diff --git a/redwood-treehouse-host/src/appsJvmTest/kotlin/app/cash/redwood/treehouse/LeaksTest.kt b/redwood-treehouse-host/src/appsJvmTest/kotlin/app/cash/redwood/treehouse/LeaksTest.kt
index 7eb474a9d0..55ed79794d 100644
--- a/redwood-treehouse-host/src/appsJvmTest/kotlin/app/cash/redwood/treehouse/LeaksTest.kt
+++ b/redwood-treehouse-host/src/appsJvmTest/kotlin/app/cash/redwood/treehouse/LeaksTest.kt
@@ -21,6 +21,7 @@ import assertk.assertions.isEmpty
 import assertk.assertions.isEqualTo
 import com.example.redwood.testapp.testing.TextInputValue
 import kotlin.test.Test
+import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.test.runTest
 
 class LeaksTest {
@@ -55,4 +56,30 @@ class LeaksTest {
 
     treehouseApp.stop()
   }
+
+  @Test
+  fun serviceNotLeaked() = runTest {
+    val tester = TreehouseTester(this)
+    val treehouseApp = tester.loadApp()
+    treehouseApp.start()
+    tester.eventLog.takeEvent("test_app.codeLoadSuccess()", skipOthers = true)
+
+    // Wait for Zipline to be ready.
+    // TODO(jwilson): consider deferring events or exposing the TreehouseApp state. As-is the
+    //     codeLoadSuccess() event occurs on the Zipline dispatcher but we don't have an instance
+    //     here until we bounce that information to the main dispatcher.
+    treehouseApp.zipline.first { it != null }
+
+    val serviceLeakWatcher = LeakWatcher {
+      tester.hostApi // The first instance of HostApi is held by the current run of the test app.
+    }
+
+    // Stop referencing this HostApi from our test harness.
+    tester.hostApi = FakeHostApi()
+
+    // Stop the app. Even though we still reference the app, it stops referencing hostApi.
+    treehouseApp.stop()
+    tester.eventLog.takeEvent("test_app.codeUnloaded()", skipOthers = true)
+    serviceLeakWatcher.assertNotLeaked()
+  }
 }
diff --git a/redwood-treehouse-host/src/appsTest/kotlin/app/cash/redwood/treehouse/FakeHostApi.kt b/redwood-treehouse-host/src/appsTest/kotlin/app/cash/redwood/treehouse/FakeHostApi.kt
new file mode 100644
index 0000000000..b82b886d0f
--- /dev/null
+++ b/redwood-treehouse-host/src/appsTest/kotlin/app/cash/redwood/treehouse/FakeHostApi.kt
@@ -0,0 +1,24 @@
+/*
+ * Copyright (C) 2024 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package app.cash.redwood.treehouse
+
+import com.example.redwood.testapp.treehouse.HostApi
+
+class FakeHostApi : HostApi {
+  override suspend fun httpCall(url: String, headers: Map<String, String>): String {
+    error("unexpected call")
+  }
+}
diff --git a/redwood-treehouse-host/src/appsTest/kotlin/app/cash/redwood/treehouse/TreehouseTester.kt b/redwood-treehouse-host/src/appsTest/kotlin/app/cash/redwood/treehouse/TreehouseTester.kt
index 036dcfd0b7..98a0cde836 100644
--- a/redwood-treehouse-host/src/appsTest/kotlin/app/cash/redwood/treehouse/TreehouseTester.kt
+++ b/redwood-treehouse-host/src/appsTest/kotlin/app/cash/redwood/treehouse/TreehouseTester.kt
@@ -43,7 +43,7 @@ import okio.SYSTEM
 internal class TreehouseTester(
   private val testScope: TestScope,
 ) {
-  private val eventLog = EventLog()
+  val eventLog = EventLog()
 
   @OptIn(ExperimentalStdlibApi::class)
   private val testDispatcher = testScope.coroutineContext[CoroutineDispatcher.Key] as TestDispatcher
@@ -96,11 +96,7 @@ internal class TreehouseTester(
     override fun create(scope: CoroutineScope, dispatchers: TreehouseDispatchers) = frameClock
   }
 
-  private val hostApi = object : HostApi {
-    override suspend fun httpCall(url: String, headers: Map<String, String>): String {
-      error("unexpected call")
-    }
-  }
+  var hostApi: HostApi = FakeHostApi()
 
   private val treehouseAppFactory = TreehouseApp.Factory(
     platform = platform,
@@ -118,14 +114,14 @@ internal class TreehouseTester(
 
   private val appSpec = object : TreehouseApp.Spec<TestAppPresenter>() {
     override val name: String
-      get() = "test-app"
+      get() = "test_app"
     override val manifestUrl: Flow<String>
       get() = this@TreehouseTester.manifestUrl
     override val loadCodeFromNetworkOnly: Boolean
       get() = true
 
     override fun bindServices(zipline: Zipline) {
-      zipline.bind<HostApi>("HostApi", hostApi)
+      zipline.bind("HostApi", hostApi)
     }
 
     override fun create(zipline: Zipline): TestAppPresenter {
diff --git a/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/CodeHost.kt b/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/CodeHost.kt
index 823a4b4bf0..93fcbe68bf 100644
--- a/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/CodeHost.kt
+++ b/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/CodeHost.kt
@@ -15,10 +15,13 @@
  */
 package app.cash.redwood.treehouse
 
