diff --git a/src/order-builder/helpers/orderAsset.ts b/src/order-builder/helpers/orderAsset.ts
index 98297ee..8d15255 100644
--- a/src/order-builder/helpers/orderAsset.ts
+++ b/src/order-builder/helpers/orderAsset.ts
@@ -6,7 +6,17 @@ export type OrderRouting = {
 };
 
 export function resolveOrderAssetID(asset: OrderAsset): string {
-	return asset.positionID ?? asset.tokenID;
+	const assetID = asset.positionID === undefined ? asset.tokenID : asset.positionID;
+	if (
+		(asset.tokenID !== undefined) === (asset.positionID !== undefined) ||
+		typeof assetID !== "string" ||
+		assetID.trim().length === 0
+	) {
+		throw new Error(
+			"Exactly one of tokenID or positionID must be provided as a non-empty string",
+		);
+	}
+	return assetID;
 }
 
 export function resolveOrderRouting(asset: OrderAsset, requestedVersion?: number): OrderRouting {
diff --git a/tests/client/positionOrders.test.ts b/tests/client/positionOrders.test.ts
new file mode 100644
index 0000000..f8b19b1
--- /dev/null
+++ b/tests/client/positionOrders.test.ts
@@ -0,0 +1,220 @@
+import { Wallet } from "@ethersproject/wallet";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { ClobClient } from "../../src/client.js";
+import { getContractConfig } from "../../src/config.js";
+import { bytes32Zero } from "../../src/constants.js";
+import { Chain, type OrderAsset, OrderType, Side } from "../../src/types/index.js";
+
+const host = "http://localhost:8080";
+const positionID = "456";
+const placed = { success: true, orderID: "0xorder", status: "live" };
+
+const makeClient = () => {
+	const signer = new Wallet("0x0000000000000000000000000000000000000000000000000000000000000001");
+	const sign = vi.spyOn(signer, "_signTypedData");
+	const client = new ClobClient({
+		host,
+		chain: Chain.AMOY,
+		signer,
+		creds: { key: "key", secret: "c2VjcmV0LXNlY3JldC1zZWNyZXQ=", passphrase: "passphrase" },
+	});
+	// Mock only HTTP: metadata caching, price/fee calculations, signing and serialization are real.
+	const get = vi.spyOn(client as any, "get").mockImplementation(async (url: unknown) => {
+		switch (url) {
+			case `${host}/version`:
+				return { version: 2 };
+			case `${host}/tick-size`:
+				return { minimum_tick_size: 0.01 };
+			case `${host}/neg-risk`:
+				return { neg_risk: false };
+			case `${host}/fee-rate`:
+				return { base_fee: 0 };
+			case `${host}/markets-by-token/${positionID}`:
+				return { condition_id: "market-a" };
+			case `${host}/clob-markets/market-a`:
+				return {
+					c: "market-a",
+					t: [{ t: positionID, o: "Yes" }],
+					mts: 0.01,
+					nr: true,
+					fd: { r: 0.2, e: 1 },
+					r: null,
+				};
+			case `${host}/book`:
+				return {
+					asks: [{ price: "0.5", size: "100" }],
+					bids: [{ price: "0.4", size: "100" }],
+				};
+			default:
+				throw new Error(`Unexpected GET ${url}`);
+		}
+	});
+	const post = vi.spyOn(client as any, "post").mockResolvedValue(placed);
+	return { client, signer, sign, get, post };
+};
+
+afterEach(() => vi.restoreAllMocks());
+
+describe.each(["limit", "market"] as const)("public %s position orders", kind => {
+	it.each([undefined, 1, 2, 3])("signs against V3 with version override %s", async version => {
+		const { client, sign, get } = makeClient();
+		const options = { version, negRisk: true };
+		const order =
+			kind === "limit"
+				? await client.createOrder(
+						{ positionID, side: Side.BUY, price: 0.5, size: 20 },
+						options,
+					)
+				: await client.createMarketOrder(
+						{ positionID, side: Side.BUY, price: 0.5, amount: 10 },
+						options,
+					);
+
+		expect(sign).toHaveBeenCalledExactlyOnceWith(
+			{
+				name: "Polymarket CTF Exchange",
+				version: "3",
+				chainId: Chain.AMOY,
+				verifyingContract: getContractConfig(Chain.AMOY).exchangeV3,
+			},
+			expect.any(Object),
+			expect.objectContaining({
+				tokenId: positionID,
+				makerAmount: "10000000",
+				takerAmount: "20000000",
+			}),
+		);
+		expect(order.tokenId).toBe(positionID);
+		expect(order.signature).toMatch(/^0x[0-9a-f]{130}$/i);
+		expect(get.mock.calls.map(([url]) => url)).not.toContain(`${host}/version`);
+		expect(get.mock.calls.map(([url]) => url)).not.toContain(`${host}/neg-risk`);
+		expect(get.mock.calls.map(([url]) => url)).not.toContain(`${host}/fee-rate`);
+		if (kind === "limit") {
+			expect(get).toHaveBeenCalledWith(`${host}/tick-size`, {
+				params: { token_id: positionID },
+			});
+		}
+	});
+
+	it("uses the position ID for fee and book lookups", async () => {
+		const { client, get } = makeClient();
+		const order =
+			kind === "limit"
+				? await client.createOrder({
+						positionID,
+						side: Side.BUY,
+						price: 0.5,
+						size: 20,
+						userUSDCBalance: 10,
+					})
+				: await client.createMarketOrder({
+						positionID,
+						side: Side.BUY,
+						amount: 10,
+						userUSDCBalance: 10,
+					});
+
