diff --git a/docs/content/docs/limitations.mdx b/docs/content/docs/limitations.mdx
index 420af8200..ed2c5b1ec 100644
--- a/docs/content/docs/limitations.mdx
+++ b/docs/content/docs/limitations.mdx
@@ -208,6 +208,10 @@ title: "Limitations"
 
 <include>../limitations/dns-lookup-errors.mdx</include>
 
+### Socket data encoding
+
+<include>../limitations/socket-data-encoding.mdx</include>
+
 ### UDP socket options
 
 <include>../limitations/udp-socket-options.mdx</include>
diff --git a/docs/content/limitations/socket-data-encoding.mdx b/docs/content/limitations/socket-data-encoding.mdx
new file mode 100644
index 000000000..8094f8143
--- /dev/null
+++ b/docs/content/limitations/socket-data-encoding.mdx
@@ -0,0 +1 @@
+After `socket.setEncoding("utf8")`, an inline `'data'` listener receives strings, as in Node, whatever type its chunk parameter declares. A listener passed by reference, such as a named function whose parameter is typed `Buffer`, keeps its `Buffer` parameter, so a string chunk throws `TypeError: expected Uint8Array at $[0], got string`. Declare that parameter as `string` or `string | Buffer`, or pass an inline function.
diff --git a/packages/compiler/src/frontend/lowering/generic-functions.ts b/packages/compiler/src/frontend/lowering/generic-functions.ts
index 4cddcf5fd..bd785a256 100644
--- a/packages/compiler/src/frontend/lowering/generic-functions.ts
+++ b/packages/compiler/src/frontend/lowering/generic-functions.ts
@@ -94,6 +94,11 @@ export interface GenericFnInfo {
    * checker types key an instantiation, exactly the generic machinery —
    * the untyped params ARE the type parameters (see implicitCallInstance). */
   implicitParams?: (ts.Symbol | null)[];
+  /** Implicit-any monomorphization of destructured object parameters
+   * (implicitPatternsOf): parallel to decl.parameters, the destructured
+   * names where the slot binds the call site's object type. Computed on
+   * first use. */
+  implicitPatterns?: (ImplicitPattern | null)[];
 }
 
 export interface GenericInstance {
@@ -1125,6 +1130,114 @@ function broadPromiseParam(lowerer: Lowerer, type: ts.Type): boolean {
   );
 }
 
