diff --git a/packages/chess/src/modifiers/primitives/add-aura.test.ts b/packages/chess/src/modifiers/primitives/add-aura.test.ts
new file mode 100644
index 0000000..58d889b
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/add-aura.test.ts
@@ -0,0 +1,87 @@
+import { Session } from "@paratype/rete";
+import { describe, expect, it } from "vitest";
+import { ChessEngine } from "../../engine.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import { ADD_AURA_PRIMITIVE } from "./add-aura.js";
+import type { PrimitiveApplyContext } from "./types.js";
+import "./add-aura.js";
+
+function makeContext(): { ctx: PrimitiveApplyContext; session: Session } {
+  const session = new Session();
+  const pieceId = session.nextId();
+  const ctx: PrimitiveApplyContext = {
+    engine: new ChessEngine(),
+    session,
+    pieceId,
+    depth: 0,
+    descriptor: {
+      id: "custom:test-add-aura",
+      type: "data",
+      version: 1,
+    },
+  };
+  return { ctx, session };
+}
+
+describe("add-aura primitive — registry", () => {
+  it("registers in PRIMITIVE_REGISTRY under key 'add-aura'", () => {
+    expect(PRIMITIVE_REGISTRY.has("add-aura")).toBe(true);
+    expect(PRIMITIVE_REGISTRY.get("add-aura")).toBe(ADD_AURA_PRIMITIVE);
+  });
+});
+
+describe("add-aura primitive — apply()", () => {
+  it("seeds one aura spec entry", () => {
+    const { ctx, session } = makeContext();
+
+    ADD_AURA_PRIMITIVE.apply(ctx, {
+      radius: 2,
+      targetAttr: "HpBonus",
+      delta: 1,
+    });
+
+    expect(session.get(ctx.pieceId, "AuraSpec")).toEqual([
+      { radius: 2, targetAttr: "HpBonus", delta: 1 },
+    ]);
+  });
+
+  it("composes multiple aura specs into an array", () => {
+    const { ctx, session } = makeContext();
+
+    ADD_AURA_PRIMITIVE.apply(ctx, {
+      radius: 1,
+      targetAttr: "RangeBonus",
+      delta: 2,
+    });
+    ADD_AURA_PRIMITIVE.apply(ctx, {
+      radius: 3,
+      targetAttr: "DamageResistance",
+      delta: -0.25,
+    });
+
+    expect(session.get(ctx.pieceId, "AuraSpec")).toEqual([
+      { radius: 1, targetAttr: "RangeBonus", delta: 2 },
+      { radius: 3, targetAttr: "DamageResistance", delta: -0.25 },
+    ]);
+  });
+});
+
+describe("add-aura primitive — paramsSchema", () => {
+  it("rejects radii outside the [1, 7] range", () => {
+    expect(() =>
+      ADD_AURA_PRIMITIVE.paramsSchema.parse({
+        radius: 0,
+        targetAttr: "HpBonus",
+        delta: 1,
+      }),
+    ).toThrow();
+
+    expect(() =>
+      ADD_AURA_PRIMITIVE.paramsSchema.parse({
+        radius: 8,
+        targetAttr: "HpBonus",
+        delta: 1,
+      }),
+    ).toThrow();
+  });
+});
diff --git a/packages/chess/src/modifiers/primitives/add-aura.ts b/packages/chess/src/modifiers/primitives/add-aura.ts
new file mode 100644
index 0000000..5962c8e
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/add-aura.ts
@@ -0,0 +1,37 @@
+import { z } from "zod";
+import type { ChessAttrMap } from "../../schema.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { EffectPrimitive, PrimitiveApplyContext } from "./types.js";
+
+const schema = z.object({
+  radius: z.number().int().min(1).max(7),
+  targetAttr: z.string(),
+  delta: z.number(),
+});
+type Params = z.infer<typeof schema>;
+
+const descriptor: EffectPrimitive<Params> = {
