diff --git a/apps/web/src/components/chat/SideChatPane.tsx b/apps/web/src/components/chat/SideChatPane.tsx
index 1a5bd1141b31f..2f430e563925e 100644
--- a/apps/web/src/components/chat/SideChatPane.tsx
+++ b/apps/web/src/components/chat/SideChatPane.tsx
@@ -14,8 +14,10 @@ import {
 import { Trash2Icon, XIcon } from "lucide-react";
 import { useEffect, useState } from "react";
 
+import { useComposerDraftStore } from "../../composerDraftStore";
 import { ComposerHandleContext } from "../../composerHandleContext";
 import { useClientSettings } from "../../hooks/useSettings";
+import { releaseComposerDraftUploads } from "../../lib/composerDraftUploads";
 import type { SideChatParentPanel } from "../../lib/sideChatPane";
 import { readLocalApi } from "../../localApi";
 import { hostRightPanel, useRightPanelStore } from "../../rightPanelStore";
@@ -111,6 +113,8 @@ export default function SideChatPane(props: {
     });
     setDeleting(false);
     if (result._tag === "Success") {
+      releaseComposerDraftUploads(threadRef);
+      useComposerDraftStore.getState().clearDraftThread(threadRef);
       useRightPanelStore.getState().closeSurface(parentRef, `side-chat:${threadId}`);
     } else if (!isAtomCommandInterrupted(result)) {
       const error = squashAtomCommandFailure(result);
diff --git a/apps/web/src/hooks/useThreadActions.permissions.test.ts b/apps/web/src/hooks/useThreadActions.permissions.test.ts
index de5056c4af696..0dea57a8a651f 100644
--- a/apps/web/src/hooks/useThreadActions.permissions.test.ts
+++ b/apps/web/src/hooks/useThreadActions.permissions.test.ts
@@ -24,6 +24,8 @@ function deferred<T>() {
 }
 
 const state = vi.hoisted(() => ({
+  serverConfig: { shellThreadFamilies: false },
+  familyLookupFails: false,
   scopes: new Map<string, Set<string>>(),
   threads: [] as {
     environmentId: EnvironmentId;
@@ -31,9 +33,14 @@ const state = vi.hoisted(() => ({
     projectId: ProjectId;
     title: string;
     worktreePath: string | null;
+    lineage: { parentThreadId: ThreadId | null; sideChat?: boolean };
     session: { status: "ready" | "stopped" } | null;
     latestTurn: null;
   }[],
+  familyThreads: [] as {
+    id: ThreadId;
+    lineage: { parentThreadId: ThreadId | null; sideChat?: boolean };
+  }[],
   requests: [] as { action: string; environmentId: string; input: { threadId?: string } }[],
   localEffects: [] as string[],
   confirm: vi.fn<(message: string) => Promise<boolean>>(),
@@ -70,15 +77,30 @@ vi.mock("../state/use-atom-command", () => ({
     },
 }));
 vi.mock("../state/use-atom-query-runner", () => ({
-  useAtomQueryRunner: () => async (environmentId: string) =>
-    state.sessionLookupFails
-      ? AsyncResult.failure(Cause.fail(new Error("Session lookup failed")))
-      : AsyncResult.success({
-          authenticated: true,
-          scopes: [...(state.scopes.get(environmentId) ?? [])],
-          auth: { serverUpdateScope: "environment:maintain" },
-        }),
+  useAtomQueryRunner: (family: unknown) => async (environmentId: string) =>
+    family === "threadFamily"
+      ? state.familyLookupFails
+        ? AsyncResult.failure(Cause.fail(new Error("Family lookup failed")))
+        : AsyncResult.success({ threads: state.familyThreads })
+      : state.sessionLookupFails
+        ? AsyncResult.failure(Cause.fail(new Error("Session lookup failed")))
+        : AsyncResult.success({
+            authenticated: true,
+            scopes: [...(state.scopes.get(environmentId) ?? [])],
+            auth: { serverUpdateScope: "environment:maintain" },
+          }),
 }));
+vi.mock("../state/orchestration", () => ({
+  orchestrationEnvironment: { threadFamily: "threadFamily" },
+}));
+vi.mock("../state/server", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../state/server")>();
+  const { Atom } = await import("effect/reactivity");
+  return {
+    ...actual,
+    environmentServerConfigsAtom: Atom.make(new Map([["secondary", state.serverConfig]])),
+  };
+});
 vi.mock("../state/threads", () => ({
   threadEnvironment: Object.fromEntries(
     [
@@ -119,7 +141,8 @@ vi.mock("../components/Sidebar.logic", () => ({
 vi.mock("../composerDraftStore", () => ({
   useComposerDraftStore: (select: (store: unknown) => unknown) =>
     select({
-      clearDraftThread: () => state.localEffects.push("clear-draft"),
+      clearDraftThread: (ref: ScopedThreadRef) =>
+        state.localEffects.push(`clear-draft:${ref.threadId}`),
       clearProjectDraftThreadById: () => state.localEffects.push("clear-project-draft"),
     }),
 }));
@@ -139,7 +162,8 @@ vi.mock("../lib/archivedThreadsState", () => ({
   refreshArchivedThreadsForEnvironment: () => state.localEffects.push("refresh-archive"),
 }));
 vi.mock("../lib/composerDraftUploads", () => ({
-  releaseComposerDraftUploads: () => state.localEffects.push("release-uploads"),
+  releaseComposerDraftUploads: (ref: ScopedThreadRef) =>
+    state.localEffects.push(`release-uploads:${ref.threadId}`),
 }));
 vi.mock("../localApi", () => ({
   readLocalApi: () => ({ dialogs: { confirm: state.confirm } }),
@@ -227,6 +251,9 @@ const operations = [
 ] as const;
 
