diff --git a/apps/server/src/provider/CodexAccountSwitch.test.ts b/apps/server/src/provider/CodexAccountSwitch.test.ts
new file mode 100644
index 0000000000000..e72872eeaa25e
--- /dev/null
+++ b/apps/server/src/provider/CodexAccountSwitch.test.ts
@@ -0,0 +1,88 @@
+import { assert, it } from "@effect/vitest";
+import {
+  ProviderDriverKind,
+  ProviderInstanceId,
+  type ServerProvider,
+  type ServerProviderAuth,
+} from "@t3tools/contracts";
+import * as Effect from "effect/Effect";
+import * as Layer from "effect/Layer";
+import * as Queue from "effect/Queue";
+import * as Stream from "effect/Stream";
+
+import * as ProviderSessionManager from "../orchestration-v2/ProviderSessionManager.ts";
+import * as CodexAccountSwitch from "./CodexAccountSwitch.ts";
+import * as ProviderRegistry from "./ProviderRegistry.ts";
+
+const codex = ProviderInstanceId.make("codex");
+const codexWork = ProviderInstanceId.make("codex-work");
+const claude = ProviderInstanceId.make("claude");
+
+const provider = (
+  instanceId: ProviderInstanceId,
+  auth: ServerProviderAuth,
+  driver = ProviderDriverKind.make("codex"),
+): ServerProvider => ({
+  instanceId,
+  driver,
+  enabled: true,
+  installed: true,
+  version: "1.0.0",
+  status: "ready",
+  auth,
+  checkedAt: "2026-11-12T00:00:00.000Z",
+  models: [],
+  slashCommands: [],
+  skills: [],
+});
+
+const signedIn = (email: string, workspaceId?: string): ServerProviderAuth => ({
+  status: "authenticated",
+  email,
+  ...(workspaceId === undefined ? {} : { workspaceId }),
+});
+
+it.effect("closes a Codex instance's sessions only when its signed-in account changes", () =>
+  Effect.gen(function* () {
+    const changes = yield* Queue.unbounded<ReadonlyArray<ServerProvider>>();
+    const closed = yield* Queue.unbounded<ProviderInstanceId>();
+    const claudeDriver = ProviderDriverKind.make("claude");
+    yield* Layer.build(
+      CodexAccountSwitch.layer.pipe(
+        Layer.provide(
+          Layer.mergeAll(
+            Layer.mock(ProviderRegistry.ProviderRegistry)({
+              getProviders: Effect.succeed([
+                provider(codex, signedIn("alice@example.com", "ws-alice")),
+                provider(codexWork, signedIn("team@example.com", "ws-personal")),
+              ]),
+              streamChanges: Stream.fromQueue(changes),
+            }),
+            Layer.mock(ProviderSessionManager.ProviderSessionManagerV2)({
+              closeInstance: (instanceId) => Queue.offer(closed, instanceId).pipe(Effect.asVoid),
+            }),
+          ),
+        ),
+      ),
+    );
+
+    yield* Queue.offerAll(changes, [
+      // A token refresh rewrites auth.json without changing the account.
+      [provider(codex, signedIn("alice@example.com", "ws-alice"))],
+      [provider(codex, signedIn("alice@example.com"))],
+      [provider(codex, { status: "unknown" })],
+      [provider(claude, signedIn("one@example.com"), claudeDriver)],
+      [provider(claude, signedIn("two@example.com"), claudeDriver)],
+      [
+        provider(codex, signedIn("bob@example.com")),
+        provider(codexWork, signedIn("team@example.com", "ws-team")),
+      ],
+    ]);
+
+    assert.deepStrictEqual(
+      [yield* Queue.take(closed), yield* Queue.take(closed)],
+      [codex, codexWork],
+    );
+    assert.strictEqual(yield* Queue.size(closed), 0);
+  }).pipe(Effect.scoped),
+);
diff --git a/apps/server/src/provider/CodexAccountSwitch.ts b/apps/server/src/provider/CodexAccountSwitch.ts
new file mode 100644
index 0000000000000..5bb4721448e6c
--- /dev/null
+++ b/apps/server/src/provider/CodexAccountSwitch.ts
@@ -0,0 +1,75 @@
+import {
+  ProviderDriverKind,
+  type ProviderInstanceId,
+  type ServerProvider,
+} from "@t3tools/contracts";
+import * as Effect from "effect/Effect";
+import * as Layer from "effect/Layer";
+import * as Stream from "effect/Stream";
+
+import * as ProviderSessionManager from "../orchestration-v2/ProviderSessionManager.ts";
+import * as ProviderRegistry from "./ProviderRegistry.ts";
+
+const CODEX_DRIVER = ProviderDriverKind.make("codex");
+
+interface CodexAccount {
+  readonly email: string;
+  readonly workspaceId: string | undefined;
+}
+
+const signedInAccount = (provider: ServerProvider): CodexAccount | undefined =>
+  provider.driver === CODEX_DRIVER &&
+  provider.auth.status === "authenticated" &&
+  provider.auth.email !== undefined
+    ? { email: provider.auth.email, workspaceId: provider.auth.workspaceId }
+    : undefined;
+
+// The workspace id comes from an optional rate-limit read, so compare it only when both have one.
