diff --git a/.gitignore b/.gitignore
index 7016f18e53..5e03a51bf3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -45,3 +45,6 @@ Untitled*.ipynb
 /.ms-playwright
 
 **/.claude/settings.local.json
+
+# pyenv
+/.python-version
diff --git a/ext/web/06_streams.js b/ext/web/06_streams.js
index 83a6ac13d0..7dd0c1fb8a 100644
--- a/ext/web/06_streams.js
+++ b/ext/web/06_streams.js
@@ -60,6 +60,7 @@ const {
   PromisePrototypeThen,
   PromiseReject,
   PromiseResolve,
+  PromiseWithResolvers,
   RangeError,
   ReflectHas,
   SafeFinalizationRegistry,
@@ -5157,6 +5158,8 @@ class ReadableStream {
   /** @type {Deferred<void>} */
   [_isClosedPromise];
 
+  [core.hostObjectBrand] = "ReadableStream";
+
   /**
    * @param {UnderlyingSource<R>=} underlyingSource
    * @param {QueuingStrategy<R>=} strategy
@@ -6164,6 +6167,8 @@ class TransformStream {
   /** @type {WritableStream<I>} */
   [_writable];
 
+  [core.hostObjectBrand] = "TransformStream";
+
   /**
    * @param {Transformer<I, O>} transformer
    * @param {QueuingStrategy<I>} writableStrategy
@@ -6174,6 +6179,10 @@ class TransformStream {
     writableStrategy = { __proto__: null },
     readableStrategy = { __proto__: null },
   ) {
+    if (transformer === _brand) {
+      this[_brand] = _brand;
+      return;
+    }
     const prefix = "Failed to construct 'TransformStream'";
     if (transformer !== undefined) {
       transformer = webidl.converters.object(transformer, prefix, "Argument 1");
@@ -6374,6 +6383,8 @@ class WritableStream {
   /** @type {Deferred<void>[]} */
   [_writeRequests];
 
+  [core.hostObjectBrand] = "WritableStream";
+
   /**
    * @param {UnderlyingSink<W>=} underlyingSink
    * @param {QueuingStrategy<W>=} strategy
@@ -6740,6 +6751,283 @@ function createProxy(stream) {
   return stream.pipeThrough(new TransformStream());
 }
 