 beforeEach(() => {
+  state.serverConfig.shellThreadFamilies = false;
+  state.familyLookupFails = false;
+  state.familyThreads = [];
   state.scopes = new Map([
     [primary, new Set([AuthOrchestrationOperateScope])],
     [secondary, new Set<string>()],
@@ -238,6 +265,7 @@ beforeEach(() => {
       projectId: ProjectId.make("project"),
       title: "Thread",
       worktreePath: null,
+      lineage: { parentThreadId: null },
       session: null,
       latestTurn: null,
     },
@@ -308,6 +336,18 @@ describe("thread action permissions", () => {
   );
 
   it("deletes an archived thread only with its own environment's grant", async () => {
+    state.serverConfig.shellThreadFamilies = true;
+    state.familyThreads = [
+      {
+        id: ThreadId.make("side-chat"),
+        lineage: { parentThreadId: target.threadId, sideChat: true },
+      },
+      { id: ThreadId.make("fork"), lineage: { parentThreadId: target.threadId } },
+      {
+        id: ThreadId.make("other-side-chat"),
+        lineage: { parentThreadId: ThreadId.make("other-parent"), sideChat: true },
+      },
+    ];
     state.threads = [];
     seedThreadPanelState(target);
     const actions = useThreadActions();
@@ -320,6 +360,26 @@ describe("thread action permissions", () => {
       expect.objectContaining({ action: "delete", environmentId: secondary }),
     ]);
     expect(threadPanelStatePresence(target)).toEqual(panelStateCleared);
+    expect(state.localEffects).toEqual(
+      expect.arrayContaining([
+        "release-uploads:thread",
+        "clear-draft:thread",
+        "release-uploads:side-chat",
+        "clear-draft:side-chat",
+      ]),
+    );
+    expect(state.localEffects).not.toContain("clear-draft:fork");
+    expect(state.localEffects).not.toContain("clear-draft:other-side-chat");
+
+    // The family read only finds drafts to clear; it never blocks the delete.
+    state.familyLookupFails = true;
+    state.requests = [];
+    state.localEffects = [];
+    expect((await actions.deleteThread(target))._tag).toBe("Success");
+    expect(state.requests).toEqual([
+      expect.objectContaining({ action: "delete", environmentId: secondary }),
+    ]);
+    expect(state.localEffects).toContain("clear-draft:thread");
   });
 
   it("stops before delete and local cleanup when permission is revoked during session stop", async () => {
@@ -348,6 +408,24 @@ describe("thread action permissions", () => {
     expect(threadPanelStatePresence(target)).toEqual(panelStateCleared);
   });
 
