fix: use configured model provider on session resume

When resuming or loading a session, getResumeModelProvider() now reads
the model_provider value from Codex config (config/read) before falling
back to "openai". This fixes a v1.0.0 regression where sessions resumed
via ACP always used OpenAI even when ~/.codex/config.toml specified
model_provider = "azure".

Author: david-melville

src/CodexAcpClient.ts

@@ -178,7 +178,7 @@ export class CodexAcpClient {
             return sessionModelProvider;
         }
         const settingsModelProvider = await this.codexClient.configRead({includeLayers: false});
-        return settingsModelProvider.config.model_provider ?? null;
+        return settingsModelProvider?.config?.model_provider ?? null;
     }
 
     async logout(): Promise<void> {
@@ -211,7 +211,7 @@ export class CodexAcpClient {
         const response = await this.codexClient.threadResume({
             config: await this.createSessionConfig(request.cwd, additionalDirectories, request.mcpServers ?? []),
             cwd: request.cwd,
-            modelProvider: this.getResumeModelProvider(),
+            modelProvider: await this.getResumeModelProvider(),
             threadId: request.sessionId,
         });
         onSubscribed?.();
@@ -233,7 +233,7 @@ export class CodexAcpClient {
         const response = await this.codexClient.threadResume({
             config: await this.createSessionConfig(request.cwd, additionalDirectories, request.mcpServers ?? []),
             cwd: request.cwd,
-            modelProvider: this.getResumeModelProvider(),
+            modelProvider: await this.getResumeModelProvider(),
             threadId: request.sessionId,
         });
         onSubscribed?.();
@@ -363,10 +363,10 @@ export class CodexAcpClient {
         return this.gatewayConfig?.modelProvider ?? this.modelProvider;
     }
 
-    private getResumeModelProvider(): string {
-        // Passing `null` forces codex to use the persisted provider for resumed session instead of default one
-        // Explicit fallback to "openai" fixes error `Model provider not found` at least for ChatGPT authentication
-        return this.getModelProvider() ?? "openai";
+    private async getResumeModelProvider(): Promise<string> {
+        // Prefer an explicit/gateway provider, then the provider persisted in Codex config.
+        // Keep OpenAI as the final fallback for ChatGPT-authenticated sessions without a configured provider.
+        return (await this.getCurrentModelProvider()) ?? "openai";
     }
 
     private async refreshSkills(

src/__tests__/CodexACPAgent/CodexAcpClient.test.ts

@@ -365,6 +365,46 @@ describe('ACP server test', { timeout: 40_000 }, () => {
         });
     });
 
+    it('uses configured model provider when resuming sessions without an explicit provider', async () => {
+        const mockFixture = createCodexMockTestFixture();
+        const codexAcpClient = mockFixture.getCodexAcpClient();
+        const codexAppServerClient = mockFixture.getCodexAppServerClient();
+
+        vi.spyOn(codexAppServerClient, "skillsExtraRootsSet").mockResolvedValue(undefined);
+        vi.spyOn(codexAppServerClient, "listSkills").mockResolvedValue({data: []});
+        vi.spyOn(codexAppServerClient, "configRead").mockResolvedValue({
+            config: {
+                model_provider: "azure",
+            },
+        } as any);
+        const threadResumeSpy = vi.spyOn(codexAppServerClient, "threadResume").mockResolvedValue({
+            thread: {id: "thread-id"} as any,
+            model: "gpt-5",
+            reasoningEffort: "medium",
+            serviceTier: null,
+        } as any);
+        vi.spyOn(codexAppServerClient, "threadRead").mockResolvedValue({
+            thread: {id: "thread-id"} as any,
+        });
+        vi.spyOn(codexAppServerClient, "listModels").mockResolvedValue({
+            data: [createTestModel({id: "gpt-5"})],
+            nextCursor: null,
+        });
+
+        await codexAcpClient.resumeSession({
+            sessionId: "resume-id",
+            cwd: "/workspace",
+        });
+        await codexAcpClient.loadSession({
+            sessionId: "load-id",
+            cwd: "/workspace",
+            mcpServers: [],
+        });
+
+        expect(threadResumeSpy.mock.calls[0]![0].modelProvider).toBe("azure");
+        expect(threadResumeSpy.mock.calls[1]![0].modelProvider).toBe("azure");
+    });
+
     it('rejects malformed ACP additional directories', async () => {
         const mockFixture = createCodexMockTestFixture();
         const codexAcpClient = mockFixture.getCodexAcpClient();