ckb-ccc-udt

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name: ckb-ccc-udt description: Covers issuing, transferring, and reading metadata for UDT / xUDT fungible tokens on CKB with the current CCC UDT API (ccc.udt.Udt, @ckb-ccc/udt). NOTE: ckb-devrel is releasing a replacement, @ckb-ccc/coin, within the next few weeks — once it ships, @ckb-ccc/udt will be removed and unmaintained, not kept for compatibility. This skill still teaches the current @ckb-ccc/udt-based pattern since it is the only working option until then, and this file will be replaced in place once @ckb-ccc/coin ships. Use when the user asks about UDT, xUDT, fungible tokens, token issuance, or token transfers on CKB — even if they just say "token" without naming UDT specifically. Builds on the standard transaction pattern in ckb-ccc-transactions. metadata: author: ckb-devrel version: "1.0.0" role: spoke depends-on: "ckb-ccc-fundamentals, ckb-ccc-transactions" priority: normal status: "current API — @ckb-ccc/coin expected within weeks; @ckb-ccc/udt will then be removed/unmaintained. This file becomes a redirect stub for one release cycle (new content goes in ckb-ccc-coin/, this directory is deleted afterward)"

CKB CCC — UDT Tokens

Covers xUDT (extensible User Defined Token) issuance, transfer, and metadata reads. Assumes a connected Signer (see ckb-ccc-signer-setup) and the standard transaction pattern (see ckb-ccc-transactions) — this skill only adds the UDT-specific steps on top of that pattern.

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Issuing xUDT tokens (Single-Use-Seal pattern)

xUDT issuance requires the Single-Use-Seal (SUS) pattern, which involves three sequential transactions to ensure token uniqueness and authority.

import { ccc } from "@ckb-ccc/ccc";
import { signer } from "@ckb-ccc/playground";

/* ============================================================
 * Types
 * ============================================================ */

/**
 * @public
 * @category Token
 */
interface TokenMetadata {
  decimals: number;
  symbol: string;
  /** Falls back to `symbol` when omitted/blank */
  name?: string;
}

/**
 * The 3 on-chain steps of the SingleUseLock (SUS) issuance flow, in order.
 * @public
 * @category Token
 */
type IssueStep = "seal" | "owner" | "mint";

/**
 * @public
 * @category Token
 */
interface IssueXudtSusParams extends TokenMetadata {
  /** Human-readable total supply, e.g. 100_000_000 (converted to Shannon internally) */
  totalSupply: ccc.FixedPointLike;
  /** Optional hook, fired after each of the 3 on-chain steps is broadcast */
  onProgress?: (step: IssueStep, txHash: ccc.Hex) => void;
}

/**
 * @public
 * @category Token
 */
interface IssueXudtSusResult {
  sealTxHash: ccc.Hex;
  ownerTxHash: ccc.Hex;
  mintTxHash: ccc.Hex;
  typeScriptHash: ccc.Hex;
}

/* ============================================================
 * Token metadata encoding (pure, no chain I/O)
 * ============================================================ */

/**
 * Encodes xUDT token metadata (decimals + name + symbol) into the
 * `UniqueType` cell data layout.
 * @public
 * @category Token
 *
 * @param metadata - The token's decimals, symbol, and optional name.
 * @returns The encoded bytes to be used as `outputsData` for the `UniqueType` cell.
 * @throws If `symbol` is missing, `decimals` is out of the 0-255 range, or
 * either `symbol`/`name` exceeds 255 UTF-8 bytes.
 */
function encodeTokenInfo({
  decimals,
  symbol,
  name,
}: TokenMetadata): Uint8Array {
  if (!symbol) {
    throw new Error("symbol is required");
  }
  if (!Number.isInteger(decimals) || decimals < 0 || decimals > 255) {
    throw new Error("decimals must be an integer within 0-255 (1-byte field)");
  }

  const symbolBytes = ccc.bytesFrom(symbol, "utf8");
  const nameBytes = ccc.bytesFrom(name?.trim() ? name : symbol, "utf8");

  // Each string is length-prefixed with a single byte -> hard cap at 255 bytes
  if (symbolBytes.length > 255 || nameBytes.length > 255) {
    throw new Error("symbol/name must each be <= 255 bytes when UTF-8 encoded");
  }

  return ccc.bytesConcat(
    ccc.numToBytes(decimals, 1),
    ccc.numToBytes(nameBytes.length, 1),
    nameBytes,
    ccc.numToBytes(symbolBytes.length, 1),
    symbolBytes,
  );
}

/**
 * Issues an xUDT token using the Single-Use-Seal (SUS) pattern: a seal cell
 * anchors a `SingleUseLock` owner cell, which is then consumed together with
 * the seal to mint the xUDT cell and its `UniqueType` metadata cell.
 *
 * @public
 * @category Blockchain
 * @category Token
 */
class XudtSusIssuer {
  /**
   * @param signer - The signer used to fund and sign all 3 transactions.
   */
  constructor(private readonly signer: ccc.Signer) {}

