diff --git a/changelog.d/10649-stream-subclass-heritage.md b/changelog.d/10649-stream-subclass-heritage.md
new file mode 100644
index 0000000000..5e9bb57991
--- /dev/null
+++ b/changelog.d/10649-stream-subclass-heritage.md
@@ -0,0 +1,10 @@
+### Fixed
+
+- **`node:stream` subclass overrides (`_transform`/`_write`/`_read`) are no
+  longer ignored when the heritage reaching `class X extends <base>` is a
+  local alias, an indirect subclass, a class expression, or a CJS
+  destructured `require('stream')` — the shape nodemailer uses in every
+  stream class it defines. `write()`/`push()` used to throw
+  `ERR_METHOD_NOT_IMPLEMENTED` because the override was never installed on
+  `this`; the dynamic `super()` dispatch now recognizes the resolved
+  bound-export value regardless of how the heritage expression reached it.
diff --git a/crates/perry-runtime/src/object/global_this/fetch_globals.rs b/crates/perry-runtime/src/object/global_this/fetch_globals.rs
index 13cbe05c23..b8e6d4a3bd 100644
--- a/crates/perry-runtime/src/object/global_this/fetch_globals.rs
+++ b/crates/perry-runtime/src/object/global_this/fetch_globals.rs
@@ -755,6 +755,56 @@ pub unsafe extern "C" fn js_fetch_or_value_super(
             return dispatch(this_box);
         }
     }
+    // #10448: `class X extends Transform` (and Readable/Writable/Duplex)
+    // never called the subclass's `_transform`/`_write`/`_read` override
+    // unless the heritage was a shape `is_genuine_node_stream_parent`
+    // recognizes statically (`crates/perry-hir/src/lower_decl/class_decl.rs`)
+    // — a local alias (`const Alias = Transform`), a namespace member reached
+    // through a CJS destructured `require('stream')`, or an indirect
+    // subclass all fell through to the ordinary-call dispatch below,
+    // which invokes the bound `stream` export as a plain constructor and
+    // drops the result — `this` stayed an empty object, so `write()` threw
+    // `ERR_METHOD_NOT_IMPLEMENTED`. Recognize the resolved bound-export
+    // value here, exactly as the WASI arm above does, and run the same
+    // runtime shim the static `extends Transform` path already uses
+    // (`js_node_stream_*_subclass_init`, `crates/perry-codegen/src/expr/write_barrier.rs`'s
+    // `lower_node_stream_super_init`), so every heritage shape installs the
+    // override onto `this` identically.
+    //
+    // `PassThrough` is deliberately NOT handled here: HIR never recognizes
+    // it as a node:stream native parent at all, even via a bare import
+    // (`canonical_native_parent_name` lists Readable/Writable/Duplex/
+    // Transform but not PassThrough), so the hidden `_transform` field this
+    // shim reads is never pre-seeded for ANY `PassThrough` heritage shape —
+    // that's a separate, deeper HIR-level gap needing its own fix; adding an
+    // arm here alone was confirmed (empirically) to change nothing.
+    if let Some((module, method)) = bound_native_parent.as_ref() {
+        if super::super::native_module::normalize_native_module_alias(module.as_str()) == "stream" {
+            let opts = if args_len >= 1 && !args_ptr.is_null() {
+                *args_ptr
+            } else {
+                undef
+            };
+            let handled = match method.as_str() {
+                "Readable" => Some(crate::node_stream::js_node_stream_readable_subclass_init(
+                    this_box, opts,
+                )),
+                "Writable" => Some(crate::node_stream::js_node_stream_writable_subclass_init(
+                    this_box, opts,
+                )),
+                "Duplex" => Some(crate::node_stream::js_node_stream_duplex_subclass_init(
+                    this_box, opts,
+                )),
+                "Transform" => Some(crate::node_stream::js_node_stream_transform_subclass_init(
+                    this_box, opts,
+                )),
+                _ => None,
+            };
+            if handled.is_some() {
+                return undef;
+            }
+        }
+    }
     // `class X extends Temporal.<Type>` (non-spread `super(a, b)`): a Temporal
     // constructor returns a fresh NaN-boxed cell and does NOT mutate the
     // implicit `this`, so the ordinary dispatch below would drop that cell and
diff --git a/test-files/gap_10448_stream_subclass_heritage_helper.cjs b/test-files/gap_10448_stream_subclass_heritage_helper.cjs
new file mode 100644
index 0000000000..ecd23333f5
--- /dev/null
+++ b/test-files/gap_10448_stream_subclass_heritage_helper.cjs
@@ -0,0 +1,68 @@
+'use strict';
+// CommonJS half of test_gap_10448_stream_subclass_heritage.ts: the exact
+// shape nodemailer uses everywhere (`const { Transform } =
+// require('stream'); class X extends Transform`), plus the sibling
+// Writable/Readable/Duplex destructured shapes the issue lists as broken
+// the same way, and a namespace-member export for comparison.
