fix(731): StreamableHTTPClientTransport Fails to Reconnect on Non-Resumable Streams

src/client/streamableHttp.test.ts

@@ -1,6 +1,6 @@
 import { StreamableHTTPClientTransport, StreamableHTTPReconnectionOptions, StartSSEOptions } from "./streamableHttp.js";
 import { OAuthClientProvider, UnauthorizedError } from "./auth.js";
-import { JSONRPCMessage } from "../types.js";
+import { JSONRPCMessage, JSONRPCRequest } from "../types.js";
 import { InvalidClientError, InvalidGrantError, UnauthorizedClientError } from "../server/auth/errors.js";
 
 
@@ -594,6 +594,111 @@ describe("StreamableHTTPClientTransport", () => {
     await expect(transport.send(message)).rejects.toThrow(UnauthorizedError);
     expect(mockAuthProvider.redirectToAuthorization.mock.calls).toHaveLength(1);
   });
+
+  describe('Reconnection Logic', () => {
+    let transport: StreamableHTTPClientTransport;
+
+    // Use fake timers to control setTimeout and make the test instant.
+    beforeEach(() => jest.useFakeTimers());
+    afterEach(() => jest.useRealTimers());
+
+    it('should reconnect a GET-initiated notification stream that fails', async () => {
+      // ARRANGE
+      transport = new StreamableHTTPClientTransport(new URL("http://localhost:1234/mcp"), {
+        reconnectionOptions: {
+          initialReconnectionDelay: 10, 
+          maxRetries: 1, 
+          maxReconnectionDelay: 1000,  // Ensure it doesn't retry indefinitely
+          reconnectionDelayGrowFactor: 1  // No exponential backoff for simplicity
+         }
+      });
+
+      const errorSpy = jest.fn();
+      transport.onerror = errorSpy;
+
+      const failingStream = new ReadableStream({
+        start(controller) { controller.error(new Error("Network failure")); }
+      });
+
+      const fetchMock = global.fetch as jest.Mock;
+      // Mock the initial GET request, which will fail.
+      fetchMock.mockResolvedValueOnce({
+        ok: true, status: 200,
+        headers: new Headers({ "content-type": "text/event-stream" }),
+        body: failingStream,
+      });
+      // Mock the reconnection GET request, which will succeed.
+      fetchMock.mockResolvedValueOnce({
+        ok: true, status: 200,
+        headers: new Headers({ "content-type": "text/event-stream" }),
+        body: new ReadableStream(),
+      });
+
+      // ACT
+      await transport.start();
+      // Trigger the GET stream directly using the internal method for a clean test.
+      await transport["_startOrAuthSse"]({});
+      await jest.advanceTimersByTimeAsync(20); // Trigger reconnection timeout
+
+      // ASSERT
+      expect(errorSpy).toHaveBeenCalledWith(expect.objectContaining({
+        message: expect.stringContaining('SSE stream disconnected: Error: Network failure'),
+      }));
+      // THE KEY ASSERTION: A second fetch call proves reconnection was attempted.
+      expect(fetchMock).toHaveBeenCalledTimes(2);
+      expect(fetchMock.mock.calls[0][1]?.method).toBe('GET');
+      expect(fetchMock.mock.calls[1][1]?.method).toBe('GET');
+    });
+
+    it('should NOT reconnect a POST-initiated stream that fails', async () => {
+      // ARRANGE
+      transport = new StreamableHTTPClientTransport(new URL("http://localhost:1234/mcp"), {
+        reconnectionOptions: { 
+          initialReconnectionDelay: 10, 
+          maxRetries: 1, 
+          maxReconnectionDelay: 1000,  // Ensure it doesn't retry indefinitely
+          reconnectionDelayGrowFactor: 1  // No exponential backoff for simplicity
+         }
+      });
+
+      const errorSpy = jest.fn();
+      transport.onerror = errorSpy;
+
+      const failingStream = new ReadableStream({
+        start(controller) { controller.error(new Error("Network failure")); }
+      });
+
+      const fetchMock = global.fetch as jest.Mock;
+      // Mock the POST request. It returns a streaming content-type but a failing body.
+      fetchMock.mockResolvedValueOnce({
+        ok: true, status: 200,
+        headers: new Headers({ "content-type": "text/event-stream" }),
+        body: failingStream,
+      });
+
+      // A dummy request message to trigger the `send` logic.
+      const requestMessage: JSONRPCRequest = {
+        jsonrpc: '2.0',
+        method: 'long_running_tool',
+        id: 'request-1',
+        params: {},
+      };
+
+      // ACT
+      await transport.start();
+      // Use the public `send` method to initiate a POST that gets a stream response.
+      await transport.send(requestMessage);
+      await jest.advanceTimersByTimeAsync(20); // Advance time to check for reconnections
+
+      // ASSERT
+      expect(errorSpy).toHaveBeenCalledWith(expect.objectContaining({
+        message: expect.stringContaining('SSE stream disconnected: Error: Network failure'),
+      }));
+      // THE KEY ASSERTION: Fetch was only called ONCE. No reconnection was attempted.
+      expect(fetchMock).toHaveBeenCalledTimes(1);
+      expect(fetchMock.mock.calls[0][1]?.method).toBe('POST');
+    });
+  });
 
