diff --git a/server/src/__tests__/computer-routes.test.ts b/server/src/__tests__/computer-routes.test.ts
new file mode 100644
index 0000000000..771cf2dbea
--- /dev/null
+++ b/server/src/__tests__/computer-routes.test.ts
@@ -0,0 +1,98 @@
+import { randomUUID } from "node:crypto";
+import express from "express";
+import request from "supertest";
+import { eq } from "drizzle-orm";
+import { afterAll, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
+import { agents, companies, createDb, environmentLeases, environments, issues, type Db, EMBEDDED_POSTGRES_TEST_TIMEOUT_MS } from "@paperclipai/db";
+import { computerRoutes } from "../routes/computers.js";
+import type { computerService } from "../modules/computers/index.js";
+import { errorHandler } from "../middleware/index.js";
+import { getEmbeddedPostgresTestSupport, startEmbeddedPostgresTestDatabase } from "./helpers/embedded-postgres.js";
+
+const settings = vi.hoisted(() => ({ get: vi.fn(), getExperimental: vi.fn() }));
+vi.mock("../services/instance-settings.js", () => ({ instanceSettingsService: () => settings }));
+vi.mock("../services/activity-log.js", () => ({ logActivity: vi.fn() }));
+
+const support = await getEmbeddedPostgresTestSupport();
+if (!support.supported) console.warn(`Skipping computer route database tests: ${support.reason}`);
+describe.skipIf(!support.supported)("task computer environment selection", () => {
+  let database: Awaited<ReturnType<typeof startEmbeddedPostgresTestDatabase>>;
+  let db: Db;
+  beforeAll(async () => {
+    database = await startEmbeddedPostgresTestDatabase("paperclip-computer-routes");
+    db = createDb(database.connectionString);
+  }, EMBEDDED_POSTGRES_TEST_TIMEOUT_MS);
+  afterAll(async () => { await db?.$client.end(); await database?.cleanup(); });
+  beforeEach(() => {
+    settings.get.mockResolvedValue({ defaultEnvironmentId: null });
+    settings.getExperimental.mockResolvedValue({ enableBoatEnvironments: true });
+  });
+
+  async function fixture(driver = "computer") {
+    const [company] = await db.insert(companies).values({ name: "Computer routes", issuePrefix: randomUUID().slice(0, 8) }).returning();
+    const [oldEnvironment] = await db.insert(environments).values({ name: randomUUID(), driver: "computer", metadata: { computerCompanyId: company.id } }).returning();
+    const [created] = await db.insert(environments).values({ name: randomUUID(), driver, metadata: driver === "computer" ? { computerCompanyId: company.id } : {} }).onConflictDoNothing().returning();
+    const selected = created ?? (await db.select().from(environments).where(eq(environments.driver, "local")))[0]!;
+    const [agent] = await db.insert(agents).values({ companyId: company.id, name: "Assigned", adapterType: "process", defaultEnvironmentId: selected.id }).returning();
+    const [issue] = await db.insert(issues).values({ companyId: company.id, title: "Moved task", assigneeAgentId: agent.id }).returning();
+    const owner = { computerId: randomUUID(), ownerId: randomUUID(), generation: 1 };
+    await db.insert(environmentLeases).values({ companyId: company.id, issueId: issue.id, environmentId: oldEnvironment.id,
+      provider: "boat", status: "released", providerLeaseId: owner.ownerId, metadata: { computerOwner: owner } });
+    const computers = { connect: vi.fn(async () => ({ owner })), renewViewer: vi.fn(), disconnectViewer: vi.fn(), preview: vi.fn() };
+    const app = express();
+    app.use(express.json());
+    app.use((req, _res, next) => { req.actor = { type: "board", source: "local_implicit", userId: "board" }; next(); });
+    app.use("/api", computerRoutes(db, computers as unknown as ReturnType<typeof computerService>));
+    app.use(errorHandler);
+    return { app, company, issue, agent, oldEnvironment, selected, owner, computers, base: `/api/issues/${issue.id}/computer` };
+  }
+
+  it.each(["local", "ssh", "sandbox"])("hides the old Boat after selecting %s and refuses reconnect", async (driver) => {
+    const f = await fixture(driver);
+    await db.insert(environmentLeases).values({ companyId: f.company.id, issueId: f.issue.id, environmentId: f.selected.id, provider: driver, status: "released" });
+    expect((await request(f.app).get(f.base)).body).toBeNull();
+    expect((await request(f.app).post(`${f.base}/connect`).send({ environmentId: f.oldEnvironment.id })).status).toBe(404);
+    expect(f.computers.connect).not.toHaveBeenCalled();
+  });
+
+  it("selects Boat B before its first run and cannot preview Boat A's lease", async () => {
+    const f = await fixture();
+    expect((await request(f.app).get(f.base)).body).toEqual({ environmentId: f.selected.id, name: f.selected.name });
+    expect((await request(f.app).post(`${f.base}/connect`).send({ environmentId: f.oldEnvironment.id })).status).toBe(404);
+    expect((await request(f.app).post(`${f.base}/connect`).send({ environmentId: f.selected.id })).status).toBe(200);