+function packAndPostMessage(port, type, value) {
+  port.postMessage({ type, value, __proto__: null });
+}
+
+function crossRealmTransformSendError(port, error) {
+  packAndPostMessage(port, "error", error);
+}
+
+function packAndPostMessageHandlingError(port, type, value) {
+  try {
+    packAndPostMessage(port, type, value);
+  } catch (e) {
+    crossRealmTransformSendError(port, e);
+    throw e;
+  }
+}
+
+/**
+ * @param stream {ReadableStream<any>}
+ * @param port {MessagePort}
+ */
+function setUpCrossRealmTransformReadable(stream, port) {
+  initializeReadableStream(stream);
+  const controller = new ReadableStreamDefaultController(_brand);
+  port.addEventListener("message", (event) => {
+    if (event.data.type === "chunk") {
+      readableStreamDefaultControllerEnqueue(controller, event.data.value);
+    } else if (event.data.type === "close") {
+      readableStreamDefaultControllerClose(controller);
+      port.close();
+    } else if (event.data.type === "error") {
+      readableStreamDefaultControllerError(controller, event.data.value);
+      port.close();
+    }
+  });
+  port.addEventListener("messageerror", (event) => {
+    crossRealmTransformSendError(port, event.error);
+    readableStreamDefaultControllerError(controller, event.error);
+    port.close();
+  });
+  port.start();
+  const startAlgorithm = () => undefined;
+  const pullAlgorithm = () => {
+    packAndPostMessage(port, "pull", undefined);
+    return PromiseResolve(undefined);
+  };
+  const cancelAlgorithm = (reason) => {
+    try {
+      packAndPostMessageHandlingError(port, "error", reason);
+    } catch (e) {
+      return PromiseReject(e);
+    } finally {
+      port.close();
+    }
+    return PromiseResolve(undefined);
+  };
+  const sizeAlgorithm = () => 1;
+  setUpReadableStreamDefaultController(
+    stream,
+    controller,
+    startAlgorithm,
+    pullAlgorithm,
+    cancelAlgorithm,
+    0,
+    sizeAlgorithm,
+  );
+}
+
+/**
+ * @param stream {WritableStream<any>}
+ * @param port {MessagePort}
+ */
+function setUpCrossRealmTransformWritable(stream, port) {
+  initializeWritableStream(stream);
+  const controller = new WritableStreamDefaultController(_brand);
+  let backpressurePromise = PromiseWithResolvers();
+  port.addEventListener("message", (event) => {
+    if (event.data.type === "pull") {
+      if (backpressurePromise) {
+        backpressurePromise.resolve();
+        backpressurePromise = undefined;
+      }
+    } else if (event.data.type === "error") {
+      writableStreamDefaultControllerErrorIfNeeded(
+        controller,
+        event.data.value,
+      );
+      if (backpressurePromise) {
+        backpressurePromise.resolve();
+        backpressurePromise = undefined;
+      }
+    }
+  });
+  port.addEventListener("messageerror", (event) => {
+    crossRealmTransformSendError(port, event.error);
+    writableStreamDefaultControllerErrorIfNeeded(controller, event.error);
+    port.close();
+  });
+  port.start();
+  const startAlgorithm = () => undefined;
+  const writeAlgorithm = (chunk) => {
+    if (!backpressurePromise) {
+      backpressurePromise = PromiseWithResolvers();
+      backpressurePromise.resolve();
+    }
+    return PromisePrototypeThen(backpressurePromise.promise, () => {
+      backpressurePromise = PromiseWithResolvers();
+      try {
+        packAndPostMessageHandlingError(port, "chunk", chunk);
+      } catch (e) {
+        port.close();
+        throw e;
+      }
+    });
+  };
+  const closeAlgorithm = () => {
+    packAndPostMessage(port, "close", undefined);
+    port.close();
+    return PromiseResolve(undefined);
+  };
+  const abortAlgorithm = (reason) => {
+    try {
+      packAndPostMessageHandlingError(port, "error", reason);
+      return PromiseResolve(undefined);
+    } catch (error) {
+      return PromiseReject(error);
+    } finally {
+      port.close();
+    }
+  };
+  const sizeAlgorithm = () => 1;
+  setUpWritableStreamDefaultController(
+    stream,
+    controller,
+    startAlgorithm,
+    writeAlgorithm,
+    closeAlgorithm,
+    abortAlgorithm,
+    1,
+    sizeAlgorithm,
+  );
+}
+
+/**
+ * @param value {ReadableStream<any>}
+ * @param port {MessagePort}
+ */
+function readableStreamTransferSteps(value, port) {
+  if (isReadableStreamLocked(value)) {
+    throw new DOMException(
+      "Cannot transfer a locked ReadableStream",
+      "DataCloneError",
+    );
+  }
+  const writable = new WritableStream(_brand);
+  setUpCrossRealmTransformWritable(writable, port);
+  const promise = readableStreamPipeTo(value, writable, false, false, false);
+  setPromiseIsHandledToTrue(promise);
+}
+
+/**
+ * @param port {MessagePort}
+ * @returns {ReadableStream<any>}
+ */
+function readableStreamTransferReceivingSteps(port) {
+  const stream = new ReadableStream(_brand);
+  setUpCrossRealmTransformReadable(stream, port);
+  return stream;
+}
+
+/**
+ * @param value {WritableStream<any>}
+ * @param port {MessagePort}
+ */
+function writableStreamTransferSteps(value, port) {
+  if (isWritableStreamLocked(value)) {
+    throw new DOMException(
