diff --git a/integrations/mcp-server/README.md b/integrations/mcp-server/README.md
index 422efae..c261125 100644
--- a/integrations/mcp-server/README.md
+++ b/integrations/mcp-server/README.md
@@ -16,6 +16,10 @@ node integrations/mcp-server/src/index.mjs
 Set `ANDROID_SERIAL` to target a specific ADB device or emulator. Set
 `OPENPHONE_DRY_RUN=1` for parser/protocol tests without ADB.
 
+The server speaks the MCP stdio transport: newline-delimited JSON-RPC (one
+message per line, no `Content-Length` headers) and negotiates
+`protocolVersion` during `initialize` (latest supported: `2025-06-18`).
+
 The same ADB transport works with the OpenPhone SDK phone emulator:
 
 ```sh
diff --git a/integrations/mcp-server/src/index.mjs b/integrations/mcp-server/src/index.mjs
index 37306e6..cc6a94f 100644
--- a/integrations/mcp-server/src/index.mjs
+++ b/integrations/mcp-server/src/index.mjs
@@ -8,11 +8,19 @@ import {
   textContent,
 } from "../runtime/protocol/openphone-runtime-tools.mjs";
 
-const PROTOCOL_VERSION = "2025-11-25";
+export const SUPPORTED_PROTOCOL_VERSIONS = ["2025-06-18"];
+const LATEST_PROTOCOL_VERSION = SUPPORTED_PROTOCOL_VERSIONS[0];
 const SERVER_INFO = { name: "openphone-runtime", version: "0.1.0" };
 
 const commands = loadCommands();
 
+export function negotiateProtocolVersion(requested) {
+  if (SUPPORTED_PROTOCOL_VERSIONS.includes(requested)) {
+    return requested;
+  }
+  return LATEST_PROTOCOL_VERSION;
+}
+
 export async function handleRequest(message, context = {}) {
   if (!message || typeof message !== "object") {
     return errorResponse(null, -32600, "Invalid Request");
@@ -26,7 +34,7 @@ export async function handleRequest(message, context = {}) {
     switch (message.method) {
       case "initialize":
         return resultResponse(id, {
-          protocolVersion: PROTOCOL_VERSION,
+          protocolVersion: negotiateProtocolVersion(message.params?.protocolVersion),
           capabilities: {
             tools: { listChanged: false },
           },
@@ -48,16 +56,39 @@ export async function handleRequest(message, context = {}) {
 
 export async function serve(options = {}) {
   const transport = options.transport ?? new OpenPhoneAdbTransport();
-  let buffer = Buffer.alloc(0);
-  process.stdin.on("data", async (chunk) => {
-    buffer = Buffer.concat([buffer, chunk]);
-    const parsed = parseMessages(buffer);
-    buffer = parsed.remaining;
-    for (const message of parsed.messages) {
-      const response = await handleRequest(message, { transport });
-      if (response) {
-        writeMessage(response);
+  let buffer = "";
+  let queue = Promise.resolve();
+  process.stdin.setEncoding("utf8");
+  process.stdin.on("data", (chunk) => {
+    // Buffer mutation must stay synchronous: extract every complete line
+    // before awaiting anything, then process messages serially on a promise
+    // chain so interleaved chunks cannot drop or reorder responses.
+    buffer += chunk;
+    const lines = [];
+    let newline = buffer.indexOf("\n");
+    while (newline >= 0) {
+      lines.push(buffer.slice(0, newline));
+      buffer = buffer.slice(newline + 1);
+      newline = buffer.indexOf("\n");
+    }
+    for (const rawLine of lines) {
+      const line = rawLine.trim();
+      if (!line) {
+        continue;
       }
+      queue = queue.then(async () => {
+        let message;
+        try {
+          message = JSON.parse(line);
+        } catch {
+          writeMessage(errorResponse(null, -32700, "Parse error"));
+          return;
+        }
+        const response = await handleRequest(message, { transport });
+        if (response) {
+          writeMessage(response);
+        }
+      });
     }
   });
   process.stdin.resume();
@@ -98,44 +129,9 @@ function errorResponse(id, code, message) {
   };
 }
 
-function parseMessages(buffer) {
-  const messages = [];
-  let rest = buffer;
-  while (rest.length > 0) {
-    const headerEnd = rest.indexOf("\r\n\r\n");
-    if (headerEnd >= 0) {
-      const header = rest.slice(0, headerEnd).toString("utf8");
-      const match = header.match(/content-length:\s*(\d+)/iu);
-      if (!match) {
-        break;
-      }
-      const length = Number(match[1]);
-      const bodyStart = headerEnd + 4;
-      const bodyEnd = bodyStart + length;
-      if (rest.length < bodyEnd) {
-        break;
-      }
-      messages.push(JSON.parse(rest.slice(bodyStart, bodyEnd).toString("utf8")));
-      rest = rest.slice(bodyEnd);
-      continue;
-    }
-
-    const newline = rest.indexOf("\n");
-    if (newline < 0) {
-      break;
-    }
-    const line = rest.slice(0, newline).toString("utf8").trim();
-    rest = rest.slice(newline + 1);
-    if (line) {
-      messages.push(JSON.parse(line));
-    }
-  }
-  return { messages, remaining: rest };
-}
-
 function writeMessage(message) {
-  const body = JSON.stringify(message);
-  process.stdout.write(`Content-Length: ${Buffer.byteLength(body, "utf8")}\r\n\r\n${body}`);
+  // MCP stdio transport framing: one JSON-RPC message per line.
