diff --git a/fetch/api/body/textstream.tentative.any.js b/fetch/api/body/textstream.tentative.any.js
new file mode 100644
index 00000000000000..42f11893991c0a
--- /dev/null
+++ b/fetch/api/body/textstream.tentative.any.js
@@ -0,0 +1,161 @@
+// META: global=window,worker
+
+async function readAllChunks(stream) {
+  const reader = stream.getReader();
+  const chunks = [];
+  while (true) {
+    const {done, value} = await reader.read();
+    if (done) {
+      break;
+    }
+    chunks.push(value);
+  }
+  return chunks;
+}
+
+test(() => {
+  assert_true('textStream' in Response.prototype, "textStream exists on Response.prototype");
+  assert_true('textStream' in Request.prototype, "textStream exists on Request.prototype");
+  assert_equals(typeof Response.prototype.textStream, "function", "Response.prototype.textStream is a function");
+  assert_equals(typeof Request.prototype.textStream, "function", "Request.prototype.textStream is a function");
+}, "textStream method existence");
+
+promise_test(async () => {
+  const response = new Response("hello world");
+  assert_false(response.bodyUsed, "bodyUsed is false initially");
+  const stream = response.textStream();
+  assert_true(stream instanceof ReadableStream, "textStream() returns a ReadableStream");
+  assert_true(response.bodyUsed, "bodyUsed becomes true immediately after calling textStream()");
+
+  const chunks = await readAllChunks(stream);
+  assert_greater_than(chunks.length, 0);
+  for (const chunk of chunks) {
+    assert_equals(typeof chunk, "string", "each chunk should be a string");
+  }
+  assert_equals(chunks.join(""), "hello world", "concatenated chunks match the body content");
+}, "Response.textStream() basic functionality");
+
+promise_test(async () => {
+  const request = new Request("https://example.com", {
+    method: "POST",
+    body: "hello world"
+  });
+  assert_false(request.bodyUsed, "bodyUsed is false initially");
+  const stream = request.textStream();
+  assert_true(stream instanceof ReadableStream, "textStream() returns a ReadableStream");
+  assert_true(request.bodyUsed, "bodyUsed becomes true immediately after calling textStream()");
+
+  const chunks = await readAllChunks(stream);
+  assert_greater_than(chunks.length, 0);
+  for (const chunk of chunks) {
+    assert_equals(typeof chunk, "string", "each chunk should be a string");
+  }
+  assert_equals(chunks.join(""), "hello world", "concatenated chunks match the body content");
+}, "Request.textStream() basic functionality");
+
+promise_test(async () => {
+  const stream = new ReadableStream({
+    start(controller) {
+      controller.enqueue(new TextEncoder().encode("hello "));
+      controller.enqueue(new TextEncoder().encode("world"));
+      controller.close();
+    }
+  });
+  const response = new Response(stream);
+  const textStream = response.textStream();
+  const chunks = await readAllChunks(textStream);
+  assert_greater_than(chunks.length, 0);
+  for (const chunk of chunks) {
+    assert_equals(typeof chunk, "string");
+  }
+  assert_equals(chunks.join(""), "hello world");
+}, "textStream() handles chunked byte stream input");
+
+test(() => {
+  const response = new Response("hello");
+  response.text(); // consumes body
+  assert_throws_js(TypeError, () => response.textStream());
+}, "Response.textStream() on consumed body throws TypeError");
+
+test(() => {
+  const request = new Request("https://example.com", {
+    method: "POST",
+    body: "hello"
+  });
+  request.text(); // consumes body
+  assert_throws_js(TypeError, () => request.textStream());
+}, "Request.textStream() on consumed body throws TypeError");
+
+test(() => {
+  const response = new Response("hello");
+  const reader = response.body.getReader(); // locks body
+  assert_throws_js(TypeError, () => response.textStream());
+}, "Response.textStream() on locked body throws TypeError");
+
+test(() => {
+  const request = new Request("https://example.com", {
+    method: "POST",
+    body: "hello"
+  });
+  const reader = request.body.getReader(); // locks body
+  assert_throws_js(TypeError, () => request.textStream());
+}, "Request.textStream() on locked body throws TypeError");
+
+promise_test(async () => {
+  const response = new Response();
+  assert_equals(response.body, null);
+  const stream = response.textStream();
+  assert_true(stream instanceof ReadableStream);
+  const chunks = await readAllChunks(stream);
+  assert_equals(chunks.length, 0, "no chunks should be read from null body textStream");
+}, "Response.textStream() with null body");
+
+promise_test(async () => {
+  const request = new Request("https://example.com");
+  assert_equals(request.body, null);
+  const stream = request.textStream();
+  assert_true(stream instanceof ReadableStream);
+  const chunks = await readAllChunks(stream);
+  assert_equals(chunks.length, 0, "no chunks should be read from null body textStream");
+}, "Request.textStream() with null body");
+
+promise_test(async () => {
+  const response = new Response("");
+  assert_false(response.bodyUsed);
+  const stream = response.textStream();
+  assert_true(stream instanceof ReadableStream);
+  assert_true(response.bodyUsed);
+  const chunks = await readAllChunks(stream);
+  assert_equals(chunks.length, 0, "no chunks should be read from empty stream");
+}, "Response.textStream() with empty body");
+
+promise_test(async () => {
+  const buffer = new Uint8Array([0x68, 0x00, 0x65, 0x00, 0x6c, 0x00, 0x6c, 0x00, 0x6f, 0x00]); // "hello" in UTF-16LE
+  const response = new Response(buffer, {
+    headers: { "Content-Type": "text/plain; charset=utf-16le" }
+  });
+  const stream = response.textStream();
+  const chunks = await readAllChunks(stream);
+  assert_equals(chunks.join(""), "h\0e\0l\0l\0o\0", "ignores charset=utf-16le and decodes as UTF-8");
+}, "Response.textStream() ignores Content-Type charset (UTF-16LE)");
+
+promise_test(async () => {
+  const buffer = new Uint8Array([0x68, 0x00, 0x65, 0x00, 0x6c, 0x00, 0x6c, 0x00, 0x6f, 0x00]); // "hello" in UTF-16LE
+  const request = new Request("https://example.com", {
+    method: "POST",
+    body: buffer,
+    headers: { "Content-Type": "text/plain; charset=utf-16le" }
+  });
+  const stream = request.textStream();
+  const chunks = await readAllChunks(stream);
+  assert_equals(chunks.join(""), "h\0e\0l\0l\0o\0", "ignores charset=utf-16le and decodes as UTF-8");
+}, "Request.textStream() ignores Content-Type charset (UTF-16LE)");
+
+promise_test(async () => {
+  const response = new Response(new TextEncoder().encode("hello"), {
+    headers: { "Content-Type": "text/plain; charset=invalid-charset" }
+  });
+  const stream = response.textStream();
+  const chunks = await readAllChunks(stream);
+  assert_equals(chunks.join(""), "hello", "ignores invalid-charset and decodes as UTF-8");
+}, "Response.textStream() ignores invalid Content-Type charset (invalid-charset)");