+      "Cannot transfer a locked WritableStream",
+      "DataCloneError",
+    );
+  }
+  const readable = new ReadableStream(_brand);
+  setUpCrossRealmTransformReadable(readable, port);
+  const promise = readableStreamPipeTo(readable, value, false, false, false);
+  setPromiseIsHandledToTrue(promise);
+}
+
+/**
+ * @param port {MessagePort}
+ * @returns {WritableStream<any>}
+ */
+function writableStreamTransferReceivingSteps(port) {
+  const stream = new WritableStream(_brand);
+  setUpCrossRealmTransformWritable(stream, port);
+  return stream;
+}
+
+/**
+ * @param value {TransformStream<any>}
+ * @param portR {MessagePort}
+ * @param portW {MessagePort}
+ */
+function transformStreamTransferSteps(value, portR, portW) {
+  if (isReadableStreamLocked(value.readable)) {
+    throw new DOMException(
+      "Cannot transfer a locked ReadableStream",
+      "DataCloneError",
+    );
+  }
+  if (isWritableStreamLocked(value.writable)) {
+    throw new DOMException(
+      "Cannot transfer a locked WritableStream",
+      "DataCloneError",
+    );
+  }
+  readableStreamTransferSteps(value.readable, portR);
+  writableStreamTransferSteps(value.writable, portW);
+}
+
+/**
+ * @param portR {MessagePort}
+ * @param portW {MessagePort}
+ * @returns {TransformStream<any>}
+ */
+function transformStreamTransferReceivingSteps(portR, portW) {
+  const stream = new TransformStream(_brand);
+  stream[_readable] = new ReadableStream(_brand);
+  setUpCrossRealmTransformReadable(stream[_readable], portR);
+  stream[_writable] = new WritableStream(_brand);
+  setUpCrossRealmTransformWritable(stream[_writable], portW);
+  return stream;
+}
+
+core.registerTransferableResource(
+  "ReadableStream",
+  (value) => {
+    const { port1, port2 } = new MessageChannel();
+    readableStreamTransferSteps(value, port1);
+    return core.getTransferableResource("MessagePort").send(port2);
+  },
+  (rid) => {
+    const port = core.getTransferableResource("MessagePort").receive(rid);
+    return readableStreamTransferReceivingSteps(port);
+  },
+);
+
+core.registerTransferableResource(
+  "WritableStream",
+  (value) => {
+    const { port1, port2 } = new MessageChannel();
+    writableStreamTransferSteps(value, port1);
+    return core.getTransferableResource("MessagePort").send(port2);
+  },
+  (rid) => {
+    const port = core.getTransferableResource("MessagePort").receive(rid);
+    return writableStreamTransferReceivingSteps(port);
+  },
+);
+
+core.registerTransferableResource(
+  "TransformStream",
+  (value) => {
+    const { port1: portR1, port2: portR2 } = new MessageChannel();
+    const { port1: portW1, port2: portW2 } = new MessageChannel();
+    transformStreamTransferSteps(value, portR1, portW1);
+    return [
+      core.getTransferableResource("MessagePort").send(portR2),
+      core.getTransferableResource("MessagePort").send(portW2),
+    ];
+  },
+  (rids) => {
+    const portR = core.getTransferableResource("MessagePort").receive(rids[0]);
+    const portW = core.getTransferableResource("MessagePort").receive(rids[1]);
+    return transformStreamTransferReceivingSteps(portR, portW);
+  },
+);
+
 webidl.converters.ReadableStream = webidl
   .createInterfaceConverter("ReadableStream", ReadableStream.prototype);
 webidl.converters.WritableStream = webidl
diff --git a/ext/web/13_message_port.js b/ext/web/13_message_port.js
index 18b64a52fe..ce0dc74318 100644
--- a/ext/web/13_message_port.js
+++ b/ext/web/13_message_port.js
@@ -385,6 +385,13 @@ function deserializeJsMessageData(messageData) {
           ArrayPrototypePush(hostObjects, hostObj);
           break;
         }
+        case "multiResource": {
+          const { 0: type, 1: rids } = transferable.data;
+          const hostObj = core.getTransferableResource(type).receive(rids);
+          ArrayPrototypePush(transferables, hostObj);
+          ArrayPrototypePush(hostObjects, hostObj);
+          break;
+        }
         case "arrayBuffer": {
           ArrayPrototypePush(transferredArrayBuffers, transferable.data);
           const index = ArrayPrototypePush(transferables, null);
@@ -460,10 +467,17 @@ function serializeJsMessageData(data, transferables) {
     if (transferable[core.hostObjectBrand]) {
       const type = transferable[core.hostObjectBrand];
       const rid = core.getTransferableResource(type).send(transferable);
-      ArrayPrototypePush(serializedTransferables, {
-        kind: "resource",
-        data: [type, rid],
-      });
+      if (typeof rid === "number") {
+        ArrayPrototypePush(serializedTransferables, {
+          kind: "resource",
+          data: [type, rid],
+        });
+      } else {
+        ArrayPrototypePush(serializedTransferables, {
+          kind: "multiResource",
+          data: [type, rid],
+        });
+      }
     } else if (isArrayBuffer(transferable)) {
       ArrayPrototypePush(serializedTransferables, {
         kind: "arrayBuffer",
diff --git a/ext/web/message_port.rs b/ext/web/message_port.rs
index 5c0ad22e23..3b20a15a48 100644
--- a/ext/web/message_port.rs
+++ b/ext/web/message_port.rs
@@ -46,6 +46,7 @@ pub enum MessagePortError {
 