+		expect(get).toHaveBeenCalledWith(`${host}/markets-by-token/${positionID}`);
+		expect(get).toHaveBeenCalledWith(`${host}/clob-markets/market-a`);
+		expect(client.feeInfos[positionID]).toEqual({ rate: 0.2, exponent: 1 });
+		// A $10 balance reserves $1 in fees at a 0.5 price, leaving $9 for 18 shares.
+		expect(order).toMatchObject({
+			tokenId: positionID,
+			makerAmount: "9000000",
+			takerAmount: "18000000",
+		});
+		if (kind === "market") {
+			expect(get).toHaveBeenCalledWith(`${host}/book`, { params: { token_id: positionID } });
+		}
+	});
+
+	it("posts the signed position under the wire tokenId field", async () => {
+		const { client, signer, sign, post } = makeClient();
+		const response =
+			kind === "limit"
+				? await client.createAndPostOrder({
+						positionID,
+						side: Side.BUY,
+						price: 0.5,
+						size: 20,
+					})
+				: await client.createAndPostMarketOrder({ positionID, side: Side.BUY, amount: 10 });
+		const signedMessage = sign.mock.calls[0][2];
+
+		expect(response).toEqual(placed);
+		expect(post).toHaveBeenCalledExactlyOnceWith(
+			`${host}/order`,
+			{
+				headers: expect.any(Object),
+				data: {
+					owner: "key",
+					orderType: kind === "limit" ? OrderType.GTC : OrderType.FOK,
+					postOnly: false,
+					deferExec: false,
+					order: {
+						salt: Number(signedMessage.salt),
+						maker: signer.address,
+						signer: signer.address,
+						taker: undefined,
+						tokenId: positionID,
+						makerAmount: "10000000",
+						takerAmount: "20000000",
+						side: Side.BUY,
+						signatureType: 0,
+						timestamp: signedMessage.timestamp,
+						expiration: "0",
+						metadata: bytes32Zero,
+						builder: bytes32Zero,
+						signature: await sign.mock.results[0].value,
+					},
+				},
+			},
+			true,
+		);
+	});
+
+	it.each([
+		{},
+		{ tokenID: "123", positionID },
+		{ tokenID: "123", positionID: null },
+		{ tokenID: null, positionID },
+		{ positionID: null },
+		{ positionID: "" },
+		{ positionID: 456 },
+	])("rejects invalid identifiers before HTTP or signing: %j", async identifiers => {
+		const { client, get, post, sign } = makeClient();
+		const asset = identifiers as unknown as OrderAsset;
+		const creating =
+			kind === "limit"
+				? client.createAndPostOrder({ ...asset, side: Side.BUY, price: 0.5, size: 20 })
+				: client.createAndPostMarketOrder({ ...asset, side: Side.BUY, amount: 10 });
+		await expect(creating).rejects.toThrow(
+			"Exactly one of tokenID or positionID must be provided as a non-empty string",
+		);
+		expect(get).not.toHaveBeenCalled();
+		expect(post).not.toHaveBeenCalled();
+		expect(sign).not.toHaveBeenCalled();
+	});
+});
+
+describe("token order compatibility", () => {
+	it.each([undefined, 1, 2, 3])("preserves version selection %s", async version => {
+		const { client, sign } = makeClient();
+		await client.createOrder(
+			{ tokenID: "123", side: Side.BUY, price: 0.5, size: 20 },
+			{ version },
+		);
+		const selectedVersion = version ?? 2;
+		const contracts = getContractConfig(Chain.AMOY);
+		expect(sign.mock.calls[0][0]).toMatchObject({
+			version: String(selectedVersion),
+			verifyingContract:
+				selectedVersion === 1
+					? contracts.exchange
+					: selectedVersion === 2
+						? contracts.exchangeV2
+						: contracts.exchangeV3,
+		});
+		expect(sign.mock.calls[0][2].tokenId).toBe("123");
+	});
+});
diff --git a/tests/order-builder/helpers/orderAsset.test.ts b/tests/order-builder/helpers/orderAsset.test.ts
index be7aa82..fb4d053 100644
--- a/tests/order-builder/helpers/orderAsset.test.ts
+++ b/tests/order-builder/helpers/orderAsset.test.ts
@@ -1,5 +1,39 @@
 import { describe, expect, it } from "vitest";
-import { resolveOrderRouting } from "../../../src/order-builder/helpers/orderAsset";
+import {
+	resolveOrderAssetID,
+	resolveOrderRouting,
+} from "../../../src/order-builder/helpers/orderAsset";
+import type { OrderAsset } from "../../../src/types/index.js";
+
+describe("order asset validation", () => {
+	it.each([
+		{},
+		{ tokenID: "123", positionID: "456" },
+		{ tokenID: "123", positionID: null },
+		{ tokenID: null, positionID: "456" },
+		{ tokenID: null },
+		{ positionID: null },
+		{ tokenID: "" },
+		{ positionID: " " },
+		{ tokenID: 123 },
+		{ positionID: 456 },
+		{ tokenID: false },
+		{ positionID: {} },
+	])("rejects invalid identifiers: %j", asset => {
+		for (const resolve of [resolveOrderAssetID, resolveOrderRouting]) {
+			expect(() => resolve(asset as unknown as OrderAsset)).toThrow(
+				"Exactly one of tokenID or positionID must be provided as a non-empty string",
+			);
+		}
+	});
+
+	it.each([
+		{ tokenID: "123", positionID: undefined },
+		{ positionID: "123", tokenID: undefined },
+	])("accepts an undefined unused identifier: %j", asset => {
+		expect(resolveOrderAssetID(asset)).toBe("123");
+	});
+});
 
 describe("resolveOrderRouting", () => {
 	it("routes position IDs through Exchange V3", () => {