+  process.stdout.write(`${JSON.stringify(message)}\n`);
 }
 
 if (import.meta.url === `file://${process.argv[1]}`) {
diff --git a/tests/integrations/runtime-mcp-contract.mjs b/tests/integrations/runtime-mcp-contract.mjs
index 8037b75..02d4097 100644
--- a/tests/integrations/runtime-mcp-contract.mjs
+++ b/tests/integrations/runtime-mcp-contract.mjs
@@ -1,7 +1,17 @@
 #!/usr/bin/env node
 
 import assert from "node:assert/strict";
-import { handleRequest } from "../../integrations/mcp-server/src/index.mjs";
+import { once } from "node:events";
+import { spawn } from "node:child_process";
+import { fileURLToPath } from "node:url";
+import {
+  handleRequest,
+  SUPPORTED_PROTOCOL_VERSIONS,
+} from "../../integrations/mcp-server/src/index.mjs";
+
+const serverPath = fileURLToPath(
+  new URL("../../integrations/mcp-server/src/index.mjs", import.meta.url),
+);
 
 const calls = [];
 const fakeTransport = {
@@ -20,6 +30,29 @@ const fakeTransport = {
   }, { transport: fakeTransport });
   assert.equal(response.result.serverInfo.name, "openphone-runtime");
   assert.ok(response.result.capabilities.tools);
+  assert.equal(response.result.protocolVersion, SUPPORTED_PROTOCOL_VERSIONS[0]);
+}
+
+{
+  // A supported client protocolVersion is echoed back.
+  const response = await handleRequest({
+    jsonrpc: "2.0",
+    id: 1,
+    method: "initialize",
+    params: { protocolVersion: "2025-06-18" },
+  }, { transport: fakeTransport });
+  assert.equal(response.result.protocolVersion, "2025-06-18");
+}
+
+{
+  // An unsupported client protocolVersion falls back to the latest we support.
+  const response = await handleRequest({
+    jsonrpc: "2.0",
+    id: 1,
+    method: "initialize",
+    params: { protocolVersion: "1999-01-01" },
+  }, { transport: fakeTransport });
+  assert.equal(response.result.protocolVersion, SUPPORTED_PROTOCOL_VERSIONS[0]);
 }
 
 {
@@ -119,3 +152,72 @@ const fakeTransport = {
   const response = await handleRequest("not an object", { transport: fakeTransport });
   assert.equal(response.error.code, -32600);
 }
+
+{
+  // Stdio smoke test: a real newline-delimited JSON handshake against the
+  // server process (initialize -> notifications/initialized -> tools/list).
+  const server = spawn(process.execPath, [serverPath], {
+    env: { ...process.env, OPENPHONE_DRY_RUN: "1" },
+    stdio: ["pipe", "pipe", "inherit"],
+  });
+
+  const responses = [];
+  let stdoutBuffer = "";
+  const waiters = [];
+  server.stdout.setEncoding("utf8");
+  server.stdout.on("data", (chunk) => {
+    stdoutBuffer += chunk;
+    let newline = stdoutBuffer.indexOf("\n");
+    while (newline >= 0) {
+      const line = stdoutBuffer.slice(0, newline).trim();
+      stdoutBuffer = stdoutBuffer.slice(newline + 1);
+      if (line) {
+        responses.push(JSON.parse(line));
+        const waiter = waiters.shift();
+        if (waiter) {
+          waiter();
+        }
+      }
+      newline = stdoutBuffer.indexOf("\n");
+    }
+  });
+
+  const nextResponse = async () => {
+    if (responses.length === 0) {
+      await new Promise((resolve) => waiters.push(resolve));
+    }
+    return responses.shift();
+  };
+
+  const send = (message) => {
+    server.stdin.write(`${JSON.stringify(message)}\n`);
+  };
+
+  try {
+    send({
+      jsonrpc: "2.0",
+      id: 1,
+      method: "initialize",
+      params: {
+        protocolVersion: "2025-06-18",
+        capabilities: {},
+        clientInfo: { name: "contract-test", version: "0.0.0" },
+      },
+    });
+    const initialize = await nextResponse();
+    assert.equal(initialize.id, 1);
+    assert.equal(initialize.result.protocolVersion, "2025-06-18");
+    assert.equal(initialize.result.serverInfo.name, "openphone-runtime");
+
+    send({ jsonrpc: "2.0", method: "notifications/initialized" });
+
+    send({ jsonrpc: "2.0", id: 2, method: "tools/list", params: {} });
+    const toolsList = await nextResponse();
+    assert.equal(toolsList.id, 2);
+    assert.ok(toolsList.result.tools.length > 10);
+    assert.equal(responses.length, 0);
+  } finally {
+    server.kill();
+    await once(server, "exit");
+  }
+}