diff --git a/test-files/test_harness_class_mixins.ts b/test-files/test_harness_class_mixins.ts
new file mode 100644
index 00000000..0da2c7d9
--- /dev/null
+++ b/test-files/test_harness_class_mixins.ts
@@ -0,0 +1,189 @@
+// #806 — systematic harness for `class X extends Fn(...)<...>` patterns.
+//
+// Effect's TaggedError, Schema.Class, and similar factory-shaped bases
+// exercise a corner of TS+JS that historically broke perry in subtle
+// ways (e.g. #740 captured-factory + #809 cross-module spread). This
+// file covers the family with small, independent assertions, each
+// printing a stable line so the parity runner can diff vs Node.
+//
+// All assertions print a "section: result" line so a single grep tells
+// you which sub-case regressed.
+
+// ── 1. Bare factory: `class X extends Fn()` ────────────────────────────────
+// The simplest dynamic-extends shape: a 0-arg factory returns a class,
+// the user subclasses the result. Caught the #740 base-shape regression.
+function makeBare() {
+  return class {
+    kind = "bare";
+    hello(): string {
+      return "Hi from Bare";
+    }
+  };
+}
+class Bare extends makeBare() {
+  extra = 7;
+}
+const bare = new Bare();
+console.log("bare.kind:", bare.kind);
+console.log("bare.hello:", bare.hello());
+console.log("bare.extra:", bare.extra);
+
+// ── 2. Parameterized factory: `class X extends Fn("tag", schema)` ──────────
+// Effect's `Schema.Class` / `TaggedError` shape. The factory takes
+// runtime args that get baked into the produced class's prototype.
+function tagged(tag: string, fields: Record<string, string>) {
+  return class {
+    readonly _tag = tag;
+    readonly _fields = fields;
+  };
+}
+class MyTagged extends tagged("MyTag", { a: "string", b: "number" }) {
+  // Subclasses freely add fields/methods. The parameter-baked
+  // properties survive the subclass step.
+}
+const t = new MyTagged();
+console.log("tagged._tag:", t._tag);
+console.log("tagged._fields.a:", t._fields.a);
+console.log("tagged._fields.b:", t._fields.b);
+
+// ── 3. Captured-factory: `class X extends F<T>()` ──────────────────────────
+// Effect's TaggedError closes around a Constructable. The factory is
+// captured-then-called — `F<T>()` evaluates the call AFTER reading the
+// generic, and Perry must preserve that. #740 root cause was the captured
+// form silently aliasing to the un-captured base.
+function makeNamed<T>(name: T) {
+  return class {
+    name: T = name;
+    greet(): string {
+      // Cast the name to a primitive for printing. The test fixes T = string
+      // so this is a no-op cast in the type system but keeps the lowering
+      // honest about T's runtime shape.
+      return `Hello, ${String(this.name)}`;
+    }
+  };
+}
+class Greeter extends makeNamed<string>("world") {
+  emphasize(): string {
+    return this.greet() + "!";
+  }
+}
+const g = new Greeter();
+console.log("captured.name:", g.name);
+console.log("captured.greet:", g.greet());
+console.log("captured.emphasize:", g.emphasize());
+
+// ── 4. Chained mixins: `class X extends M1(M2(M3(Base)))` ──────────────────
+// Layered mixin pattern from older TypeScript codebases (sequelize-typescript,
+// older Nest, etc.). Each layer wraps the prior class and adds a method.
+class CoreBase {
+  core(): string {
+    return "core";
+  }
+}
+type Ctor<T = {}> = new (...args: any[]) => T;
+function WithA<TBase extends Ctor>(B: TBase) {
+  return class extends B {
+    a(): string {
+      return "a";
+    }
+  };
+}
+function WithB<TBase extends Ctor>(B: TBase) {
+  return class extends B {
+    b(): string {
+      return "b";
+    }
+  };
+}
+function WithC<TBase extends Ctor>(B: TBase) {
+  return class extends B {
+    c(): string {
+      return "c";
+    }
+  };
+}
+class Chained extends WithA(WithB(WithC(CoreBase))) {}
+const chained = new Chained();
+console.log("chained.core:", chained.core());
+console.log("chained.a:", chained.a());
+console.log("chained.b:", chained.b());
+console.log("chained.c:", chained.c());
+
+// ── 5. Mixin that calls super() with args ──────────────────────────────────
+// Constructor-arg forwarding through a mixin layer. Both Perry and Node
+// must propagate the super-constructor's side effects.
