diff --git a/packages/compiler/src/backend/llvm/lib-shared.ts b/packages/compiler/src/backend/llvm/lib-shared.ts
index 2d91386b3..d01374235 100644
--- a/packages/compiler/src/backend/llvm/lib-shared.ts
+++ b/packages/compiler/src/backend/llvm/lib-shared.ts
@@ -994,6 +994,7 @@ export const LIB_FN_SYMS: Record<string, string> = {
   // family have non-generic shapes (adapter/variadic) — special-cased in
   // emitLibCall.
   "emitter.new": "scr_emitter_new",
+  "emitter.className": "scr_emitter_class_name",
   "emitter.ctor": "scr_emitter_init",
   "emitter.off": "scr_emitter_off",
   "emitter.offDyn": "scr_emitter_off_dyn",
diff --git a/packages/compiler/src/frontend/lowering/builtins/runtime-constructor.ts b/packages/compiler/src/frontend/lowering/builtins/runtime-constructor.ts
new file mode 100644
index 000000000..4b61f98c1
--- /dev/null
+++ b/packages/compiler/src/frontend/lowering/builtins/runtime-constructor.ts
@@ -0,0 +1,181 @@
+import * as ts from "../../ts7/adapter.js";
+import { type Lowerer } from "../lowerer.js";
+import { type ClassInfo } from "../lower-classes.js";
+import { isJsSourceFile, locOf } from "../../program.js";
+import { varRef } from "../../../ir/build.js";
+import { BOOL, STRING, type IrExpr, type IrType, type SrcLoc } from "../../../ir/ir.js";
+
+/** `x.constructor` on an instance typed as EventEmitter or a stream class
+ * in a TypeScript file: the receiver's runtime ClassInfo, or null. The
+ * runtime records each instance's Node class name (EventEmitter, Readable,
+ * ReadStream, Gzip, Socket, a program subclass's name), which the
+ * lowerings below read with emitter.className. */
+export function runtimeConstructorReceiver(
+  lowerer: Lowerer,
+  expr: ts.Expression,
+): { receiver: ts.Expression; info: ClassInfo } | null {
+  let e = expr;
+  while (ts.isParenthesizedExpression(e)) e = e.expression;
+  if (!ts.isPropertyAccessExpression(e) || e.questionDotToken) return null;
+  if (e.name.text !== "constructor" || isJsSourceFile(e.getSourceFile())) return null;
+  const mapped = lowerer.mapTypeOf(lowerer.typeOf(e.expression));
+  if (mapped?.kind !== "object") return null;
+  const info = lowerer.classes.get(mapped.className);
+  if (!info?.def.runtime || (!info.builtinEmitter && info.builtinStream === undefined)) return null;
+  return { receiver: e.expression, info };
+}
+
+/** `x.constructor.name`: the instance's class name. */
+export function lowerRuntimeConstructorName(
+  lowerer: Lowerer,
+  receiver: ts.Expression,
+  loc: SrcLoc,
+): IrExpr {
+  return {
+    kind: "libCall",
+    fn: "emitter.className",
+    args: [lowerer.lowerExpr(receiver)],
+    type: STRING,
+    loc,
+  };
+}
+
+/** `x.constructor === C` for EventEmitter or a stream class C: true when
+ * the instance's own class is C, which needs both the vtable (C's own
+ * interval, not a subclass's) and the class name (fs, zlib, crypto, child
+ * process and worker streams share the base classes' vtables). */
+export function lowerRuntimeConstructorIs(
+  lowerer: Lowerer,
+  receiver: ts.Expression,
+  target: ClassInfo,
+  negated: boolean,
+  loc: SrcLoc,
+): IrExpr {
+  const value = lowerer.lowerExpr(receiver);
+  const local = lowerer.declareHiddenLocal("%constructorOf", value.type);
+  const ref = varRef(local.id, value.type, loc);
+  const test = exactClassTest(lowerer, ref, target, loc);
+  return {
+    kind: "seqExpr",
+    stmts: [{ kind: "varDecl", localId: local.id, init: value, loc }],
+    result: negated ? { kind: "unary", op: "!", operand: test, type: BOOL, loc } : test,
+    type: BOOL,
+    loc,
+  };
+}
+
