diff --git a/packages/chess/src/modifiers/descriptors/hp-bonus.test.ts b/packages/chess/src/modifiers/descriptors/hp-bonus.test.ts
new file mode 100644
index 0000000..6f6079e
--- /dev/null
+++ b/packages/chess/src/modifiers/descriptors/hp-bonus.test.ts
@@ -0,0 +1,58 @@
+import { describe, it, expect } from "vitest";
+import { Session } from "@paratype/rete";
+import { MODIFIER_REGISTRY } from "../registry.js";
+import "./hp-bonus.js";
+import { HP_BONUS_DESCRIPTOR } from "./hp-bonus.js";
+
+describe("hp-bonus descriptor — registry", () => {
+  it("registers in MODIFIER_REGISTRY under key 'hp-bonus'", () => {
+    expect(MODIFIER_REGISTRY.has("hp-bonus")).toBe(true);
+    expect(MODIFIER_REGISTRY.get("hp-bonus")).toBe(HP_BONUS_DESCRIPTOR);
+  });
+});
+
+describe("hp-bonus descriptor — describe()", () => {
+  it("formats positive values with a leading '+'", () => {
+    expect(HP_BONUS_DESCRIPTOR.describe(3)).toBe("HP +3");
+  });
+
+  it("formats negative values without an extra sign", () => {
+    expect(HP_BONUS_DESCRIPTOR.describe(-1)).toBe("HP -1");
+  });
+
+  it("formats zero as '+'", () => {
+    expect(HP_BONUS_DESCRIPTOR.describe(0)).toBe("HP +0");
+  });
+});
+
+describe("hp-bonus descriptor — apply()", () => {
+  it("seeds HpBonus fact on piece entity in session", () => {
+    const session = new Session();
+    const pieceId = session.nextId();
+    // Seed a PieceType fact so it's a "real" piece entity.
+    session.insert(pieceId, "PieceType", "pawn");
+
+    HP_BONUS_DESCRIPTOR.apply(session, pieceId, 3);
+
+    expect(session.contains(pieceId, "HpBonus")).toBe(true);
+    expect(session.get(pieceId, "HpBonus")).toBe(3);
+  });
+});
+
+describe("hp-bonus descriptor — valueSchema", () => {
+  const schema = HP_BONUS_DESCRIPTOR.valueSchema;
+
+  it("accepts values within range [-10, 10]", () => {
+    expect(() => schema.parse(10)).not.toThrow();
+    expect(() => schema.parse(-10)).not.toThrow();
+    expect(() => schema.parse(0)).not.toThrow();
+  });
+
+  it("rejects values above 10", () => {
+    expect(() => schema.parse(11)).toThrow();
+  });
+
+  it("rejects non-integer values", () => {
+    expect(() => schema.parse(1.5)).toThrow();
+  });
+});
diff --git a/packages/chess/src/modifiers/descriptors/hp-bonus.ts b/packages/chess/src/modifiers/descriptors/hp-bonus.ts
new file mode 100644
index 0000000..5ccffee
--- /dev/null
+++ b/packages/chess/src/modifiers/descriptors/hp-bonus.ts
@@ -0,0 +1,25 @@
+import { z } from "zod";
+import type { Session, EntityId } from "@paratype/rete";
+import { MODIFIER_REGISTRY } from "../registry.js";
+import type { ModifierDescriptor } from "../types.js";
+
+const schema = z.number().int().min(-10).max(10);
+type Value = z.infer<typeof schema>;
+
+const descriptor: ModifierDescriptor<Value> = {
+  id: "hp-bonus",
+  attrName: "HpBonus",
+  label: "HP Bonus",
+  valueSchema: schema,
+  stackingRule: "additive",
+  uiForm: "number",
+  apply(session: Session, pieceId: EntityId, effectiveValue: Value): void {
+    session.insert(pieceId, "HpBonus", effectiveValue);
+  },
+  describe(value: Value): string {
+    return `HP ${value >= 0 ? "+" : ""}${value}`;
+  },
+};
+
+MODIFIER_REGISTRY.register(descriptor);
+export { descriptor as HP_BONUS_DESCRIPTOR };
diff --git a/packages/chess/src/modifiers/registry.ts b/packages/chess/src/modifiers/registry.ts
index b7aeeb1..cc9571e 100644
--- a/packages/chess/src/modifiers/registry.ts
+++ b/packages/chess/src/modifiers/registry.ts
@@ -28,15 +28,25 @@ class ModifierRegistryClass {
    * taken — defensive check because silently overwriting would create
    * confusing debugging ("which of my two hp-bonus descriptors won?"
    * races on module import order).
+   *
+   * Generic over V so descriptors with concrete value types
+   * (e.g. `ModifierDescriptor<number>`) can be passed without a
+   * type assertion at the call site. The registry stores them as
+   * `ModifierDescriptor<unknown>` — a heterogeneous container that
+   * loses V intentionally; callers that need the typed value should
+   * import the descriptor directly from its own module.
    */
-  register(descriptor: ModifierDescriptor): void {
+  register<V>(descriptor: ModifierDescriptor<V>): void {
     if (this.#byId.has(descriptor.id)) {
       throw new Error(
         `ModifierRegistry: duplicate modifier id "${descriptor.id}". ` +
           `Each modifier descriptor must have a unique id.`,
       );
     }
-    this.#byId.set(descriptor.id, descriptor);
+    // Widen to the internal unknown-typed representation. The double-cast
+    // through `unknown` is intentional: the registry is a heterogeneous
+    // store and V is intentionally erased at storage time.
+    this.#byId.set(descriptor.id, descriptor as unknown as ModifierDescriptor);
   }
 
   /**
diff --git a/packages/chess/src/modifiers/types.ts b/packages/chess/src/modifiers/types.ts
index ffdeb0c..d668135 100644
--- a/packages/chess/src/modifiers/types.ts
+++ b/packages/chess/src/modifiers/types.ts
@@ -80,7 +80,7 @@ export interface ModifierProfile {
   readonly name: string;
   readonly description: string;
   /** Layout this profile's per-instance modifiers are bound to. Optional. */
-  readonly layoutId?: string;
+  readonly layoutId?: string | undefined;
   readonly perType: readonly TypeModifier[];
   readonly perInstance: readonly InstanceModifier[];
   readonly version: 1;