 pub enum Transferable {
   Resource(String, Box<dyn TransferredResource>),
+  MultiResource(String, Vec<Box<dyn TransferredResource>>),
   ArrayBuffer(u32),
 }
 
@@ -180,6 +181,7 @@ pub fn op_message_port_create_entangled(
 pub enum JsTransferable {
   ArrayBuffer(u32),
   Resource(String, ResourceId),
+  MultiResource(String, Vec<ResourceId>),
 }
 
 pub fn deserialize_js_transferables(
@@ -197,6 +199,18 @@ pub fn deserialize_js_transferables(
         let tx = resource.transfer().map_err(MessagePortError::Generic)?;
         transferables.push(Transferable::Resource(name, tx));
       }
+      JsTransferable::MultiResource(name, rids) => {
+        let mut txs = Vec::with_capacity(rids.len());
+        for rid in rids {
+          let resource = state
+            .resource_table
+            .take_any(rid)
+            .map_err(|_| MessagePortError::InvalidTransfer)?;
+          let tx = resource.transfer().map_err(MessagePortError::Generic)?;
+          txs.push(tx);
+        }
+        transferables.push(Transferable::MultiResource(name, txs));
+      }
       JsTransferable::ArrayBuffer(id) => {
         transferables.push(Transferable::ArrayBuffer(id));
       }
@@ -217,6 +231,13 @@ pub fn serialize_transferables(
         let rid = state.resource_table.add_rc_dyn(rx);
         js_transferables.push(JsTransferable::Resource(name, rid));
       }
+      Transferable::MultiResource(name, txs) => {
+        let rids = txs
+          .into_iter()
+          .map(|tx| state.resource_table.add_rc_dyn(tx.receive()))
+          .collect();
+        js_transferables.push(JsTransferable::MultiResource(name, rids));
+      }
       Transferable::ArrayBuffer(id) => {
         js_transferables.push(JsTransferable::ArrayBuffer(id));
       }
diff --git a/tests/wpt/runner/expectation.json b/tests/wpt/runner/expectation.json
index 932c788dd7..91a24d7d67 100644
--- a/tests/wpt/runner/expectation.json
+++ b/tests/wpt/runner/expectation.json
@@ -5371,17 +5371,27 @@
     "queuing-strategies-size-function-per-global.window.html": false,
     "transferable": {
       "deserialize-error.window.html": false,
-      "transfer-with-messageport.window.html": false,
-      "readable-stream.html": false,
-      "reason.html": false,
+      "transfer-with-messageport.window.html": true,
+      "readable-stream.html": [
+        "cancel should be propagated to the original",
+        "cancel should abort a pending read()",
+        "transferring a non-serializable chunk should error both sides"
+      ],
+      "reason.html": [
+        "DOMException errors should be preserved"
+      ],
       "service-worker.https.html": false,
       "shared-worker.html": false,
       "transform-stream-members.any.html": true,
       "transform-stream-members.any.worker.html": true,
-      "transform-stream.html": false,
-      "window.html": false,
-      "worker.html": false,
-      "writable-stream.html": false
+      "transform-stream.html": true,
+      "window.html": [
+        "transfer to and from an iframe should work"
+      ],
+      "worker.html": true,
+      "writable-stream.html": [
+        "writing a unclonable object should error the stream"
+      ]
     }
   },
   "user-timing": {
@@ -13093,7 +13103,6 @@
           "An object whose interface is deleted from the global must still deserialize",
           "A subclass instance will deserialize as its closest serializable superclass",
           "Growable SharedArrayBuffer",
-          "A subclass instance will be received as its closest transferable superclass",
           "Transferring OOB TypedArray throws"
         ],
         "structured-clone.any.worker.html": [
@@ -13121,7 +13130,6 @@
           "An object whose interface is deleted from the global must still deserialize",
           "A subclass instance will deserialize as its closest serializable superclass",
           "Growable SharedArrayBuffer",
-          "A subclass instance will be received as its closest transferable superclass",
           "Transferring OOB TypedArray throws"
         ],
         "structured-clone-cross-realm-method.html": false
diff --git a/tests/wpt/runner/runner.ts b/tests/wpt/runner/runner.ts
index b7e56f7205..dc84ffca2e 100644
--- a/tests/wpt/runner/runner.ts
+++ b/tests/wpt/runner/runner.ts
@@ -189,6 +189,35 @@ export async function runSingleTest(
   }
 }
 