+  kind: "add-aura",
+  label: "Add Aura",
+  description:
+    "Seeds an AuraSpec fact entry consumed by the aura engine's recomputation phase.",
+  paramsSchema: schema,
+  apply(ctx: PrimitiveApplyContext, params: Params): void {
+    const existing =
+      (ctx.session.get(ctx.pieceId, "AuraSpec") as
+        | ChessAttrMap["AuraSpec"]
+        | undefined) ?? [];
+
+    ctx.session.insert(ctx.pieceId, "AuraSpec", [
+      ...existing,
+      {
+        radius: params.radius,
+        targetAttr: params.targetAttr,
+        delta: params.delta,
+      },
+    ]);
+  },
+};
+
+PRIMITIVE_REGISTRY.register(descriptor);
+export { descriptor as ADD_AURA_PRIMITIVE };
diff --git a/packages/chess/src/modifiers/primitives/add-direction.test.ts b/packages/chess/src/modifiers/primitives/add-direction.test.ts
new file mode 100644
index 0000000..d9c6573
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/add-direction.test.ts
@@ -0,0 +1,109 @@
+import { describe, expect, it } from "vitest";
+import { Session } from "@paratype/rete";
+import { ChessEngine } from "../../engine.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { PrimitiveApplyContext } from "./types.js";
+import "./add-direction.js";
+import { ADD_DIRECTION_PRIMITIVE } from "./add-direction.js";
+
+function makeContext(): { ctx: PrimitiveApplyContext; session: Session } {
+  const session = new Session();
+  const pieceId = session.nextId();
+  const ctx: PrimitiveApplyContext = {
+    engine: new ChessEngine(),
+    session,
+    pieceId,
+    depth: 0,
+    descriptor: {
+      id: "custom:test-add-direction",
+      type: "data",
+      version: 1,
+    },
+  };
+  return { ctx, session };
+}
+
+describe("add-direction primitive — registry", () => {
+  it("registers in PRIMITIVE_REGISTRY under key 'add-direction'", () => {
+    expect(PRIMITIVE_REGISTRY.has("add-direction")).toBe(true);
+    expect(PRIMITIVE_REGISTRY.get("add-direction")).toBe(ADD_DIRECTION_PRIMITIVE);
+  });
+});
+
+describe("add-direction primitive — apply()", () => {
+  it("adds one direction to empty state", () => {
+    const { ctx, session } = makeContext();
+
+    ADD_DIRECTION_PRIMITIVE.apply(ctx, { directions: ["forward"] });
+
+    expect(session.get(ctx.pieceId, "DirectionAdditions")).toEqual(["forward"]);
+  });
+
+  it("concatenates with existing directions", () => {
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "DirectionAdditions", ["right"]);
+
+    ADD_DIRECTION_PRIMITIVE.apply(ctx, { directions: ["left"] });
+
+    expect(session.get(ctx.pieceId, "DirectionAdditions")).toEqual([
+      "right",
+      "left",
+    ]);
+  });
+
+  it("deduplicates overlapping directions by name", () => {
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "DirectionAdditions", ["forward"]);
+
+    ADD_DIRECTION_PRIMITIVE.apply(ctx, {
+      directions: ["forward", "diagonal-fr"],
+    });
+
+    expect(session.get(ctx.pieceId, "DirectionAdditions")).toEqual([
+      "forward",
+      "diagonal-fr",
+    ]);
+  });
+
+  it("composes additively with the T1 direction-additions descriptor's writes", () => {
+    // The T1 modifier writes `string[]` of named directions. This primitive
+    // must read+merge that exact shape so the engine's existing
+    // `generateDirectionMoves` walker handles both contributions.