+const isSameAccount = (previous: CodexAccount, next: CodexAccount) =>
+  previous.email === next.email &&
+  (previous.workspaceId === undefined ||
+    next.workspaceId === undefined ||
+    previous.workspaceId === next.workspaceId);
+
+const recordAccounts = (
+  accounts: Map<ProviderInstanceId, CodexAccount>,
+  providers: ReadonlyArray<ServerProvider>,
+): ReadonlyArray<ProviderInstanceId> => {
+  const switched: Array<ProviderInstanceId> = [];
+  for (const provider of providers) {
+    const next = signedInAccount(provider);
+    if (next === undefined) continue;
+    const previous = accounts.get(provider.instanceId);
+    if (previous !== undefined && !isSameAccount(previous, next)) {
+      switched.push(provider.instanceId);
+    }
+    accounts.set(provider.instanceId, {
+      email: next.email,
+      workspaceId:
+        next.workspaceId ?? (previous?.email === next.email ? previous.workspaceId : undefined),
+    });
+  }
+  return switched;
+};
+
+// A Codex app-server reads auth.json once, so after `codex login` switches accounts outside T3
+// its live sessions keep the old account until they are closed.
+export const layer = Layer.effectDiscard(
+  Effect.gen(function* () {
+    const providers = yield* ProviderRegistry.ProviderRegistry;
+    const sessions = yield* ProviderSessionManager.ProviderSessionManagerV2;
+    const accounts = new Map<ProviderInstanceId, CodexAccount>();
+    recordAccounts(accounts, yield* providers.getProviders);
+    yield* providers.streamChanges.pipe(
+      Stream.runForEach((snapshot) =>
+        Effect.forEach(
+          recordAccounts(accounts, snapshot),
+          (instanceId) =>
+            sessions.closeInstance(instanceId).pipe(Effect.ignoreCause({ log: true })),
+          { discard: true },
+        ),
+      ),
+      Effect.forkScoped,
+    );
+  }),
+);
diff --git a/apps/server/src/server.ts b/apps/server/src/server.ts
index 293d6670bbf0b..b4b541e73115c 100644
--- a/apps/server/src/server.ts
+++ b/apps/server/src/server.ts
@@ -114,6 +114,7 @@ import * as ProviderInstallationGate from "./provider/ProviderInstallationGate.t
 import * as ProviderInstanceRegistry from "./provider/ProviderInstanceRegistry.ts";
 import * as ProviderAdapterRegistry from "./orchestration-v2/ProviderAdapterRegistry.ts";
 import * as ProviderRegistry from "./provider/ProviderRegistry.ts";
+import * as CodexAccountSwitch from "./provider/CodexAccountSwitch.ts";
 import * as ProviderUsageLimitsIngestion from "./provider/ProviderUsageLimitsIngestion.ts";
 import * as UsageLimitSources from "./usage/UsageLimitSources.ts";
 import * as ProjectFaviconResolver from "./project/ProjectFaviconResolver.ts";
@@ -682,6 +683,7 @@ const layerRuntimeCoreDependenciesBase = Layer.mergeAll(
   ).pipe(Layer.provide(DelegatedTaskQuestionReactor.layer), Layer.provide(ProjectionStoreV2.layer)),
   ProviderUsageLimitsIngestion.layer,
   layerProviderInstallationRefresh,
+  CodexAccountSwitch.layer,
   ProviderInstallation.layer,
   ReplayMarkers.layer,
   Layer.effectDiscard(ProjectSettingsOverrideSweep.sweep),