+/** A destructured object parameter (`function f({ Base, name })`) whose
+ * names bind the call site argument's property types. */
+export interface ImplicitPattern {
+  elements: { property: string; symbol: ts.Symbol }[];
+}
+
+/** The destructured object parameters of a JS function-like that bind like
+ * implicit type parameters: every element a plain name (optionally renamed
+ * from an identifier or string key) with no default, no rest element, and
+ * no write in the body, under a broad inferred type. Parallel to
+ * decl.parameters. */
+export function implicitPatternsOf(
+  lowerer: Lowerer,
+  decl: ts.FunctionDeclaration | ts.MethodDeclaration | ts.FunctionExpression | ts.ArrowFunction,
+): (ImplicitPattern | null)[] {
+  return decl.parameters.map((param): ImplicitPattern | null => {
+    if (!ts.isObjectBindingPattern(param.name) || !decl.body) return null;
+    if (param.dotDotDotToken || param.questionToken || param.initializer) return null;
+    const elements: ImplicitPattern["elements"] = [];
+    for (const element of param.name.elements) {
+      if (element.dotDotDotToken || element.initializer || !ts.isIdentifier(element.name))
+        return null;
+      const key = element.propertyName ?? element.name;
+      if (!ts.isIdentifier(key) && !ts.isStringLiteral(key)) return null;
+      const symbol = lowerer.checker.getSymbolAtLocation(element.name);
+      if (!symbol || paramWrittenInBody(lowerer, decl.body, symbol, element.name.text)) return null;
+      // A JSDoc or contextual type names the properties: keep its ABI.
+      if ((lowerer.typeOf(element.name).flags & ts.TypeFlags.Any) === 0) return null;
+      elements.push({ property: key.text, symbol });
+    }
+    return elements.length > 0 ? { elements } : null;
+  });
+}
+
+/** The type a destructured parameter takes at one call site (the object's
+ * own representation, a record or a checked value) and the checker type
+ * each destructured name binds. An object literal argument types the names
+ * from its property expressions, which see the enclosing instance's
+ * bindings (`makeClass({ Base, name })` inside a specialized
+ * `extend(Base, name)`); any other argument uses its checker type. Null
+ * when the argument lacks a destructured property, or passes a class the
+ * callee itself defines: each such class would instantiate the callee
+ * again (a static `extend` calling `makeClass({ Base: this })`). */
+function implicitPatternBinding(
+  lowerer: Lowerer,
+  callee: ts.Node,
+  pattern: ImplicitPattern,
+  arg: ts.Expression,
+): { record: IrType; types: Map<ts.Symbol, ts.Type> } | null {
+  let literal: ts.Expression = arg;
+  while (ts.isParenthesizedExpression(literal)) literal = literal.expression;
+  const types = new Map<ts.Symbol, ts.Type>();
+  if (ts.isObjectLiteralExpression(literal)) {
+    const values = new Map<string, ts.Expression>();
+    /** Shorthand properties name the binding they read. */
+    const shorthand = new Map<string, ts.Symbol>();
+    for (const property of literal.properties) {
+      if (ts.isShorthandPropertyAssignment(property) && ts.isIdentifier(property.name)) {
+        values.set(property.name.text, property.name);
+        const read = lowerer.checker.getShorthandAssignmentValueSymbol(property);
+        if (read) shorthand.set(property.name.text, read);
+      } else if (
+        ts.isPropertyAssignment(property) &&
+        (ts.isIdentifier(property.name) || ts.isStringLiteral(property.name))
+      )
+        values.set(property.name.text, property.initializer);
+      else return null;
+    }
+    for (const element of pattern.elements) {
+      const value = values.get(element.property);
+      if (!value) return null;
+      const read = shorthand.get(element.property);
+      const type = (read && lowerer.implicitParamTypes?.get(read)) ?? lowerer.typeOf(value);
+      types.set(element.symbol, lowerer.checker.getBaseTypeOfLiteralType(type));
+    }
+    // The object keeps the representation the call site lowers it to; the
+    // destructured names read their static types back out of it.
+    const lowered = tryLowerExpression(lowerer, literal);
+    if (lowered?.type.kind !== "record" && lowered?.type.kind !== "dyn") return null;
+    if (lowered.type.kind === "record") {
+      for (const field of lowerer.shapes.get(lowered.type.shapeId)?.fields ?? [])
+        if (field.type.kind === "classval" && classDefinedIn(lowerer, field.type.className, callee))
+          return null;
+    }
+    return { record: lowered.type, types };
+  }
+  const argType = lowerer.checker.getBaseTypeOfLiteralType(lowerer.typeOf(arg));
+  const record = lowerer.mapTypeOf(argType);
+  if (record?.kind !== "record") return null;
+  for (const element of pattern.elements) {
+    const property = lowerer.checker.getPropertyOfType(argType, element.property);
+    if (!property) return null;
+    types.set(element.symbol, lowerer.checker.getTypeOfSymbol(property));
+  }
+  return { record, types };
+}
+
+/** The class is declared inside `scope`. */
+function classDefinedIn(lowerer: Lowerer, className: string, scope: ts.Node): boolean {
+  for (
+    let n: ts.Node | undefined = lowerer.classes.get(className)?.decl ?? undefined;
+    n;
+    n = n.parent
+  )
+    if (n === scope) return true;
+  return false;
+}
+
 /** The implicit-type-parameter slots of a JS function-like: parallel to
  * decl.parameters, the param SYMBOL where the slot is a bindable
  * implicit param (identifier-named, broad inferred/JSDoc type, not
@@ -1169,6 +1282,10 @@ export function implicitAnyParamSymbolsOf(
     hasImplicitParams = true;
     return sym;
   });
+  // Destructured object parameters monomorphize too (implicitPatternsOf);
+  // their slots stay null here.
+  if (!hasImplicitParams && isJsSourceFile(decl.getSourceFile()))
+    hasImplicitParams = implicitPatternsOf(lowerer, decl).some((pattern) => pattern !== null);
   return hasImplicitParams ? out : null;
 }
 
