feat: report the model of each native subagent session

src/CodexAcpServer.ts

@@ -170,7 +170,11 @@ import {
     clientSupportsSubagents,
     type SubagentAwareSessionCapabilities,
 } from "./subagents/AcpSubagents";
-import {CodexSubagentEventRouter} from "./subagents/CodexSubagentEventRouter";
+import {
+    CodexSubagentEventRouter,
+    reportSubagentModel,
+    type SubagentModels,
+} from "./subagents/CodexSubagentEventRouter";
 import {nameFromAgentPath} from "./subagents/CodexAgentPath";
 import {listCodexHooks, trustCodexHooks, type CodexHookIdentity} from "./CodexHookTrust";
 import type {HooksListEntry} from "./app-server/v2/HooksListEntry";
@@ -1213,6 +1217,7 @@ export class CodexAcpServer {
                 clientSupportsSubagents(this.clientCapabilities),
                 new ACPSessionConnection(this.connection, sessionId),
                 childSessionId => this.reportingConnection.reports.releaseOpen(childSessionId),
+                () => this.subagentModels(sessionState),
             ),
             asyncTasks: this.createAsyncTasks(sessionId),
             compactions: new CodexSessionCompactions(),
@@ -3122,6 +3127,19 @@ export class CodexAcpServer {
         await new ACPSessionConnection(this.connection, sessionState.sessionId).update(update);
     }
 
+    /** The models of a root session, for the configuration of its native subagent sessions. */
+    private subagentModels(sessionState: SessionState): SubagentModels | null {
+        if (!this.isSessionConfigEnabled()) return null;
+        let currentModel: string | null = null;
+        try {
+            currentModel = ModelId.fromString(sessionState.currentModelId).model;
+        }
+        catch {
+            // A session without a known model gives no current entry to match.
+        }
+        return {currentModel, availableModels: sessionState.availableModels};
+    }
+
     private findCurrentModel(models: Model[], currentModelId: string): Model | undefined {
         const modelId = ModelId.fromString(currentModelId);
         return models.find(m => m.id === modelId.model);
@@ -3271,6 +3289,7 @@ export class CodexAcpServer {
                 clientSupportsSubagents(this.clientCapabilities),
                 new ACPSessionConnection(this.connection, sessionId),
                 childSessionId => this.reportingConnection.reports.releaseOpen(childSessionId),
+                () => this.subagentModels(sessionState),
             ),
             asyncTasks: this.createAsyncTasks(sessionId),
             compactions: new CodexSessionCompactions(),
@@ -3404,7 +3423,11 @@ export class CodexAcpServer {
         isOpen: () => boolean,
     ): Promise<void> {
         const session = new ACPSessionConnection(this.connection, sessionId);
-        const announced = new Map<string, {generation: number; sessionId: string; terminal: boolean}>();
+        const announced = new Map<string, {generation: number; sessionId: string; terminal: boolean; model?: string}>();
+        // Sends the model of a replayed child after its announcement, as the live session does. Replay sends only
+        // the resolved model that the activity items name for the child, so an older record gets no model option.
+        const reportModel = (childSessionId: string, model: string | undefined) =>
+            reportSubagentModel(session, childSessionId, this.subagentModels(sessionState), model);
         // v2 only (this path also serves v1's native replay, unchanged there).
         const startedReplayMessages = new Set<string>();
         for await (const items of itemPages) {
@@ -3436,14 +3459,21 @@ export class CodexAcpServer {
                         // An interaction that started no child turn is not a new generation.
                         if (activityKind === "interacted" && !childItems) continue;
                         const name = nameFromAgentPath(item.agentPath, `Agent ${item.agentThreadId.slice(-8)}`);
+                        const model = item.model || previous?.model;
                         await session.update({
                             sessionUpdate: "subagent_spawned",
                             subagentSessionId: childSessionId,
                             name,
                             task: `Delegated task for ${name}`,
                             capabilities: {},
                         });
-                        announced.set(item.agentThreadId, {generation, sessionId: childSessionId, terminal: false});
+                        announced.set(item.agentThreadId, {
+                            generation,
+                            sessionId: childSessionId,
+                            terminal: false,
+                            ...(model ? {model} : {}),
+                        });
+                        await reportModel(childSessionId, model);
                         if (childItems) {
                             const commandIds = new Set<string>();
                             try {
@@ -3477,6 +3507,7 @@ export class CodexAcpServer {
                         const child = announced.get(item.agentThreadId);
                         if (!child) {
                             const name = nameFromAgentPath(item.agentPath, `Agent ${item.agentThreadId.slice(-8)}`);
+                            const model = item.model || undefined;
                             await session.update({
                                 sessionUpdate: "subagent_spawned",
                                 subagentSessionId: item.agentThreadId,
@@ -3488,7 +3519,9 @@ export class CodexAcpServer {
                                 generation: 1,
                                 sessionId: item.agentThreadId,
                                 terminal: false,
+                                ...(model ? {model} : {}),
                             });
+                            await reportModel(item.agentThreadId, model);
                             continue;
                         }
                         if (child.terminal) continue;

src/ModelConfigOption.ts

@@ -33,23 +33,31 @@ export function findSupportedEffort(
     return options.find(o => o.reasoningEffort === effort)?.reasoningEffort;
 }
 