+/** `x.constructor` as a value: the class object of the instance's own class
+ * among EventEmitter and the stream classes, as the receiver's static
+ * class's class value. Instances of the other runtime classes (fs, zlib,
+ * crypto, child process and worker streams) and of program subclasses
+ * report SC1090 at run time. */
+export function lowerRuntimeConstructor(
+  lowerer: Lowerer,
+  access: ts.PropertyAccessExpression,
+  info: ClassInfo,
+): IrExpr {
+  const loc = locOf(access);
+  const resultT: IrType = { kind: "classval", className: info.def.name };
+  const value = lowerer.lowerExpr(access.expression);
+  const local = lowerer.declareHiddenLocal("%constructorOf", value.type);
+  const ref = varRef(local.id, value.type, loc);
+  let result: IrExpr = refusalCall(lowerer, resultT, loc);
+  for (const candidate of valueClassesUnder(info)) {
+    lowerer.defineClassValueConstruct(candidate);
+    lowerer.defineClassValueConstruct(info);
+    const classRef: IrExpr = {
+      kind: "classRef",
+      className: candidate.def.name,
+      type: { kind: "classval", className: candidate.def.name },
+      loc,
+    };
+    result = {
+      kind: "ternary",
+      cond: exactClassTest(lowerer, ref, candidate, loc),
+      then: candidate === info ? classRef : { kind: "upcast", value: classRef, type: resultT, loc },
+      else_: result,
+      type: resultT,
+      loc,
+    };
+  }
+  return {
+    kind: "seqExpr",
+    stmts: [{ kind: "varDecl", localId: local.id, init: value, loc }],
+    result,
+    type: resultT,
+    loc,
+  };
+}
+
+/** `info` and its runtime descendants whose values construct. */
+function valueClassesUnder(info: ClassInfo): ClassInfo[] {
+  const out: ClassInfo[] = [];
+  const visit = (c: ClassInfo): void => {
+    if (!c.def.runtime) return;
+    if (c.valueCtorParams !== undefined) out.push(c);
+    for (const s of c.subclasses) visit(s);
+  };
+  visit(info);
+  return out;
+}
+
+/** The instance is of class `target` itself: inside its interval, outside
+ * each direct subclass's (runtime or program), and named as it. */
+function exactClassTest(lowerer: Lowerer, ref: IrExpr, target: ClassInfo, loc: SrcLoc): IrExpr {
+  let test: IrExpr = {
+    kind: "instanceOf",
+    value: ref,
+    className: target.def.name,
+    type: BOOL,
+    loc,
+  };
+  for (const sub of target.subclasses) {
+    test = {
+      kind: "logical",
+      op: "&&",
+      left: test,
+      right: {
+        kind: "unary",
+        op: "!",
+        operand: { kind: "instanceOf", value: ref, className: sub.def.name, type: BOOL, loc },
+        type: BOOL,
+        loc,
+      },
+      type: BOOL,
+      loc,
+    };
+  }
+  const name: IrExpr = {
+    kind: "strEq",
+    negated: false,
+    left: { kind: "libCall", fn: "emitter.className", args: [ref], type: STRING, loc },
+    right: { kind: "strLit", value: target.def.name.slice(1), type: STRING, loc },
+    type: BOOL,
+    loc,
+  };
+  return { kind: "logical", op: "&&", left: test, right: name, type: BOOL, loc };
+}
+
+/** The run-time refusal for an instance whose class has no class value. */
+function refusalCall(lowerer: Lowerer, type: IrType, loc: SrcLoc): IrExpr {
+  const name = `%constructor.refusal:${type.kind === "classval" ? type.className : ""}`;
+  if (!lowerer.liftedFns.some((fn) => fn.name === name)) {
+    lowerer.liftedFns.push({
+      name,
+      params: [],
+      locals: [],
+      returnType: type,
+      body: [
+        {
+          kind: "runtimeFence",
+          code: "SC1090",
+          message:
+            "reading 'constructor' of an fs, zlib, crypto, child process or worker stream, or of an instance of a program subclass of EventEmitter or a stream class, is not supported yet",
+          loc,
+        },
+      ],
+      loc,
+    });
+  }
+  return { kind: "call", callee: name, args: [], type, loc };
+}
diff --git a/packages/compiler/src/frontend/lowering/expressions/binary.ts b/packages/compiler/src/frontend/lowering/expressions/binary.ts
index 2a8de78b9..cecbde717 100644
--- a/packages/compiler/src/frontend/lowering/expressions/binary.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/binary.ts
@@ -66,6 +66,10 @@ import { lowerAbstractEquality } from "../abstract-equality.js";
 import { lowerInExpression } from "./relational.js";
 import { lowerDynTypeofTest, lowerErrorCodeTypeofTest } from "./typeof.js";
 import { builtinClassRefOf, sameBuiltinClass } from "../builtins/class-values.js";
+import {
+  lowerRuntimeConstructorIs,
+  runtimeConstructorReceiver,
+} from "../builtins/runtime-constructor.js";
 import { tryLowerIndexedComparison } from "../indexed-comparison.js";
 import { lowerUnionEquality, tagEqualityMayMissAlias } from "../strict-equality.js";
 import { withUnitArm } from "../../type-mapper.js";
@@ -1322,6 +1326,24 @@ export function lowerBinary(lowerer: Lowerer, expr: ts.BinaryExpression): IrExpr
       lowerUnionTypeofTest(lowerer, expr, loc);
     if (test) return test;
   }
+  // `x.constructor === Readable` for an instance of EventEmitter or a
+  // stream class: the instance's own class, compared without a class value.
+  if (
+    op === ts.SyntaxKind.EqualsEqualsEqualsToken ||
+    op === ts.SyntaxKind.ExclamationEqualsEqualsToken
+  ) {
+    for (const [side, other] of [
+      [expr.left, expr.right],
+      [expr.right, expr.left],
+    ] as const) {
+      const ctor = runtimeConstructorReceiver(lowerer, side);
+      const target = ctor ? builtinClassRefOf(lowerer, other) : null;
+      if (ctor && (target?.kind === "emitter" || target?.kind === "stream")) {
+        const negated = op === ts.SyntaxKind.ExclamationEqualsEqualsToken;
+        return lowerRuntimeConstructorIs(lowerer, ctor.receiver, target.info, negated, loc);
+      }
+    }
+  }
   // `C === EventEmitter` between two builtin class references: the
   // classes are compile-time values, so the answer folds.
   if (
diff --git a/packages/compiler/src/frontend/lowering/expressions/property-access.ts b/packages/compiler/src/frontend/lowering/expressions/property-access.ts
index 7f0b2ddbb..1a5298379 100644
--- a/packages/compiler/src/frontend/lowering/expressions/property-access.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/property-access.ts
@@ -76,6 +76,11 @@ import { lifetimeRegionMemberOf } from "../../experimental-import.js";
 import { lowerInternTableProperty } from "../intern-tables.js";
 import { lowerThreadSlotsProperty } from "../thread-slots.js";
 import { builtinClassRefOf, runtimeClassValueOfType } from "../builtins/class-values.js";
+import {
+  lowerRuntimeConstructor,
+  lowerRuntimeConstructorName,
+  runtimeConstructorReceiver,
+} from "../builtins/runtime-constructor.js";
 import { findGenericMethodOn, lowerStaticFieldRead } from "../lower-classes.js";
 import { lowerClassPrototypeData } from "../class-prototypes.js";
 import { lowerEnumAccess } from "../lower-enums.js";
@@ -290,6 +295,16 @@ export function lowerPropertyAccessExpr(
     const ref = builtinClassRefOf(lowerer, expr.expression);
     if (ref !== null) return { kind: "strLit", value: ref.name, type: STRING, loc };
   }
+  // `x.constructor.name` and `x.constructor` on an instance of
+  // EventEmitter or a stream class (builtins/runtime-constructor.ts).