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "DirectionAdditions", ["forward", "left"]);
+
+    ADD_DIRECTION_PRIMITIVE.apply(ctx, {
+      directions: ["right", "diagonal-fl"],
+    });
+
+    expect(session.get(ctx.pieceId, "DirectionAdditions")).toEqual([
+      "forward",
+      "left",
+      "right",
+      "diagonal-fl",
+    ]);
+  });
+
+  it("rejects unknown direction names at the schema layer", () => {
+    const parsed = ADD_DIRECTION_PRIMITIVE.paramsSchema.safeParse({
+      directions: ["northwest"],
+    });
+    expect(parsed.success).toBe(false);
+  });
+
+  it("throws when the existing DirectionAdditions fact has a wrong shape", () => {
+    // If a stale or upstream-buggy fact seeds DirectionAdditions with a
+    // non-string entry, the primitive must fail loudly rather than silently
+    // mixing two incompatible shapes into the same array. We bypass the
+    // chess-typed insert by going through the underlying rete session API
+    // (which accepts `unknown` values).
+    const { ctx, session } = makeContext();
+    const factId = ctx.pieceId;
+    // Insert via the untyped rete path so we can plant a wrong-shape value
+    // without triggering the chess attr-map type check.
+    session.insert(factId, "DirectionAdditions", [{ dx: 1, dy: 0 }]);
+    expect(() => {
+      ADD_DIRECTION_PRIMITIVE.apply(ctx, { directions: ["forward"] });
+    }).toThrow();
+  });
+});
diff --git a/packages/chess/src/modifiers/primitives/add-direction.ts b/packages/chess/src/modifiers/primitives/add-direction.ts
new file mode 100644
index 0000000..7424b30
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/add-direction.ts
@@ -0,0 +1,66 @@
+import { z } from "zod";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { EffectPrimitive, PrimitiveApplyContext } from "./types.js";
+
+/**
+ * `DirectionAdditions` is the existing T1 modifier attribute (see
+ * `descriptors/direction-additions.ts`). Its value is a `readonly string[]`
+ * of named, color-relative directions interpreted by `generateDirectionMoves`.
+ * This primitive composes additively with that descriptor — appending names
+ * into the same array — so the engine's existing direction-walker handles
+ * both T1-built-in and T3-DSL contributions identically.
+ */
+const directionNameSchema = z.enum([
+  "forward",
+  "backward",
+  "left",
+  "right",
+  "diagonal-fl",
+  "diagonal-fr",
+  "diagonal-bl",
+  "diagonal-br",
+]);
+const schema = z.object({
+  directions: z.array(directionNameSchema).min(1),
+});
+
+type DirectionName = z.infer<typeof directionNameSchema>;
+type Params = z.infer<typeof schema>;
+
+function dedupe(directions: readonly DirectionName[]): DirectionName[] {
+  const seen = new Set<string>();
+  const out: DirectionName[] = [];
+  for (const d of directions) {
+    if (seen.has(d)) continue;
+    seen.add(d);
+    out.push(d);
+  }
+  return out;
+}
+
+const descriptor: EffectPrimitive<Params> = {
+  kind: "add-direction",
+  label: "Add Direction",
+  description: "Appends movement directions into DirectionAdditions with dedupe.",
+  paramsSchema: schema,
+  apply(ctx: PrimitiveApplyContext, params: Params): void {
+    const existing = ctx.session.get(ctx.pieceId, "DirectionAdditions");
+    const existingDirections = existing ?? [];
+    if (!Array.isArray(existingDirections)) {
+      throw new Error("add-direction expected DirectionAdditions to be an array");
+    }
+
+    const parsed = z.array(directionNameSchema).safeParse(existingDirections);
