diff --git a/packages/server/src/server/agent/providers/codex-app-server-agent.test.ts b/packages/server/src/server/agent/providers/codex-app-server-agent.test.ts
index 2c74912a47..d55a0b2a79 100644
--- a/packages/server/src/server/agent/providers/codex-app-server-agent.test.ts
+++ b/packages/server/src/server/agent/providers/codex-app-server-agent.test.ts
@@ -1170,6 +1170,36 @@ describe("Codex app-server provider", () => {
     }
   });
 
+  test("keeps the default effort when a session without one is resumed after a restart", async () => {
+    const beforeRestart = createFakeCodexAppServer();
+    const firstSession = await createProviderWithFakeAppServer(beforeRestart).createSession(
+      createConfig({ thinkingOptionId: undefined }),
+    );
+    await firstSession.startTurn("first turn");
+    await expect(beforeRestart.waitForTurnStart()).resolves.toMatchObject({ effort: "medium" });
+    const handle = firstSession.describePersistence()!;
+    await firstSession.close();
+
+    const afterRestart = createFakeCodexAppServer();
+    // The daemon resumes with the stored agent config, which never recorded an effort.
+    const resumed = await createProviderWithFakeAppServer(afterRestart).resumeSession(handle, {
+      model: "gpt-5.4",
+      thinkingOptionId: undefined,
+    });
+
+    try {
+      await expect(resumed.getRuntimeInfo()).resolves.toMatchObject({
+        thinkingOptionId: "medium",
+      });
+      await resumed.startTurn("turn after restart");
+      await expect(afterRestart.waitForTurnStart()).resolves.toMatchObject({ effort: "medium" });
+      beforeRestart.assertNoErrors();
+      afterRestart.assertNoErrors();
+    } finally {
+      await resumed.close();
+    }
+  });
+
   test("preapproves only granted tools on the injected Codex MCP server", async () => {
     const session = createSession({
       modeId: undefined,
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 f16c5fb6d8..f86d5de653 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
@@ -3611,6 +3611,7 @@ export class CodexAppServerAgentSession implements AgentSession {
       if (this.currentThreadId) {
         await this.ensureThreadLoaded();
         await this.loadPersistedHistory(this.client);
+        await this.applyDefaultModelAndThinking();
       }
 
       if (this.closed) {
@@ -5208,13 +5209,20 @@ export class CodexAppServerAgentSession implements AgentSession {
     return { model, thinkingOptionId };
   }
 
+  // A session without a model or effort runs on Codex's configured defaults, whether its
+  // thread is new or resumed after a restart.
+  private async applyDefaultModelAndThinking(): Promise<string> {
+    const { model, thinkingOptionId } = await this.resolveModelAndThinking();
+    this.config.model = model;
+    this.config.thinkingOptionId = thinkingOptionId;
+    return model;
+  }
+
   private async ensureThread(): Promise<void> {
     if (!this.client) return;
     if (this.currentThreadId) return;
 
-    const { model, thinkingOptionId } = await this.resolveModelAndThinking();
-    this.config.model = model;
-    this.config.thinkingOptionId = thinkingOptionId;
+    const model = await this.applyDefaultModelAndThinking();
 
     const { params, approvalPolicy, sandbox } = this.buildThreadStartRequest(model);
     const rawResponse = await this.client.request("thread/start", params);