+  if (expr.name.text === "name" && !expr.questionDotToken) {
+    const ctor = runtimeConstructorReceiver(lowerer, expr.expression);
+    if (ctor) return lowerRuntimeConstructorName(lowerer, ctor.receiver, loc);
+  }
+  {
+    const ctor = runtimeConstructorReceiver(lowerer, expr);
+    if (ctor) return lowerRuntimeConstructor(lowerer, expr, ctor.info);
+  }
   // `C.name` through a runtime-provided class VALUE (a `typeof Readable`
   // parameter): the class object's stored name.
   if (
diff --git a/packages/compiler/src/ir/builtin-signatures.ts b/packages/compiler/src/ir/builtin-signatures.ts
index 697a185a5..2a0e5124a 100644
--- a/packages/compiler/src/ir/builtin-signatures.ts
+++ b/packages/compiler/src/ir/builtin-signatures.ts
@@ -3499,6 +3499,9 @@ export const LIB_FN_SIGS = defineLibFnSignatures({
    * throws err when unhandled. count/countFn/names/listeners/getMax/
    * setMax/setDefaultMax/getDefaultMax are the introspection surface. */
   "emitter.new": { argTypes: [], result: VOID },
+  /** `x.constructor.name` of an emitter or stream (a borrowed object of
+   * EventEmitter's hierarchy): the +1 class name the instance records. */
+  "emitter.className": { argTypes: [null], result: STRING },
   "emitter.ctor": { argTypes: [null], result: VOID },
   "emitter.on": { argTypes: [null, STRING, null, BOOL, BOOL], result: VOID },
   "emitter.off": { argTypes: [null, STRING, null], result: VOID },
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5983-runtime-constructor.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5983-runtime-constructor.ts.json
new file mode 100644
index 000000000..004b1e350
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5983-runtime-constructor.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/5983-runtime-constructor.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/runtime/src/scr_child.c b/packages/runtime/src/scr_child.c
index 87ead5755..c714fb555 100644
--- a/packages/runtime/src/scr_child.c
+++ b/packages/runtime/src/scr_child.c
@@ -6458,7 +6458,7 @@ static ScrDyn *scr_child_bridge_readable(ScrChildStream *reader, ScrDyn **cached
     return NULL;
   }
   if (!*cached) {
-    ScrStream *stream = scr_child_stream_ops->readable();
+    ScrStream *stream = scr_child_stream_ops->readable("Socket");
     scr_child_stream_on_data(reader, scr_child_bridge_callback(stream, (void *)scr_child_bridge_data),
         scr_child_bridge_data, false);
     scr_child_stream_on_end(reader, scr_child_bridge_callback(stream, (void *)scr_child_bridge_end), false);
@@ -6578,7 +6578,7 @@ static ScrDyn *scr_child_bridge_writable(ScrChildWriter *writer, ScrDyn **cached
     return NULL;
   }
   if (!*cached) {
-    ScrStream *stream = scr_child_stream_ops->writable(
+    ScrStream *stream = scr_child_stream_ops->writable("Socket",
         scr_child_writer_bridge_callback(writer, (void *)scr_child_bridge_write), scr_child_bridge_write,
         scr_child_writer_bridge_callback(writer, (void *)scr_child_bridge_final), scr_child_bridge_final);
     *cached = scr_child_writable_box(stream);
diff --git a/packages/runtime/src/scr_child.h b/packages/runtime/src/scr_child.h
index 1187985c8..bbb50e0cb 100644
--- a/packages/runtime/src/scr_child.h
+++ b/packages/runtime/src/scr_child.h
@@ -253,9 +253,13 @@ void scr_child_dyn_install(void);
