diff --git a/packages/chess/src/modifiers/custom/schema.test.ts b/packages/chess/src/modifiers/custom/schema.test.ts
new file mode 100644
index 0000000..6748cc7
--- /dev/null
+++ b/packages/chess/src/modifiers/custom/schema.test.ts
@@ -0,0 +1,147 @@
+import { describe, expect, it } from "vitest";
+import {
+  EffectPrimitiveNodeSchema,
+  parseCustomModifierDescriptor,
+  safeParseCustomModifierDescriptor,
+  serializeCustomModifierDescriptor,
+} from "./schema.js";
+import { asCustomModifierId, type CustomModifierDescriptor } from "./types.js";
+
+function validDescriptor(
+  overrides: Partial<CustomModifierDescriptor> = {},
+): CustomModifierDescriptor {
+  return {
+    type: "data",
+    id: asCustomModifierId("custom:test"),
+    name: "Test",
+    description: "",
+    version: 1,
+    primitives: [],
+    targetAttrs: [],
+    uiForm: "primitive-composer",
+    source: "custom",
+    ...overrides,
+  };
+}
+
+describe("EffectPrimitiveNodeSchema", () => {
+  it("accepts a leaf node", () => {
+    const node = { kind: "seed-attribute", params: { attr: "Hp", value: 5 } };
+    expect(EffectPrimitiveNodeSchema.parse(node)).toEqual(node);
+  });
+
+  it("accepts a nested-tree node (params holds more nodes)", () => {
+    const nested = {
+      kind: "on-turn-start",
+      params: {
+        primitives: [
+          { kind: "add-to-attribute", params: { attr: "Hp", delta: 1 } },
+        ],
+      },
+    };
+    expect(EffectPrimitiveNodeSchema.parse(nested)).toEqual(nested);
+  });
+
+  it("rejects a node missing the kind field", () => {
+    expect(() =>
+      EffectPrimitiveNodeSchema.parse({ params: {} }),
+    ).toThrow();
+  });
+
+  it("rejects a node with an empty kind string", () => {
+    expect(() =>
+      EffectPrimitiveNodeSchema.parse({ kind: "", params: {} }),
+    ).toThrow();
+  });
+});
+
+describe("CustomModifierDescriptorSchema — happy path", () => {
+  it("round-trips a minimal valid descriptor", () => {
+    const desc = validDescriptor();
+    const parsed = parseCustomModifierDescriptor(desc);
+    expect(parsed).toEqual(desc);
+  });
+
+  it("round-trips a descriptor with primitives + author + createdAt", () => {
+    const desc = validDescriptor({
+      name: "Shield",
+      description: "Absorbs damage.",
+      primitives: [
+        { kind: "seed-attribute", params: { attr: "ShieldCharges", value: 3 } },
+      ],
+      targetAttrs: ["AbsorbDamageAttr", "AbsorbDamageRate"],
+      author: "alice",
+      createdAt: 1700000000000,
+    });
+    const parsed = parseCustomModifierDescriptor(desc);
+    expect(parsed.author).toBe("alice");
+    expect(parsed.createdAt).toBe(1700000000000);
+  });
+
+  it("safeParse returns success on a valid descriptor", () => {
+    const result = safeParseCustomModifierDescriptor(validDescriptor());
+    expect(result.success).toBe(true);
+  });
+
+  it("serialize round-trips the same shape", () => {
+    const desc = validDescriptor({ name: "Roundtrip" });
+    const out = serializeCustomModifierDescriptor(desc) as CustomModifierDescriptor;
+    expect(out).toEqual(desc);
+  });
+});
+
+describe("CustomModifierDescriptorSchema — rejections", () => {
+  it("rejects type !== 'data'", () => {
+    const bad = { ...validDescriptor(), type: "scripted" };
+    expect(() => parseCustomModifierDescriptor(bad)).toThrow();
+  });
+
+  it("rejects version !== 1", () => {
+    const bad = { ...validDescriptor(), version: 2 };
+    expect(() => parseCustomModifierDescriptor(bad)).toThrow();
+  });
+
+  it("rejects name longer than 40 chars", () => {
+    const bad = validDescriptor({ name: "x".repeat(41) });
+    expect(() => parseCustomModifierDescriptor(bad)).toThrow();
+  });
+
+  it("rejects description longer than 200 chars", () => {
+    const bad = validDescriptor({ description: "x".repeat(201) });
+    expect(() => parseCustomModifierDescriptor(bad)).toThrow();
+  });
+
+  it("rejects empty id", () => {
+    const bad = { ...validDescriptor(), id: "" };
+    expect(() => parseCustomModifierDescriptor(bad)).toThrow();
+  });
+
+  it("rejects negative createdAt", () => {
+    const bad = validDescriptor({ createdAt: -1 });
+    expect(() => parseCustomModifierDescriptor(bad)).toThrow();
+  });
+
+  it("rejects uiForm !== 'primitive-composer'", () => {
+    const bad = { ...validDescriptor(), uiForm: "number" };
+    expect(() => parseCustomModifierDescriptor(bad)).toThrow();
+  });
+
+  it("rejects source !== 'custom'", () => {
+    const bad = { ...validDescriptor(), source: "premade" };
+    expect(() => parseCustomModifierDescriptor(bad)).toThrow();
+  });
+
+  it("rejects when primitives is missing", () => {
+    const { primitives: _drop, ...rest } = validDescriptor();
+    expect(() => parseCustomModifierDescriptor(rest)).toThrow();
+  });
+});
+
+describe("CustomModifierDescriptorSchema — optional fields", () => {
+  it("accepts a descriptor without author / createdAt", () => {
+    const desc = validDescriptor();
+    const parsed = parseCustomModifierDescriptor(desc);
+    expect(parsed.author).toBeUndefined();
+    expect(parsed.createdAt).toBeUndefined();
+  });
+});
diff --git a/packages/chess/src/modifiers/custom/schema.ts b/packages/chess/src/modifiers/custom/schema.ts
new file mode 100644
index 0000000..3e899ef
--- /dev/null
+++ b/packages/chess/src/modifiers/custom/schema.ts
@@ -0,0 +1,94 @@
+/**
+ * Zod schemas for CustomModifierDescriptor serialization (T20).