  /**
   * Runs the full seal -> owner -> mint flow.
   *
   * @param params - Token metadata, total supply, and an optional progress hook.
   * @returns The tx hashes of all 3 steps and the resulting xUDT type script hash.
   *
   * @example
   * ```typescript
   * const result = await new XudtSusIssuer(signer).issue({
   *   decimals: 8,
   *   symbol: "SPARK",
   *   totalSupply: 100_000_000,
   *   onProgress: (step, txHash) => console.log(step, txHash),
   * });
   * ```
   */
  async issue(params: IssueXudtSusParams): Promise<IssueXudtSusResult> {
    const { decimals, totalSupply } = params;
    if (!Number.isInteger(decimals) || decimals < 0 || decimals > 255) {
      throw new Error(
        "decimals must be an integer within 0-255 (1-byte field)",
      );
    }
    const tokenInfo = encodeTokenInfo(params);
    const supply = ccc.fixedPointFrom(totalSupply, decimals);
    if (supply < 0n || supply >= 1n << 128n) {
      throw new Error("totalSupply must encode to a non-negative uint128");
    }

    const { script: ownerLock } = await this.signer.getRecommendedAddressObj();

    const sealTxHash = await this.createSealCell(ownerLock);
    params.onProgress?.("seal", sealTxHash);

    const singleUseLock = await ccc.Script.fromKnownScript(
      this.signer.client,
      ccc.KnownScript.SingleUseLock,
      ccc.OutPoint.from({ txHash: sealTxHash, index: 0 }).toBytes(),
    );

    const ownerTxHash = await this.createOwnerCell(singleUseLock);
    params.onProgress?.("owner", ownerTxHash);

    const { mintTxHash, typeScriptHash } = await this.mintToken({
      sealTxHash,
      ownerTxHash,
      singleUseLock,
      ownerLock,
      supply,
      tokenInfo,
    });
    params.onProgress?.("mint", mintTxHash);

    return { sealTxHash, ownerTxHash, mintTxHash, typeScriptHash };
  }

  /** Step 1 - the "seal" cell that SingleUseLock will bind to and that mint() later consumes as proof */
  private async createSealCell(lock: ccc.Script): Promise<ccc.Hex> {
    const tx = ccc.Transaction.from({ outputs: [{ lock }] });
    await tx.completeInputsByCapacity(this.signer);
    await tx.completeFeeBy(this.signer);
    const txHash = await this.signer.sendTransaction(tx);

    // Reserve this output so later completeInputsByCapacity() calls in this
    // flow don't accidentally spend it as generic fee-funding - it must
    // survive untouched until the mint transaction consumes it as the seal.
    await this.signer.client.cache.markUnusable({ txHash, index: 0 });

    return txHash;
  }

  /** Step 2 - the owner cell, locked with SingleUseLock bound to the seal's outpoint */
  private async createOwnerCell(singleUseLock: ccc.Script): Promise<ccc.Hex> {
    const tx = ccc.Transaction.from({ outputs: [{ lock: singleUseLock }] });
    await tx.completeInputsByCapacity(this.signer);
    await tx.completeFeeBy(this.signer);
    return this.signer.sendTransaction(tx);
  }

  /** Step 3 - consume seal + owner cells to mint the xUDT cell and its UniqueType metadata cell */
  private async mintToken(args: {
    sealTxHash: ccc.Hex;
    ownerTxHash: ccc.Hex;
    singleUseLock: ccc.Script;
    ownerLock: ccc.Script;
    supply: bigint;
    tokenInfo: Uint8Array;
  }): Promise<{ mintTxHash: ccc.Hex; typeScriptHash: ccc.Hex }> {
    const {
      sealTxHash,
      ownerTxHash,
      singleUseLock,
      ownerLock,
      supply,
      tokenInfo,
    } = args;

    const xudtType = await ccc.Script.fromKnownScript(
      this.signer.client,
      ccc.KnownScript.XUdt,
      singleUseLock.hash(),
    );
    const uniqueType = await ccc.Script.fromKnownScript(
      this.signer.client,
      ccc.KnownScript.UniqueType,
      "00".repeat(32), // placeholder - patched below with the real TypeId once inputs are known
    );

    const tx = ccc.Transaction.from({
      inputs: [
        { previousOutput: { txHash: sealTxHash, index: 0 } },
        { previousOutput: { txHash: ownerTxHash, index: 0 } },
      ],
      outputs: [
        { lock: ownerLock, type: xudtType },
        { lock: ownerLock, type: uniqueType },
      ],
      outputsData: [ccc.numLeToBytes(supply, 16), tokenInfo],
    });

    await tx.addCellDepsOfKnownScripts(
      this.signer.client,
      ccc.KnownScript.SingleUseLock,
      ccc.KnownScript.XUdt,
      ccc.KnownScript.UniqueType,
    );

    await tx.completeInputsByCapacity(this.signer);