 void scr_child_on_dyn(ScrChild *child, ScrStr *event, ScrDyn *listener); /* borrowed */
 /* Optional native-stream bridge. The table avoids a stream dependency
  * in programs that only use the child-process surface. */
+/* readable/writable take the stream's class name (the constructor name
+ * Node reports: Socket for child stdio, ReadableWorkerStdio and
+ * WritableWorkerStdio for worker stdio). */
 typedef struct ScrChildStreamOps {
-  ScrStream *(*readable)(void);
-  ScrStream *(*writable)(ScrClosure *, ScrStreamChunkInv, ScrClosure *, ScrStreamPlainInv);
+  ScrStream *(*readable)(const char *cls);
+  ScrStream *(*writable)(const char *cls, ScrClosure *, ScrStreamChunkInv, ScrClosure *,
+      ScrStreamPlainInv);
   void *(*retain)(void *);
   void (*release)(void *);
   ScrTraceFn trace;
diff --git a/packages/runtime/src/scr_crypto.h b/packages/runtime/src/scr_crypto.h
index 4b6b6a6a6..d71f46b13 100644
--- a/packages/runtime/src/scr_crypto.h
+++ b/packages/runtime/src/scr_crypto.h
@@ -71,6 +71,7 @@ void scr_crypto_hash_stream_set(ScrCryptoHash *hash, void *stream, void (*releas
 void scr_crypto_hash_stream_detach(void *hash);
 void scr_crypto_hash_stream_close(ScrCryptoHash *hash);
 bool scr_crypto_hash_stream_usable(const ScrCryptoHash *hash);
+bool scr_crypto_hash_is_hmac(const ScrCryptoHash *hash);
 void scr_crypto_hash_stream_hold(ScrCryptoHash *hash);
 void scr_crypto_hash_stream_unhold(ScrCryptoHash *hash);
 ScrStream *scr_crypto_hash_stream_piped(ScrCryptoHash *hash);
diff --git a/packages/runtime/src/scr_events_emitter.c b/packages/runtime/src/scr_events_emitter.c
index ceecb15e3..e0e305bde 100644
--- a/packages/runtime/src/scr_events_emitter.c
+++ b/packages/runtime/src/scr_events_emitter.c
@@ -324,6 +324,13 @@ ScrEmitter *scr_emitter_new(void) {
   return em;
 }
 
+/* x.constructor.name of an emitter, a stream, or an instance of an emitted
+ * subclass: the class name every allocation stamps (EventEmitter,
+ * Readable, ReadStream, Gzip, Socket, the program class's name). +1. */
+ScrStr *scr_emitter_class_name(const ScrEmitter *em) {
+  return scr_str_new(em->cls, strlen(em->cls));
+}
+
 /* super() into the emitter prefix of an emitted subclass: the allocation
  * already zeroed reg and stamped the concrete class's display name, so
  * construction has nothing left to do — the call exists so the super()
diff --git a/packages/runtime/src/scr_events_emitter.h b/packages/runtime/src/scr_events_emitter.h
index 99da57a50..a92ae1864 100644
--- a/packages/runtime/src/scr_events_emitter.h
+++ b/packages/runtime/src/scr_events_emitter.h
@@ -38,6 +38,7 @@ extern SCR_TL ScrVt scr_emitter_vt; /* main() stamps pre/post */
 extern SCR_TL double scr_emitter_default_max;
 
 ScrEmitter *scr_emitter_new(void); /* +1, collector-headered */
+ScrStr *scr_emitter_class_name(const ScrEmitter *em); /* +1 */
 /* Internal owner hook; observes listener presence without running script. */
 void scr_emitter_observe(ScrEmitter *em,
                         void (*observe)(void *, const ScrStr *, bool), void *context);
diff --git a/packages/runtime/src/scr_fs_stream.c b/packages/runtime/src/scr_fs_stream.c
index 97bf4ebaa..70a5ab075 100644
--- a/packages/runtime/src/scr_fs_stream.c
+++ b/packages/runtime/src/scr_fs_stream.c
@@ -562,6 +562,7 @@ static ScrStream *scr_fss_new(const ScrDyn *path, const ScrDyn *opts_in, bool re
     scr_dyn_release(eo);
   }
