fix: protocol version validation in worker transport

.changeset/large-squids-cut.md

@@ -0,0 +1,5 @@
+---
+"agents": patch
+---
+
+MCP WorkerTransport accepts any supported protocol version in request headers and only rejects truly unsupported versions. This aligns with the move by MCP community to stateless transports and fixes an isse with 'mcp-protocol-version': '2025-11-25'

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

@@ -9,7 +9,6 @@ import type {
 import type {
   JSONRPCMessage,
   RequestId,
-  InitializeRequest,
   RequestInfo,
   MessageExtraInfo
 } from "@modelcontextprotocol/sdk/types.js";
@@ -18,16 +17,13 @@ import {
   isJSONRPCError,
   isJSONRPCRequest,
   isJSONRPCResponse,
-  JSONRPCMessageSchema
+  JSONRPCMessageSchema,
+  SUPPORTED_PROTOCOL_VERSIONS
 } from "@modelcontextprotocol/sdk/types.js";
 import type { CORSOptions } from "./types";
 
-const SUPPORTED_PROTOCOL_VERSIONS = ["2025-03-26", "2025-06-18"] as const;
-const DEFAULT_PROTOCOL_VERSION = "2025-03-26";
 const MCP_PROTOCOL_VERSION_HEADER = "MCP-Protocol-Version";
 
-type ProtocolVersion = (typeof SUPPORTED_PROTOCOL_VERSIONS)[number];
-
 interface StreamMapping {
   writer?: WritableStreamDefaultWriter<Uint8Array>;
   encoder?: TextEncoder;
@@ -43,7 +39,6 @@ export interface MCPStorageApi {
 export interface TransportState {
   sessionId?: string;
   initialized: boolean;
-  protocolVersion?: ProtocolVersion;
 }
 
 export interface WorkerTransportOptions {
@@ -73,7 +68,6 @@ export class WorkerTransport implements Transport {
   private requestToStreamMapping = new Map<RequestId, string>();
   private requestResponseMap = new Map<RequestId, JSONRPCMessage>();
   private corsOptions?: CORSOptions;
-  private protocolVersion?: ProtocolVersion;
   private storage?: MCPStorageApi;
   private stateRestored = false;
 
@@ -104,7 +98,6 @@ export class WorkerTransport implements Transport {
     if (state) {
       this.sessionId = state.sessionId;
       this.initialized = state.initialized;
-      this.protocolVersion = state.protocolVersion;
     }
 
     this.stateRestored = true;
@@ -120,8 +113,7 @@ export class WorkerTransport implements Transport {
 
     const state: TransportState = {
       sessionId: this.sessionId,
-      initialized: this.initialized,
-      protocolVersion: this.protocolVersion
+      initialized: this.initialized
     };
 
     await Promise.resolve(this.storage.set(state));
@@ -134,66 +126,32 @@ export class WorkerTransport implements Transport {
     this.started = true;
   }
 
+  /**
+   * Validates the MCP-Protocol-Version header on incoming requests.
+   *
+   * This performs a simple check: if a version header is present, it must be
+   * in the SUPPORTED_PROTOCOL_VERSIONS list. We do not track the negotiated
+   * version or enforce version consistency across requests - the SDK handles
+   * version negotiation during initialization, and we simply reject any
+   * explicitly unsupported versions.
+   *
+   * - Header present and supported: Accept
+   * - Header present and unsupported: 400 Bad Request
+   * - Header missing: Accept (version validation is optional)
+   */
   private validateProtocolVersion(request: Request): Response | undefined {
-    const versionHeader = request.headers.get(MCP_PROTOCOL_VERSION_HEADER);
-
-    if (!versionHeader) {
-      if (
-        !this.protocolVersion ||
-        this.protocolVersion === DEFAULT_PROTOCOL_VERSION
-      ) {
-        return undefined;
-      }
-      // If we negotiated a different version, the client MUST send the header
-      return new Response(
-        JSON.stringify({
-          jsonrpc: "2.0",
-          error: {
-            code: -32000,
-            message: `Bad Request: ${MCP_PROTOCOL_VERSION_HEADER} header is required`
-          },
-          id: null
-        }),
-        {
-          status: 400,
-          headers: {
-            "Content-Type": "application/json",
-            ...this.getHeaders()
-          }
-        }
-      );
-    }
+    const protocolVersion = request.headers.get(MCP_PROTOCOL_VERSION_HEADER);
 
     if (
-      !SUPPORTED_PROTOCOL_VERSIONS.includes(versionHeader as ProtocolVersion)
+      protocolVersion !== null &&
+      !SUPPORTED_PROTOCOL_VERSIONS.includes(protocolVersion)
     ) {
       return new Response(
         JSON.stringify({
           jsonrpc: "2.0",
           error: {
             code: -32000,
-            message: `Bad Request: Unsupported ${MCP_PROTOCOL_VERSION_HEADER}: ${versionHeader}. Supported versions: ${SUPPORTED_PROTOCOL_VERSIONS.join(", ")}`
-          },
-          id: null
-        }),
-        {
-          status: 400,
-          headers: {
-            "Content-Type": "application/json",
-            ...this.getHeaders()
-          }
-        }
-      );
-    }
-
-    // Check if it matches the negotiated version
-    if (this.protocolVersion && versionHeader !== this.protocolVersion) {
-      return new Response(
-        JSON.stringify({
-          jsonrpc: "2.0",
-          error: {
-            code: -32000,
-            message: `Bad Request: ${MCP_PROTOCOL_VERSION_HEADER} mismatch. Expected: ${this.protocolVersion}, Got: ${versionHeader}`
+            message: `Bad Request: Unsupported protocol version: ${protocolVersion} (supported versions: ${SUPPORTED_PROTOCOL_VERSIONS.join(", ")})`
           },
           id: null
         }),
@@ -206,7 +164,6 @@ export class WorkerTransport implements Transport {
         }
       );
     }
-
     return undefined;
   }
 
@@ -504,23 +461,6 @@ export class WorkerTransport implements Transport {
         );
       }
 
-      // Capture protocol version from initialization request
-      const initRequest = messages.find(isInitializeRequest) as
-        | InitializeRequest
-        | undefined;
-      if (initRequest?.params) {
-        const version = initRequest.params.protocolVersion;
-        if (
-          version &&
-          SUPPORTED_PROTOCOL_VERSIONS.includes(version as ProtocolVersion)
-        ) {
-          this.protocolVersion = version as ProtocolVersion;
-        } else {
-          // Default to 2025-03-26 if not specified or unsupported
-          this.protocolVersion = DEFAULT_PROTOCOL_VERSION;
-        }
-      }
-
       this.sessionId = this.sessionIdGenerator?.();
       this.initialized = true;
       await this.saveState();