-export function createModelConfigOption(
-    availableModels: Array<Model>,
-    currentBaseModelId: string,
-    recommendedModelId?: string,
-): SessionConfigOption {
-    const options: Array<{ value: string; name: string; description: string | null }> = availableModels.map(model => ({
+type ModelOption = { value: string; name: string; description: string | null };
+
+/** The entries of the session `model` option. */
+function modelOptions(availableModels: ReadonlyArray<Model>, currentBaseModelId: string | null): Array<ModelOption> {
+    const options: Array<ModelOption> = availableModels.map(model => ({
         value: model.id,
         name: formatModelDisplayName(model.displayName),
         description: model.description,
     }));
-    if (!availableModels.some(model => model.id === currentBaseModelId)) {
+    if (currentBaseModelId !== null && !availableModels.some(model => model.id === currentBaseModelId)) {
         options.unshift({
             value: currentBaseModelId,
             name: formatModelDisplayName(currentBaseModelId),
             description: null,
         });
     }
+    return options;
+}
+
+export function createModelConfigOption(
+    availableModels: Array<Model>,
+    currentBaseModelId: string,
+    recommendedModelId?: string,
+): SessionConfigOption {
+    const options = modelOptions(availableModels, currentBaseModelId);
 
     const recommendation = recommendedModelId && options.some(option => option.value === recommendedModelId)
         ? recommendedModelId
@@ -91,3 +99,33 @@ export function createReasoningEffortConfigOption(
             : {}),
     };
 }
+
+/**
+ * The model option of a native subagent session. The client may not change the model of a subagent, so the
+ * option offers only the current model. Its value and name are those of the same model in the `model` option
+ * of the parent session, so that the client can compare them. A model that the parent option does not offer
+ * keeps its full id as the value and the name.
+ */
+export function createSubagentModelConfigOption(
+    availableModels: ReadonlyArray<Model>,
+    parentBaseModelId: string | null,
+    model: string,
+): SessionConfigOption {
+    // Without models the parent session has no model option, so there is nothing to match.
+    const parentOptions = availableModels.length > 0 ? modelOptions(availableModels, parentBaseModelId) : [];
+    // Codex can name a model by its `model` slug instead of its id.
+    const id = availableModels.find(candidate => candidate.id === model)?.id
+        ?? availableModels.find(candidate => candidate.model === model)?.id
+        ?? model;
+    const option = parentOptions.find(candidate => candidate.value === id)
+        ?? {value: model, name: model, description: null};
+    return {
+        id: MODEL_CONFIG_ID,
+        name: "Model",
+        description: "Model Codex uses for the subagent",
+        category: "model",
+        type: "select",
+        currentValue: option.value,
+        options: [option],
+    };
+}

src/__tests__/CodexACPAgent/agent-file-change-report.test.ts

@@ -167,6 +167,7 @@ async function setupNativeSubagentPrompt(): Promise<{
         true,
         new ACPSessionConnection(fixture.getAcpConnection(), sessionState.sessionId),
         () => {},
+        () => ({currentModel: null, availableModels: []}),
     );
     vi.spyOn(fixture.getCodexAcpAgent(), "getSessionState").mockReturnValue(sessionState);
     vi.spyOn(fixture.getCodexAppServerClient(), "turnStart")

src/__tests__/CodexACPAgent/async-tasks.test.ts

@@ -490,7 +490,7 @@ describe("Codex background terminal tasks", () => {
         });
         const sessionState = createTestSessionState({sessionId: "thread-1"});
         const session = new ACPSessionConnection(fixture.getAcpConnection(), sessionState.sessionId);
-        sessionState.subagents = new CodexSubagentEventRouter(sessionState.sessionId, true, session, () => {});
+        sessionState.subagents = new CodexSubagentEventRouter(sessionState.sessionId, true, session, () => {}, () => ({currentModel: null, availableModels: []}));
         sessionState.asyncTasks = new CodexBackgroundTerminalTasks(
             true,
             sessionState.sessionId,

