diff --git a/redwood-treehouse-guest/build.gradle b/redwood-treehouse-guest/build.gradle
index 485b23b0f3..9ba84fc7bf 100644
--- a/redwood-treehouse-guest/build.gradle
+++ b/redwood-treehouse-guest/build.gradle
@@ -25,6 +25,18 @@ kotlin {
         api projects.redwoodTreehouseGuestCompose
       }
     }
+    nonJsMain {
+      dependsOn(commonMain)
+    }
+    androidMain {
+      dependsOn(nonJsMain)
+    }
+    jvmMain {
+      dependsOn(nonJsMain)
+    }
+    nativeMain {
+      dependsOn(nonJsMain)
+    }
   }
 }
 
diff --git a/redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironment.kt b/redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironment.kt
new file mode 100644
index 0000000000..6f4b21c133
--- /dev/null
+++ b/redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironment.kt
@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2023 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 kotlinx.coroutines.CoroutineExceptionHandler
+
+/**
+ * Customize the runtime for this Treehouse application.
+ */
+internal expect fun prepareEnvironment(
+  coroutineExceptionHandler: CoroutineExceptionHandler,
+)
diff --git a/redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/StandardAppLifecycle.kt b/redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/StandardAppLifecycle.kt
index 2ca6f0264c..e08118f6ce 100644
--- a/redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/StandardAppLifecycle.kt
+++ b/redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/StandardAppLifecycle.kt
@@ -33,6 +33,7 @@ public class StandardAppLifecycle(
   internal val json: Json,
   internal val widgetVersion: UInt,
 ) : AppLifecycle {
+  private var started = false
   private lateinit var host: Host
 
   private lateinit var broadcastFrameClock: BroadcastFrameClock
@@ -60,9 +61,14 @@ public class StandardAppLifecycle(
   internal val coroutineScope = CoroutineScope(coroutineExceptionHandler)
 
   override fun start(host: Host) {
+    check(!started) { "already started" }
+    this.started = true
+
     this.host = host
     this.broadcastFrameClock = BroadcastFrameClock { host.requestFrame() }
     this.frameClock = broadcastFrameClock
+
+    prepareEnvironment(coroutineExceptionHandler)
   }
 
   override fun sendFrame(timeNanos: Long) {
diff --git a/redwood-treehouse-guest/src/jsMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironmentJs.kt b/redwood-treehouse-guest/src/jsMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironmentJs.kt
new file mode 100644
index 0000000000..68bbb31203
--- /dev/null
+++ b/redwood-treehouse-guest/src/jsMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironmentJs.kt
@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2023 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 kotlin.coroutines.CoroutineContext
+import kotlinx.coroutines.CoroutineExceptionHandler
+
+/**
+ * This installs a global coroutine exception handler using an internal API.
+ *
+ * This is necessary for uncaught exceptions to be delivered to the host application. Otherwise
+ * they're logged to the console where they're easily missed.
+ *
+ * This assumes we're the [StandardAppLifecycle] owns the entire JS runtime.
+ */
+@Suppress("INVISIBLE_MEMBER") // https://github.com/Kotlin/kotlinx.coroutines/issues/3978
+internal actual fun prepareEnvironment(
+  coroutineExceptionHandler: CoroutineExceptionHandler,
+) {
+  kotlinx.coroutines.internal.ensurePlatformExceptionHandlerLoaded(
+    object : CoroutineExceptionHandler by coroutineExceptionHandler {
+      override fun handleException(context: CoroutineContext, exception: Throwable) {
+        coroutineExceptionHandler.handleException(context, exception)
+        throw kotlinx.coroutines.internal.ExceptionSuccessfullyProcessed
+      }
+    },
+  )
+}
diff --git a/redwood-treehouse-guest/src/nonJsMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironmentNonJs.kt b/redwood-treehouse-guest/src/nonJsMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironmentNonJs.kt
new file mode 100644
index 0000000000..77327aa69a
--- /dev/null
+++ b/redwood-treehouse-guest/src/nonJsMain/kotlin/app/cash/redwood/treehouse/PrepareEnvironmentNonJs.kt
@@ -0,0 +1,24 @@
+/*
+ * Copyright (C) 2023 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 kotlinx.coroutines.CoroutineExceptionHandler
+
+internal actual fun prepareEnvironment(
+  coroutineExceptionHandler: CoroutineExceptionHandler,
+) {
+  // Do nothing.
+}