diff --git a/src/client/streamableHttp.test.ts b/src/client/streamableHttp.test.ts
index 9e8efa52..f748a2be 100644
--- a/src/client/streamableHttp.test.ts
+++ b/src/client/streamableHttp.test.ts
@@ -1,12 +1,24 @@
 import { StreamableHTTPClientTransport, StreamableHTTPReconnectionOptions } from "./streamableHttp.js";
+import { OAuthClientProvider, UnauthorizedError } from "./auth.js";
 import { JSONRPCMessage } from "../types.js";
 
 
 describe("StreamableHTTPClientTransport", () => {
   let transport: StreamableHTTPClientTransport;
+  let mockAuthProvider: jest.Mocked<OAuthClientProvider>;
 
   beforeEach(() => {
-    transport = new StreamableHTTPClientTransport(new URL("http://localhost:1234/mcp"));
+    mockAuthProvider = {
+      get redirectUrl() { return "http://localhost/callback"; },
+      get clientMetadata() { return { redirect_uris: ["http://localhost/callback"] }; },
+      clientInformation: jest.fn(() => ({ client_id: "test-client-id", client_secret: "test-client-secret" })),
+      tokens: jest.fn(),
+      saveTokens: jest.fn(),
+      redirectToAuthorization: jest.fn(),
+      saveCodeVerifier: jest.fn(),
+      codeVerifier: jest.fn(),
+    };
+    transport = new StreamableHTTPClientTransport(new URL("http://localhost:1234/mcp"), { authProvider: mockAuthProvider });
     jest.spyOn(global, "fetch");
   });
 
@@ -497,4 +509,27 @@ describe("StreamableHTTPClientTransport", () => {
     expect(getDelay(10)).toBe(5000);
   });
 
+  it("attempts auth flow on 401 during POST request", async () => {
+    const message: JSONRPCMessage = {
+      jsonrpc: "2.0",
+      method: "test",
+      params: {},
+      id: "test-id"
+    };
+
+    (global.fetch as jest.Mock)
+      .mockResolvedValueOnce({
+        ok: false,
+        status: 401,
+        statusText: "Unauthorized",
+        headers: new Headers()
+      })
+      .mockResolvedValue({
+        ok: false,
+        status: 404
+      });
+
+    await expect(transport.send(message)).rejects.toThrow(UnauthorizedError);
+    expect(mockAuthProvider.redirectToAuthorization.mock.calls).toHaveLength(1);
+  });
 });