@@ -1440,7 +1557,27 @@ export function implicitCallInstance(
 ): GenericInstance {
   const shapes: ParamShape[] = [];
   const argTypes = new Map<ts.Symbol, ts.Type>();
+  const patterns = (info.implicitPatterns ??= implicitPatternsOf(lowerer, info.decl));
+  /** The destructured names' bound types, which the object's own
+   * representation does not always carry. */
+  const patternKey: string[] = [];
   info.decl.parameters.forEach((param, i) => {
+    const pattern = patterns[i];
+    const patternArg = call.arguments[i];
+    if (pattern && patternArg && !ts.isSpreadElement(patternArg)) {
+      // The argument's object type binds each destructured name, so a
+      // class passed as `{ Base }` keeps its static class type.
+      const bound = implicitPatternBinding(lowerer, info.decl, pattern, patternArg);
+      if (bound) {
+        for (const [symbol, type] of bound.types) {
+          argTypes.set(symbol, type);
+          const mapped = lowerer.mapTypeOf(type);
+          patternKey.push(mapped ? typeKey(mapped) : "?");
+        }
+        shapes.push({ type: bound.record, mode: "required" });
+        return;
+      }
+    }
     const sym = info.implicitParams![i];
     if (!sym) {
       shapes.push(lowerer.paramShape(param));
@@ -1578,7 +1715,7 @@ export function implicitCallInstance(
     }
     shapes.push({ type: bound, mode: "required" });
   });
-  return internImplicitInstance(lowerer, call, info, shapes, argTypes);
+  return internImplicitInstance(lowerer, call, info, shapes, argTypes, patternKey.join(","));
 }
 
 /** The all-dyn DEFAULT instance — today's compiled body exactly: what a
@@ -1611,8 +1748,9 @@ function internImplicitInstance(
   info: GenericFnInfo,
   shapes: ParamShape[],
   argTypes: Map<ts.Symbol, ts.Type>,
+  patternKey = "",
 ): GenericInstance {
-  const key = shapes.map((s) => typeKey(s.type)).join(",");
+  const key = shapes.map((s) => typeKey(s.type)).join(",") + (patternKey ? `|${patternKey}` : "");
   let inst = info.instances.get(key);
   if (inst?.implicitState === "failed")
     lowerer.unsupported("SC1090", blame, `the failed specialization of '${info.baseName}'`);
diff --git a/packages/compiler/src/frontend/lowering/lower-server.ts b/packages/compiler/src/frontend/lowering/lower-server.ts
index 8c8639d43..54d9adeb5 100644
--- a/packages/compiler/src/frontend/lowering/lower-server.ts
+++ b/packages/compiler/src/frontend/lowering/lower-server.ts
@@ -2111,6 +2111,67 @@ function lowerNetServerMethodCall(
   );
 }
 