+    if (!parsed.success) {
+      throw new Error(
+        "add-direction expected DirectionAdditions to be a string[] of named directions",
+      );
+    }
+
+    const merged = [...parsed.data, ...params.directions];
+    ctx.session.insert(ctx.pieceId, "DirectionAdditions", dedupe(merged));
+  },
+};
+
+PRIMITIVE_REGISTRY.register(descriptor);
+export { descriptor as ADD_DIRECTION_PRIMITIVE };
diff --git a/packages/chess/src/modifiers/primitives/add-to-attribute.test.ts b/packages/chess/src/modifiers/primitives/add-to-attribute.test.ts
new file mode 100644
index 0000000..b061390
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/add-to-attribute.test.ts
@@ -0,0 +1,61 @@
+import { describe, expect, it } from "vitest";
+import { Session } from "@paratype/rete";
+import { ChessEngine } from "../../engine.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { PrimitiveApplyContext } from "./types.js";
+import "./add-to-attribute.js";
+import { ADD_TO_ATTRIBUTE_PRIMITIVE } from "./add-to-attribute.js";
+
+function makeContext(): { ctx: PrimitiveApplyContext; session: Session } {
+  const session = new Session();
+  const pieceId = session.nextId();
+  const ctx: PrimitiveApplyContext = {
+    engine: new ChessEngine(),
+    session,
+    pieceId,
+    depth: 0,
+    descriptor: {
+      id: "custom:test-add-to-attribute",
+      type: "data",
+      version: 1,
+    },
+  };
+  return { ctx, session };
+}
+
+describe("add-to-attribute primitive — registry", () => {
+  it("registers in PRIMITIVE_REGISTRY under key 'add-to-attribute'", () => {
+    expect(PRIMITIVE_REGISTRY.has("add-to-attribute")).toBe(true);
+    expect(PRIMITIVE_REGISTRY.get("add-to-attribute")).toBe(
+      ADD_TO_ATTRIBUTE_PRIMITIVE,
+    );
+  });
+});
+
+describe("add-to-attribute primitive — apply()", () => {
+  it("adds delta to an existing number", () => {
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "HpBonus", 2);
+
+    ADD_TO_ATTRIBUTE_PRIMITIVE.apply(ctx, { attr: "HpBonus", delta: 3 });
+
+    expect(session.get(ctx.pieceId, "HpBonus")).toBe(5);
+  });
+
+  it("treats an absent fact as 0", () => {
+    const { ctx, session } = makeContext();
+
+    ADD_TO_ATTRIBUTE_PRIMITIVE.apply(ctx, { attr: "RangeBonus", delta: 4 });
+
+    expect(session.get(ctx.pieceId, "RangeBonus")).toBe(4);
+  });
+
+  it("throws when existing value is non-numeric", () => {
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "PieceType", "pawn");
+
+    expect(() => {
+      ADD_TO_ATTRIBUTE_PRIMITIVE.apply(ctx, { attr: "PieceType", delta: 1 });
+    }).toThrow(/expected numeric value/);
+  });
+});
diff --git a/packages/chess/src/modifiers/primitives/add-to-attribute.ts b/packages/chess/src/modifiers/primitives/add-to-attribute.ts
new file mode 100644
index 0000000..9edbf90
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/add-to-attribute.ts
@@ -0,0 +1,29 @@
+import { z } from "zod";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { EffectPrimitive, PrimitiveApplyContext } from "./types.js";
+
+const schema = z.object({
+  attr: z.string(),
+  delta: z.number(),
+});
+type Params = z.infer<typeof schema>;
+
+const descriptor: EffectPrimitive<Params> = {
+  kind: "add-to-attribute",
+  label: "Add To Attribute",
+  description: "Adds delta to the current attribute value, treating missing as 0.",
+  paramsSchema: schema,
+  apply(ctx: PrimitiveApplyContext, params: Params): void {
+    const existing = ctx.session.get(ctx.pieceId, params.attr);