src/__tests__/CodexACPAgent/collab-agent-events.test.ts

@@ -44,6 +44,7 @@ describe("CodexEventHandler - collab agent tool call events", () => {
             true,
             new ACPSessionConnection(mockFixture.getAcpConnection(), sessionId),
             () => {},
+            () => ({currentModel: null, availableModels: []}),
         );
         return response;
     }
@@ -1139,6 +1140,7 @@ describe("CodexEventHandler - collab agent tool call events", () => {
             true,
             new ACPSessionConnection(mockFixture.getAcpConnection(), sessionId),
             () => {},
+            () => ({currentModel: null, availableModels: []}),
         );
         await router.handle({
             method: "item/started",
@@ -1614,6 +1616,7 @@ describe("CodexEventHandler - collab agent tool call events", () => {
             true,
             new ACPSessionConnection(mockFixture.getAcpConnection(), sessionId),
             () => {},
+            () => ({currentModel: null, availableModels: []}),
         );
         const activity = (threadId: string, path: string): ServerNotification => ({
             method: "item/started",
@@ -1690,6 +1693,7 @@ describe("CodexEventHandler - collab agent tool call events", () => {
             true,
             new ACPSessionConnection(mockFixture.getAcpConnection(), sessionId),
             () => {},
+            () => ({currentModel: null, availableModels: []}),
         );
         await router.handle({
             method: "item/started",
@@ -1751,6 +1755,7 @@ describe("CodexEventHandler - collab agent tool call events", () => {
             true,
             new ACPSessionConnection(mockFixture.getAcpConnection(), sessionId),
             () => {},
+            () => ({currentModel: null, availableModels: []}),
         );
         const spawn = (item: Record<string, unknown>) => router.handle({
             method: "item/started",
@@ -1781,6 +1786,7 @@ describe("CodexEventHandler - collab agent tool call events", () => {
             true,
             new ACPSessionConnection(mockFixture.getAcpConnection(), sessionId),
             childSessionId => ended.push(childSessionId),
+            () => ({currentModel: null, availableModels: []}),
         );
         const rootItem = (item: Record<string, unknown>) => router.handle({
             method: "item/started",
@@ -1836,6 +1842,7 @@ describe("CodexEventHandler - collab agent tool call events", () => {
             true,
             new ACPSessionConnection(mockFixture.getAcpConnection(), sessionId),
             () => {},
+            () => ({currentModel: null, availableModels: []}),
         );
         await router.handle({
             method: "item/started",
@@ -1942,6 +1949,7 @@ describe("CodexEventHandler - collab agent tool call events", () => {
                 true,
                 new ACPSessionConnection(mockFixture.getAcpConnection(), sessionId),
                 childSessionId => reports.releaseOpen(childSessionId),
+                () => ({currentModel: null, availableModels: []}),
             );
             await router.handle(spawn(childThreadId));
             if (materialize) await router.handle(activity(childThreadId));

src/__tests__/CodexACPAgent/data/subagent-model-activity.json

@@ -0,0 +1,35 @@
+[
+  {
+    "sessionId": "root",
+    "update": {
+      "sessionUpdate": "subagent_spawned",
+      "subagentSessionId": "child-1",
+      "name": "Reviewer",
+      "task": "Review the diff",
+      "capabilities": {}
+    }
+  },
+  {
+    "sessionId": "child-1",
+    "update": {
+      "sessionUpdate": "config_option_update",
+      "configOptions": [
+        {
+          "id": "model",
+          "name": "Model",
+          "description": "Model Codex uses for the subagent",
+          "category": "model",
+          "type": "select",
+          "currentValue": "gpt-5.2-codex",
+          "options": [
+            {
+              "value": "gpt-5.2-codex",
+              "name": "5.2 Codex",
+              "description": "gpt-5.2-codex model"
+            }
+          ]
+        }
+      ]
+    }
+  }
+]

src/__tests__/CodexACPAgent/data/subagent-model-reopen.json

@@ -0,0 +1,76 @@
+[
+  {
+    "sessionId": "root",
+    "update": {
+      "sessionUpdate": "subagent_spawned",
+      "subagentSessionId": "child-1",
+      "name": "Reviewer",
+      "task": "Review the diff",
+      "capabilities": {}
+    }
+  },
+  {
+    "sessionId": "child-1",
+    "update": {
+      "sessionUpdate": "config_option_update",
+      "configOptions": [
+        {
+          "id": "model",
+          "name": "Model",
+          "description": "Model Codex uses for the subagent",
+          "category": "model",
+          "type": "select",
+          "currentValue": "gpt-5.2-codex",
+          "options": [
+            {
+              "value": "gpt-5.2-codex",
+              "name": "5.2 Codex",
+              "description": "gpt-5.2-codex model"
+            }
+          ]
+        }
+      ]
+    }
+  },
+  {
+    "sessionId": "root",
+    "update": {
+      "sessionUpdate": "subagent_state_update",
+      "subagentSessionId": "child-1",
+      "state": "completed"
+    }
+  },
+  {
+    "sessionId": "root",
+    "update": {
+      "sessionUpdate": "subagent_spawned",
+      "subagentSessionId": "child-1:generation:2",
+      "name": "Reviewer",
+      "task": "Review the diff",
+      "capabilities": {}
+    }
+  },
+  {
+    "sessionId": "child-1:generation:2",
+    "update": {
+      "sessionUpdate": "config_option_update",
+      "configOptions": [
+        {
+          "id": "model",
+          "name": "Model",
+          "description": "Model Codex uses for the subagent",
+          "category": "model",
+          "type": "select",
+          "currentValue": "gpt-5.2-codex",
+          "options": [
+            {
+              "value": "gpt-5.2-codex",
+              "name": "5.2 Codex",
+              "description": "gpt-5.2-codex model"
+            }
+          ]
+        }
+      ]
+    }
+  }
+]