+function getShim(test: string): string {
+  const shim = [];
+
+  shim.push("globalThis.window = globalThis;");
+
+  if (test.includes("streams/transferable")) {
+    shim.push(`
+      {
+        const { port1, port2 } = new MessageChannel();
+        port2.addEventListener('message', (e) => {
+          queueMicrotask(() => {
+            globalThis.dispatchEvent(e);
+          });
+        });
+        port2.start();
+        globalThis.postMessage = (message, targetOriginOrOptions, transfer) => {
+          let options = targetOriginOrOptions;
+          if (transfer || typeof targetOriginOrOptions === 'string') {
+            options = { transfer };
+          }
+          return port1.postMessage(message, options);
+        };
+      }
+    `);
+  }
+
+  return shim.join("\n");
+}
+
 async function generateBundle(location: URL): Promise<string> {
   const res = await fetch(location);
   const body = await res.text();
@@ -206,6 +235,7 @@ async function generateBundle(location: URL): Promise<string> {
       `globalThis.META_TITLE=${JSON.stringify(title)}`,
     ]);
   }
+  const shim = getShim(location.pathname);
   for (const script of scripts) {
     const src = script.getAttribute("src");
     if (src === "/resources/testharnessreport.js") {
@@ -213,20 +243,20 @@ async function generateBundle(location: URL): Promise<string> {
         join(ROOT_PATH, "./tests/wpt/runner/testharnessreport.js"),
       );
       const contents = await Deno.readTextFile(url);
-      scriptContents.push([url.href, "globalThis.window = globalThis;"]);
+      scriptContents.push([url.href, shim]);
       scriptContents.push([url.href, contents]);
     } else if (src) {
       const url = new URL(src, location);
       const res = await fetch(url);
       if (res.ok) {
         const contents = await res.text();
-        scriptContents.push([url.href, "globalThis.window = globalThis;"]);
+        scriptContents.push([url.href, shim]);
         scriptContents.push([url.href, contents]);
       }
     } else {
       const url = new URL(`#${inlineScriptCount}`, location);
       inlineScriptCount++;
-      scriptContents.push([url.href, "globalThis.window = globalThis;"]);
+      scriptContents.push([url.href, shim]);
       scriptContents.push([url.href, script.textContent]);
     }
   }