Add custom fetch support to StreamableHTTP MCP transport

This enhancement significantly improves the flexibility of the StreamableHTTPClientTransport
class by enabling the injection of a custom fetch implementation. This change empowers users
to exert fine-grained control over network requests, facilitating advanced use cases such as
custom authentication flows, specialized request handling, and improved adaptability of the
transport layer.

Key improvements:
- Added optional 'fetch' parameter to MCPServerStreamableHttpOptions interface
- Updated NodeMCPServerStreamableHttp implementation to pass custom fetch to underlying transport
- Enables custom authentication flows for corporate environments
- Supports request/response logging and debugging capabilities
- Allows retry logic implementation for unreliable connections
- Facilitates testing with mock fetch implementations
- Enables custom header injection and request modification

The implementation is fully backward compatible - existing code continues to work unchanged
as the fetch parameter is optional and defaults to the standard fetch implementation.

Changes include:
- packages/agents-core/src/mcp.ts: Added fetch parameter to interface with proper documentation
- packages/agents-core/src/shims/mcp-server/node.ts: Updated transport initialization
- docs/src/content/docs/guides/mcp.mdx: Updated English documentation
- docs/src/content/docs/ja/guides/mcp.mdx: Updated Japanese documentation
- examples/mcp/streamable-http-custom-fetch-example.ts: Added comprehensive usage example

This change aligns with the MCP SDK's existing support for custom fetch implementations
and provides a clean, type-safe API for advanced network request customization.

Author: alonhar

docs/src/content/docs/guides/mcp.mdx

@@ -80,7 +80,7 @@ When your Agent talks directly to a Streamable HTTP MCP server—local or remote
   title="Run with Streamable HTTP MCP servers"
 />
 
-The constructor also accepts additional MCP TypeScript‑SDK options such as `authProvider`, `requestInit`, `reconnectionOptions`, and `sessionId`. See the [MCP TypeScript SDK repository](https://github.com/modelcontextprotocol/typescript-sdk) and its documents for details.
+The constructor also accepts additional MCP TypeScript‑SDK options such as `authProvider`, `requestInit`, `fetch`, `reconnectionOptions`, and `sessionId`. See the [MCP TypeScript SDK repository](https://github.com/modelcontextprotocol/typescript-sdk) and its documents for details.
 
 ## 3. Stdio MCP servers
 

docs/src/content/docs/ja/guides/mcp.mdx

@@ -79,7 +79,7 @@ import stdioExample from '../../../../../../examples/docs/mcp/stdio.ts?raw';
   title="Run with Streamable HTTP MCP servers"
 />
 
-コンストラクターでは `authProvider`、`requestInit`、`reconnectionOptions`、`sessionId` など、MCP TypeScript SDK の追加オプションも指定できます。詳細は [MCP TypeScript SDK リポジトリ](https://github.com/modelcontextprotocol/typescript-sdk) とそのドキュメントを参照してください。
+コンストラクターでは `authProvider`、`requestInit`、`fetch`、`reconnectionOptions`、`sessionId` など、MCP TypeScript SDK の追加オプションも指定できます。詳細は [MCP TypeScript SDK リポジトリ](https://github.com/modelcontextprotocol/typescript-sdk) とそのドキュメントを参照してください。
 
 ## 3. Stdio MCP サーバー
 

examples/mcp/streamable-http-custom-fetch-example.ts

@@ -0,0 +1,49 @@
+import { Agent, run, MCPServerStreamableHttp, withTrace } from '@openai/agents';
+
+async function main() {
+  // Example of using a custom fetch implementation
+  const customFetch = async (url: string | URL, init?: RequestInit) => {
+    console.log(`Custom fetch called for URL: ${url}`);
+    // You could add custom headers, logging, retries, etc. here
+    const response = await fetch(url, {
+      ...init,
+      headers: {
+        ...init?.headers,
+        'User-Agent': 'MyCustomAgent/1.0',
+        'X-Custom-Header': 'custom-value',
+      },
+    });
+    console.log(`Response status: ${response.status}`);
+    return response;
+  };
+
+  const mcpServer = new MCPServerStreamableHttp({
+    url: 'https://gitmcp.io/openai/codex',
+    name: 'GitMCP Documentation Server',
+    fetch: customFetch, // Pass custom fetch implementation
+  });
+
+  const agent = new Agent({
+    name: 'GitMCP Assistant',
+    instructions: 'Use the tools to respond to user requests.',
+    mcpServers: [mcpServer],
+  });
+
+  try {
+    await withTrace('GitMCP Documentation Server Example with Custom Fetch', async () => {
+      await mcpServer.connect();
+      const result = await run(
+        agent,
+        'Which language is this repo written in?',
+      );
+      console.log(result.finalOutput);
+    });
+  } finally {
+    await mcpServer.close();
+  }
+}
+
+main().catch((err) => {
+  console.error(err);
+  process.exit(1);
+});

packages/agents-core/src/mcp.ts

@@ -536,6 +536,9 @@ export interface MCPServerStreamableHttpOptions {
   authProvider?: any;
   // RequestInit
   requestInit?: any;
+  // Custom fetch implementation used for all network requests.
+  // import { FetchLike } from '@modelcontextprotocol/sdk/shared/transport.js';
+  fetch?: any;
   // import { StreamableHTTPReconnectionOptions } from '@modelcontextprotocol/sdk/client/streamableHttp.js';
   reconnectionOptions?: any;
   sessionId?: string;

packages/agents-core/src/shims/mcp-server/node.ts

@@ -322,6 +322,7 @@ export class NodeMCPServerStreamableHttp extends BaseMCPServerStreamableHttp {
         {
           authProvider: this.params.authProvider,
           requestInit: this.params.requestInit,
+          fetch: this.params.fetch,
           reconnectionOptions: this.params.reconnectionOptions,
           sessionId: this.params.sessionId,
         },