diff --git a/packages/ast-grep-mcp/src/mcp-stdio-server.test.ts b/packages/ast-grep-mcp/src/mcp-stdio-server.test.ts
new file mode 100644
index 000000000..614f34800
--- /dev/null
+++ b/packages/ast-grep-mcp/src/mcp-stdio-server.test.ts
@@ -0,0 +1,63 @@
+import { describe, expect, it } from "bun:test";
+import { PassThrough } from "node:stream";
+import { runMcpStdioServer } from "./mcp";
+
+describe("ast-grep MCP stdio server", () => {
+  it("#given Codex sends a content-length framed initialize #when stdio server handles it #then responds with a framed initialize result", async () => {
+    const input = new PassThrough();
+    const output = new PassThrough();
+    const received = nextOutput(output);
+    const server = runMcpStdioServer(input, output);
+
+    writeContentLengthFrame(input, {
+      jsonrpc: "2.0",
+      id: 5,
+      method: "initialize",
+      params: {
+        protocolVersion: "2024-11-05",
+        capabilities: {},
+        clientInfo: { name: "codex", version: "0.0.0" },
+      },
+    });
+
+    const response = parseContentLengthFrame(await received);
+    input.end();
+    await server;
+
+    expect(response).toEqual({
+      jsonrpc: "2.0",
+      id: 5,
+      result: {
+        capabilities: { tools: { listChanged: false } },
+        serverInfo: { name: "ast_grep", version: "0.1.0" },
+        protocolVersion: "2024-11-05",
+      },
+    });
+  });
+});
+
+function writeContentLengthFrame(input: PassThrough, message: unknown): void {
+  const body = JSON.stringify(message);
+  input.write(`Content-Length: ${Buffer.byteLength(body, "utf8")}\r\n\r\n${body}`);
+}
+
+function nextOutput(output: PassThrough): Promise<string> {
+  return new Promise((resolve) => {
+    output.once("data", (chunk: Buffer | string) => {
+      resolve(String(chunk));
+    });
+  });
+}
+
+function parseContentLengthFrame(raw: string): unknown {
+  const separator = raw.indexOf("\r\n\r\n");
+  expect(separator).toBeGreaterThan(0);
+  const headers = raw.slice(0, separator);
+  const match = /^Content-Length: (\d+)$/im.exec(headers);
+  expect(match).not.toBeNull();
+  if (match === null) throw new TypeError(`Missing Content-Length header: ${raw}`);
+  const lengthValue = match[1];
+  if (lengthValue === undefined) throw new TypeError(`Invalid Content-Length header: ${raw}`);
+  const bodyStart = separator + "\r\n\r\n".length;
+  return JSON.parse(raw.slice(bodyStart, bodyStart + Number(lengthValue)));
+}