+    const baseValue = existing === undefined ? 0 : existing;
+    if (typeof baseValue !== "number" || Number.isNaN(baseValue)) {
+      throw new Error(
+        `add-to-attribute expected numeric value for attr "${params.attr}" but got ${typeof baseValue}`,
+      );
+    }
+    ctx.session.insert(ctx.pieceId, params.attr, baseValue + params.delta);
+  },
+};
+
+PRIMITIVE_REGISTRY.register(descriptor);
+export { descriptor as ADD_TO_ATTRIBUTE_PRIMITIVE };
diff --git a/packages/chess/src/modifiers/primitives/multiply-attribute.test.ts b/packages/chess/src/modifiers/primitives/multiply-attribute.test.ts
new file mode 100644
index 0000000..e9ed1a0
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/multiply-attribute.test.ts
@@ -0,0 +1,61 @@
+import { describe, expect, it } from "vitest";
+import { Session } from "@paratype/rete";
+import { ChessEngine } from "../../engine.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { PrimitiveApplyContext } from "./types.js";
+import "./multiply-attribute.js";
+import { MULTIPLY_ATTRIBUTE_PRIMITIVE } from "./multiply-attribute.js";
+
+function makeContext(): { ctx: PrimitiveApplyContext; session: Session } {
+  const session = new Session();
+  const pieceId = session.nextId();
+  const ctx: PrimitiveApplyContext = {
+    engine: new ChessEngine(),
+    session,
+    pieceId,
+    depth: 0,
+    descriptor: {
+      id: "custom:test-multiply-attribute",
+      type: "data",
+      version: 1,
+    },
+  };
+  return { ctx, session };
+}
+
+describe("multiply-attribute primitive — registry", () => {
+  it("registers in PRIMITIVE_REGISTRY under key 'multiply-attribute'", () => {
+    expect(PRIMITIVE_REGISTRY.has("multiply-attribute")).toBe(true);
+    expect(PRIMITIVE_REGISTRY.get("multiply-attribute")).toBe(
+      MULTIPLY_ATTRIBUTE_PRIMITIVE,
+    );
+  });
+});
+
+describe("multiply-attribute primitive — apply()", () => {
+  it("multiplies an existing number", () => {
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "Hp", 6);
+
+    MULTIPLY_ATTRIBUTE_PRIMITIVE.apply(ctx, { attr: "Hp", factor: 1.5 });
+
+    expect(session.get(ctx.pieceId, "Hp")).toBe(9);
+  });
+
+  it("is a no-op when the attribute is absent", () => {
+    const { ctx, session } = makeContext();
+
+    MULTIPLY_ATTRIBUTE_PRIMITIVE.apply(ctx, { attr: "RangeBonus", factor: 2 });
+
+    expect(session.get(ctx.pieceId, "RangeBonus")).toBeUndefined();
+  });
+
+  it("throws when existing value is non-numeric", () => {
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "PieceType", "rook");
+
+    expect(() => {
+      MULTIPLY_ATTRIBUTE_PRIMITIVE.apply(ctx, { attr: "PieceType", factor: 2 });
+    }).toThrow(/expected numeric value/);
+  });
+});
diff --git a/packages/chess/src/modifiers/primitives/multiply-attribute.ts b/packages/chess/src/modifiers/primitives/multiply-attribute.ts
new file mode 100644
index 0000000..53fbb6f
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/multiply-attribute.ts
@@ -0,0 +1,31 @@
+import { z } from "zod";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { EffectPrimitive, PrimitiveApplyContext } from "./types.js";
+
+const schema = z.object({
+  attr: z.string(),
+  factor: z.number(),
+});
+type Params = z.infer<typeof schema>;
+
+const descriptor: EffectPrimitive<Params> = {
+  kind: "multiply-attribute",
+  label: "Multiply Attribute",
+  description: "Multiplies an existing attribute value by the provided factor.",
+  paramsSchema: schema,