   it("invalidates all credentials on InvalidClientError during auth", async () => {
     const message: JSONRPCMessage = {

src/client/streamableHttp.ts

@@ -231,7 +231,7 @@ const response = await (this._fetch ?? fetch)(this._url, {
         );
       }
 
-      this._handleSseStream(response.body, options);
+      this._handleSseStream(response.body, options, true);
     } catch (error) {
       this.onerror?.(error as Error);
       throw error;
@@ -300,7 +300,11 @@ const response = await (this._fetch ?? fetch)(this._url, {
     }, delay);
   }
 
-  private _handleSseStream(stream: ReadableStream<Uint8Array> | null, options: StartSSEOptions): void {
+  private _handleSseStream(
+    stream: ReadableStream<Uint8Array> | null, 
+    options: StartSSEOptions,
+    isReconnectable: boolean,
+  ): void {
     if (!stream) {
       return;
     }
@@ -347,20 +351,22 @@ const response = await (this._fetch ?? fetch)(this._url, {
         this.onerror?.(new Error(`SSE stream disconnected: ${error}`));
 
         // Attempt to reconnect if the stream disconnects unexpectedly and we aren't closing
-        if (this._abortController && !this._abortController.signal.aborted) {
+        if (
+          isReconnectable && 
+          this._abortController && 
+          !this._abortController.signal.aborted
+        ) {
           // Use the exponential backoff reconnection strategy
-          if (lastEventId !== undefined) {
-            try {
-              this._scheduleReconnection({
-                resumptionToken: lastEventId,
-                onresumptiontoken,
-                replayMessageId
-              }, 0);
-            }
-            catch (error) {
-              this.onerror?.(new Error(`Failed to reconnect: ${error instanceof Error ? error.message : String(error)}`));
+          try {
+            this._scheduleReconnection({
+              resumptionToken: lastEventId,
+              onresumptiontoken,
+              replayMessageId
+            }, 0);
+          }
+          catch (error) {
+            this.onerror?.(new Error(`Failed to reconnect: ${error instanceof Error ? error.message : String(error)}`));
 
-            }
           }
         }
       }
@@ -473,7 +479,7 @@ const response = await (this._fetch ?? fetch)(this._url, init);
           // Handle SSE stream responses for requests
           // We use the same handler as standalone streams, which now supports
           // reconnection with the last event ID
-          this._handleSseStream(response.body, { onresumptiontoken });
+          this._handleSseStream(response.body, { onresumptiontoken }, false);
         } else if (contentType?.includes("application/json")) {
           // For non-streaming servers, we might get direct JSON responses
           const data = await response.json();