diff --git a/src/server/streamableHttp.ts b/src/server/streamableHttp.ts
index ab1131f63..219cb00cf 100644
--- a/src/server/streamableHttp.ts
+++ b/src/server/streamableHttp.ts
@@ -873,7 +873,7 @@ export class StreamableHTTPServerTransport implements Transport {
         let requestId = options?.relatedRequestId;
         if (isJSONRPCResultResponse(message) || isJSONRPCErrorResponse(message)) {
             // If the message is a response, use the request ID from the message
-            requestId = message.id;
+            requestId = message.id ?? undefined;
         }
 
         // Check if this message should be sent on the standalone SSE stream (no request ID)
diff --git a/src/spec.types.ts b/src/spec.types.ts
index 07a1cceff..3ee9880a8 100644
--- a/src/spec.types.ts
+++ b/src/spec.types.ts
@@ -169,7 +169,7 @@ export interface JSONRPCResultResponse {
  */
 export interface JSONRPCErrorResponse {
   jsonrpc: typeof JSONRPC_VERSION;
-  id?: RequestId;
+  id: RequestId | null;
   error: Error;
 }
 
diff --git a/src/types.ts b/src/types.ts
index dc0c22353..e0cced52b 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -198,7 +198,7 @@ export enum ErrorCode {
 export const JSONRPCErrorResponseSchema = z
     .object({
         jsonrpc: z.literal(JSONRPC_VERSION),
-        id: RequestIdSchema.optional(),
+        id: RequestIdSchema.nullable(),
         error: z.object({
             /**
              * The error type that occurred.