+  apply(ctx: PrimitiveApplyContext, params: Params): void {
+    const existing = ctx.session.get(ctx.pieceId, params.attr);
+    if (existing === undefined) {
+      return;
+    }
+    if (typeof existing !== "number" || Number.isNaN(existing)) {
+      throw new Error(
+        `multiply-attribute expected numeric value for attr "${params.attr}" but got ${typeof existing}`,
+      );
+    }
+    ctx.session.insert(ctx.pieceId, params.attr, existing * params.factor);
+  },
+};
+
+PRIMITIVE_REGISTRY.register(descriptor);
+export { descriptor as MULTIPLY_ATTRIBUTE_PRIMITIVE };
diff --git a/packages/chess/src/modifiers/primitives/seed-attribute.test.ts b/packages/chess/src/modifiers/primitives/seed-attribute.test.ts
new file mode 100644
index 0000000..c9b4197
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/seed-attribute.test.ts
@@ -0,0 +1,67 @@
+import { describe, expect, it } from "vitest";
+import { Session } from "@paratype/rete";
+import { ChessEngine } from "../../engine.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { PrimitiveApplyContext } from "./types.js";
+import "./seed-attribute.js";
+import { SEED_ATTRIBUTE_PRIMITIVE } from "./seed-attribute.js";
+
+function makeContext(): { ctx: PrimitiveApplyContext; session: Session } {
+  const session = new Session();
+  const pieceId = session.nextId();
+  const ctx: PrimitiveApplyContext = {
+    engine: new ChessEngine(),
+    session,
+    pieceId,
+    depth: 0,
+    descriptor: {
+      id: "custom:test-seed-attribute",
+      type: "data",
+      version: 1,
+    },
+  };
+  return { ctx, session };
+}
+
+describe("seed-attribute primitive — registry", () => {
+  it("registers in PRIMITIVE_REGISTRY under key 'seed-attribute'", () => {
+    expect(PRIMITIVE_REGISTRY.has("seed-attribute")).toBe(true);
+    expect(PRIMITIVE_REGISTRY.get("seed-attribute")).toBe(SEED_ATTRIBUTE_PRIMITIVE);
+  });
+});
+
+describe("seed-attribute primitive — apply()", () => {
+  it("writes a string attribute", () => {
+    const { ctx, session } = makeContext();
+
+    SEED_ATTRIBUTE_PRIMITIVE.apply(ctx, {
+      attr: "PieceType",
+      value: "queen",
+    });
+
+    expect(session.get(ctx.pieceId, "PieceType")).toBe("queen");
+  });
+
+  it("writes a number attribute", () => {
+    const { ctx, session } = makeContext();
+
+    SEED_ATTRIBUTE_PRIMITIVE.apply(ctx, {
+      attr: "Hp",
+      value: 12,
+    });
+
+    expect(session.get(ctx.pieceId, "Hp")).toBe(12);
+  });
+
+  it("overwrites an existing fact", () => {
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "RangeBonus", 1);
+
+    SEED_ATTRIBUTE_PRIMITIVE.apply(ctx, {
+      attr: "RangeBonus",
+      value: 4,
+    });
+
+    expect(session.get(ctx.pieceId, "RangeBonus")).toBe(4);
+  });
+});
diff --git a/packages/chess/src/modifiers/primitives/seed-attribute.ts b/packages/chess/src/modifiers/primitives/seed-attribute.ts
new file mode 100644
index 0000000..f60ac95
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/seed-attribute.ts
@@ -0,0 +1,51 @@
+import { z } from "zod";
+import type { ChessAttrKey } from "../../schema.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { EffectPrimitive, PrimitiveApplyContext } from "./types.js";
+
+const CHESS_ATTR_KEYS: ReadonlySet<ChessAttrKey> = new Set([
+  "PieceType",
+  "Color",
+  "Position",
+  "HasMoved",
+  "Turn",
+  "HalfmoveClock",
+  "FullmoveNumber",
+  "EnPassantTarget",
+  "GameStatus",
+  "Winner",
+  "Hp",
+  "PoisonedSquare",
+  "HpBonus",
+  "RangeBonus",
