diff --git a/packages/adapter-utils/src/execution-target.test.ts b/packages/adapter-utils/src/execution-target.test.ts
index 46a4df7d6d..88919a99f3 100644
--- a/packages/adapter-utils/src/execution-target.test.ts
+++ b/packages/adapter-utils/src/execution-target.test.ts
@@ -5,6 +5,9 @@ import * as ssh from "./ssh.js";
 import * as serverUtils from "./server-utils.js";
 import {
   cleanupGitHubOperationLaunchers,
+  adapterExecutionTargetRuntimeRoot,
+  prepareAdapterExecutionTargetRuntime,
+  type AdapterComputerExecutionTarget,
   prepareGitHubOperationLaunchers,
   adapterExecutionTargetUsesManagedHome,
   ensureAdapterExecutionTargetRuntimeCommandInstalled,
@@ -437,3 +440,36 @@ describe("GitHub launcher lifecycle", () => {
     expect(runner.execute).toHaveBeenCalledTimes(1);
   });
 });
+
+
+describe("computer legacy runtime ownership", () => {
+  it.each(["claude", "codex"])("isolates %s assets and callback roots for owners sharing a checkout", async adapterKey => {
+    const execute = vi.fn(async () => ({ exitCode: 0, signal: null, timedOut: false, stdout: "", stderr: "", pid: 123, startedAt: "2026-10-10T00:00:00Z" }));
+    const target = (ownerId: string, generation = 1) => ({
+      kind: "remote", transport: "computer", providerKey: "boat", environmentId: "environment",
+      leaseId: ownerId, remoteCwd: "/shared/project", listenerPort: 43127,
+      runner: { execute }, processRunner: { execute },
+      resourceAuthority: { kind: "computer-owner", computerId: "computer", ownerId, generation },
+      fileAuthority: { kind: "remote-persistent", placementId: "shared", root: "/shared/project", agentHome: `/homes/${ownerId}` },
+      launch: vi.fn(), inspectProcess: vi.fn(), retainWarm: vi.fn(), retire: vi.fn(),
+      computerTool: { command: "cua-driver", args: ["mcp"] },
+    } satisfies AdapterComputerExecutionTarget);
+    const prepare = (owner: AdapterComputerExecutionTarget) => prepareAdapterExecutionTargetRuntime({
+      target: owner, runId: owner.leaseId!, adapterKey, workspaceLocalDir: "/unused-local-workspace", syncWorkspace: false,
+    });
+    const first = await prepare(target("agent-one"));
+    const second = await prepare(target("agent-two"));
+    expect(first.workspaceRemoteDir).toBe("/shared/project");
+    expect(second.workspaceRemoteDir).toBe(first.workspaceRemoteDir);
+    expect(first.runtimeRootDir).toBe(adapterExecutionTargetRuntimeRoot(target("agent-one"), adapterKey));
+    expect(second.runtimeRootDir).not.toBe(first.runtimeRootDir);
+    expect(adapterExecutionTargetRuntimeRoot(target("agent-one", 2), adapterKey)).not.toBe(first.runtimeRootDir);
+    expect((await prepare(target("agent-one"))).runtimeRootDir).toBe(first.runtimeRootDir);
+    expect(first.runtimeRootDir).toMatch(new RegExp(`^/shared/project/\\.paperclip-runtime/${adapterKey}/owners/[a-f0-9]{64}$`));
+  });
+
+  it("preserves the existing sandbox runtime root", () => {
+    const target = { kind: "remote", transport: "sandbox", remoteCwd: "/workspace" } as const;
+    expect(adapterExecutionTargetRuntimeRoot(target, "claude")).toBe("/workspace/.paperclip-runtime/claude");
+  });
+});
diff --git a/packages/adapter-utils/src/execution-target.ts b/packages/adapter-utils/src/execution-target.ts
index dbffa195aa..73595d5cfa 100644
--- a/packages/adapter-utils/src/execution-target.ts
+++ b/packages/adapter-utils/src/execution-target.ts
@@ -6,7 +6,7 @@ import { promisify } from "node:util";
 import net from "node:net";
 import os from "node:os";
 import path from "node:path";
-import { randomBytes, randomUUID } from "node:crypto";
+import { createHash, randomBytes, randomUUID } from "node:crypto";
 import { githubLauncherSource } from "./github-launcher.js";
 import type { SshRemoteExecutionSpec } from "./ssh.js";
 import {
@@ -1442,6 +1442,20 @@ export function readAdapterExecutionTarget(input: {
   );
 }
 
+/** Keep mutable legacy assets separate for owners sharing the same checkout. */
+export function adapterExecutionTargetRuntimeRoot(
+  target: Extract<AdapterExecutionTarget, { kind: "remote" }>,
+  adapterKey: string,
+  baseRoot = path.posix.join(target.remoteCwd, ".paperclip-runtime", adapterKey),
+): string {
+  if (target.transport !== "computer") return baseRoot;
+  const owner = target.resourceAuthority;
+  const scope = createHash("sha256").update(JSON.stringify([
+    owner.computerId, owner.ownerId, owner.generation,
+  ])).digest("hex");
+  return path.posix.join(baseRoot, "owners", scope);
+}
+
 export async function prepareAdapterExecutionTargetRuntime(input: {
   runId: string;
   target: AdapterExecutionTarget | null | undefined;
@@ -1535,7 +1549,9 @@ export async function prepareAdapterExecutionTargetRuntime(input: {
     adapterKey: input.adapterKey,
     workspaceLocalDir: input.workspaceLocalDir,
     workspaceRemoteDir: input.workspaceRemoteDir,
-    runtimeRootDir: input.runtimeRootDir,
+    runtimeRootDir: target.transport === "computer"
+      ? adapterExecutionTargetRuntimeRoot(target, input.adapterKey, input.runtimeRootDir)
+      : input.runtimeRootDir,
     syncWorkspace: input.syncWorkspace,
     workspaceInboundMode: input.workspaceInboundMode,
     workspaceDurableSeed: input.workspaceDurableSeed,
@@ -2033,7 +2049,7 @@ export async function startAdapterExecutionTargetProcessSessionBridge(input: {
       ? Math.trunc(input.timeoutSec * 1000)
       : target.timeoutMs ?? undefined;
   const bridgeRuntimeDir = path.posix.join(
-    input.runtimeRootDir?.trim() || path.posix.join(target.remoteCwd, ".paperclip-runtime", input.adapterKey),
+    input.runtimeRootDir?.trim() || adapterExecutionTargetRuntimeRoot(target, input.adapterKey),
     "process-sessions",
   );
   const sessionId = randomUUID();
@@ -4402,7 +4418,7 @@ export async function startAdapterExecutionTargetPaperclipBridge(input: {
   const runtimeRootDir =
     input.runtimeRootDir?.trim().length
       ? input.runtimeRootDir.trim()
-      : path.posix.join(target.remoteCwd, ".paperclip-runtime", input.adapterKey);
+      : adapterExecutionTargetRuntimeRoot(target, input.adapterKey);
   const bridgeRuntimeDir = path.posix.join(runtimeRootDir, "paperclip-bridge");
   const queueDir = path.posix.join(bridgeRuntimeDir, "queue");
   const assetRemoteDir = path.posix.join(bridgeRuntimeDir, "server");