diff --git a/.changeset/forbid-entity-subclassing.md b/.changeset/forbid-entity-subclassing.md
new file mode 100644
index 0000000..ac4c812
--- /dev/null
+++ b/.changeset/forbid-entity-subclassing.md
@@ -0,0 +1,17 @@
+---
+"@btravstack/entity": minor
+---
+
+**BREAKING**: subclassing an entity class is no longer supported.
+
+`class Sub extends SomeEntity {}` now fails at construction with a `Defect` —
+a bug in domain code, not bad caller input, so it is not an `InvalidEntity`.
+One `extends` is the declaration form and is unaffected; so is using the
+builder's return directly without `extends`.
+
+Behaviour belongs in the entity's own class body, which is unchanged: extra
+fields stay writable and are still absent from `toJSON()`.
+
+The prohibition is runtime-only. TypeScript has no `final`, so
+`class Sub extends SomeEntity {}` still compiles and reports on first
+construction.
diff --git a/CLAUDE.md b/CLAUDE.md
index 368b4b1..785dcb4 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -98,6 +98,11 @@ design — `contract.spec.ts` pins that both ways.
   updating the matching `@ts-expect-error` assertion.
 - **One concept, one name.** The surface is meant to stay small enough that the
   library can be "done". Resist convenience aliases.
+- **Entities are not subclassable.** One `extends` is the declaration form;
+  `construct` defects on anything deeper. Behaviour goes in the entity's own
+  class body. This is runtime-only — TypeScript has no `final`, and
+  `private`/`protected` constructors were measured to break the declaration
+  form (TS2675) and the statics (TS2684) respectively.
 - **No I/O.** The package reads no clock and generates no id — `create` takes
   domain-generated values as its second argument, by design.
 - `zod`, `unthrown` and `@unthrown/standard-schema` are peer dependencies to
diff --git a/README.md b/README.md
index 5669923..b6080ce 100644
--- a/README.md
+++ b/README.md
@@ -535,12 +535,12 @@ field whose schema yields a live mutable object is outside the guarantee.
 so assigning to _its_ keys is fine and mappers keep working unchanged (the
 values inside it are the entity's own, and stay frozen).
 
-`Object.freeze(this)` is **not** used, and cannot be: a subclass's field
+`Object.freeze(this)` is **not** used, and cannot be: a class body's field
 initialisers run after `super()` returns, so the instance itself has to stay
 extensible. Freezing the field values individually leaves that intact:
 
 ```ts
-class OrgWithCache extends Organization {
+class OrgWithCache extends Entity("OrgWithCache")({ id: OrgId, slug: Slug }) {
   cachedSummary = "";
 }
 const org = OrgWithCache.decode(raw).getOrThrow();
@@ -548,6 +548,38 @@ org.cachedSummary = "computed"; // ✓ still writable — it isn't declared data
 org.toJSON(); // does NOT include cachedSummary — toJSON() projects only the declared schema's keys
 ```
 
+## Entities are not subclassable
+
+One `extends` is the declaration form. Subclassing the result is not
+supported, and fails at construction with a `Defect`:
+
+```ts
+class Sub extends Organization {}
+Sub.decode(raw); // Defect — not an InvalidEntity: this is a bug in domain code
+```
+
+Put the behaviour in the entity's own class body, which is what it is for:
+
+```ts
+class Organization extends Entity("Organization")({ ...fields }) {
+  get greeting(): string {
+    return `Welcome, ${this.name}`;
+  }
+}
+```
+
+A subclass buys nothing the body does not, and it adds a second place to look
+for an entity's methods. Redeclaring a data field is doubly blocked — the
+compiler reports TS4114 (`must have an 'override' modifier`), and the field is
+installed non-writable and non-configurable, so construction fails with
+`TypeError: Cannot redefine property`.
+
+The prohibition is a **runtime** one: TypeScript has no `final`, and a
+`private`/`protected` constructor cannot express "extendable once" — measured,
+`TS2675` for `private` (the declaration form stops compiling) and `TS2684` for
+`protected` (the statics stop returning the subclass). So `class Sub extends
+Organization {}` compiles, and reports on first construction.
+
 ## Helper types
 
 Four generic type-level helpers name each schema by reading it off an entity
diff --git a/packages/entity/README.md b/packages/entity/README.md
index 78c2812..570e462 100644
--- a/packages/entity/README.md
+++ b/packages/entity/README.md
@@ -98,16 +98,14 @@ test in `contract.spec.ts` pins that.
   re-running the invariants; immutable fields are dropped even if smuggled
   in at runtime past the type check
 - `toJSON()` — the stored data, projected to exactly the `decoded` schema's
-  keys, even from a subclass with extra fields. This is the **only** public
+  keys, even when the class body declares extra fields. This is the **only** public
   projection: it is the hook `JSON.stringify` looks for, so it has to exist,
   and a second method returning the same value under a domain name would be
   the alias this package resists. A repository write is
   `db.insert(org.toJSON())`
 - `equals(other)` — true when both are the same entity type and their
-  stored data is deep-equal. "Same entity" means the entity the class was
-  built from, not the class itself: two subclasses of a single `Entity(...)`
-  call compare equal when their stored data matches, while two separate
-  `Entity(...)` calls never do, even with identical fields
+  stored data is deep-equal. Two separate `Entity(...)` calls never compare
+  equal, even with identical fields
 
