fix(cli): display correct model for sub-agents in sessions list (#18660)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: ba54c5a351
Co-authored-by: robbyczgw-cla <239660374+robbyczgw-cla@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

src/commands/sessions.model-resolution.test.ts

@@ -0,0 +1,108 @@
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("../config/config.js", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../config/config.js")>();
+  return {
+    ...actual,
+    loadConfig: () => ({
+      agents: {
+        defaults: {
+          model: { primary: "pi:opus" },
+          models: { "pi:opus": {} },
+          contextTokens: 32000,
+        },
+      },
+    }),
+  };
+});
+
+import { sessionsCommand } from "./sessions.js";
+
+const makeRuntime = () => {
+  const logs: string[] = [];
+  return {
+    runtime: {
+      log: (msg: unknown) => logs.push(String(msg)),
+      error: (msg: unknown) => {
+        throw new Error(String(msg));
+      },
+      exit: (code: number) => {
+        throw new Error(`exit ${code}`);
+      },
+    },
+    logs,
+  } as const;
+};
+
+const writeStore = (data: unknown) => {
+  const file = path.join(
+    os.tmpdir(),
+    `sessions-model-${Date.now()}-${Math.random().toString(16).slice(2)}.json`,
+  );
+  fs.writeFileSync(file, JSON.stringify(data, null, 2));
+  return file;
+};
+
+describe("sessionsCommand model resolution", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2025-12-06T00:00:00Z"));
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("prefers runtime model fields for subagent sessions in JSON output", async () => {
+    const store = writeStore({
+      "agent:research:subagent:demo": {
+        sessionId: "subagent-1",
+        updatedAt: Date.now() - 2 * 60_000,
+        modelProvider: "openai-codex",
+        model: "gpt-5.3-codex",
+        modelOverride: "pi:opus",
+      },
+    });
+
+    const { runtime, logs } = makeRuntime();
+    await sessionsCommand({ store, json: true }, runtime);
+
+    fs.rmSync(store);
+
+    const payload = JSON.parse(logs[0] ?? "{}") as {
+      sessions?: Array<{
+        key: string;
+        model?: string | null;
+      }>;
+    };
+    const subagent = payload.sessions?.find((row) => row.key === "agent:research:subagent:demo");
+    expect(subagent?.model).toBe("gpt-5.3-codex");
+  });
+
+  it("falls back to modelOverride when runtime model is missing", async () => {
+    const store = writeStore({
+      "agent:research:subagent:demo": {
+        sessionId: "subagent-2",
+        updatedAt: Date.now() - 2 * 60_000,
+        modelOverride: "openai-codex/gpt-5.3-codex",
+      },
+    });
+
+    const { runtime, logs } = makeRuntime();
+    await sessionsCommand({ store, json: true }, runtime);
+
+    fs.rmSync(store);
+
+    const payload = JSON.parse(logs[0] ?? "{}") as {
+      sessions?: Array<{
+        key: string;
+        model?: string | null;
+      }>;
+    };
+    const subagent = payload.sessions?.find((row) => row.key === "agent:research:subagent:demo");
+    expect(subagent?.model).toBe("gpt-5.3-codex");
+  });
+});