+class Logged {
+  log: string;
+  constructor(seed: string) {
+    this.log = "seed=" + seed;
+  }
+}
+function WithSuffix<TBase extends Ctor<Logged>>(B: TBase) {
+  return class extends B {
+    constructor(seed: string) {
+      super(seed);
+      this.log += ":wrapped";
+    }
+  };
+}
+class WrappedLogged extends WithSuffix(Logged) {}
+const wl = new WrappedLogged("alpha");
+console.log("super-args.log:", wl.log);
+
+// ── 6. Factory returning class expression with static method ──────────────
+// Statics on factory-produced classes must survive the subclass step
+// without losing their `this`-binding.
+function makeWithStatic() {
+  return class {
+    static prefix(s: string): string {
+      return "STATIC:" + s;
+    }
+    instance(): string {
+      return "instance";
+    }
+  };
+}
+class StaticHost extends makeWithStatic() {}
+console.log("static.prefix:", StaticHost.prefix("x"));
+console.log("static.instance:", new StaticHost().instance());
+
+// ── 7. Effect-shape: `class X extends TaggedError<X>()("name", schema)` ────
+// Hot path: Effect's TaggedError is a double-call factory — the outer
+// `TaggedError<X>` is a curried generic, and calling it with no args
+// returns the inner factory that *then* takes the tag + schema. This
+// is the precise shape #740 fixed (class-extends-factory(captured)).
+function TaggedError<_Self>() {
+  return <Tag extends string>(tag: Tag, schema: Record<string, string>) =>
+    class {
+      readonly _tag: Tag = tag;
+      readonly _schema = schema;
+      readonly cause?: unknown;
+    };
+}
+class MyError extends TaggedError<MyError>()("MyError", { code: "string" }) {
+  describe(): string {
+    return `${this._tag}(code:${this._schema.code})`;
+  }
+}
+const err = new MyError();
+console.log("effect._tag:", err._tag);
+console.log("effect._schema.code:", err._schema.code);
+console.log("effect.describe:", err.describe());
+
+// ── 8. Mixin + parameterized generic combo ─────────────────────────────────
+// The combination that #321 tracks for Effect end-to-end: a mixin that
+// is itself a parameterized factory.
+function ParamMixin<T>(seed: T) {
+  return <TBase extends Ctor>(B: TBase) =>
+    class extends B {
+      seed: T = seed;
+      describeSeed(): string {
+        return "seed=" + String(this.seed);
+      }
+    };
+}
+class Combined extends ParamMixin<number>(42)(CoreBase) {}
+const combined = new Combined();
+console.log("combo.core:", combined.core());
+console.log("combo.seed:", combined.seed);
+console.log("combo.describeSeed:", combined.describeSeed());
diff --git a/test-parity/known_failures.json b/test-parity/known_failures.json
index abe2bae2..66ea1361 100644
--- a/test-parity/known_failures.json
+++ b/test-parity/known_failures.json
@@ -20,6 +20,12 @@
     "category": "bug-open",
     "reason": "#807 harness for AsyncLocalStorage propagation through await/microtask/timer and async_hooks.createHook lifecycle. Today's Perry: sync nested run() works (sections 1, 6, 7) but the store evaporates when crossing an await boundary into a separately-named async fn (sections 2/3) and through queueMicrotask/setTimeout (sections 4/5). createHook returns undefined (section 8). Tracks #788 (AsyncLocalStorage real tracking) and #789 (async_hooks lifecycle); each section flips to PASS independently as those land."
   },
+  "test_harness_class_mixins": {
+    "issue": "806",
+    "added": "2026-05-16",
+    "category": "bug-stale",
+    "reason": "#806 harness for `class X extends Fn(...)<...>` patterns. Two sub-cases surface real Perry gaps today: (1) bare-factory subclass loses the base's field initializer (`bare.kind: undefined` vs Node's `bare.kind: bare`); (2) chained mixin `WithA(WithB(WithC(Base)))` throws `TypeError: value is not a function` at construction. Captured-factory + Effect double-call shapes pass post-#740 — the harness verifies that survives. Linked to #321 umbrella; will flip to PASS as each sub-case lands."
+  },
   "test_gap_array_methods": {
     "issue": null,
     "added": "2026-05-15",