream",
+            url = "https://events.example.com/mcp",
+            headers = listOf(HttpHeader("X-API-Key", "apikey456"))
+        )
+
+        val encoded = ACPJson.encodeToString(McpServer.serializer(), server)
+        assertTrue(encoded.contains("\"type\":\"sse\""))
+    }
+
+    @Test
+    fun `encodes stdio discriminator when serializing`() {
+        val server = McpServer.Stdio(
+            name = "filesystem",
+            command = "/path/to/mcp-server",
+            args = listOf("--stdio"),
+            env = listOf(EnvVariable("TOKEN", "secret"))
+        )
+
+        val encoded = ACPJson.encodeToString(McpServer.serializer(), server)
+        assertTrue(encoded.contains("\"type\":\"stdio\""))
+    }
+}