From a2c4917813a3ff4bdd3e13a92f7864b270456cf2 Mon Sep 17 00:00:00 2001
From: Tag <tag@austen.info>
Date: Wed, 25 Mar 2026 01:43:01 -0400
Subject: [PATCH] feat(agents): support dynamic model switching in Copilot SDK
 sessions

---
 src/agents/copilot-runner.ts                  |  5 +-
 .../copilot-sdk.dynamic-model-switch.test.ts  | 91 +++++++++++++++++++
 src/agents/copilot-sdk.ts                     | 20 +++-
 3 files changed, 113 insertions(+), 3 deletions(-)
 create mode 100644 src/agents/copilot-sdk.dynamic-model-switch.test.ts

diff --git a/src/agents/copilot-runner.ts b/src/agents/copilot-runner.ts
index 252708ccd43c0..baf9826fa887f 100644
--- a/src/agents/copilot-runner.ts
+++ b/src/agents/copilot-runner.ts
@@ -117,6 +117,9 @@ export async function runCopilotCliAgent(params: {
     const result = await runCopilotAgent({
       prompt: params.prompt,
       model: modelId === "default" ? undefined : modelId,
+      // When resuming an existing session, use modelOverride to dynamically
+      // switch the model via setModel() instead of requiring a new session.
+      modelOverride: params.cliSessionId && modelId !== "default" ? modelId : undefined,
       workspaceDir: resolvedWorkspace,
       systemPrompt,
       timeoutMs: params.timeoutMs,
@@ -133,7 +136,7 @@ export async function runCopilotCliAgent(params: {
         agentMeta: {
           sessionId: result.sessionId ?? params.sessionId ?? "",
           provider: "copilot-cli",
-          model: modelId,
+          model: result.model ?? modelId,
         },
       },
     };
diff --git a/src/agents/copilot-sdk.dynamic-model-switch.test.ts b/src/agents/copilot-sdk.dynamic-model-switch.test.ts
new file mode 100644
index 0000000000000..a8070574c1657
--- /dev/null
+++ b/src/agents/copilot-sdk.dynamic-model-switch.test.ts
@@ -0,0 +1,91 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+// Mock the @github/copilot-sdk module
+const mockSetModel = vi.fn().mockResolvedValue(undefined);
+const mockSendAndWait = vi.fn().mockResolvedValue({ data: { content: "hello" } });
+const mockDestroy = vi.fn().mockResolvedValue(undefined);
+const mockStop = vi.fn().mockResolvedValue(undefined);
+const mockGetAuthStatus = vi.fn().mockResolvedValue({
+  isAuthenticated: true,
+  authType: "token",
+  login: "test-user",
+});
+const mockCreateSession = vi.fn().mockResolvedValue({
+  sessionId: "sess-new",
+  setModel: mockSetModel,
+  sendAndWait: mockSendAndWait,
+  destroy: mockDestroy,
+});
+const mockResumeSession = vi.fn().mockResolvedValue({
+  sessionId: "sess-resumed",
+  setModel: mockSetModel,
+  sendAndWait: mockSendAndWait,
+  destroy: mockDestroy,
+});
+
+vi.mock("@github/copilot-sdk", () => {
+  const MockClient = vi.fn(function (this: Record<string, unknown>) {
+    this.getAuthStatus = mockGetAuthStatus;
+    this.createSession = mockCreateSession;
+    this.resumeSession = mockResumeSession;
+    this.listModels = vi.fn().mockResolvedValue([]);
+    this.stop = mockStop;
+  });
+  return { CopilotClient: MockClient };
+});
+
+// Must import after mock setup
+const { runCopilotAgent } = await import("./copilot-sdk.js");
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  mockSendAndWait.mockResolvedValue({ data: { content: "response" } });
+});
+
+describe("runCopilotAgent dynamic model switching", () => {
+  it("does not call setModel when modelOverride is not set", async () => {
+    const result = await runCopilotAgent({
+      prompt: "hi",
+      model: "gpt-4o",
+    });
+
+    expect(mockSetModel).not.toHaveBeenCalled();
+    expect(result.model).toBe("gpt-4o");
+    expect(result.text).toBe("response");
+  });
+
+  it("calls setModel when modelOverride is provided on new session", async () => {
+    const result = await runCopilotAgent({
+      prompt: "hi",
+      model: "gpt-4o",
+      modelOverride: "claude-sonnet-4",
+    });
+
+    expect(mockSetModel).toHaveBeenCalledWith("claude-sonnet-4");
+    expect(result.model).toBe("claude-sonnet-4");
+    expect(result.sessionId).toBe("sess-new");
+  });
+
+  it("calls setModel when modelOverride is provided on resumed session", async () => {
+    const result = await runCopilotAgent({
+      prompt: "hi",
+      model: "gpt-4o",
+      modelOverride: "claude-sonnet-4",
+      sessionId: "existing-session",
+    });
+
+    expect(mockResumeSession).toHaveBeenCalledWith("existing-session", expect.any(Object));
+    expect(mockSetModel).toHaveBeenCalledWith("claude-sonnet-4");
+    expect(result.model).toBe("claude-sonnet-4");
+    expect(result.sessionId).toBe("sess-resumed");
+  });
+
+  it("returns model as undefined when no model or override specified", async () => {
+    const result = await runCopilotAgent({
+      prompt: "hi",
+    });
+
+    expect(mockSetModel).not.toHaveBeenCalled();
+    expect(result.model).toBeUndefined();
+  });
+});
diff --git a/src/agents/copilot-sdk.ts b/src/agents/copilot-sdk.ts
index 067dffbb9c0b9..6616796d4a6d2 100644
--- a/src/agents/copilot-sdk.ts
+++ b/src/agents/copilot-sdk.ts
@@ -112,6 +112,8 @@ export async function listCopilotModels(options?: { cwd?: string }): Promise<Mod
 export type CopilotAgentRunOptions = {
   prompt: string;
   model?: string;
+  /** Switch to a different model mid-session (calls session.setModel() after create/resume). */
+  modelOverride?: string;
   workspaceDir?: string;
   systemPrompt?: string;
   timeoutMs?: number;
@@ -121,6 +123,8 @@ export type CopilotAgentRunOptions = {
 export type CopilotAgentRunResult = {
   text: string;
   sessionId: string;
+  /** The model actually used (may differ from requested if setModel() was called). */
+  model?: string;
 };
 
 /**
@@ -164,6 +168,18 @@ export async function runCopilotAgent(
       session = await client.createSession(sessionConfig);
     }
 
+    // Apply dynamic model switch if requested
+    let activeModel = options.model;
+    if (options.modelOverride) {
+      log.info("switching copilot session model", {
+        sessionId: session.sessionId,
+        from: options.model ?? "default",
+        to: options.modelOverride,
+      });
+      await session.setModel(options.modelOverride);
+      activeModel = options.modelOverride;
+    }
+
     const timeoutMs = options.timeoutMs ?? 120_000;
     const response = await session.sendAndWait({ prompt: options.prompt }, timeoutMs);
 
@@ -172,11 +188,11 @@ export async function runCopilotAgent(
 
     log.info(`copilot agent run completed`, {
       sessionId,
-      model: options.model,
+      model: activeModel,
       responseLength: text.length,
     });
 
-    return { text, sessionId };
+    return { text, sessionId, model: activeModel };
   } finally {
     if (session) {
       try {