+  it("clears the drafts of the side chats deleted with it", async () => {
+    state.scopes.get(secondary)!.add(AuthOrchestrationOperateScope);
+    const parent = state.threads[0]!;
+    state.threads.push(
+      {
+        ...parent,
+        id: ThreadId.make("side-chat"),
+        lineage: { parentThreadId: parent.id, sideChat: true },
+      },
+      { ...parent, id: ThreadId.make("fork"), lineage: { parentThreadId: parent.id } },
+    );
+    expect((await useThreadActions().deleteThread(target))._tag).toBe("Success");
+    expect(state.localEffects).toEqual(
+      expect.arrayContaining(["release-uploads:side-chat", "clear-draft:side-chat"]),
+    );
+    expect(state.localEffects).not.toContain("clear-draft:fork");
+  });
+
   it("does not request worktree removal after its grant is revoked during delete", async () => {
     state.scopes
       .get(secondary)!
diff --git a/apps/web/src/hooks/useThreadActions.ts b/apps/web/src/hooks/useThreadActions.ts
index e5eedffc16698..9b7f0c50bd323 100644
--- a/apps/web/src/hooks/useThreadActions.ts
+++ b/apps/web/src/hooks/useThreadActions.ts
@@ -325,6 +325,10 @@ export function useThreadActions() {
     reportFailure: false,
     refresh: true,
   });
+  const loadThreadFamily = useAtomQueryRunner(orchestrationEnvironment.threadFamily, {
+    reportFailure: false,
+    refresh: true,
+  });
   const refreshVcsStatus = useAtomCommand(vcsEnvironment.refreshStatus, {
     reportFailure: false,
   });
@@ -440,11 +444,37 @@ export function useThreadActions() {
     if (permissionFailure) return permissionFailure;
     const resolved = resolveThreadTarget(target);
     if (!resolved) {
+      let threads = readEnvironmentThreadRefs(target.environmentId).flatMap((ref) => {
+        const shell = readThreadShell(ref);
+        return shell === null ? [] : [{ id: shell.id, lineage: shell.lineage }];
+      });
+      if (
+        appAtomRegistry.get(environmentServerConfigsAtom).get(target.environmentId)
+          ?.shellThreadFamilies
+      ) {
+        const family = await loadThreadFamily({
+          environmentId: target.environmentId,
+          input: { threadId: target.threadId },
+        });
+        if (family._tag === "Success") threads = [...family.value.threads];
+      }
       const result = await deleteThreadMutation({
         environmentId: target.environmentId,
         input: { threadId: target.threadId },
       });
       if (result._tag === "Success") {
+        for (const ref of [
+          target,
+          ...threads
+            .filter(
+              (entry) =>
+                entry.lineage.sideChat === true && entry.lineage.parentThreadId === target.threadId,
+            )
+            .map((entry) => scopeThreadRef(target.environmentId, entry.id)),
+        ]) {
+          releaseComposerDraftUploads(ref);
+          clearComposerDraftForThread(ref);
+        }
         expireThreadUndos(target, [...LIST_PLACEMENT_UNDO_KINDS, "archive"]);
         refreshArchivedThreadsForEnvironment(target.environmentId);
         clearDeletedThreadPanelState(target);
@@ -566,6 +596,15 @@ export function useThreadActions() {
     refreshArchivedThreadsForEnvironment(threadRef.environmentId);
     releaseComposerDraftUploads(threadRef);
     clearComposerDraftForThread(threadRef);
+    // The server deletes the thread's side chats with it.
+    for (const entry of threads) {
+      if (entry.lineage.sideChat !== true || entry.lineage.parentThreadId !== threadRef.threadId) {
+        continue;
+      }
+      const sideChatRef = scopeThreadRef(threadRef.environmentId, entry.id);
+      releaseComposerDraftUploads(sideChatRef);
+      clearComposerDraftForThread(sideChatRef);
+    }
     clearProjectDraftThreadById(
       scopeProjectRef(threadRef.environmentId, thread.projectId),
       threadRef,
diff --git a/packages/client-runtime/src/state/orchestration.ts b/packages/client-runtime/src/state/orchestration.ts
index 94dc18582a56e..2e465aad9a47a 100644
--- a/packages/client-runtime/src/state/orchestration.ts
+++ b/packages/client-runtime/src/state/orchestration.ts
@@ -151,6 +151,12 @@ export function createOrchestrationEnvironmentAtoms<R, E>(
       label: "environment-data:orchestration:archived-shell-snapshot",
       tag: ORCHESTRATION_V2_WS_METHODS.getArchivedShellSnapshot,
     }),
+    threadFamily: createEnvironmentRpcQueryAtomFamily(runtime, {
+      label: "environment-data:orchestration:thread-family-for-delete",
+      tag: ORCHESTRATION_V2_WS_METHODS.getThreadFamily,
+      staleTimeMs: 0,
+      idleTtlMs: 0,
+    }),
     // Read right before deleting a worktree, so never served from cache.
     worktreeThreadIds: createEnvironmentRpcQueryAtomFamily(runtime, {
       label: "environment-data:orchestration:worktree-thread-ids",