+/** Whether a setEncoding call can reach some net.Socket in this program:
+ * `x.setEncoding(...)` or `x["setEncoding"](...)` where `x` is a socket
+ * or an untyped value. Sockets that never get an encoding keep the Buffer
+ * chunk path; with one, Node's 'data' chunks are strings. */
+function socketEncodingReachable(lowerer: Lowerer): boolean {
+  if (lowerer.socketEncodingReachable !== null) return lowerer.socketEncodingReachable;
+  let reachable = false;
+  for (const file of lowerer.program.getSourceFiles()) {
+    if (file.isDeclarationFile || !file.text.includes("setEncoding")) continue;
+    ts.walkPreorder(file, (node) => {
+      if (reachable) return "skip";
+      if (!ts.isCallExpression(node)) return undefined;
+      const callee = node.expression;
+      const receiver =
+        ts.isPropertyAccessExpression(callee) && callee.name.text === "setEncoding"
+          ? callee.expression
+          : ts.isElementAccessExpression(callee) &&
+              ts.isStringLiteralLike(callee.argumentExpression) &&
+              callee.argumentExpression.text === "setEncoding"
+            ? callee.expression
+            : null;
+      if (receiver === null) return undefined;
+      const kind = lowerer.mapTypeOf(lowerer.checker.getTypeAtLocation(receiver))?.kind;
+      if (kind === undefined || kind === "netSocket" || kind === "dyn") reachable = true;
+      return undefined;
+    });
+    if (reachable) break;
+  }
+  lowerer.socketEncodingReachable = reachable;
+  return reachable;
+}
+
+/** An inline 'data' listener whose chunk parameter is typed Buffer, for a
+ * socket that may have an encoding set: the chunk binds as a checked
+ * dynamic value, so the body sees what Node passes (a string after
+ * setEncoding, a Buffer otherwise) whatever the annotation says. Null
+ * for other listener shapes. */
+function lowerEncodedDataListener(lowerer: Lowerer, node: ts.Expression): IrExpr | null {
+  let fn = node;
+  while (ts.isParenthesizedExpression(fn)) fn = fn.expression;
+  if (!ts.isArrowFunction(fn) && !ts.isFunctionExpression(fn)) return null;
+  if (fn.parameters.length !== 1) return null;
+  const param = fn.parameters[0]!;
+  if (!ts.isIdentifier(param.name) || param.initializer || param.dotDotDotToken) return null;
+  const symbol = lowerer.checker.getSymbolAtLocation(param.name);
+  if (symbol === undefined) return null;
+  const previousBinding = lowerer.runtimeOptionalBindingTypes.get(symbol);
+  const previousImplicit = lowerer.implicitParamTypes;
+  const implicit = new Map(previousImplicit ?? []);
+  implicit.set(symbol, lowerer.checker.getUnknownType());
+  lowerer.runtimeOptionalBindingTypes.set(symbol, DYN);
+  lowerer.implicitParamTypes = implicit;
+  try {
+    return lowerer.lowerExpr(fn);
+  } finally {
+    if (previousBinding === undefined) lowerer.runtimeOptionalBindingTypes.delete(symbol);
+    else lowerer.runtimeOptionalBindingTypes.set(symbol, previousBinding);
+    lowerer.implicitParamTypes = previousImplicit;
+  }
+}
+
 /** Method calls on net.Socket receivers: write/end/destroy/pipe and
  * on/once("data" | "end" | "close" | "error" | "connect"). Null for
  * other receivers. */
@@ -2565,6 +2626,27 @@ function lowerNetSocketMethodCall(
           loc,
         };
       }
+      if (param !== null && param.kind === "bytes" && socketEncodingReachable(lowerer)) {
+        const encoded = lowerEncodedDataListener(lowerer, args[1]!);
+        // A listener defined elsewhere keeps its Buffer parameter; it takes
+        // the chunk through the checked boundary, so a string chunk fails
+        // the check instead of arriving as a Buffer.
+        const cb: IrExpr =
+          encoded ??
+          ({
+            kind: "dynCheck",
+            value: lowerSocketDynListener(lowerer, args[1]!, "data listeners"),
+            type: funcOf([DYN], VOID),
+            loc,
+          } as IrExpr);
+        return {
+          kind: "libCall",
+          fn: "net.sockOnData",
+          args: [receiver, cb, once],
+          type: VOID,
+          loc,
+        };
+      }
       const { cb } = lowerCallbackArg(
         lowerer,
         args[1]!,
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index 61ccc76ce..d4cf82752 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -341,6 +341,7 @@ import {
 } from "./call-signatures.js";
 import {
   implicitAnyParamSymbolsOf,
+  implicitLocalFnNodeOf,
   type GenericFnInfo,
   type GenericInstance,
   implicitMonoFile,
@@ -1525,6 +1526,28 @@ function panicSafe<T>(query: () => T, fallback: T): T {
   }
 }
 