+  "DirectionAdditions",
+  "CaptureFlags",
+  "PromotionOverride",
+  "DamageResistance",
+]);
+
+function isChessAttrKey(attr: string): attr is ChessAttrKey {
+  return CHESS_ATTR_KEYS.has(attr as ChessAttrKey);
+}
+
+const schema = z.object({
+  attr: z.string(),
+  value: z.unknown(),
+});
+type Params = z.infer<typeof schema>;
+
+const descriptor: EffectPrimitive<Params> = {
+  kind: "seed-attribute",
+  label: "Seed Attribute",
+  description: "Seeds a fact on the target piece, overwriting existing value.",
+  paramsSchema: schema,
+  apply(ctx: PrimitiveApplyContext, params: Params): void {
+    if (!isChessAttrKey(params.attr)) {
+      return;
+    }
+    ctx.session.insert(ctx.pieceId, params.attr, params.value);
+  },
+};
+
+PRIMITIVE_REGISTRY.register(descriptor);
+export { descriptor as SEED_ATTRIBUTE_PRIMITIVE };
diff --git a/packages/chess/src/modifiers/primitives/set-capture-flag.test.ts b/packages/chess/src/modifiers/primitives/set-capture-flag.test.ts
new file mode 100644
index 0000000..0c35810
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/set-capture-flag.test.ts
@@ -0,0 +1,76 @@
+import { describe, expect, it } from "vitest";
+import { Session } from "@paratype/rete";
+import { ChessEngine } from "../../engine.js";
+import { CaptureFlag } from "../../schema.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { PrimitiveApplyContext } from "./types.js";
+import "./set-capture-flag.js";
+import { SET_CAPTURE_FLAG_PRIMITIVE } from "./set-capture-flag.js";
+
+function makeContext(): { ctx: PrimitiveApplyContext; session: Session } {
+  const session = new Session();
+  const pieceId = session.nextId();
+  const ctx: PrimitiveApplyContext = {
+    engine: new ChessEngine(),
+    session,
+    pieceId,
+    depth: 0,
+    descriptor: {
+      id: "custom:test-set-capture-flag",
+      type: "data",
+      version: 1,
+    },
+  };
+  return { ctx, session };
+}
+
+describe("set-capture-flag primitive — registry", () => {
+  it("registers in PRIMITIVE_REGISTRY under key 'set-capture-flag'", () => {
+    expect(PRIMITIVE_REGISTRY.has("set-capture-flag")).toBe(true);
+    expect(PRIMITIVE_REGISTRY.get("set-capture-flag")).toBe(
+      SET_CAPTURE_FLAG_PRIMITIVE,
+    );
+  });
+});
+
+describe("set-capture-flag primitive — apply()", () => {
+  it("sets a flag from a clean state", () => {
+    const { ctx, session } = makeContext();
+
+    SET_CAPTURE_FLAG_PRIMITIVE.apply(ctx, {
+      flag: CaptureFlag.CANNOT_BE_CAPTURED,
+    });
+
+    expect(session.get(ctx.pieceId, "CaptureFlags")).toBe(
+      CaptureFlag.CANNOT_BE_CAPTURED,
+    );
+  });
+
+  it("preserves existing flags when adding a new one", () => {
+    const { ctx, session } = makeContext();
+    session.insert(ctx.pieceId, "CaptureFlags", CaptureFlag.CAN_CAPTURE_OWN);
+
+    SET_CAPTURE_FLAG_PRIMITIVE.apply(ctx, {
+      flag: CaptureFlag.EN_PASSANT,
+    });
+
+    expect(session.get(ctx.pieceId, "CaptureFlags")).toBe(
+      CaptureFlag.CAN_CAPTURE_OWN | CaptureFlag.EN_PASSANT,
+    );
+  });
+
+  it("is idempotent when setting the same flag twice", () => {
+    const { ctx, session } = makeContext();
+
+    SET_CAPTURE_FLAG_PRIMITIVE.apply(ctx, {
+      flag: CaptureFlag.CAN_CAPTURE_OWN,
+    });
+    SET_CAPTURE_FLAG_PRIMITIVE.apply(ctx, {
+      flag: CaptureFlag.CAN_CAPTURE_OWN,
+    });
+
+    expect(session.get(ctx.pieceId, "CaptureFlags")).toBe(
+      CaptureFlag.CAN_CAPTURE_OWN,
+    );
+  });
+});