   if (!s) goto fail;
+  s->cls = readable ? "ReadStream" : "WriteStream";
   bool unknown_encoding = false;
   if (!readable) {
     /* setDefaultEncoding(options.encoding) after the Writable constructor:
diff --git a/packages/runtime/src/scr_lib.c b/packages/runtime/src/scr_lib.c
index 930d44818..52036e02c 100644
--- a/packages/runtime/src/scr_lib.c
+++ b/packages/runtime/src/scr_lib.c
@@ -8473,6 +8473,8 @@ void scr_crypto_hash_stream_close(ScrCryptoHash *hash) {
   if (stream) stream_release(stream);
 }
 
+bool scr_crypto_hash_is_hmac(const ScrCryptoHash *hash) { return hash->hmac; }
+
 bool scr_crypto_hash_stream_usable(const ScrCryptoHash *hash) {
   if (!hash->stream_closed) return true;
   static const char msg[] = "Stream use of a Hash or Hmac after its stream closed has no lowering";
diff --git a/packages/runtime/src/scr_stream.c b/packages/runtime/src/scr_stream.c
index 244e2ec5d..c2d27e7ce 100644
--- a/packages/runtime/src/scr_stream.c
+++ b/packages/runtime/src/scr_stream.c
@@ -6350,15 +6350,19 @@ void scr_stream_install(void) {
 static void scr_stream_child_read(ScrClosure *cb, ScrStream *s, double size) {
   (void)cb; (void)s; (void)size;
 }
-static ScrStream *scr_stream_child_readable(void) {
-  return scr_stream_new_readable(-1, true, true,
+static ScrStream *scr_stream_child_readable(const char *cls) {
+  ScrStream *s = scr_stream_new_readable(-1, true, true,
       scr_closure_new((void *)scr_stream_child_read, 0), scr_stream_child_read,
       NULL, NULL);
+  s->cls = cls;
+  return s;
 }
-static ScrStream *scr_stream_child_writable(ScrClosure *write, ScrStreamChunkInv write_inv,
-    ScrClosure *final_cb, ScrStreamPlainInv final_inv) {
-  return scr_stream_new_writable(-1, true, true, write, write_inv,
+static ScrStream *scr_stream_child_writable(const char *cls, ScrClosure *write,
+    ScrStreamChunkInv write_inv, ScrClosure *final_cb, ScrStreamPlainInv final_inv) {
+  ScrStream *s = scr_stream_new_writable(-1, true, true, write, write_inv,
       final_cb, final_inv, NULL, NULL);
+  s->cls = cls;
+  return s;
 }
 const ScrChildStreamOps scr_stream_child_ops = {
   scr_stream_child_readable, scr_stream_child_writable,
@@ -6578,6 +6582,7 @@ ScrStream *scr_crypto_hash_stream(ScrCryptoHash *hash) {
                                scr_hash_stream_step(box), scr_hash_stream_transform,
                                scr_hash_stream_step(box), scr_hash_stream_flush,
                                scr_hash_stream_step(box), scr_hash_stream_destroy);
+  s->cls = scr_crypto_hash_is_hmac(hash) ? "Hmac" : "Hash";
   /* The steps keep the box alive as long as the stream. */
   scr_crypto_hash_stream_set(hash, scr_stream_retain(s), scr_stream_release_v, box);
   scr_box_release(box);
diff --git a/packages/runtime/src/scr_worker_events.c b/packages/runtime/src/scr_worker_events.c
index 53041f3d5..b4efa3e0d 100644
--- a/packages/runtime/src/scr_worker_events.c
+++ b/packages/runtime/src/scr_worker_events.c
@@ -122,7 +122,7 @@ static ScrClosure *scr_worker_stdin_callback(ScrWorkerThread *thread, void *fire
 }
 
 static ScrStream *scr_worker_stdin_new(ScrWorkerThread *thread) {
-  return scr_worker_stdio_ops->writable(
+  return scr_worker_stdio_ops->writable("WritableWorkerStdio",
       scr_worker_stdin_callback(thread, (void *)scr_worker_stdin_write), scr_worker_stdin_write,
