diff --git a/apps/server/src/routes/threads/actions.ts b/apps/server/src/routes/threads/actions.ts
index a59673fa2..154e9982d 100644
--- a/apps/server/src/routes/threads/actions.ts
+++ b/apps/server/src/routes/threads/actions.ts
@@ -40,6 +40,10 @@ import {
 } from "../../services/environments/environment-cleanup-internal.js";
 import { applyLoggedEnvironmentLifecycleEvent } from "../../services/environments/lifecycle-outcome.js";
 import { requirePublicThread } from "../../services/lib/entity-lookup.js";
+import {
+  goneThreadEnvironmentDetails,
+  throwThreadEnvironmentUnavailable,
+} from "../../services/lib/lifecycle-api-errors.js";
 import { parseSafeRelativeRoutePath } from "../relative-route-path.js";
 import { validatePromptAttachmentReferences } from "../../services/projects/attachments.js";
 import {
@@ -252,12 +256,33 @@ function queuedMessagePayloadFromSendRequest(
   };
 }
 
+/**
+ * A queued message can only ever drain into the thread's environment. A gone
+ * environment (destroying/destroyed) is never reprovisioned, so accepting the
+ * message would park it in the queue forever while the thread keeps reporting
+ * `idle` (#1789). Refuse with the same 409 the direct send path returns.
+ */
+function ensureThreadEnvironmentIsNotGone(deps: AppDeps, thread: Thread): void {
+  if (thread.environmentId === null) {
+    return;
+  }
+  const environment = getEnvironment(deps.db, thread.environmentId);
+  if (!environment) {
+    return;
+  }
+  const goneDetails = goneThreadEnvironmentDetails(environment);
+  if (goneDetails) {
+    throwThreadEnvironmentUnavailable(goneDetails);
+  }
+}
+
 async function createQueuedMessageForThread(
   deps: AppDeps,
   args: CreateQueuedMessageForThreadArgs,
 ): Promise<ThreadQueuedMessage> {
   const { payload, thread } = args;
   ensureThreadIsWritable(thread);
+  ensureThreadEnvironmentIsNotGone(deps, thread);
   await validatePromptAttachmentReferences({
     dataDir: deps.config.dataDir,
     input: payload.input,