diff --git a/packages/chess/src/modifiers/primitives/set-capture-flag.ts b/packages/chess/src/modifiers/primitives/set-capture-flag.ts
new file mode 100644
index 0000000..53e9704
--- /dev/null
+++ b/packages/chess/src/modifiers/primitives/set-capture-flag.ts
@@ -0,0 +1,36 @@
+import { z } from "zod";
+import {
+  CaptureFlag,
+  type CaptureFlag as CaptureFlagValue,
+} from "../../schema.js";
+import { PRIMITIVE_REGISTRY } from "./registry.js";
+import type { EffectPrimitive, PrimitiveApplyContext } from "./types.js";
+
+const schema = z.object({
+  flag: z.union([
+    z.literal(CaptureFlag.CAN_CAPTURE_OWN),
+    z.literal(CaptureFlag.CANNOT_BE_CAPTURED),
+    z.literal(CaptureFlag.EN_PASSANT),
+  ]),
+});
+type Params = {
+  flag: CaptureFlagValue;
+};
+
+const descriptor: EffectPrimitive<Params> = {
+  kind: "set-capture-flag",
+  label: "Set Capture Flag",
+  description: "Bitwise-ORs one capture flag into CaptureFlags.",
+  paramsSchema: schema,
+  apply(ctx: PrimitiveApplyContext, params: Params): void {
+    const existing = ctx.session.get(ctx.pieceId, "CaptureFlags");
+    const baseFlags = existing === undefined ? 0 : existing;
+    if (typeof baseFlags !== "number" || Number.isNaN(baseFlags)) {
+      throw new Error("set-capture-flag expected CaptureFlags to be numeric");
+    }
+    ctx.session.insert(ctx.pieceId, "CaptureFlags", baseFlags | params.flag);
+  },
+};
+
+PRIMITIVE_REGISTRY.register(descriptor);
+export { descriptor as SET_CAPTURE_FLAG_PRIMITIVE };
diff --git a/packages/chess/src/schema.ts b/packages/chess/src/schema.ts
index 11f0769..36c96f7 100644
--- a/packages/chess/src/schema.ts
+++ b/packages/chess/src/schema.ts
@@ -7,12 +7,24 @@
  *   e.g. a1=0, h1=7, a8=56, h8=63, e4=28
  */
 import type { EntityId } from "@paratype/rete";
+import type { EffectPrimitiveNode } from "./modifiers/primitives/types.js";
 
 export type PieceType = "pawn" | "knight" | "bishop" | "rook" | "queen" | "king";
 export type PieceColor = "white" | "black";
+export type MoveType = "capture" | "step" | "slide";
 export type Square = number; // 0..63
 export type GameStatus = "active" | "checkmate" | "stalemate" | "draw";
 export type GameResult = PieceColor | "draw";
+export type ConditionSpec =
+  | { readonly type: "attr-lt"; readonly attr: string; readonly value: number }
+  | { readonly type: "attr-gt"; readonly attr: string; readonly value: number }
+  | {
+      readonly type: "attr-eq";
+      readonly attr: string;
+      readonly value: string | number | boolean | null;
+    }
+  | { readonly type: "always" }
+  | { readonly type: "never" };
 
 /** EntityId reserved for game-level facts (Turn, HalfmoveClock, etc.). */
 export const GAME_ENTITY: EntityId = 0 as EntityId;
@@ -56,6 +68,23 @@ export interface ChessAttrMap {
   CaptureFlags: number;
   PromotionOverride: PieceType | "disabled";
   DamageResistance: number;
+  AbsorbDamageAttr: string;
+  AbsorbDamageRate: number;
+  ReflectDamagePercent: number;
+  BlockedMoveTypes: readonly MoveType[];
+  AuraSpec: readonly {
+    readonly radius: number;
+    readonly targetAttr: string;
+    readonly delta: number;
+  }[];
+  OnTurnStartHooks: readonly EffectPrimitiveNode[][];
+  OnCaptureHooks: readonly EffectPrimitiveNode[][];
+  OnDamagedHooks: readonly EffectPrimitiveNode[][];
+  ConditionalHooks: readonly {
+    readonly condition: ConditionSpec;
+    readonly then: readonly EffectPrimitiveNode[];
+    readonly else?: readonly EffectPrimitiveNode[];
+  }[];
 }
 
 export type ChessAttrKey = keyof ChessAttrMap;