diff --git a/packages/server/src/server/agent/plugin-provider.test.ts b/packages/server/src/server/agent/plugin-provider.test.ts
index 043058692c..fd15cadafb 100644
--- a/packages/server/src/server/agent/plugin-provider.test.ts
+++ b/packages/server/src/server/agent/plugin-provider.test.ts
@@ -1136,6 +1136,18 @@ describe("PluginAgentClientRegistry", () => {
       await session.close();
     });
 
+    test("rejects a key the plugin does not report as a setting, and configures none of a mixed patch", async () => {
+      const { harness, session } = await openStubbornVoiceSession({ launch_key: "original" });
+      const configureBefore = lastConfigure(harness.inputs);
+
+      await expect(session.applyOptions?.({ loud: true, launch_key: "changed" })).rejects.toThrow(
+        /launch_key.*cannot change on a running session/,
+      );
+
+      expect(lastConfigure(harness.inputs)).toBe(configureBefore);
+      await session.close();
+    });
+
     test("applying options returns the settings that ended up different, and emits nothing", async () => {
       const { session, events } = await openStubbornVoiceSession({});
 
diff --git a/packages/server/src/server/agent/plugin-provider.ts b/packages/server/src/server/agent/plugin-provider.ts
index f2a09991f3..d21ce06fbb 100644
--- a/packages/server/src/server/agent/plugin-provider.ts
+++ b/packages/server/src/server/agent/plugin-provider.ts
@@ -64,6 +64,7 @@ import {
   createProviderEnv,
   type ProviderRuntimeSettings,
 } from "./provider-launch-config.js";
+import { ProviderOptionsValidationError } from "./provider-options.js";
 import type { RegisteredProviderDefinition } from "./provider-registry.js";
 import { runProviderTurn } from "./providers/provider-runner.js";
 import { StaleProviderSessionError } from "./stale-provider-session-error.js";
@@ -1437,11 +1438,20 @@ class PluginAgentSession implements AgentSession {
   async applyOptions(options: ProviderOptions): Promise<Record<string, JsonValue>> {
     this.assertReplayComplete();
     const before = this.currentSettings();
+    // A key the plugin does not report as a setting is one it only reads when the session opens.
+    const unsupported = Object.keys(options).filter((key) => !(key in before));
+    if (unsupported.length > 0) {
+      throw new ProviderOptionsValidationError(
+        this.provider,
+        unsupported.map((key) => ({
+          path: [key],
+          message: "cannot change on a running session; it applies when the agent is opened",
+        })),
+      );
+    }
     const settings: Record<string, JsonValue> = {};
-    for (const setting of this.bridge.config.settings) {
-      const value = options[setting.id];
-      if (value === undefined) continue;
-      settings[setting.id] = toJsonValue(value, `setting '${setting.id}'`);
+    for (const [id, value] of Object.entries(options)) {
+      settings[id] = toJsonValue(value, `setting '${id}'`);
     }
     if (Object.keys(settings).length === 0) return {};
     this.applyingOptions = true;
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 6b39b7fde2..6fe71f9616 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
@@ -558,6 +558,21 @@ describe("Codex app-server provider controls", () => {
     });
   });
 
+  test("opening on a model without the saved tier runs Normal and the open patch says so", async () => {
+    const models = [
+      { id: "plain", isDefault: true, defaultReasoningEffort: "medium", serviceTiers: [] },
+    ];
+    const { session, appServer } = await createConnectedSession(
+      { model: "plain", providerOptions: { service_tier: "priority" } },
+      { models },
+    );
+
+    expect(session.openOptionsPatch).toEqual({ service_tier: "default", fast_mode: null });
+
+    await session.startTurn("hello");
+    await expect(appServer.waitForTurnStart()).resolves.toMatchObject({ serviceTier: "default" });
+  });
+
   test("a saved service_tier: fast runs as the model's Fast tier and the open patch names that tier", async () => {
     const { session, appServer } = await createConnectedSession({
       providerOptions: { service_tier: "fast" },
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 dbe67f4fbc..412fbcbc7d 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
@@ -3873,11 +3873,15 @@ export class CodexAppServerAgentSession implements AgentSession {
     const requested = this.requestedServiceTier;
     const resolved = this.resolveServiceTier(requested);
     const legacy = this.providerOptions.fast_mode !== undefined || requested !== resolved;
-    if (!legacy || resolved === null) return;
+    if (!legacy) return;
+    // A tier the opening model lacks runs as Normal, and the options say so (as when the model
+    // is switched later).
+    const runs = resolved ?? (requested === null ? null : "default");
+    if (runs === null) return;
     const { fast_mode: _legacy, ...rest } = this.providerOptions;
-    this.providerOptions = { ...rest, service_tier: resolved };
-    this.requestedServiceTier = resolved;
-    this.openOptionsPatch = { service_tier: resolved, fast_mode: null };
+    this.providerOptions = { ...rest, service_tier: runs };
+    this.requestedServiceTier = runs;
+    this.openOptionsPatch = { service_tier: runs, fast_mode: null };
   }
 
   private get serviceTier(): string | null {