fix: capture transport reference at request time to fix response routing

The Protocol class was overwriting its transport reference when new clients
connected, causing responses to be sent to the wrong transport. This fix
captures the transport reference when a request arrives and uses that
captured reference throughout the request lifecycle.

This ensures:
- Each request's response is sent back through the correct transport
- Multiple clients can connect without interfering with each other
- Async request handling works correctly even when new connections arrive

Author: grimmerk

src/shared/protocol-transport-handling.test.ts

@@ -102,24 +102,23 @@ describe("Protocol transport handling bug", () => {
     console.log("Transport A received:", transportA.sentMessages);
     console.log("Transport B received:", transportB.sentMessages);
 
-    // BUG: The response for client A's request will be sent to transport B
-    // because the protocol's transport reference was overwritten
+    // FIXED: Each transport now receives its own response
 
-    // What happens (bug):
-    // - Transport A should have received the response for request ID 1, but it's empty
-    expect(transportA.sentMessages.length).toBe(0);
-    
-    // - Transport B incorrectly receives BOTH responses
-    expect(transportB.sentMessages.length).toBe(2);
-    expect(transportB.sentMessages[0]).toMatchObject({
+    // Transport A should receive response for request ID 1
+    expect(transportA.sentMessages.length).toBe(1);
+    expect(transportA.sentMessages[0]).toMatchObject({
       jsonrpc: "2.0",
-      id: 1, // This is A's request ID!
+      id: 1,
       result: { data: "responseForA" }
     });
 
-    // What SHOULD happen (after fix):
-    // - Transport A should receive response for request ID 1
-    // - Transport B should receive response for request ID 2
+    // Transport B should only receive its own response (when implemented)
+    expect(transportB.sentMessages.length).toBe(1);
+    expect(transportB.sentMessages[0]).toMatchObject({
+      jsonrpc: "2.0",
+      id: 2,
+      result: { data: "responseForA" } // Same handler result in this test
+    });
   });
 
   test("demonstrates the timing issue with multiple rapid connections", async () => {
@@ -183,8 +182,8 @@ describe("Protocol transport handling bug", () => {
 
     console.log("Timing test results:", results);
 
-    // BUG: All responses go to transport B
-    expect(transportA.sentMessages.length).toBe(0);
-    expect(transportB.sentMessages.length).toBe(2); // Gets both responses
+    // FIXED: Each transport receives its own responses
+    expect(transportA.sentMessages.length).toBe(1);
+    expect(transportB.sentMessages.length).toBe(1);
   });
 });

src/shared/protocol.ts

@@ -367,8 +367,11 @@ export abstract class Protocol<
     const handler =
       this._requestHandlers.get(request.method) ?? this.fallbackRequestHandler;
 
+    // Capture the current transport at request time to ensure responses go to the correct client
+    const capturedTransport = this._transport;
+
     if (handler === undefined) {
-      this._transport
+      capturedTransport
         ?.send({
           jsonrpc: "2.0",
           id: request.id,
@@ -390,7 +393,7 @@ export abstract class Protocol<
 
     const fullExtra: RequestHandlerExtra<SendRequestT, SendNotificationT> = {
       signal: abortController.signal,
-      sessionId: this._transport?.sessionId,
+      sessionId: capturedTransport?.sessionId,
       _meta: request.params?._meta,
       sendNotification:
         (notification) =>
@@ -411,7 +414,7 @@ export abstract class Protocol<
             return;
           }
 
-          return this._transport?.send({
+          return capturedTransport?.send({
             result,
             jsonrpc: "2.0",
             id: request.id,
@@ -422,7 +425,7 @@ export abstract class Protocol<
             return;
           }
 
-          return this._transport?.send({
+          return capturedTransport?.send({
             jsonrpc: "2.0",
             id: request.id,
             error: {