diff --git a/apps/server/src/services/threads/thread-archive.ts b/apps/server/src/services/threads/thread-archive.ts
index 16f30ead3d..46744addc4 100644
--- a/apps/server/src/services/threads/thread-archive.ts
+++ b/apps/server/src/services/threads/thread-archive.ts
@@ -1,4 +1,5 @@
 import {
+  archiveThread,
   listLiveThreadsInEnvironment,
   listUnarchivedAssignedChildThreads,
   listUnarchivedHiddenSourceThreads,
@@ -33,11 +34,15 @@ interface ArchiveThreadEnvironment {
   id: string;
 }
 
-interface ArchiveThreadWithLifecycleEffectsArgs {
+interface ArchiveThreadTarget {
   environment: ArchiveThreadEnvironment | null;
   thread: Pick<Thread, "environmentId" | "id" | "status">;
 }
 
+interface ArchiveThreadWithLifecycleEffectsArgs extends ArchiveThreadTarget {
+  releaseChildren: boolean;
+}
+
 interface ResolveArchiveThreadEnvironmentArgs {
   thread: ArchiveThreadWithLifecycleEffectsArgs["thread"];
 }
@@ -85,9 +90,9 @@ function archiveThreadWithLifecycleEffects(
   deps: AppDeps,
   args: ArchiveThreadWithLifecycleEffectsArgs,
 ): Thread | null {
-  const archivedThread = archiveThreadAndReleaseChildren(deps, {
-    threadId: args.thread.id,
-  });
+  const archivedThread = args.releaseChildren
+    ? archiveThreadAndReleaseChildren(deps, { threadId: args.thread.id })
+    : archiveThread(deps.db, deps.hub, args.thread.id);
   if (!archivedThread) {
     return null;
   }
@@ -126,9 +131,12 @@ function archiveThreadWithLifecycleEffects(
  */
 export function archiveThreadAndHiddenSourceForks(
   deps: AppDeps,
-  args: ArchiveThreadWithLifecycleEffectsArgs,
+  args: ArchiveThreadTarget,
 ): Thread | null {
-  const archivedThread = archiveThreadWithLifecycleEffects(deps, args);
+  const archivedThread = archiveThreadWithLifecycleEffects(deps, {
+    ...args,
+    releaseChildren: true,
+  });
   if (!archivedThread) {
     return null;
   }
@@ -137,6 +145,7 @@ export function archiveThreadAndHiddenSourceForks(
   })) {
     archiveThreadWithLifecycleEffects(deps, {
       environment: resolveArchiveThreadEnvironment(deps, { thread: fork }),
+      releaseChildren: true,
       thread: fork,
     });
   }
@@ -155,6 +164,9 @@ export function archiveEnvironmentThreads(
   for (const thread of threads) {
     const result = archiveThreadWithLifecycleEffects(deps, {
       environment: args.environment,
+      // This operation is scoped to one worktree. Releasing a live child in
+      // another environment would mutate ownership outside that scope.
+      releaseChildren: false,
       thread,
     });
     if (!result) {
@@ -206,6 +218,7 @@ export function archiveThreadAndChildren(
     const environment = resolveArchiveThreadEnvironment(deps, { thread });
     const result = archiveThreadWithLifecycleEffects(deps, {
       environment,
+      releaseChildren: true,
       thread,
     });
     if (!result) {
diff --git a/apps/server/test/public/public-thread-parenting.test.ts b/apps/server/test/public/public-thread-parenting.test.ts
index fc6d439cd8..577601273f 100644
--- a/apps/server/test/public/public-thread-parenting.test.ts
+++ b/apps/server/test/public/public-thread-parenting.test.ts
@@ -2,6 +2,7 @@ import { getThread } from "@bb/db";
 import { threadSchema } from "@bb/domain";
 import {
   apiErrorSchema,
+  environmentArchiveThreadsResponseSchema,
   sidebarBootstrapResponseSchema,
   threadArchiveAllResponseSchema,
   threadChildSummaryResponseSchema,
@@ -407,6 +408,54 @@ describe("public thread parenting routes", () => {
     });
   });
 
+  it("preserves child ownership outside an archived worktree", async () => {
+    await withTestHarness(async (harness) => {
+      const { host } = seedHostSession(harness.deps);
+      const { project } = seedProjectWithSource(harness.deps, {
+        hostId: host.id,
+      });
+      const parentEnvironment = seedEnvironment(harness.deps, {
+        hostId: host.id,
+        managed: true,
+        projectId: project.id,
+        workspaceProvisionType: "managed-worktree",
+      });
+      const childEnvironment = seedEnvironment(harness.deps, {
+        hostId: host.id,
+        managed: true,
+        path: "/tmp/child-test-environment",
+        projectId: project.id,
+        workspaceProvisionType: "managed-worktree",
+      });
+      const parentThread = seedThread(harness.deps, {
+        environmentId: parentEnvironment.id,
+        projectId: project.id,
+      });
+      const childThread = seedThread(harness.deps, {
+        environmentId: childEnvironment.id,
+        parentThreadId: parentThread.id,
+        projectId: project.id,
+      });
+
+      const response = await harness.app.request(
+        `/api/v1/environments/${parentEnvironment.id}/archive-threads`,
+        { method: "POST" },
+      );
+
+      expect(response.status).toBe(200);
+      expect(
+        environmentArchiveThreadsResponseSchema.parse(await readJson(response)),
+      ).toEqual({
+        ok: true,
+        archivedThreadIds: [parentThread.id],
+      });
+      expect(getThread(harness.db, parentThread.id)?.archivedAt).not.toBeNull();
+      const preservedChild = getThread(harness.db, childThread.id);
+      expect(preservedChild?.archivedAt).toBeNull();
+      expect(preservedChild?.parentThreadId).toBe(parentThread.id);
+    });
+  });
+
   // Archiving one thread cascades too, not just archive-all: the plugin's
   // `thread.archived` listener used to cover this route.
   it("archives hidden source-derived forks when archiving a single thread", async () => {