+import app.cash.zipline.Zipline
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.SupervisorJob
 import kotlinx.coroutines.cancel
 import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.job
 import kotlinx.coroutines.launch
 
@@ -56,6 +59,12 @@ internal abstract class CodeHost<A : AppService>(
 
   private var state: State<A> = State.Idle()
 
+  /** Updated with [State]. */
+  private val mutableZipline = MutableStateFlow<Zipline?>(null)
+
+  val zipline: StateFlow<Zipline?>
+    get() = mutableZipline
+
   private val codeSessionListener = object : CodeSession.Listener<A> {
     override fun onUncaughtException(codeSession: CodeSession<A>, exception: Throwable) {
     }
@@ -69,6 +78,7 @@ internal abstract class CodeHost<A : AppService>(
       val previous = state
       if (previous is State.Running) {
         state = State.Crashed(previous.codeUpdatesScope)
+        mutableZipline.value = null
       }
     }
   }
@@ -104,6 +114,7 @@ internal abstract class CodeHost<A : AppService>(
     previous.codeSession?.stop()
 
     state = State.Idle()
+    mutableZipline.value = null
   }
 
   fun restart() {
@@ -118,6 +129,7 @@ internal abstract class CodeHost<A : AppService>(
 
     val codeUpdatesScope = newCodeUpdatesScope()
     state = State.Starting(codeUpdatesScope)
+    mutableZipline.value = null
     codeUpdatesScope.collectCodeUpdates()
   }
 
@@ -152,7 +164,7 @@ internal abstract class CodeHost<A : AppService>(
       previous.codeSession?.stop()
 
       // If the codeUpdatesScope is null, we're stopped. Discard the newly-loaded code.
-      val codeUpdatesScope = state.codeUpdatesScope
+      val codeUpdatesScope = previous.codeUpdatesScope
       if (codeUpdatesScope == null) {
         next.stop()
         return@launch
@@ -160,6 +172,7 @@ internal abstract class CodeHost<A : AppService>(
 
       // Boot up the new code.
       state = State.Running(codeUpdatesScope, next)
+      mutableZipline.value = (next as? ZiplineCodeSession)?.zipline
       next.addListener(codeSessionListener)
       next.start()
 
diff --git a/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/TreehouseApp.kt b/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/TreehouseApp.kt
index e4ca849674..724c063490 100644
--- a/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/TreehouseApp.kt
+++ b/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/TreehouseApp.kt
@@ -27,6 +27,7 @@ import app.cash.zipline.loader.ZiplineLoader
 import kotlin.native.ObjCName
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.mapNotNull
 import kotlinx.serialization.modules.EmptySerializersModule
 import kotlinx.serialization.modules.SerializersModule
@@ -74,8 +75,8 @@ public class TreehouseApp<A : AppService> private constructor(
    * It is unwise to use this instance for anything beyond measurement and monitoring, because the
    * instance may be replaced if new code is loaded.
    */
-  public val zipline: Zipline?
-    get() = (codeHost.codeSession as? ZiplineCodeSession)?.zipline
+  public val zipline: StateFlow<Zipline?>
+    get() = codeHost.zipline
 
   /**
    * Create content for [source].
diff --git a/redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/EventLog.kt b/redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/EventLog.kt
index 284c9d2ddb..cff0baf1d2 100644
--- a/redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/EventLog.kt
+++ b/redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/EventLog.kt
@@ -32,8 +32,14 @@ class EventLog {
     return events.receive()
   }
 
-  suspend fun takeEvent(event: String) {
-    assertThat(takeEvent()).isEqualTo(event)
+  suspend fun takeEvent(event: String, skipOthers: Boolean = false) {
+    while (true) {
+      val actual = takeEvent()
+      if (skipOthers && actual != event) continue
+
+      assertThat(actual).isEqualTo(event)
+      return
+    }
   }
 
   /**
diff --git a/redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/FakeEventListener.kt b/redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/FakeEventListener.kt
index c2c74d96c4..7597c115f2 100644
--- a/redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/FakeEventListener.kt
+++ b/redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/FakeEventListener.kt
@@ -17,6 +17,8 @@ package app.cash.redwood.treehouse
 
 import app.cash.redwood.protocol.EventTag
 import app.cash.redwood.protocol.WidgetTag
+import app.cash.zipline.Zipline
+import app.cash.zipline.ZiplineManifest
 
 class FakeEventListener(
   private val eventLog: EventLog,
@@ -29,6 +31,23 @@ class FakeEventListener(
       FakeEventListener(eventLog, app)
   }
 
+  override fun codeLoadStart(): Any? {
+    eventLog += "${app.spec.name}.codeLoadStart()"
+    return null
+  }
+
+  override fun codeLoadSuccess(manifest: ZiplineManifest, zipline: Zipline, startValue: Any?) {
+    eventLog += "${app.spec.name}.codeLoadSuccess()"
+  }
+
+  override fun codeLoadFailed(exception: Exception, startValue: Any?) {
+    eventLog += "${app.spec.name}.codeLoadFailed()"
+  }
+
+  override fun codeUnloaded() {
+    eventLog += "${app.spec.name}.codeUnloaded()"
+  }
+
   override fun unknownEvent(widgetTag: WidgetTag, tag: EventTag) {
     eventLog += "${app.spec.name}.unknownEvent($widgetTag, $tag)"
   }