+/** The function's class expressions lower once per implicit-any instance.
+ * A function whose only implicit slots are destructured options and that
+ * is bound to a variable the implicit-instance probe rejects runs as one
+ * shared body, so its classes keep a single name. */
+function monomorphizedImplicit(
+  lowerer: Lowerer,
+  fn: ts.FunctionDeclaration | ts.FunctionExpression | ts.ArrowFunction | ts.MethodDeclaration,
+): boolean {
+  const slots = implicitAnyParamSymbolsOf(lowerer, fn);
+  if (!slots) return false;
+  if (slots.some((slot) => slot !== null)) return true;
+  let binding: ts.Node | undefined = fn.parent;
+  while (binding && ts.isParenthesizedExpression(binding)) binding = binding.parent;
+  if (
+    (ts.isArrowFunction(fn) || ts.isFunctionExpression(fn)) &&
+    binding &&
+    ts.isVariableDeclaration(binding)
+  )
+    return implicitLocalFnNodeOf(lowerer, binding) !== null;
+  return true;
+}
+
 export class Lowerer {
   readonly frontendServices: FrontendServices | undefined;
   readonly checker: ts.TypeChecker;
@@ -1769,6 +1792,10 @@ export class Lowerer {
    * typeParamBindings — the checker has no `T` to substitute, so the
    * binding rides the node-type accessor instead of mapType. */
   implicitParamTypes: Map<ts.Symbol, ts.Type> | null = null;
+  /** Whether some net.Socket in this program may get an encoding: a
+   * setEncoding call whose receiver is a socket or an untyped value.
+   * Computed once on first use (null until then). */
+  socketEncodingReachable: boolean | null = null;
   /** IMPLICIT-ANY instances lowered EAGERLY at first demand (their return
    * types are inferred from the body — the call site needs them settled),
    * collected here for run()'s function list (the liftedFns discipline). */
@@ -2787,7 +2814,7 @@ export class Lowerer {
           if (this.localClassInstantiations.some((entry) => entry.owner === parent)) break;
           if (
             parent.typeParameters?.length ||
-            (implicitMonoFile(parent.getSourceFile()) && implicitAnyParamSymbolsOf(this, parent))
+            (implicitMonoFile(parent.getSourceFile()) && monomorphizedImplicit(this, parent))
           )
             return null;
         }
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5972-net-set-encoding-listeners.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5972-net-set-encoding-listeners.ts.json
new file mode 100644
index 000000000..efab76852
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5972-net-set-encoding-listeners.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/5972-net-set-encoding-listeners.ts"
+ ],
+ "diags": []
+}
diff --git a/scripts/bench-runtime.mjs b/scripts/bench-runtime.mjs
index ff9c9d690..4e4f2c245 100644
--- a/scripts/bench-runtime.mjs
+++ b/scripts/bench-runtime.mjs
@@ -56,9 +56,10 @@ Each checkout must have a built CLI (packages/cli/dist/bootstrap.js) and native
 artifacts for this host. Without --baseline, reports absolute times only. Node is
 timed in the same interleaved loop as the reference to beat (--no-node skips it).
 
---layouts=N (macOS/Linux) links every executable N times, shifting the program
-object by 0, 16, 32, ... bytes of padding, and spreads the timed runs evenly over
-those layouts, so verdicts cover code-placement luck instead of one placement.
+--layouts=N (macOS/Linux) links every executable N times, shifting the runtime
+and program code by 0, 16, 32, ... bytes of padding, and spreads the timed runs
+evenly over those layouts, so verdicts cover code-placement luck instead of one
+placement.
 Use it for small codegen changes (e.g. --layouts=4 --runs=16).
 
 Workload names are listed in benchmarks/runtime/workloads.json; for example
@@ -103,10 +104,11 @@ const work = mkdtempSync(
 );
 
