diff --git a/crates/perry-codegen/src/expr/helpers.rs b/crates/perry-codegen/src/expr/helpers.rs
index ecb8a3757..6ea7f144e 100644
--- a/crates/perry-codegen/src/expr/helpers.rs
+++ b/crates/perry-codegen/src/expr/helpers.rs
@@ -307,6 +307,23 @@ pub(crate) fn is_global_this_builtin_name(name: &str) -> bool {
             | "TextDecoderStream"
             | "CompressionStream"
             | "DecompressionStream"
+            // The three core Web Streams constructors. Perry already implements
+            // them (`new ReadableStream(…)` and `x instanceof ReadableStream` are
+            // both lowered, and `ReadableStream` has a class id), but the NAMES
+            // were never registered as globalThis builtins — so a bare
+            // `ReadableStream` identifier did not resolve: `typeof
+            // ReadableStream === "undefined"` and `"ReadableStream" in globalThis`
+            // was false, while Node exposes all three as functions.
+            //
+            // Libraries feature-detect exactly this (`typeof ReadableStream !==
+            // "undefined" ? … : …`) to decide how to consume a `fetch()` body.
+            // Getting "undefined" sent a large esbuild-bundled CLI app down the
+            // wrong branch, which then threw "The first argument must be a
+            // Readable, a ReadableStream, or an async iterable" and aborted its
+            // background tar-stream downloads entirely.
+            | "ReadableStream"
+            | "WritableStream"
+            | "TransformStream"
             | "Navigator"
             | "URL"
             | "URLSearchParams"
diff --git a/crates/perry-runtime/src/object/global_this_tables.rs b/crates/perry-runtime/src/object/global_this_tables.rs
index c65c8ee49..907d4f249 100644
--- a/crates/perry-runtime/src/object/global_this_tables.rs
+++ b/crates/perry-runtime/src/object/global_this_tables.rs
@@ -55,6 +55,17 @@ pub(crate) const GLOBAL_THIS_BUILTIN_CONSTRUCTORS: &[&str] = &[
     "TextDecoderStream",
     "CompressionStream",
     "DecompressionStream",
+    // The three core Web Streams constructors. Perry implements them (codegen
+    // lowers `new ReadableStream(…)` and `x instanceof ReadableStream`, and the
+    // class has an id), but the NAMES were never registered here or in codegen's
+    // globalThis-builtin list — so a bare `ReadableStream` identifier resolved to
+    // nothing: `typeof ReadableStream === "undefined"` and `"ReadableStream" in
+    // globalThis` was false, whereas Node exposes all three as functions.
+    // Libraries feature-detect precisely that (`typeof ReadableStream !==
+    // "undefined" ? … : …`) when deciding how to consume a `fetch()` body.
+    "ReadableStream",
+    "WritableStream",
+    "TransformStream",
     "Navigator",
     "URL",
     "URLSearchParams",
diff --git a/crates/perry-runtime/src/object/instanceof.rs b/crates/perry-runtime/src/object/instanceof.rs
index 9c4f902e2..9a3a8bce8 100644
--- a/crates/perry-runtime/src/object/instanceof.rs
+++ b/crates/perry-runtime/src/object/instanceof.rs
@@ -93,6 +93,52 @@ fn is_native_module_namespace_value(value: f64, expected: &str) -> bool {
 /// `js_instanceof`. Returns FALSE for non-class-ref type values (matches
 /// JS spec: `1 instanceof 2` throws, but Perry returns false defensively).
 /// Refs #420 / #618 followup.
+#[no_mangle]
+/// Map a builtin constructor VALUE (the `ClosureHeader`-backed function installed
+/// on `globalThis`) back to the class id that codegen passes to `js_instanceof`
+/// for the static `x instanceof <Identifier>` form.
+///
+/// Only the natively-backed builtins need this: their instances are handles
+/// (stream / fetch registries), not heap objects with a real prototype chain, so
+/// `js_instanceof` brand-checks them via the kind probes rather than a chain walk.
+/// Heap-backed builtins already resolve through the class-id / prototype paths.
+fn builtin_ctor_class_id_from_value(type_ref: f64) -> Option<u32> {
+    let closure =
+        crate::value::js_nanbox_get_pointer(type_ref) as *const crate::closure::ClosureHeader;
+    if closure.is_null() {
+        return None;
+    }
+    if unsafe { (*closure).type_tag } != crate::closure::CLOSURE_MAGIC {
+        return None;
+    }
+    let name_value = crate::closure::closure_get_dynamic_prop(closure as usize, "name");
+    let mut scratch = [0u8; crate::value::SHORT_STRING_MAX_LEN];
+    let (ptr, len) = crate::string::str_bytes_from_jsvalue(name_value, &mut scratch)?;
+    if ptr.is_null() {
+        return None;
+    }
+    let name =
+        std::str::from_utf8(unsafe { std::slice::from_raw_parts(ptr, len as usize) }).ok()?;
+    let class_id = match name {
+        "ReadableStream" => 0xFFFF_0060,
+        "WritableStream" => 0xFFFF_0061,
+        "TransformStream" => 0xFFFF_0062,
+        "Response" => 0xFFFF_0028,
+        "Request" => 0xFFFF_0029,
+        "Headers" => 0xFFFF_002A,
+        "Blob" => 0xFFFF_0026,
+        _ => return None,
+    };
+    // The name alone is forgeable (`function Response() {}` in user code).
+    // Require IDENTITY with the builtin constructor installed on
+    // `globalThis`: only the genuine builtin value brand-checks instances.
+    let global_ctor = crate::object::js_get_global_this_builtin_value(name.as_ptr(), name.len());
+    if crate::value::js_nanbox_get_pointer(global_ctor) as usize != closure as usize {
+        return None;
+    }
+    Some(class_id)
+}
+
 #[no_mangle]
 pub extern "C" fn js_instanceof_dynamic(value: f64, type_ref: f64) -> f64 {
     const TAG_FALSE: u64 = 0x7FFC_0000_0000_0003;
@@ -206,6 +252,29 @@ pub extern "C" fn js_instanceof_dynamic(value: f64, type_ref: f64) -> f64 {
             return js_instanceof(value, class_id);
         }
     }
+    // A builtin constructor held in a VARIABLE — `const RS = ReadableStream; body
+    // instanceof RS` — arrives here as the ClosureHeader-backed function installed
+    // on `globalThis`, so none of the class-id paths above match and the prototype
+    // walk below returns false. Codegen only special-cases the *static identifier*
+    // form (`body instanceof ReadableStream`), where it hands the builtin class id
+    // straight to `js_instanceof`, which brand-checks these natively-backed values
+    // via the stream / fetch kind probes (their instances are handles, not heap
+    // objects with a real prototype chain).
+    //
+    // Minified bundles almost always alias constructors into locals, so the
+    // variable form is the common one in the wild: `x instanceof <alias>` for
+    // ReadableStream / Response / Headers silently returned `false` while Node
+    // returns `true`. That made a large esbuild-bundled CLI app mis-detect its
+    // `fetch()` body, throw "The first argument must be a Readable, a
+    // ReadableStream, or an async iterable", and abort its background
+    // tar-stream downloads entirely.
+    //
+    // Recover the builtin's name from the constructor closure (recorded by
+    // `set_bound_native_closure_name` when globalThis is populated) and reuse the
+    // static path's class id, so both spellings agree.
+    if let Some(class_id) = builtin_ctor_class_id_from_value(type_ref) {
+        return js_instanceof(value, class_id);
+    }
     if let Some((module, method)) = unsafe { bound_native_callable_module_and_method(type_ref) } {
         if module == "stream"
             && matches!(