perf: parallelize new-session model discovery

src/CodexAcpClient.ts

@@ -600,13 +600,15 @@ export class CodexAcpClient {
         const additionalDirectories = readAdditionalDirectories(request.cwd, request.additionalDirectories, request._meta);
         await this.refreshSkills(request.cwd, additionalDirectories);
 
-        const response = await this.codexClient.threadStart({
-            config: await this.createSessionConfig(request.cwd, additionalDirectories, request.mcpServers),
-            modelProvider: this.getModelProvider(),
-            cwd: request.cwd,
-        });
-
-        const codexModels = await this.fetchAvailableModels();
+        const config = await this.createSessionConfig(request.cwd, additionalDirectories, request.mcpServers);
+        const [response, codexModels] = await Promise.all([
+            this.codexClient.threadStart({
+                config,
+                modelProvider: this.getModelProvider(),
+                cwd: request.cwd,
+            }),
+            this.fetchAvailableModels(),
+        ]);
         if (codexModels.length === 0) {
             throw new Error("Codex did not return any models");
         }

src/__tests__/CodexACPAgent/CodexAcpClient.test.ts

@@ -98,8 +98,8 @@ describe('ACP server test', { timeout: 40_000 }, () => {
             "account/read",
             "thread/start",
             "model/list",
-            "thread/started",
             "account/read",
+            "thread/started",
             "skills/list",
         ]);
         expect(loginRequest).toEqual({
@@ -475,6 +475,35 @@ describe('ACP server test', { timeout: 40_000 }, () => {
         expect(listSkillsSpy.mock.invocationCallOrder[0]!).toBeLessThan(threadStartSpy.mock.invocationCallOrder[0]!);
     });
 
+    it('fetches models while a new thread is starting', async () => {
+        const mockFixture = createCodexMockTestFixture();
+        const codexAcpClient = mockFixture.getCodexAcpClient();
+        const codexAppServerClient = mockFixture.getCodexAppServerClient();
+        const threadStart = deferred<Awaited<ReturnType<typeof codexAppServerClient.threadStart>>>();
+
+        vi.spyOn(codexAppServerClient, "listSkills").mockResolvedValue({data: []});
+        vi.spyOn(codexAppServerClient, "threadStart").mockReturnValue(threadStart.promise);
+        const listModelsSpy = vi.spyOn(codexAppServerClient, "listModels").mockResolvedValue({
+            data: [createTestModel({id: "gpt-5"})],
+            nextCursor: null,
+        });
+
+        const session = codexAcpClient.newSession({cwd: "/workspace", mcpServers: []});
+        await vi.waitFor(() => expect(listModelsSpy).toHaveBeenCalledOnce());
+
+        threadStart.resolve({
+            thread: {id: "thread-id"} as any,
+            model: "gpt-5",
+            reasoningEffort: "medium",
+            serviceTier: null,
+        } as Awaited<ReturnType<typeof codexAppServerClient.threadStart>>);
+
+        await expect(session).resolves.toMatchObject({
+            sessionId: "thread-id",
+            currentModelId: "gpt-5[medium]",
+        });
+    });
+
     it('prefers ACP additional directories over legacy meta roots for new session skill discovery', async () => {
         const mockFixture = createCodexMockTestFixture();
         const codexAcpClient = mockFixture.getCodexAcpClient();