       scr_worker_stdin_callback(thread, (void *)scr_worker_stdin_final), scr_worker_stdin_final);
 }
@@ -1350,7 +1350,8 @@ static ScrDyn *scr_worker_new_options(double root, ScrStr *filename, ScrDyn *opt
   for (int i = 0; i < 2; i++) {
     view->stdio_captured[i] = (capture >> i) & 1;
     /* without the stream unit nothing can read the stream: the output is dropped */
-    if (view->stdio_captured[i] && scr_worker_stdio_ops) view->stdio[i] = scr_worker_stdio_ops->readable();
+    if (view->stdio_captured[i] && scr_worker_stdio_ops)
+      view->stdio[i] = scr_worker_stdio_ops->readable("ReadableWorkerStdio");
   }
   view->stdin_captured = capture & 4;
   if (view->stdin_captured && scr_worker_stdio_ops) view->stdin_stream = scr_worker_stdin_new(thread);
diff --git a/tests/corpus/5983-runtime-constructor.ts b/tests/corpus/5983-runtime-constructor.ts
new file mode 100644
index 000000000..3fb9672d0
--- /dev/null
+++ b/tests/corpus/5983-runtime-constructor.ts
@@ -0,0 +1,48 @@
+// x.constructor on instances of EventEmitter and the stream classes:
+// .name is the class Node reports (fs, zlib and program subclasses
+// included), === compares with the instance's own class, and the value
+// is the class object for EventEmitter and the stream classes.
+import { EventEmitter } from "node:events";
+import { Duplex, PassThrough, Readable, Transform, Writable } from "node:stream";
+import * as fs from "node:fs";
+import * as zlib from "node:zlib";
+
+class Bus extends EventEmitter {}
+
+const emitter = new EventEmitter();
+const bus: EventEmitter = new Bus();
+console.log("emitter", emitter.constructor === EventEmitter, emitter.constructor.name);
+console.log("subclass", bus.constructor === EventEmitter, bus.constructor.name);
+
+const streams: Readable[] = [new Readable(), new Duplex(), new Transform(), new PassThrough()];
+for (const s of streams) {
+  console.log(
+    "stream",
+    s.constructor.name,
+    s.constructor === Readable,
+    s.constructor === Duplex,
+    s.constructor === Transform,
+    s.constructor === PassThrough,
+    s.constructor !== Readable,
+  );
+}
+
+const from = Readable.from(["a"]);
+console.log("from", from.constructor.name, from.constructor === Readable);
+
+const readStream: Readable = fs.createReadStream(import.meta.filename);
+const writeStream: Writable = fs.createWriteStream("/dev/null");
+console.log("fs", readStream.constructor.name, readStream.constructor === Readable);
+console.log("fs", writeStream.constructor.name, writeStream.constructor === Writable);
+readStream.destroy();
+writeStream.destroy();
+
+const gzip: Transform = zlib.createGzip();
+const inflate: Transform = zlib.createInflateRaw();
+console.log("zlib", gzip.constructor.name, inflate.constructor.name, gzip.constructor === Transform);
+
+const kinds: EventEmitter[] = [new EventEmitter(), new Writable(), new PassThrough()];
+for (const k of kinds) {
+  const Kind = k.constructor as typeof EventEmitter;
+  console.log("value", Kind.name, Kind === EventEmitter, k instanceof Kind, new Kind() instanceof EventEmitter);
+}
diff --git a/tests/harness/corpus-costs/5983-runtime-constructor.ts.json b/tests/harness/corpus-costs/5983-runtime-constructor.ts.json
new file mode 100644
index 000000000..5506cd9a8
--- /dev/null
+++ b/tests/harness/corpus-costs/5983-runtime-constructor.ts.json
@@ -0,0 +1 @@
+{ "plain": 3500, "san": 6900 }