diff --git a/src/mcp/server.ts b/src/mcp/server.ts
index 168fab1..a4c7290 100644
--- a/src/mcp/server.ts
+++ b/src/mcp/server.ts
@@ -78,7 +78,7 @@ export class McpStdioServer {
     process.stdin.setEncoding("utf8")
     process.stdin.on("data", (chunk: string) => {
       buffer += chunk
-      if (buffer.length > McpStdioServer.MAX_BUFFER_BYTES) {
+      if (Buffer.byteLength(buffer, "utf8") > McpStdioServer.MAX_BUFFER_BYTES) {
         // Oversized frame flood: drop the buffered bytes and signal a parse
         // error, but keep the connection alive so the client can recover.
         this.log(`inbound buffer exceeded ${McpStdioServer.MAX_BUFFER_BYTES} bytes; discarding`)
@@ -95,7 +95,7 @@ export class McpStdioServer {
         const line = buffer.slice(0, idx).trim()
         buffer = buffer.slice(idx + 1)
         if (!line) continue
-        if (line.length > McpStdioServer.MAX_LINE_BYTES) {
+        if (Buffer.byteLength(line, "utf8") > McpStdioServer.MAX_LINE_BYTES) {
           this.write({
             jsonrpc: "2.0",
             id: null,
diff --git a/test/unit/core/mcp-size.test.ts b/test/unit/core/mcp-size.test.ts
new file mode 100644
index 0000000..fff7e4e
--- /dev/null
+++ b/test/unit/core/mcp-size.test.ts
@@ -0,0 +1,74 @@
+import { test } from "node:test"
+import assert from "node:assert/strict"
+import { spawnSync } from "node:child_process"
+
+const MAX_LINE_BYTES = 1_048_576
+
+function pingFrame(bytes: number, character: string): string {
+  const empty = JSON.stringify({ jsonrpc: "2.0", id: 1, method: "ping", params: { text: "" } })
+  const available = bytes - Buffer.byteLength(empty, "utf8")
+  const width = Buffer.byteLength(character, "utf8")
+  return empty.replace(
+    '"text":""',
+    `"text":"${character.repeat(Math.floor(available / width))}${"x".repeat(available % width)}"`,
+  )
+}
+
+// Feed deterministic decoded chunks in a child process so a pipe's OS-specific
+// chunk boundaries cannot hide the buffer-limit branch. All parsing, dispatch,
+// and output still run through the real server.
+function feedChunks(
+  chunks: string[],
+): Array<{ id: number | null; result?: unknown; error?: { code: number; message: string } }> {
+  const moduleUrl = new URL("../../../src/mcp/server.js", import.meta.url).href
+  const script = `
+    import { readFileSync } from 'node:fs';
+    import { McpStdioServer } from ${JSON.stringify(moduleUrl)};
+    const chunks = JSON.parse(readFileSync(0, 'utf8'));
+    let receive;
+    process.stdin.setEncoding = () => process.stdin;
+    process.stdin.on = (event, callback) => {
+      if (event === 'data') receive = callback;
+      return process.stdin;
+    };
+    process.stdin.resume = () => process.stdin;
+    new McpStdioServer({name:'test',version:'1',tools:[]}).listen();
+    for (const chunk of chunks) receive(chunk);
+    await new Promise(resolve => setImmediate(resolve));
+  `
+  const result = spawnSync(process.execPath, ["--input-type=module", "-e", script], {
+    input: JSON.stringify(chunks),
+    encoding: "utf8",
+    timeout: 10_000,
+  })
+  assert.equal(result.status, 0, result.stderr)
+  return result.stdout
+    .trim()
+    .split("\n")
+    .map((line) => JSON.parse(line))
+}
+
+for (const character of ["x", "😀", "界"]) {
+  test(`MCP accepts a ${character} frame exactly at the UTF-8 byte limit`, () => {
+    const frame = pingFrame(MAX_LINE_BYTES, character)
+    assert.equal(Buffer.byteLength(frame, "utf8"), MAX_LINE_BYTES)
+    assert.deepEqual(feedChunks([`${frame}\n`]), [{ jsonrpc: "2.0", id: 1, result: {} }])
+  })
+
+  test(`MCP rejects a ${character} frame one byte over the limit and recovers`, () => {
+    const frame = pingFrame(MAX_LINE_BYTES + 1, character)
+    const replies = feedChunks([`${frame}\n`, '{"jsonrpc":"2.0","id":2,"method":"ping"}\n'])
+    assert.equal(replies.length, 2)
+    assert.equal(replies[0]?.error?.code, -32700)
+    assert.match(replies[0]?.error?.message ?? "", /Oversized/)
+    assert.deepEqual(replies[1], { jsonrpc: "2.0", id: 2, result: {} })
+  })
+}
+
+test("MCP accumulates UTF-8 bytes across chunks for the buffer limit and recovers", () => {
+  const chunk = "界".repeat(Math.floor(MAX_LINE_BYTES / 3))
+  const replies = feedChunks([chunk, chunk, "界", '{"jsonrpc":"2.0","id":2,"method":"ping"}\n'])
+  assert.equal(replies.length, 2)
+  assert.match(replies[0]?.error?.message ?? "", /Oversized/)
+  assert.deepEqual(replies[1], { jsonrpc: "2.0", id: 2, result: {} })
+})