diff --git a/packages/core/src/types/types.ts b/packages/core/src/types/types.ts
index f3e1b92a8..bf89cf732 100644
--- a/packages/core/src/types/types.ts
+++ b/packages/core/src/types/types.ts
@@ -2384,11 +2384,6 @@ type Flatten<T> = T extends Primitive
 
 type Infer<Schema extends z.ZodTypeAny> = Flatten<z.infer<Schema>>;
 
-/**
- * Headers that are compatible with both Node.js and the browser.
- */
-export type IsomorphicHeaders = Record<string, string | string[] | undefined>;
-
 /**
  * Information about the incoming request.
  */
@@ -2396,7 +2391,7 @@ export interface RequestInfo {
     /**
      * The headers of the request.
      */
-    headers: IsomorphicHeaders;
+    headers: Headers;
 }
 
 /**
diff --git a/packages/middleware/node/test/streamableHttp.test.ts b/packages/middleware/node/test/streamableHttp.test.ts
index b406b80c0..5f865ef49 100644
--- a/packages/middleware/node/test/streamableHttp.test.ts
+++ b/packages/middleware/node/test/streamableHttp.test.ts
@@ -12,13 +12,13 @@ import type {
     JSONRPCResultResponse,
     RequestId
 } from '@modelcontextprotocol/core';
+import type { EventId, EventStore, StreamId } from '@modelcontextprotocol/server';
+import { McpServer } from '@modelcontextprotocol/server';
 import type { ZodMatrixEntry } from '@modelcontextprotocol/test-helpers';
 import { listenOnRandomPort, zodTestMatrix } from '@modelcontextprotocol/test-helpers';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
 
 import { NodeStreamableHTTPServerTransport } from '../src/streamableHttp.js';
-import { McpServer } from '@modelcontextprotocol/server';
-import type { EventId, EventStore, StreamId } from '@modelcontextprotocol/server';
-import { describe, expect, beforeEach, afterEach, it } from 'vitest';
 
 async function getFreePort() {
     return new Promise(res => {
@@ -402,10 +402,15 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
                 'A simple test tool with request info',
                 { name: z.string().describe('Name to greet') },
                 async ({ name }, { requestInfo }): Promise<CallToolResult> => {
+                    // Convert Headers object to plain object for JSON serialization
+                    // Headers is a Web API class that doesn't serialize with JSON.stringify
+                    const serializedRequestInfo = {
+                        headers: Object.fromEntries(requestInfo?.headers ?? new Headers())
+                    };
                     return {
                         content: [
                             { type: 'text', text: `Hello, ${name}!` },
-                            { type: 'text', text: `${JSON.stringify(requestInfo)}` }
+                            { type: 'text', text: `${JSON.stringify(serializedRequestInfo)}` }
                         ]
                     };
                 }
diff --git a/packages/server/src/server/server.ts b/packages/server/src/server/server.ts
index 8132e342b..c0e0541e8 100644
--- a/packages/server/src/server/server.ts
+++ b/packages/server/src/server/server.ts
@@ -168,7 +168,7 @@ export class Server<
         if (this._capabilities.logging) {
             this.setRequestHandler(SetLevelRequestSchema, async (request, extra) => {
                 const transportSessionId: string | undefined =
-                    extra.sessionId || (extra.requestInfo?.headers['mcp-session-id'] as string) || undefined;
+                    extra.sessionId || (extra.requestInfo?.headers.get('mcp-session-id') as string) || undefined;
                 const { level } = request.params;
                 const parseResult = LoggingLevelSchema.safeParse(level);
                 if (parseResult.success) {
diff --git a/packages/server/src/server/streamableHttp.ts b/packages/server/src/server/streamableHttp.ts
index 7034ce64a..19360310e 100644
--- a/packages/server/src/server/streamableHttp.ts
+++ b/packages/server/src/server/streamableHttp.ts
@@ -596,7 +596,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
 
             // Build request info from headers
             const requestInfo: RequestInfo = {
-                headers: Object.fromEntries(req.headers.entries())
+                headers: req.headers
             };
 
             let rawMessage;