diff --git a/packages/server/src/server/agent/providers/codex-app-server-agent.features.test.ts b/packages/server/src/server/agent/providers/codex-app-server-agent.features.test.ts
index 6fe71f9616..ff0dab49a6 100644
--- a/packages/server/src/server/agent/providers/codex-app-server-agent.features.test.ts
+++ b/packages/server/src/server/agent/providers/codex-app-server-agent.features.test.ts
@@ -521,6 +521,19 @@ describe("Codex app-server provider controls", () => {
     });
   });
 
+  test("removing the legacy Fast key after opening leaves the speed the session opened with", async () => {
+    const { session, appServer } = await createConnectedSession({
+      providerOptions: { fast_mode: true },
+    });
+    expect(session.openOptionsPatch).toEqual({ service_tier: "priority", fast_mode: null });
+
+    // The stored options now hold `service_tier`; `fast_mode` is an alias no longer in them.
+    await expect(session.applyOptions?.({ fast_mode: null })).resolves.toEqual({});
+
+    await session.startTurn("hello");
+    await expect(appServer.waitForTurnStart()).resolves.toMatchObject({ serviceTier: "priority" });
+  });
+
   test("a patch that sets another option keeps the running sandbox and an explicit speed", async () => {
     const { session, appServer } = await createConnectedSession({
       providerOptions: { sandbox_mode: "read-only", service_tier: "default", fast_mode: true },
diff --git a/packages/server/src/server/agent/providers/codex-app-server-agent.ts b/packages/server/src/server/agent/providers/codex-app-server-agent.ts
index e6f205c508..82dab386cc 100644
--- a/packages/server/src/server/agent/providers/codex-app-server-agent.ts
+++ b/packages/server/src/server/agent/providers/codex-app-server-agent.ts
@@ -4752,7 +4752,8 @@ export class CodexAppServerAgentSession implements AgentSession {
 
   private requestedTierFrom(options: ProviderOptions): string | null {
     if ("service_tier" in options) return readCodexServiceTier(options);
-    if (options.fast_mode === null) return null;
+    // Opening moved a legacy Fast into `service_tier`, so removing the alias changes no speed.
+    if (options.fast_mode === null) return this.requestedServiceTier;
     return options.fast_mode ? "fast" : "default";
   }