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‎doc/api/deprecations.md‎
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‎doc/api/deprecations.md‎
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‎lib/internal/webstreams/adapters.js‎
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‎lib/internal/webstreams/adapters.js‎
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‎test/parallel/test-stream-duplex.js‎
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‎test/parallel/test-stream-duplex.js‎
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@@ -4431,12 +4431,15 @@ import { opendir } from 'node:fs/promises';
 
 <!-- YAML
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/62173
+    description: Runtime deprecation.
   - version: v25.7.0
     pr-url: https://github.com/nodejs/node/pull/61632
     description: Documentation-only deprecation.
 -->
 
-Type: Documentation-only
+Type: Runtime
 
 Passing the `type` option to [`Duplex.toWeb()`][] is deprecated. To specify the
 type of the readable half of the constructed readable-writable pair, use the
 
@@ -71,6 +71,7 @@ const {
 } = require('internal/errors');
 
 const {
+  getDeprecationWarningEmitter,
   kEmptyObject,
   normalizeEncoding,
 } = require('internal/util');
@@ -650,6 +651,12 @@ function newStreamReadableFromReadableStream(readableStream, options = kEmptyObj
   return readable;
 }
 
+const emitDEP0201 = getDeprecationWarningEmitter(
+  'DEP0201',
+  "Passing 'options.type' to Duplex.toWeb() is deprecated. " +
+      "To specify the ReadableStream type, use 'options.readableType'.",
+  newReadableWritablePairFromDuplex);
+
 /**
  * @typedef {import('./readablestream').ReadableWritablePair
  * } ReadableWritablePair
@@ -677,10 +684,15 @@ function newReadableWritablePairFromDuplex(duplex, options = kEmptyObject) {
 
   const readableOptions = {
     __proto__: null,
-    // DEP0201: 'options.type' is a deprecated alias for 'options.readableType'
-    type: options.readableType ?? options.type,
+    type: options.readableType,
   };
 
+  if (options.readableType == null && options.type != null) {
+    // 'options.type' is a deprecated alias for 'options.readableType'
+    emitDEP0201();
+    readableOptions.type = options.type;
+  }
+
   if (isDestroyed(duplex)) {
     const writable = new WritableStream();
     const readable = new ReadableStream({ type: readableOptions.type });
 
@@ -157,4 +157,6 @@ process.on('exit', () => {
   // Ensure that the originally-named `options.type` still works as an alias for `options.readableType`
   // `getReader({ mode: 'byob' })` throws if the underlying ReadableStream is not a byte stream
   Duplex.toWeb(duplex, { type: 'bytes' }).readable.getReader({ mode: 'byob' });
+  common.expectWarning('DeprecationWarning',
+                       [/Passing 'options\.type' to Duplex\.toWeb\(\) is deprecated/, 'DEP0201']);
 }
Original file line number	Diff line number	Diff line change
`@@ -157,4 +157,6 @@ process.on('exit', () => {`
`157`	`157`	// Ensure that the originally-named `options.type` still works as an alias for `options.readableType`
`158`	`158`	// `getReader({ mode: 'byob' })` throws if the underlying ReadableStream is not a byte stream
`159`	`159`	`Duplex.toWeb(duplex, { type: 'bytes' }).readable.getReader({ mode: 'byob' });`
	`160`	`+ common.expectWarning('DeprecationWarning',`
	`161`	`+ [/Passing 'options\.type' to Duplex\.toWeb\(\) is deprecated/, 'DEP0201']);`
`160`	`162`	`}`