diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/class-id-instances-as-streams.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/class-id-instances-as-streams.ts.json
new file mode 100644
index 000000000..01d871cb3
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/class-id-instances-as-streams.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/class-id-instances-as-streams.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/runtime/src/scr_dyn_builtin_stream.c b/packages/runtime/src/scr_dyn_builtin_stream.c
index b7295f59a..209c8c39a 100644
--- a/packages/runtime/src/scr_dyn_builtin_stream.c
+++ b/packages/runtime/src/scr_dyn_builtin_stream.c
@@ -140,9 +140,11 @@ static void *bcs_capsule_retain(void *p) { return scr_stream_retain(p); }
 /* Also the capsule's tag. */
 static void bcs_capsule_release(void *p) { scr_stream_release(p); }
 
-static bool bcs_stream_vt(const ScrVt *vt) {
-  return (vt->pre >= scr_readable_vt.pre && vt->pre <= scr_readable_vt.post) ||
-         (vt->pre >= scr_writable_vt.pre && vt->pre <= scr_writable_vt.post);
+/* Whether `stream`, boxed as an "object:" typed ref, is a runtime stream. */
+static bool bcs_stream_vt(const ScrStream *stream) {
+  const ScrVt *vt = scr_stream_vt_of(stream);
+  return vt != NULL && ((vt->pre >= scr_readable_vt.pre && vt->pre <= scr_readable_vt.post) ||
+                        (vt->pre >= scr_writable_vt.pre && vt->pre <= scr_writable_vt.post));
 }
 
 static void bcs_rec_release(void *p);
@@ -154,7 +156,7 @@ static ScrStream *bcs_of(const ScrDyn *value) {
   if (value != NULL && value->kind == SCR_DYN_TYPED_REF && value->v.typed_ref.type_key_len > 7 &&
       memcmp(value->v.typed_ref.type_key, "object:", 7) == 0) {
     ScrStream *stream = value->v.typed_ref.ptr;
-    return bcs_stream_vt(stream->vt) && scr_stream_native(stream, bcs_rec_release) ? stream : NULL;
+    return bcs_stream_vt(stream) && scr_stream_native(stream, bcs_rec_release) ? stream : NULL;
   }
   if (value == NULL || value->kind != SCR_DYN_OBJ || value->symbol_properties == NULL) return NULL;
   ScrDyn *capsule = scr_dyn_obj_get(value->symbol_properties, bcs_hidden_key, BCS_HIDDEN_LEN);
@@ -184,7 +186,7 @@ static ScrStream *bcs_native_of(const ScrDyn *value) {
       memcmp(value->v.typed_ref.type_key, "object:", 7) != 0)
     return NULL;
   ScrStream *stream = scr_dyn_typed_ref_unbox(value);
-  if (bcs_stream_vt(stream->vt)) return stream;
+  if (bcs_stream_vt(stream)) return stream;
   value->v.typed_ref.release(stream);
   return NULL;
 }
diff --git a/packages/runtime/src/scr_stream.c b/packages/runtime/src/scr_stream.c
index da28cec7a..7d7ec04ed 100644
--- a/packages/runtime/src/scr_stream.c
+++ b/packages/runtime/src/scr_stream.c
@@ -4740,12 +4740,14 @@ ScrStream *scr_stream_rl_from_dyn(const ScrDyn *value, bool output) {
   if (value->kind == SCR_DYN_TYPED_REF && value->v.typed_ref.type_key_len > 7 &&
       !memcmp(value->v.typed_ref.type_key, "object:", 7)) {
     ScrStream *stream = scr_dyn_typed_ref_unbox(value);
-    const ScrVt *vt = stream->vt;
-    bool duplex = vt == &scr_duplex_vt || vt == &scr_transform_vt || vt == &scr_passthrough_vt ||
-                  scr_vt_within(vt, &scr_duplex_vt);
-    bool ok = duplex || (output ? vt == &scr_writable_vt || scr_vt_within(vt, &scr_writable_vt)
-                                : vt == &scr_readable_vt || scr_vt_within(vt, &scr_readable_vt));
-    if (ok) return stream;
+    const ScrVt *vt = scr_stream_vt_of(stream);
+    if (vt) {
+      bool duplex = vt == &scr_duplex_vt || vt == &scr_transform_vt || vt == &scr_passthrough_vt ||
+                    scr_vt_within(vt, &scr_duplex_vt);
+      bool ok = duplex || (output ? vt == &scr_writable_vt || scr_vt_within(vt, &scr_writable_vt)
+                                  : vt == &scr_readable_vt || scr_vt_within(vt, &scr_readable_vt));
+      if (ok) return stream;
+    }
     value->v.typed_ref.release(stream);
   }
   static const char in_m[] = "createInterface with an input that is not a readable stream has no lowering";
@@ -6411,9 +6413,7 @@ static ScrStream *scr_stream_view_receiver(const ScrDyn *value) {
   if (value->kind == SCR_DYN_TYPED_REF && value->v.typed_ref.type_key_len > 7 &&
       !memcmp(value->v.typed_ref.type_key, "object:", 7)) {
     ScrStream *stream = scr_dyn_typed_ref_unbox(value);
-    /* A class instance with a class id in its rc word (scr_rc.h) has no
-     * vtable word: its class is no stream. */
-    const ScrVt *vt = scr_rc_class_id(stream) == 0 ? stream->vt : NULL;
+    const ScrVt *vt = scr_stream_vt_of(stream);
     if (vt && ((vt->pre >= scr_readable_vt.pre && vt->pre <= scr_readable_vt.post) ||
                (vt->pre >= scr_writable_vt.pre && vt->pre <= scr_writable_vt.post))) return stream;
     value->v.typed_ref.release(stream);
@@ -6718,9 +6718,9 @@ ScrStream *scr_stream_unbox_dyn(const ScrDyn *value, const ScrStr *message) {
   if (value->kind == SCR_DYN_TYPED_REF && value->v.typed_ref.type_key_len > 7 &&
       !memcmp(value->v.typed_ref.type_key, "object:", 7)) {
     ScrStream *stream = scr_dyn_typed_ref_unbox(value);
-    const ScrVt *vt = stream->vt;
-    if ((vt->pre >= scr_readable_vt.pre && vt->pre <= scr_readable_vt.post) ||
-        (vt->pre >= scr_writable_vt.pre && vt->pre <= scr_writable_vt.post)) return stream;
+    const ScrVt *vt = scr_stream_vt_of(stream);
+    if (vt && ((vt->pre >= scr_readable_vt.pre && vt->pre <= scr_readable_vt.post) ||
+               (vt->pre >= scr_writable_vt.pre && vt->pre <= scr_writable_vt.post))) return stream;
     value->v.typed_ref.release(stream);
   }
   scr_throw_error_msg_code(SCR_ERR_ERROR, message->data, message->len, "SC2020");
diff --git a/packages/runtime/src/scr_stream.h b/packages/runtime/src/scr_stream.h
index 78290d8c4..99f58331c 100644
--- a/packages/runtime/src/scr_stream.h
+++ b/packages/runtime/src/scr_stream.h
@@ -48,6 +48,13 @@ typedef struct ScrStream {
   ScrStreamState *st;
 } ScrStream;
 
