test(coding-agent): stabilize MCP stream resumption

(cherry picked from commit 88bbd031b2230fc51ebc0ab38a953cefc722e4e8)

packages/coding-agent/test/mcp-http-transport.test.ts

@@ -1,4 +1,4 @@
-import { afterEach, describe, expect, it } from "bun:test";
+import { afterEach, describe, expect, it, vi } from "bun:test";
 import { HttpTransport } from "@oh-my-pi/pi-coding-agent/mcp/transports/http";
 
 const encoder = new TextEncoder();
@@ -258,41 +258,41 @@ describe("MCP Streamable HTTP POST response resumption", () => {
 	});
 
 	it("resumes after an abrupt stream drop once an event ID exists", async () => {
-		// Bun.serve cannot produce a genuine mid-body transport failure in-process
-		// (stream errors surface as clean EOF client-side), so speak raw HTTP: a
-		// chunked response without the terminal chunk, closed mid-body, makes the
-		// client's body read throw.
-		const observed = { posts: 0, lastEventId: null as string | null };
-		const sseChunk = (payload: string): string =>
-			`HTTP/1.1 200 OK\r\nContent-Type: text/event-stream\r\nTransfer-Encoding: chunked\r\n\r\n${payload.length.toString(16)}\r\n${payload}\r\n`;
-		const listener = Bun.listen({
-			hostname: "127.0.0.1",
-			port: 0,
-			socket: {
-				data(socket, data) {
-					const request = new TextDecoder().decode(data);
-					if (request.startsWith("POST")) {
-						observed.posts++;
-						// Priming event, then close without the terminal 0-chunk.
-						socket.write(sseChunk("id: stream-1\nretry: 10\ndata:\n\n"));
-						socket.end();
-						return;
-					}
-					if (!request.startsWith("GET")) return;
-					observed.lastEventId = /^Last-Event-ID:\s*(.+)$/im.exec(request)?.[1]?.trim() ?? null;
-					socket.write(
-						`${sseChunk(
-							'id: stream-2\ndata: {"jsonrpc":"2.0","id":1,"result":{"tools":[{"name":"resumed","inputSchema":{"type":"object"}}]}}\n\n',
-						)}0\r\n\r\n`,
+		const observed = { posts: 0, gets: 0, lastEventId: null as string | null };
+		const fetchMock: typeof globalThis.fetch = Object.assign(
+			async (_input: string | URL | Request, init?: RequestInit | BunFetchRequestInit): Promise<Response> => {
+				if (init?.method === "POST") {
+					observed.posts++;
+					let deliveredEvent = false;
+					return new Response(
+						new ReadableStream<Uint8Array>({
+							pull(controller) {
+								if (!deliveredEvent) {
+									deliveredEvent = true;
+									controller.enqueue(encoder.encode("id: stream-1\nretry: 10\ndata:\n\n"));
+									return;
+								}
+								controller.error(new Error("simulated abrupt stream drop"));
+							},
+						}),
+						{ headers: { "Content-Type": "text/event-stream" } },
 					);
-					socket.end();
-				},
+				}
+				if (init?.method !== "GET") throw new Error(`Unexpected method: ${init?.method}`);
+				observed.gets++;
+				observed.lastEventId = new Headers(init.headers).get("Last-Event-ID");
+				return new Response(
+					'id: stream-2\ndata: {"jsonrpc":"2.0","id":1,"result":{"tools":[{"name":"resumed","inputSchema":{"type":"object"}}]}}\n\n',
+					{ headers: { "Content-Type": "text/event-stream" } },
+				);
 			},
-		});
+			{ preconnect: globalThis.fetch.preconnect },
+		);
+		const fetchSpy = vi.spyOn(globalThis, "fetch").mockImplementation(fetchMock);
 		try {
 			const transport = new HttpTransport({
 				type: "http",
-				url: `http://127.0.0.1:${listener.port}/mcp`,
+				url: "http://mcp.test/mcp",
 				timeout: GUARD_TIMEOUT_MS,
 			});
 			await transport.connect();
@@ -301,9 +301,10 @@ describe("MCP Streamable HTTP POST response resumption", () => {
 				tools: [{ name: "resumed", inputSchema: { type: "object" } }],
 			});
 			expect(observed.posts).toBe(1);
+			expect(observed.gets).toBe(1);
 			expect(observed.lastEventId).toBe("stream-1");
 		} finally {
-			listener.stop(true);
+			fetchSpy.mockRestore();
 		}
 	});
 });