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@@ -2882,6 +2882,13 @@ An attempt was made to call [`stream.pipe()`][] on a [`Writable`][] stream.
 A stream method was called that cannot complete because the stream was
 destroyed using `stream.destroy()`.
 
+<a id="ERR_STREAM_ITER_MISSING_FLAG"></a>
+
+### `ERR_STREAM_ITER_MISSING_FLAG`
+
+A stream/iter API was used without the `--experimental-stream-iter` CLI flag
+enabled.
+
 <a id="ERR_STREAM_NULL_VALUES"></a>
 
 ### `ERR_STREAM_NULL_VALUES`
 
@@ -1998,6 +1998,61 @@ option. In the code example above, data will be in a single chunk if the file
 has less then 64 KiB of data because no `highWaterMark` option is provided to
 [`fs.createReadStream()`][].
 
+##### `readable[Symbol.for('Stream.toAsyncStreamable')]()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* Returns: {AsyncIterable} An `AsyncIterable<Uint8Array[]>` that yields
+  batched chunks from the stream.
+
+When the `--experimental-stream-iter` flag is enabled, `Readable` streams
+implement the [`Stream.toAsyncStreamable`][] protocol, enabling efficient
+consumption by the [`stream/iter`][] API.
+
+This provides a batched async iterator that drains the stream's internal
+buffer into `Uint8Array[]` batches, amortizing the per-chunk Promise overhead
+of the standard `Symbol.asyncIterator` path. For byte-mode streams, chunks
+are yielded directly as `Buffer` instances (which are `Uint8Array` subclasses).
+For object-mode or encoded streams, each chunk is normalized to `Uint8Array`
+before batching.
+
+The returned iterator is tagged as a validated source, so [`from()`][stream-iter-from]
+passes it through without additional normalization.
+
+```mjs
+import { Readable } from 'node:stream';
+import { text, from } from 'node:stream/iter';
+
+const readable = new Readable({
+  read() { this.push('hello'); this.push(null); },
+});
+
+// Readable is automatically consumed via toAsyncStreamable
+console.log(await text(from(readable))); // 'hello'
+```
+
+```cjs
+const { Readable } = require('node:stream');
+const { text, from } = require('node:stream/iter');
+
+async function run() {
+  const readable = new Readable({
+    read() { this.push('hello'); this.push(null); },
+  });
+
+  console.log(await text(from(readable))); // 'hello'
+}
+
+run().catch(console.error);
+```
+
+Without the `--experimental-stream-iter` flag, calling this method throws
+[`ERR_STREAM_ITER_MISSING_FLAG`][].
+
 ##### `readable[Symbol.asyncDispose]()`
 
 <!-- YAML
@@ -4997,8 +5052,10 @@ contain multi-byte characters.
 [`'finish'`]: #event-finish
 [`'readable'`]: #event-readable
 [`Duplex`]: #class-streamduplex
+[`ERR_STREAM_ITER_MISSING_FLAG`]: errors.md#err_stream_iter_missing_flag
 [`EventEmitter`]: events.md#class-eventemitter
 [`Readable`]: #class-streamreadable
+[`Stream.toAsyncStreamable`]: stream_iter.md#streamtoasyncstreamable
 [`Symbol.hasInstance`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/hasInstance
 [`Transform`]: #class-streamtransform
 [`Writable`]: #class-streamwritable
@@ -5024,6 +5081,7 @@ contain multi-byte characters.
 [`stream.uncork()`]: #writableuncork
 [`stream.unpipe()`]: #readableunpipedestination
 [`stream.wrap()`]: #readablewrapstream
+[`stream/iter`]: stream_iter.md
 [`writable._final()`]: #writable_finalcallback
 [`writable._write()`]: #writable_writechunk-encoding-callback
 [`writable._writev()`]: #writable_writevchunks-callback
@@ -5052,6 +5110,7 @@ contain multi-byte characters.
 [stream-end]: #writableendchunk-encoding-callback
 [stream-finished]: #streamfinishedstream-options-callback
 [stream-finished-promise]: #streamfinishedstream-options
+[stream-iter-from]: stream_iter.md#frominput
 [stream-pause]: #readablepause
 [stream-pipeline]: #streampipelinesource-transforms-destination-callback
 [stream-pipeline-promise]: #streampipelinesource-transforms-destination-options
 
@@ -1424,6 +1424,258 @@ Compression and decompression transforms for use with `pull()`, `pullSync()`,
 `pipeTo()`, and `pipeToSync()` are available via the [`node:zlib/iter`][]
 module. See the [`node:zlib/iter` documentation][] for details.
 