 /* ── code-layout perturbation ──────────────────────────────────────────── */
-// Layout k > 0 links a retained k*16-byte padding object directly before the
-// program object, through a SCRIPTC_LINKER wrapper that forwards to clang (the
-// default linker driver on macOS and Linux). Code after that point moves as if
-// earlier code had grown; any checkout's compiler can be perturbed this way.
+// Layout k > 0 links a retained k*16-byte padding object before every other
+// input, through a SCRIPTC_LINKER wrapper that forwards to clang (the default
+// linker driver on macOS and Linux). The runtime is linked before the program
+// object, so both move as if earlier code had grown; any checkout's compiler can
+// be perturbed this way.
 function layoutTooling() {
   if (layouts === 1) return null;
   const root = join(work, "layouts");
@@ -137,12 +139,11 @@ const args = process.argv.slice(2);
 const pad = process.env.SCRIPTC_BENCH_LAYOUT_PAD;
 let out = args;
 if (pad && !args.some((a) => a.startsWith("-print-prog-name"))) {
-  const at = args.findIndex((a) => basename(a).startsWith(".scriptc-native-program-object-"));
-  if (at < 0) {
+  if (!args.some((a) => basename(a).startsWith(".scriptc-native-program-object-"))) {
     process.stderr.write("bench layout wrapper: no program object on the link line\\n");
     process.exit(2);
   }
-  out = ["-Wl,-u,${symbol}", ...args.slice(0, at), pad, ...args.slice(at)];
+  out = ["-Wl,-u,${symbol}", pad, ...args];
 }
 const result = spawnSync("clang", out, { stdio: "inherit" });
 process.exit(result.status ?? 1);
diff --git a/tests/corpus/5972-net-set-encoding-listeners.ts b/tests/corpus/5972-net-set-encoding-listeners.ts
new file mode 100644
index 000000000..a7b49adaa
--- /dev/null
+++ b/tests/corpus/5972-net-set-encoding-listeners.ts
@@ -0,0 +1,42 @@
+// After socket.setEncoding("utf8"), every 'data' listener receives the
+// string Node passes, whether its chunk parameter is unannotated,
+// annotated Buffer, string or string | Buffer. A Buffer-only method on
+// the string throws Node's TypeError. A socket without an encoding in
+// the same program still receives Buffers.
+import * as net from "node:net";
+
+const server = net.createServer((socket) => {
+  socket.on("data", (chunk) => {
+    console.log("server chunk:", typeof chunk, chunk.length, "bytes:", chunk[0]);
+    socket.end("hello wörld");
+  });
+});
+
+server.listen(0, "127.0.0.1", () => {
+  const { port } = server.address() as net.AddressInfo;
+  const client = net.connect(port, "127.0.0.1", () => client.write("hi"));
+  client.setEncoding("utf8");
+  let text = "";
+  client.on("data", (chunk) => {
+    text += chunk;
+    console.log("unannotated:", typeof chunk, chunk.length, chunk);
+    console.log("unannotated upper:", chunk.toString().toUpperCase());
+  });
+  client.on("data", (chunk: Buffer) => {
+    console.log("annotated Buffer:", typeof chunk, chunk);
+    console.log("annotated concat:", "<" + chunk + ">");
+    try {
+      console.log(chunk.readUInt8(0));
+    } catch (error) {
+      console.log("readUInt8:", (error as Error).name, (error as Error).message);
+    }
+  });
+  client.once("data", (chunk: string) => console.log("string:", chunk.split(" ")));
+  client.on("data", (chunk: string | Buffer) =>
+    console.log("union:", typeof chunk === "string" ? "string " + chunk : "buffer"),
+  );
+  client.on("close", () => {
+    console.log("text:", text);
+    server.close();
+  });
+});
diff --git a/tests/fixtures/npm-static/class-options-cli.ts b/tests/fixtures/npm-static/class-options-cli.ts
new file mode 100644
index 000000000..9d9c4f902
--- /dev/null
+++ b/tests/fixtures/npm-static/class-options-cli.ts
@@ -0,0 +1,15 @@
+// A package factory that receives its base class through a destructured
+// options object builds subclasses of program classes that construct and
+// dispatch like Node.
+import { Class, extend } from "class-options/classes.js";
+
+class Animal extends Class("Animal") {}
+class Dog extends extend(Animal, "Dog") {}
+class Puppy extends extend(Dog, "Puppy") {}
+
+const rex = new Dog("Rex");
+const ada = new Animal("Ada");
+const bit = new Puppy("Bit");
+console.log(rex instanceof Dog, rex instanceof Animal, ada instanceof Dog, bit instanceof Animal);
+console.log(rex.describe(), ada.describe(), bit.describe());
+console.log(rex.kind, rex.value, ada.kind, bit.label);
diff --git a/tests/fixtures/npm-static/node_modules/class-options/classes.js b/tests/fixtures/npm-static/node_modules/class-options/classes.js
new file mode 100644
index 000000000..e2ae647f8
--- /dev/null
+++ b/tests/fixtures/npm-static/node_modules/class-options/classes.js
@@ -0,0 +1,31 @@
+// A class factory that takes its base and settings as one options object,
+// as Effect's makeClass does: `makeClass({ Base, name })` destructures them
+// and builds `class extends Base`.
+function makeClass({ Base, name, tag: label }) {
+  return class extends Base {
+    constructor(value) {
+      super(value);
+      this.kind = name;
+      this.label = label;
+    }
+    describe() {
+      return `${this.label}:${this.kind}:${this.value}`;
+    }
+    // Each class passes itself back as a base: a call site must not
+    // specialize makeClass on the classes makeClass itself defines.
+    static extend(name) {
+      return makeClass({ Base: this, name, tag: label });
+    }
+  };
+}
+class Root {
+  constructor(value) {
+    this.value = value;
+  }
+}
+export function Class(name) {
+  return makeClass({ Base: Root, name, tag: "root" });
+}
+export function extend(Base, name) {
+  return makeClass({ Base, name, tag: "sub" });
+}
diff --git a/tests/fixtures/npm-static/node_modules/class-options/package.json b/tests/fixtures/npm-static/node_modules/class-options/package.json
new file mode 100644
index 000000000..3e2fb2b49
--- /dev/null
+++ b/tests/fixtures/npm-static/node_modules/class-options/package.json
@@ -0,0 +1 @@
+{ "name": "class-options", "version": "1.0.0", "type": "module", "exports": { "./classes.js": "./classes.js" } }
diff --git a/tests/harness/corpus-costs.json b/tests/harness/corpus-costs.json
index cf5c31ce1..d5a11992e 100644
--- a/tests/harness/corpus-costs.json
+++ b/tests/harness/corpus-costs.json
@@ -1850,6 +1850,9 @@
     "5886-inspect-dates.ts": 2559,
     "5887-inspect-typed-arrays.ts": 3448,
     "5888-inspect-sorted.ts": 2998,
+    "5970-net-connect-host-name.ts": 2400,
+    "5971-buffer-chunks-to-string.ts": 2700,
+    "5972-net-set-encoding-listeners.ts": 2550,
     "600-closures-basic.ts": 2349,
     "601-closures-loops.ts": 1788,
     "602-closures-identity-recursion.ts": 2372,
@@ -4483,6 +4486,9 @@
     "5886-inspect-dates.ts": 2736,
     "5887-inspect-typed-arrays.ts": 2997,
     "5888-inspect-sorted.ts": 2861,
+    "5970-net-connect-host-name.ts": 2800,
+    "5971-buffer-chunks-to-string.ts": 3100,
+    "5972-net-set-encoding-listeners.ts": 3000,
     "600-closures-basic.ts": 2534,
     "601-closures-loops.ts": 2297,
     "602-closures-identity-recursion.ts": 2427,
diff --git a/tests/harness/npm-static.test.ts b/tests/harness/npm-static.test.ts
index 9324d1c7f..89bd2e0b8 100644
--- a/tests/harness/npm-static.test.ts
+++ b/tests/harness/npm-static.test.ts
@@ -618,6 +618,40 @@ describe(`npm-static pilots${sanitize ? " (sanitized)" : ""}`, () => {
     },
   );
 
