RelatedRequestId routing for MCP streams (cloudflare#654)

* feat: implement relatedRequestId routing for bidirectional MCP streams

- Added support for TransportSendOptions to route server-to-client requests through the same stream as the originating client request
- Updated send() method to prioritize relatedRequestId for routing, with message.id override for responses/errors
- Added comprehensive test coverage for stream routing scenarios including multiple streams, error cases, and response handling

* changeset

* fix: review suggestions

* add related id to ellicitation call

* fix: optional chaining

* update changeset

Author: mattzcarey

.changeset/spotty-crabs-allow.md

@@ -0,0 +1,5 @@
+---
+"agents": patch
+---
+
+When handling MCP server requests use relatedRequestId in TransportOptions to send the response down a POST stream if supported (streamable-http)

examples/mcp-elicitation/src/index.ts

@@ -54,28 +54,31 @@ export class MyAgent extends Agent<Env, State> {
           confirm: z.boolean().describe("Do you want to increase the counter?")
         }
       },
-      async ({ confirm }) => {
+      async ({ confirm }, extra) => {
         if (!confirm) {
           return {
             content: [{ type: "text", text: "Counter increase cancelled." }]
           };
         }
         try {
-          const basicInfo = await this.server.server.elicitInput({
-            message: "By how much do you want to increase the counter?",
-            requestedSchema: {
-              type: "object",
-              properties: {
-                amount: {
-                  type: "number",
-                  title: "Amount",
-                  description: "The amount to increase the counter by",
-                  minLength: 1
-                }
-              },
-              required: ["amount"]
-            }
-          });
+          const basicInfo = await this.server.server.elicitInput(
+            {
+              message: "By how much do you want to increase the counter?",
+              requestedSchema: {
+                type: "object",
+                properties: {
+                  amount: {
+                    type: "number",
+                    title: "Amount",
+                    description: "The amount to increase the counter by",
+                    minLength: 1
+                  }
+                },
+                required: ["amount"]
+              }
+            },
+            { relatedRequestId: extra.requestId }
+          );
 
           if (basicInfo.action !== "accept" || !basicInfo.content) {
             return {

packages/agents/src/mcp/worker-transport.ts

@@ -2,7 +2,10 @@
  * Based on @hono/mcp transport implementation (https://github.com/honojs/middleware/tree/main/packages/mcp)
  */
 
-import type { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
+import type {
+  Transport,
+  TransportSendOptions
+} from "@modelcontextprotocol/sdk/shared/transport.js";
 import type {
   JSONRPCMessage,
   RequestId,
@@ -357,7 +360,7 @@ export class WorkerTransport implements Transport {
     const acceptHeader = request.headers.get("Accept");
     if (
       !acceptHeader?.includes("application/json") ||
-      !acceptHeader.includes("text/event-stream")
+      !acceptHeader?.includes("text/event-stream")
     ) {
       return new Response(
         JSON.stringify({
@@ -738,9 +741,14 @@ export class WorkerTransport implements Transport {
     this.onclose?.();
   }
 
-  async send(message: JSONRPCMessage): Promise<void> {
-    let requestId: RequestId | undefined;
+  async send(
+    message: JSONRPCMessage,
+    options?: TransportSendOptions
+  ): Promise<void> {
+    // Check relatedRequestId FIRST to route server-to-client requests through the same stream as the originating client request
+    let requestId: RequestId | undefined = options?.relatedRequestId;
 
+    // Then override with message.id for responses/errors
     if (isJSONRPCResponse(message) || isJSONRPCError(message)) {
       requestId = message.id;
     }