+## Classic stream interop
+
+These utility functions bridge between classic
+[`stream.Readable`][]/[`stream.Writable`][] streams and the `stream/iter`
+API.
+
+Both `fromReadable()` and `fromWritable()` accept duck-typed objects -- they
+do not require the input to extend `stream.Readable` or `stream.Writable`
+directly. The minimum contract is described below for each function.
+
+### `fromReadable(readable)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `readable` {stream.Readable|Object} A classic Readable stream or any object
+  with `read()` and `on()` methods.
+* Returns: {AsyncIterable\<Uint8Array\[]>} A stream/iter async iterable source.
+
+Converts a classic Readable stream (or duck-typed equivalent) into a
+stream/iter async iterable source that can be passed to [`from()`][],
+[`pull()`][], [`text()`][], etc.
+
+If the object implements the [`toAsyncStreamable`][] protocol (as
+`stream.Readable` does), that protocol is used. Otherwise, the function
+duck-types on `read()` and `on()` (EventEmitter) and wraps the stream with
+a batched async iterator.
+
+The result is cached per instance -- calling `fromReadable()` twice with the
+same stream returns the same iterable.
+
+For object-mode or encoded Readable streams, chunks are automatically
+normalized to `Uint8Array`.
+
+```mjs
+import { Readable } from 'node:stream';
+import { fromReadable, text } from 'node:stream/iter';
+
+const readable = new Readable({
+  read() { this.push('hello world'); this.push(null); },
+});
+
+const result = await text(fromReadable(readable));
+console.log(result); // 'hello world'
+```
+
+```cjs
+const { Readable } = require('node:stream');
+const { fromReadable, text } = require('node:stream/iter');
+
+const readable = new Readable({
+  read() { this.push('hello world'); this.push(null); },
+});
+
+async function run() {
+  const result = await text(fromReadable(readable));
+  console.log(result); // 'hello world'
+}
+run();
+```
+
+### `fromWritable(writable[, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `writable` {stream.Writable|Object} A classic Writable stream or any object
+  with `write()` and `on()` methods.
+* `options` {Object}
+  * `backpressure` {string} Backpressure policy. **Default:** `'strict'`.
+    * `'strict'` -- writes are rejected when the buffer is full. Catches
+      callers that ignore backpressure.
+    * `'block'` -- writes wait for drain when the buffer is full. Recommended
+      for use with [`pipeTo()`][].
+    * `'drop-newest'` -- writes are silently discarded when the buffer is full.
+    * `'drop-oldest'` -- **not supported**. Throws `ERR_INVALID_ARG_VALUE`.
+* Returns: {Object} A stream/iter Writer adapter.
+
+Creates a stream/iter Writer adapter from a classic Writable stream (or
+duck-typed equivalent). The adapter can be passed to [`pipeTo()`][] as a
+destination.
+
+Since all writes on a classic Writable are fundamentally asynchronous,
+the synchronous Writer methods (`writeSync`, `writevSync`, `endSync`) always
+return `false` or `-1`, deferring to the async path. The per-write
+`options.signal` parameter from the Writer interface is also ignored.
+
+The result is cached per instance -- calling `fromWritable()` twice with the
+same stream returns the same Writer.
+
+For duck-typed streams that do not expose `writableHighWaterMark`,
+`writableLength`, or similar properties, sensible defaults are used.
+Object-mode writables (if detectable) are rejected since the Writer
+interface is bytes-only.
+
+```mjs
+import { Writable } from 'node:stream';
+import { from, fromWritable, pipeTo } from 'node:stream/iter';
+
+const writable = new Writable({
+  write(chunk, encoding, cb) { console.log(chunk.toString()); cb(); },
+});
+
+await pipeTo(from('hello world'),
+             fromWritable(writable, { backpressure: 'block' }));
+```
+
+```cjs
+const { Writable } = require('node:stream');
+const { from, fromWritable, pipeTo } = require('node:stream/iter');
+
+async function run() {
+  const writable = new Writable({
+    write(chunk, encoding, cb) { console.log(chunk.toString()); cb(); },
+  });
+
+  await pipeTo(from('hello world'),
+               fromWritable(writable, { backpressure: 'block' }));
+}
+run();
+```
+
+### `toReadable(source[, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `source` {AsyncIterable} An `AsyncIterable<Uint8Array[]>` source, such as
+  the return value of [`pull()`][] or [`from()`][].
+* `options` {Object}
+  * `highWaterMark` {number} The internal buffer size in bytes before
+    backpressure is applied. **Default:** `65536` (64 KB).
+  * `signal` {AbortSignal} An optional signal to abort the readable.
+* Returns: {stream.Readable}
+
+Creates a byte-mode [`stream.Readable`][] from an `AsyncIterable<Uint8Array[]>`
+(the native batch format used by the stream/iter API). Each `Uint8Array` in a
+yielded batch is pushed as a separate chunk into the Readable.
+
+```mjs
+import { createWriteStream } from 'node:fs';
+import { from, pull, toReadable } from 'node:stream/iter';
+import { compressGzip } from 'node:zlib/iter';
+
+const source = pull(from('hello world'), compressGzip());
+const readable = toReadable(source);
+
+readable.pipe(createWriteStream('output.gz'));
+```
+
+```cjs
+const { createWriteStream } = require('node:fs');
+const { from, pull, toReadable } = require('node:stream/iter');
+const { compressGzip } = require('node:zlib/iter');
+
+const source = pull(from('hello world'), compressGzip());
+const readable = toReadable(source);
+
+readable.pipe(createWriteStream('output.gz'));
+```
+
+### `toReadableSync(source[, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `source` {Iterable} An `Iterable<Uint8Array[]>` source, such as the
+  return value of [`pullSync()`][] or [`fromSync()`][].
+* `options` {Object}
+  * `highWaterMark` {number} The internal buffer size in bytes before
+    backpressure is applied. **Default:** `65536` (64 KB).
+* Returns: {stream.Readable}
+
+Creates a byte-mode [`stream.Readable`][] from a synchronous
+`Iterable<Uint8Array[]>`. The `_read()` method pulls from the iterator
+synchronously, so data is available immediately via `readable.read()`.
+
+```mjs
+import { fromSync, toReadableSync } from 'node:stream/iter';
+
+const source = fromSync('hello world');
+const readable = toReadableSync(source);
+
+console.log(readable.read().toString()); // 'hello world'
+```
+
+```cjs
+const { fromSync, toReadableSync } = require('node:stream/iter');
+
+const source = fromSync('hello world');
+const readable = toReadableSync(source);
+
+console.log(readable.read().toString()); // 'hello world'
+```
+
+### `toWritable(writer)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `writer` {Object} A stream/iter Writer. Only the `write()` method is
+  required; `end()`, `fail()`, `writeSync()`, `writevSync()`, `endSync()`,
+  and `writev()` are optional.
+* Returns: {stream.Writable}
+
+Creates a classic [`stream.Writable`][] backed by a stream/iter Writer.
+
+Each `_write()` / `_writev()` call attempts the Writer's synchronous method
+first (`writeSync` / `writevSync`), falling back to the async method if the
+sync path returns `false` or throws. Similarly, `_final()` tries `endSync()`
+before `end()`. When the sync path succeeds, the callback is deferred via
+`queueMicrotask` to preserve the async resolution contract.
+
+The Writable's `highWaterMark` is set to `Number.MAX_SAFE_INTEGER` to
+effectively disable its internal buffering, allowing the underlying Writer
+to manage backpressure directly.
+
+```mjs
+import { push, toWritable } from 'node:stream/iter';
+
+const { writer, readable } = push();
+const writable = toWritable(writer);
+
+writable.write('hello');
+writable.end();
+```
+
+```cjs
+const { push, toWritable } = require('node:stream/iter');
+
+const { writer, readable } = push();
+const writable = toWritable(writer);
+
+writable.write('hello');
+writable.end();
+```
+
 ## Protocol symbols
 