+ *
+ * Validates STRUCTURE only — kind-in-registry and per-primitive params
+ * checks live in `validate.ts` (T19), which is the semantic layer that
+ * runs after a clean structural parse.
+ *
+ * The `EffectPrimitiveNode` schema is recursive: a primitive's `params`
+ * may embed more nodes (e.g. `on-turn-start` carries a `primitives: []`
+ * inside its params). We model this with `z.lazy()` and treat `params`
+ * as `z.unknown()` at the schema layer — the validator drills into it
+ * per-kind.
+ */
+import { z } from "zod";
+import {
+  asCustomModifierId,
+  type CustomModifierDescriptor,
+} from "./types.js";
+
+// ---------------------------------------------------------------------------
+// Primitive node — recursive
+// ---------------------------------------------------------------------------
+
+/**
+ * Structural shape of a primitive node. `kind` is `string` here (not the
+ * `PrimitiveKind` literal union) because the schema is structure-only;
+ * the validator (T19) checks `kind ∈ PRIMITIVE_REGISTRY`. `params` is
+ * `unknown` so nested trees pass through this schema cleanly.
+ *
+ * Inferred output: `{ kind: string; params: unknown }`. Consumers that
+ * want the branded `PrimitiveKind` shape go through `parseCustomModifierDescriptor`
+ * which performs the narrowing at the boundary.
+ */
+export const EffectPrimitiveNodeSchema = z.lazy(() =>
+  z.object({
+    kind: z.string().min(1),
+    params: z.unknown(),
+  }),
+);
+
+// ---------------------------------------------------------------------------
+// CustomModifierDescriptor
+// ---------------------------------------------------------------------------
+
+const CustomModifierIdSchema = z
+  .string()
+  .min(1)
+  .transform((s) => asCustomModifierId(s));
+
+export const CustomModifierDescriptorSchema = z.object({
+  type: z.literal("data"),
+  id: CustomModifierIdSchema,
+  name: z.string().min(1).max(40),
+  description: z.string().max(200),
+  version: z.literal(1),
+  primitives: z.array(EffectPrimitiveNodeSchema),
+  targetAttrs: z.array(z.string()),
+  uiForm: z.literal("primitive-composer"),
+  source: z.literal("custom"),
+  author: z.string().optional(),
+  createdAt: z.number().int().nonnegative().optional(),
+});
+
+// ---------------------------------------------------------------------------
+// Public API
+// ---------------------------------------------------------------------------
+
+/**
+ * Parse an unknown value as a CustomModifierDescriptor. Throws ZodError.
+ *
+ * The inferred Zod output type is structurally compatible but Zod's
+ * representation of optional fields and `string`-typed `kind` doesn't
+ * exactly match the `CustomModifierDescriptor` interface (which has
+ * `kind: PrimitiveKind` and uses `?` not `| undefined`). The single
+ * boundary cast below is the pragmatic way to bridge this — every
+ * narrowing it implies is enforced separately by T19's validator.
+ */
+export function parseCustomModifierDescriptor(
+  raw: unknown,
+): CustomModifierDescriptor {
+  return CustomModifierDescriptorSchema.parse(raw) as CustomModifierDescriptor;
+}
+
+/** Non-throwing variant, returns Zod's discriminated SafeParseReturnType. */
+export function safeParseCustomModifierDescriptor(raw: unknown) {
+  return CustomModifierDescriptorSchema.safeParse(raw);
+}
+
+/** Serialize a CustomModifierDescriptor to a JSON-safe plain object. */
+export function serializeCustomModifierDescriptor(
+  descriptor: CustomModifierDescriptor,
+): unknown {
+  return CustomModifierDescriptorSchema.parse(descriptor);
+}