diff --git a/cli/tests/unit/flash_test.ts b/cli/tests/unit/flash_test.ts
index 78340a3901..5e1e55d863 100644
--- a/cli/tests/unit/flash_test.ts
+++ b/cli/tests/unit/flash_test.ts
@@ -495,6 +495,43 @@ Deno.test(
   },
 );
 
+Deno.test(
+  { permissions: { net: true } },
+  async function httpServerCorrectLengthForUnicodeString() {
+    const ac = new AbortController();
+    const listeningPromise = deferred();
+
+    const server = Deno.serve({
+      handler: () => new Response("韓國".repeat(10)),
+      port: 4503,
+      signal: ac.signal,
+      onListen: onListen(listeningPromise),
+      onError: createOnErrorCb(ac),
+    });
+
+    await listeningPromise;
+    const conn = await Deno.connect({ port: 4503 });
+    const encoder = new TextEncoder();
+    const decoder = new TextDecoder();
+
+    const body =
+      `GET / HTTP/1.1\r\nHost: example.domain\r\nConnection: close\r\n\r\n`;
+    const writeResult = await conn.write(encoder.encode(body));
+    assertEquals(body.length, writeResult);
+
+    const buf = new Uint8Array(1024);
+    const readResult = await conn.read(buf);
+    assert(readResult);
+    const msg = decoder.decode(buf.subarray(0, readResult));
+
+    conn.close();
+
+    ac.abort();
+    await server;
+    assert(msg.includes("Content-Length: 60"));
+  },
+);
+
 Deno.test({ permissions: { net: true } }, async function httpServerWebSocket() {
   const ac = new AbortController();
   const listeningPromise = deferred();
diff --git a/core/01_core.js b/core/01_core.js
index e5e2c9fd5d..ddd3ac82d6 100644
--- a/core/01_core.js
+++ b/core/01_core.js
@@ -323,6 +323,7 @@
     opNames: () => ops.op_op_names(),
     eventLoopHasMoreWork: () => ops.op_event_loop_has_more_work(),
     setPromiseRejectCallback: (fn) => ops.op_set_promise_reject_callback(fn),
+    byteLength: (str) => ops.op_str_byte_length(str),
   });
 
   ObjectAssign(globalThis.__bootstrap, { core });
diff --git a/core/ops_builtin.rs b/core/ops_builtin.rs
index bd19b74f6b..26ab4bed5a 100644
--- a/core/ops_builtin.rs
+++ b/core/ops_builtin.rs
@@ -39,6 +39,7 @@ pub(crate) fn init_builtins() -> Extension {
       op_metrics::decl(),
       op_format_file_name::decl(),
       op_is_proxy::decl(),
+      op_str_byte_length::decl(),
     ])
     .ops(crate::ops_builtin_v8::init_builtins_v8())
     .build()
@@ -195,3 +196,15 @@ fn op_format_file_name(file_name: String) -> String {
 fn op_is_proxy(value: serde_v8::Value) -> bool {
   value.v8_value.is_proxy()
 }
+
+#[op(v8)]
+fn op_str_byte_length(
+  scope: &mut v8::HandleScope,
+  value: serde_v8::Value,
+) -> u32 {
+  if let Ok(string) = v8::Local::<v8::String>::try_from(value.v8_value) {
+    string.utf8_length(scope) as u32
+  } else {
+    0
+  }
+}
diff --git a/ext/flash/01_http.js b/ext/flash/01_http.js
index b00c9f8e49..949eb0ac31 100644
--- a/ext/flash/01_http.js
+++ b/ext/flash/01_http.js
@@ -122,7 +122,14 @@
   //    CRLF
   //    [ message-body ]
   //
-  function http1Response(method, status, headerList, body, earlyEnd = false) {
+  function http1Response(
+    method,
+    status,
+    headerList,
+    body,
+    bodyLen,
+    earlyEnd = false,
+  ) {
     // HTTP uses a "<major>.<minor>" numbering scheme
     //   HTTP-version  = HTTP-name "/" DIGIT "." DIGIT
     //   HTTP-name     = %x48.54.54.50 ; "HTTP", case-sensitive
@@ -155,7 +162,7 @@
 
     // null body status is validated by inititalizeAResponse in ext/fetch
     if (body !== null && body !== undefined) {
-      str += `Content-Length: ${body.length}\r\n\r\n`;
+      str += `Content-Length: ${bodyLen}\r\n\r\n`;
     } else {
       str += "Transfer-Encoding: chunked\r\n\r\n";
       return str;
@@ -192,6 +199,7 @@
     server,
     requestId,
     response,
+    responseLen,
     end,
     respondFast,
   ) {
@@ -215,7 +223,7 @@
       }
     }
 
-    if (nwritten < response.length) {
+    if (nwritten < responseLen) {
       core.opAsync(
         "op_flash_respond_async",
         server,
@@ -421,16 +429,19 @@
 
             const ws = resp[_ws];
             if (isStreamingResponseBody === false) {
+              const length = respBody.byteLength || core.byteLength(respBody);
               const responseStr = http1Response(
                 method,
                 innerResp.status ?? 200,
                 innerResp.headerList,
                 respBody,
+                length,
               );
               writeFixedResponse(
                 serverId,
                 i,
                 responseStr,
+                length,
                 !ws, // Don't close socket if there is a deferred websocket upgrade.
                 respondFast,
               );
@@ -460,6 +471,7 @@
                         method,
                         innerResp.status ?? 200,
                         innerResp.headerList,
+                        0, // Content-Length will be set by the op.
                         null,
                         true,
                       ),
@@ -483,8 +495,10 @@
                       method,
                       innerResp.status ?? 200,
                       innerResp.headerList,
+                      respBody.byteLength,
                       null,
                     ),
+                    respBody.byteLength,
                     false,
                     respondFast,
                   );