+/* The vtable of an object boxed as an "object:" typed ref, which may be any
+ * program class instance: NULL for one with a class id in its rc word
+ * (scr_rc.h), which has no vtable word and is no stream. */
+static inline const ScrVt *scr_stream_vt_of(const ScrStream *s) {
+  return scr_rc_class_id(s) == 0 ? s->vt : NULL;
+}
+
 extern SCR_TL ScrVt scr_readable_vt, scr_writable_vt, scr_duplex_vt,
     scr_transform_vt, scr_passthrough_vt; /* main() stamps pre/post */
 
diff --git a/packages/runtime/src/scr_stream_dyn.c b/packages/runtime/src/scr_stream_dyn.c
index 0859c4e07..11dcf6ee2 100644
--- a/packages/runtime/src/scr_stream_dyn.c
+++ b/packages/runtime/src/scr_stream_dyn.c
@@ -23,7 +23,11 @@ ScrStream *scr_stream_dyn_native(const ScrDyn *value, double side) {
       memcmp(value->v.typed_ref.type_key, "object:", 7) != 0)
     return NULL;
   ScrStream *stream = scr_dyn_typed_ref_unbox(value);
-  const ScrVt *vt = stream->vt;
+  const ScrVt *vt = scr_stream_vt_of(stream);
+  if (vt == NULL) {
+    value->v.typed_ref.release(stream);
+    return NULL;
+  }
   bool duplex = scr_stream_dyn_within(vt, &scr_duplex_vt) ||
                 scr_stream_dyn_within(vt, &scr_transform_vt) ||
                 scr_stream_dyn_within(vt, &scr_passthrough_vt);
diff --git a/tests/corpus/class-id-instances-as-streams.ts b/tests/corpus/class-id-instances-as-streams.ts
new file mode 100644
index 000000000..813e54ed2
--- /dev/null
+++ b/tests/corpus/class-id-instances-as-streams.ts
@@ -0,0 +1,39 @@
+// Instances of a class hierarchy, held as untyped values, where the stream
+// adapters probe for a native stream: pipe, pipeline, readline, and
+// instanceof checks against the stream classes. Their first field is a
+// number, which sits where a stream's vtable word would be.
+import { Duplex, PassThrough, Readable, Transform, Writable, pipeline } from "node:stream";
+import { createInterface } from "node:readline";
+
+class Shape {
+  size = 3.5;
+  area(): number {
+    return 0;
+  }
+}
+class Square extends Shape {
+  override area(): number {
+    return this.size * this.size;
+  }
+}
+
+function attempt(label: string, run: () => unknown): void {
+  try {
+    run();
+    console.log(label, "ok");
+  } catch (error) {
+    console.log(label, "throws", error instanceof Error);
+  }
+}
+
+const shape: any = new Square();
+try {
+  Readable.from(["a"]).pipe(shape);
+} catch (error) {
+  console.log("pipe", (error as Error).name);
+}
+attempt("pipeline", () => pipeline(shape, new PassThrough(), () => {}));
+attempt("readline-input", () => createInterface({ input: shape }));
+console.log("instanceof", shape instanceof Readable, shape instanceof Writable, shape instanceof Duplex);
+console.log("instanceof-transform", shape instanceof Transform, shape instanceof PassThrough);
+console.log("area", (shape as Square).area());
diff --git a/tests/harness/corpus-costs/class-id-instances-as-streams.ts.json b/tests/harness/corpus-costs/class-id-instances-as-streams.ts.json
new file mode 100644
index 000000000..07189e390
--- /dev/null
+++ b/tests/harness/corpus-costs/class-id-instances-as-streams.ts.json
@@ -0,0 +1 @@
+{ "plain": 4000, "san": 4500 }