 ## Computed fields: `add`
 
diff --git a/packages/entity/src/entity.spec.ts b/packages/entity/src/entity.spec.ts
index d02f9a0..544d497 100644
--- a/packages/entity/src/entity.spec.ts
+++ b/packages/entity/src/entity.spec.ts
@@ -131,23 +131,44 @@ test("data fields are locked against mutation at runtime", () => {
   expect(org.slug).toBe("acme");
 });
 
-test("locking data fields leaves subclass instance fields writable", () => {
-  // Object.freeze(this) would break this: subclass field initialisers run
+// The entity's own class body — the supported place for behaviour and extra
+// fields, now that subclassing an entity is not.
+class OrgWithCache extends Entity("OrgWithCache")({
+  id: OrgId,
+  slug: Slug,
+  name: DisplayName,
+}) {
+  cachedSummary = "";
+}
+
+test("locking data fields leaves class-body instance fields writable", () => {
+  // Object.freeze(this) would break this: class-body field initialisers run
   // after super() returns, so the object must stay extensible.
-  class OrgWithCache extends Organization {
-    cachedSummary = "";
-  }
   const org = OrgWithCache.decode(raw).getOrThrow();
   org.cachedSummary = "computed";
   expect(org.cachedSummary).toBe("computed");
   expect(org.slug).toBe("acme");
 });
 
-test("toJSON does not leak subclass instance fields", () => {
-  class OrgWithCache extends Organization {
-    cachedSummary = "leak me";
-  }
-  expect(OrgWithCache.decode(raw).getOrThrow().toJSON()).not.toHaveProperty("cachedSummary");
+test("toJSON does not leak class-body instance fields", () => {
+  const org = OrgWithCache.decode(raw).getOrThrow();
+  org.cachedSummary = "leak me";
+  expect(org.toJSON()).not.toHaveProperty("cachedSummary");
+});
+
+test("subclassing an entity is a defect, not a silent success", () => {
+  class Sub extends Organization {}
+  const outcome = Sub.decode(raw).match({
+    ok: () => "WRONGLY ACCEPTED",
+    errCases: (m) => m.with(P.tag("InvalidEntity"), () => "invalid"),
+    defect: () => "defect",
+  });
+  expect(outcome).toBe("defect");
+});
+
+test("using the builder's return directly, without extends, still works", () => {
+  const Anon = Entity("Anon")({ id: OrgId, slug: Slug, name: DisplayName });
+  expect(Anon.decode(raw).getOrThrow().slug).toBe("acme");
 });
 
 const Instant = z.iso.datetime().brand("Instant");
diff --git a/packages/entity/src/entity.ts b/packages/entity/src/entity.ts
index 42b152b..113940c 100644
--- a/packages/entity/src/entity.ts
+++ b/packages/entity/src/entity.ts
@@ -1,5 +1,5 @@
 import { fromSchema, type SchemaIssues } from "@unthrown/standard-schema";
-import { Err, Ok, P, type Result } from "unthrown";
+import { Err, Ok, P, fromThrowable, type Result } from "unthrown";
 import type { z } from "zod";
 
 import type { AddSpec } from "./add.js";
@@ -147,9 +147,27 @@ export function Entity<Tag extends string>(tag: Tag) {
     ): Result<T, InvalidEntity> => {
       const broken = invariants?.(d) ?? [];
       // no `path` — an invariant spans the entity, not one field
-      return broken.length > 0
-        ? Err(new InvalidEntity({ entity: tag, issues: broken.map((message) => ({ message })) }))
-        : Ok(new Ctor(d as Sealed<DecodedShape>));
+      if (broken.length > 0) {
+        return Err(
+          new InvalidEntity({ entity: tag, issues: broken.map((message) => ({ message })) }),
+        );
+      }
+      // A defect, not an `InvalidEntity`: subclassing is a bug in domain code,
+      // not bad caller input. `fromThrowable` is what keeps it inside the
+      // Result channel — a bare throw would escape `decode()` entirely.
+      return fromThrowable(
+        () => {
+          const ctor = Ctor as unknown as object;
+          if (ctor !== Base && (Object.getPrototypeOf(ctor) as unknown) !== Base) {
+            // oxlint-disable-next-line unthrown/no-throw
+            throw new Error(
+              `${tag}: subclassing an entity class is not supported — put the behaviour in the entity's own class body.`,
+            );
+          }
+          return new Ctor(d as Sealed<DecodedShape>);
+        },
+        (cause, defect) => defect(cause),
+      )() as Result<T, InvalidEntity>;
     };
 
     class Base {
diff --git a/packages/entity/src/equality.spec.ts b/packages/entity/src/equality.spec.ts
index ab6d129..1c06dce 100644
--- a/packages/entity/src/equality.spec.ts
+++ b/packages/entity/src/equality.spec.ts
@@ -41,14 +41,6 @@ test("different entity types with identical data are unequal", () => {
   expect(Organization.decode(raw).getOrThrow().equals(Team.decode(raw).getOrThrow())).toBe(false);
 });
 
-test("sibling subclasses of one entity with the same data are equal", () => {
-  // Identity is the entity a class was built from, not the class itself: both
-  // sides are still an `Organization`, so equal stored data means equal entity.
-  class Vendor extends Organization {}
-  class Customer extends Organization {}
-  expect(Vendor.decode(raw).getOrThrow().equals(Customer.decode(raw).getOrThrow())).toBe(true);
-});
-
 test("comparing against a non-entity is false, not a throw", () => {
   const org = Organization.decode(raw).getOrThrow();
   expect(org.equals(raw)).toBe(false);