+//
+// State is captured via public class fields, not a constructor-body
+// assignment after `super(...args)` with a rest-param spread — that shape
+// (`constructor(...args) { super(...args); ... }`) hits a separate,
+// pre-existing gap (native stream methods go missing) independent of
+// heritage shape or this issue; not exercised here to keep this test
+// isolated to #10448's own defect.
+const { Transform, Writable, Readable, Duplex } = require('stream');
+const stream = require('stream');
+
+class CjsTransform extends Transform {
+  _transform(chunk, _enc, cb) {
+    cb(null, String(chunk).toUpperCase());
+  }
+}
+
+// `require('stream').Transform` reached via a namespace member on a plain
+// `require()` result (not destructured) — control: this shape is already
+// recognized statically (`is_genuine_node_stream_parent`).
+class CjsViaMember extends stream.Transform {
+  _transform(chunk, _enc, cb) {
+    cb(null, String(chunk).toUpperCase());
+  }
+}
+
+class CjsWritable extends Writable {
+  captured = '';
+  _write(chunk, _enc, cb) {
+    this.captured += String(chunk).toUpperCase();
+    cb();
+  }
+}
+
+class CjsReadable extends Readable {
+  _done = false;
+  _read() {
+    if (this._done) return;
+    this._done = true;
+    this.push('x');
+    this.push('y');
+    this.push(null);
+  }
+}
+
+// Write-half only (no `_read`/push): proves the destructured `Duplex`
+// heritage installs `_write` the same way `Writable` does, without
+// depending on read/write event-ordering across engines.
+class CjsDuplex extends Duplex {
+  captured = '';
+  _write(chunk, _enc, cb) {
+    this.captured += String(chunk).toUpperCase();
+    cb();
+  }
+}
+
+module.exports = {
+  CjsTransform,
+  CjsViaMember,
+  CjsWritable,
+  CjsReadable,
+  CjsDuplex,
+};
diff --git a/test-files/test_gap_10448_stream_subclass_heritage.ts b/test-files/test_gap_10448_stream_subclass_heritage.ts
new file mode 100644
index 0000000000..e07d8d7fee
--- /dev/null
+++ b/test-files/test_gap_10448_stream_subclass_heritage.ts
@@ -0,0 +1,167 @@
+// #10448: node:stream subclass overrides (`_transform`/`_write`/`_read`)
+// were ignored whenever the heritage reaching `class X extends <base>` was
+// anything OTHER than a shape `is_genuine_node_stream_parent` recognizes
+// statically at HIR-lowering time
+// (`crates/perry-hir/src/lower_decl/class_decl.rs`) — a local alias
+// (`const Alias = Transform`), an indirect subclass, a class expression, or
+// a CJS destructured `require('stream')` (the shape nodemailer uses in
+// every stream class it defines). `write()`/`push()` then threw
+// `ERR_METHOD_NOT_IMPLEMENTED` because the override was never installed on
+// `this`.
+//
+// (`PassThrough` is a separate, deeper gap — HIR never recognizes it
+// statically even via a bare import, unlike Readable/Writable/Duplex/
+// Transform, so it needs its own follow-up; not covered by this test, see
+// the PR body.)
+//
+// Each check awaits its own stream before starting the next so output order
+// is deterministic regardless of engine event-loop/nextTick scheduling
+// differences — only the per-check content is the thing under test. State
+// is captured via public class fields (not a `constructor(...args) {
+// super(...args); ... }` rest-spread pattern, which hits an unrelated
+// pre-existing gap independent of heritage shape).