+    expect(f.computers.connect).toHaveBeenCalledWith(expect.objectContaining({ companyId: f.company.id, environmentId: f.selected.id }));
+    expect((await request(f.app).post(`${f.base}/preview`).send({ environmentId: f.selected.id, port: 5173 })).status).toBe(409);
+    expect(f.computers.preview).not.toHaveBeenCalled();
+  });
+
+  it("uses the current instance default when the agent has no selection", async () => {
+    const f = await fixture();
+    await db.update(agents).set({ defaultEnvironmentId: null }).where(eq(agents.id, f.agent.id));
+    settings.get.mockResolvedValue({ defaultEnvironmentId: f.selected.id });
+    expect((await request(f.app).get(f.base)).body).toEqual({ environmentId: f.selected.id, name: f.selected.name });
+  });
+
+  it("previews only the current environment's lease, even when an old computer's lease is newer", async () => {
+    const f = await fixture();
+    const owner = { computerId: randomUUID(), ownerId: randomUUID(), generation: 2 };
+    await db.insert(environmentLeases).values({ companyId: f.company.id, issueId: f.issue.id, environmentId: f.selected.id,
+      provider: "boat", providerLeaseId: owner.ownerId, metadata: { computerOwner: owner }, createdAt: new Date(0) });
+    f.computers.preview.mockResolvedValue({ url: "https://example.test/preview" });
+    expect((await request(f.app).post(`${f.base}/preview`).send({ environmentId: f.selected.id, port: 5173 })).status).toBe(200);
+    expect(f.computers.preview).toHaveBeenCalledWith({ companyId: f.company.id, environmentId: f.selected.id, owner, port: 5173 });
+  });
+
+  it("hides historical Boat leases when both current defaults select local implicitly", async () => {
+    const f = await fixture();
+    await db.update(agents).set({ defaultEnvironmentId: null }).where(eq(agents.id, f.agent.id));
+    expect((await request(f.app).get(f.base)).body).toBeNull();
+  });
+
+  it("keeps disconnect scoped to the posted former owner after reassignment and feature disable", async () => {
+    const f = await fixture("ssh");
+    settings.getExperimental.mockResolvedValue({ enableBoatEnvironments: false });
+    expect((await request(f.app).post(`${f.base}/disconnect`).send({ environmentId: f.oldEnvironment.id, owner: f.owner })).status).toBe(204);
+    expect(f.computers.disconnectViewer).toHaveBeenCalledWith({ companyId: f.company.id, environmentId: f.oldEnvironment.id, owner: f.owner, userId: "board" });
+    expect(f.computers.connect).not.toHaveBeenCalled();
+  });
+});
diff --git a/server/src/routes/computers.ts b/server/src/routes/computers.ts
index e00115136e..702c01469c 100644
--- a/server/src/routes/computers.ts
+++ b/server/src/routes/computers.ts
@@ -39,19 +39,20 @@ export function computerRoutes(db: Db, computers = computerService(db)) {
       if (req.method === "GET") { res.json(null); return; }
       throw unprocessable("Boat environments are disabled.");
     }
-    const [lease] = await db.select().from(environmentLeases)
-      .where(and(eq(environmentLeases.companyId, issue.companyId), eq(environmentLeases.issueId, issue.id), eq(environmentLeases.provider, "boat")))
-      // Every computer admission inserts a lease. Finalization can update an older
-      // lease later, but must not make it the task's current authority again.
-      .orderBy(desc(environmentLeases.createdAt), desc(environmentLeases.id)).limit(1);
     const [agent] = issue.assigneeAgentId ? await db.select().from(agents)
       .where(and(eq(agents.companyId, issue.companyId), eq(agents.id, issue.assigneeAgentId))).limit(1) : [];
-    const environmentId = lease?.environmentId ?? agent?.defaultEnvironmentId ?? (await settings.get()).defaultEnvironmentId;
+    const environmentId = agent?.defaultEnvironmentId ?? (await settings.get()).defaultEnvironmentId;
     const [environment] = environmentId ? await db.select().from(environments).where(eq(environments.id, environmentId)).limit(1) : [];
     if (!environment || environment.driver !== "computer" || environment.metadata?.computerCompanyId !== issue.companyId || environment.status !== "active") {
       if (req.method === "GET") { res.json(null); return; }
       throw notFound("Computer not available for this task");
     }
+    const [lease] = await db.select().from(environmentLeases)
+      .where(and(eq(environmentLeases.companyId, issue.companyId), eq(environmentLeases.issueId, issue.id),
+        eq(environmentLeases.environmentId, environment.id), eq(environmentLeases.provider, "boat")))
+      // A historical lease cannot select the task's computer. Within the current
+      // selection, later finalization must not promote an older admission.
+      .orderBy(desc(environmentLeases.createdAt), desc(environmentLeases.id)).limit(1);
     res.locals.computer = { environment, issue, lease, agent };
     next();
   });