+  test.each(["release", "dev"] as const)(
+    "a factory that destructures its base class from an options object builds static subclasses (%s)",
+    async (optimization) => {
+      const entry = join(pilotRoot, "class-options-cli.ts");
+      const outDir = join(cacheDir, `class-options-${optimization}-${sanitize ? "san" : "plain"}`);
+      const result = await compile(entry, {
+        backend: "llvm",
+        npmStatic: ["class-options"],
+        sanitize,
+        optimization,
+        outDir,
+        outPath: join(outDir, "program"),
+        emitIr: true,
+      });
+      if (!result.ok)
+        throw new Error(result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"));
+      const [reference, native] = await Promise.all([
+        runBinary(process.execPath, [entry]),
+        runBinary(result.binaryPath, []),
+      ]);
+      expect(reference.exitCode).toBe(0);
+      expect(native.stdout).toEqual(reference.stdout);
+      expect(comparableStderr(native.stderr)).toEqual(reference.stderr);
+      expect(native.exitCode).toBe(reference.exitCode);
+      // One makeClass instance per distinct base (Root, Animal, Dog) plus
+      // the generic one a class's own `extend` passing itself back reuses.
+      const ir = JSON.parse(readFileSync(result.irPath!, "utf8")) as {
+        functions: { name: string }[];
+      };
+      const instances = ir.functions.filter((fn) => /^%[\w.]*makeClass%\d+$/.test(fn.name));
+      expect(instances.length).toBeLessThanOrEqual(4);
+    },
+  );
+
   test.for([
     ["escape-string-regexp", "escape-cli.ts"],
     ["slash", "slash-cli.ts"],