+import { Transform, Writable, Readable, Duplex } from "stream";
+import * as streamNs from "stream";
+import {
+  CjsTransform,
+  CjsViaMember,
+  CjsWritable,
+  CjsReadable,
+  CjsDuplex,
+} from "./gap_10448_stream_subclass_heritage_helper.cjs";
+
+const AliasTransform = Transform;
+
+class ViaImport extends Transform {
+  _transform(chunk: any, _enc: string, cb: any) {
+    cb(null, String(chunk).toUpperCase());
+  }
+}
+class ViaAlias extends AliasTransform {
+  _transform(chunk: any, _enc: string, cb: any) {
+    cb(null, String(chunk).toUpperCase());
+  }
+}
+class ViaNamespaceMember extends streamNs.Transform {
+  _transform(chunk: any, _enc: string, cb: any) {
+    cb(null, String(chunk).toUpperCase());
+  }
+}
+class Mid extends AliasTransform {}
+class ViaIndirect extends Mid {
+  _transform(chunk: any, _enc: string, cb: any) {
+    cb(null, String(chunk).toUpperCase());
+  }
+}
+const ViaClassExpr = class extends AliasTransform {
+  _transform(chunk: any, _enc: string, cb: any) {
+    cb(null, String(chunk).toUpperCase());
+  }
+};
+
+function runTransform(name: string, T: any): Promise<void> {
+  return new Promise((resolve) => {
+    const t = new T();
+    let out = "";
+    t.on("data", (c: any) => (out += c));
+    t.on("end", () => {
+      console.log(name, JSON.stringify(out));
+      resolve();
+    });
+    try {
+      t.write("ab");
+      t.end("c");
+    } catch (e: any) {
+      console.log(name, "threw", e.code);
+      resolve();
+    }
+  });
+}
+
+function runWritable(name: string, W: any): Promise<void> {
+  return new Promise((resolve) => {
+    let w: any;
+    try {
+      w = new W();
+    } catch (e: any) {
+      console.log(name, "threw (construct)", e.message);
+      resolve();
+      return;
+    }
+    w.on("finish", () => {
+      console.log(name, JSON.stringify(w.captured));
+      resolve();
+    });
+    try {
+      w.write("ab");
+      w.end("c");
+    } catch (e: any) {
+      console.log(name, "threw", e.code);
+      resolve();
+    }
+  });
+}
+
+function runReadable(name: string, R: any): Promise<void> {
+  return new Promise((resolve) => {
+    let r: any;
+    try {
+      r = new R();
+    } catch (e: any) {
+      console.log(name, "threw (construct)", e.message);
+      resolve();
+      return;
+    }
+    let out = "";
+    r.on("data", (c: any) => (out += c));
+    r.on("end", () => {
+      console.log(name, JSON.stringify(out));
+      resolve();
+    });
+  });
+}
+
+class WViaWritable extends Writable {
+  captured = "";
+  _write(chunk: any, _enc: string, cb: any) {
+    this.captured += String(chunk).toUpperCase();
+    cb();
+  }
+}
+
+class RViaReadable extends Readable {
+  private _done = false;
+  _read() {
+    if (this._done) return;
+    this._done = true;
+    this.push("m");
+    this.push("n");
+    this.push(null);
+  }
+}
+
+class DViaDuplex extends Duplex {
+  captured = "";
+  _write(chunk: any, _enc: string, cb: any) {
+    this.captured += String(chunk).toUpperCase();
+    cb();
+  }
+}
+
+async function main() {
+  await runTransform("Transform    via import          ", ViaImport);
+  await runTransform("Transform    via alias           ", ViaAlias);
+  await runTransform("Transform    via namespace member", ViaNamespaceMember);
+  await runTransform("Transform    via indirect subclas", ViaIndirect);
+  await runTransform("Transform    via class expression", ViaClassExpr);
+  await runTransform("Transform    CJS destructured    ", CjsTransform);
+  await runTransform("Transform    CJS namespace member", CjsViaMember);
+  await runWritable("Writable     CJS destructured    ", CjsWritable);
+  await runWritable("Duplex       CJS destructured    ", CjsDuplex);
+  await runReadable("Readable     CJS destructured    ", CjsReadable);
+  await runWritable("Writable     via import          ", WViaWritable);
+  await runReadable("Readable     via import          ", RViaReadable);
+  await runWritable("Duplex       via import          ", DViaDuplex);
+}
+
+main();