 These well-known symbols allow third-party objects to participate in the
@@ -1816,10 +2068,15 @@ console.log(textSync(stream)); // 'hello world'
 [`arrayBuffer()`]: #arraybuffersource-options
 [`bytes()`]: #bytessource-options
 [`from()`]: #frominput
+[`fromSync()`]: #fromsyncinput
 [`node:zlib/iter`]: zlib_iter.md
 [`node:zlib/iter` documentation]: zlib_iter.md
 [`pipeTo()`]: #pipetosource-transforms-writer-options
 [`pull()`]: #pullsource-transforms-options
+[`pullSync()`]: #pullsyncsource-transforms-options
 [`share()`]: #sharesource-options
+[`stream.Readable`]: stream.md#class-streamreadable
+[`stream.Writable`]: stream.md#class-streamwritable
 [`tap()`]: #tapcallback
 [`text()`]: #textsource-options
+[`toAsyncStreamable`]: #streamtoasyncstreamable
@@ -1770,6 +1770,8 @@ E('ERR_STREAM_ALREADY_FINISHED',
   Error);
 E('ERR_STREAM_CANNOT_PIPE', 'Cannot pipe, not readable', Error);
 E('ERR_STREAM_DESTROYED', 'Cannot call %s after a stream was destroyed', Error);
+E('ERR_STREAM_ITER_MISSING_FLAG',
+  'The stream/iter API requires the --experimental-stream-iter flag', TypeError);
 E('ERR_STREAM_NULL_VALUES', 'May not write null values to stream', TypeError);
 E('ERR_STREAM_PREMATURE_CLOSE', 'Premature close', Error);
 E('ERR_STREAM_PUSH_AFTER_EOF', 'stream.push() after EOF', Error);