    // UniqueType's args must be a TypeId derived from the first input + output
    // index, which is only known once inputs are finalized above.
    if (!tx.outputs[1].type) {
      throw new Error("UniqueType output unexpectedly missing before TypeId patch");
    }
    tx.outputs[1].type.args = ccc.hexFrom(
      ccc.bytesFrom(ccc.hashTypeId(tx.inputs[0], 1)).slice(0, 20),
    );

    await tx.completeFeeBy(this.signer);
    const mintTxHash = await this.signer.sendTransaction(tx);
    await this.signer.client.waitTransaction(mintTxHash);

    return { mintTxHash, typeScriptHash: tx.outputs[0].type!.hash() };
  }
}

/* ============================================================
 * Convenience entrypoint - keeps the same call shape as before
 * ============================================================ */

/**
 * Issues an xUDT token via the Single-Use-Seal pattern using the
 * playground's default signer.
 * @public
 * @category Token
 *
 * @example
 * ```typescript
 * const result = await issueXudtWithSUS({
 *   decimals: 8,
 *   symbol: "SPARK",
 *   totalSupply: 100_000_000,
 *   onProgress: (step, txHash) => console.log(step, txHash),
 * });
 * console.log(result);
 * ```
 */
export async function issueXudtWithSUS(
  params: IssueXudtSusParams,
): Promise<IssueXudtSusResult> {
  return new XudtSusIssuer(signer).issue(params);
}

// Example usage in the playground:
const result = await issueXudtWithSUS({
  decimals: 8,
  symbol: "ZXMOTO",
  totalSupply: 100_000_000,
  onProgress: (step, txHash) => console.log(step, txHash),
});
console.log(result);

Transferring UDT tokens

1. Construct UDT instance — Resolve type script: await ccc.Script.fromKnownScript(client, ccc.KnownScript.XUdt, args). Get code OutPoint from cell deps. Create: new ccc.udt.Udt(code, typeScript).
2. Build transfer — const { res: tx } = await udt.transfer(signer, [{ to: lock, amount: 100n }]).
3. Complete UDT inputs — tx = await udt.completeBy(tx, signer) — adds UDT inputs and change output. Do not skip: omitting this loses tokens permanently.
4. Complete CKB capacity — await tx.completeInputsByCapacity(signer).
5. Pay fee and send — await tx.completeFeeBy(signer), then await signer.sendTransaction(tx).
6. Read metadata (SSRI tokens only) — udt.name(), udt.symbol(), udt.decimals(), udt.icon(). Always check return value is not undefined — legacy sUDT tokens do not implement SSRI.

async function transferUdt(signer: ccc.Signer, receiverAddress: string) {
  const { script: recipientLock } = await ccc.Address.fromString(receiverAddress, signer.client);
  
  const type = await ccc.Script.fromKnownScript(
    signer.client, ccc.KnownScript.XUdt, "0x<xudt-cell-typescript-args>"
  );
  const code = (await signer.client.getCellDeps(
    (await signer.client.getKnownScript(ccc.KnownScript.XUdt)).cellDeps
  ))[0].outPoint;

  // Construct a Udt instance
  const udt = new ccc.udt.Udt(code, type);

  // Transfer tokens — always three steps after udt.transfer
  const decimals = 8;  
  let { res: tx } = await udt.transfer(signer, [{ to: recipientLock, amount: ccc.fixedPointFrom(100, decimals)}]);
  tx = await udt.completeBy(tx, signer);         // fill UDT inputs + change
  await tx.completeInputsByCapacity(signer);     // fill CKB capacity
  await tx.completeFeeBy(signer);
  const txHash = await signer.sendTransaction(tx);

  return txHash;
}

Rule: UDT transfers need completeBy (UDT) then completeInputsByCapacity (CKB) — order matters.

---

Gotchas (UDT-specific)

Symptom / Error	Cause	Fix
UDT tokens lost after transfer	udt.completeBy() not called	Always call udt.completeBy(tx, signer) before completeInputsByCapacity; it adds the token change output
udt.name() / symbol() returns undefined	Token doesn't implement SSRI	Only xUDT with SSRI support returns metadata; always guard with ?? "unknown"
On-chain amount is 10^decimals smaller than expected	Used human-facing amount directly instead of scaling by decimals	Always use ccc.fixedPointFrom(humanFacingAmount, decimals) for both mint and transfer — the on-chain integer is display * 10^decimals, not the human-facing number

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Q&A

Q: How do I find the args for a specific xUDT?

A: You can view it in the CKB explorer:

1. Find the xUDT in the CKB explorer (e.g., https://testnet.explorer.nervos.org/xudt/0xb5378a3a22ed158233a4493ec0994483aada2bc6de9446952184653d90bdf889)
2. Navigate to the xUDT's detail page
3. In the info section, look for the Type Script section and find the Args field
4. This Args value is the xUDT's args, used when constructing ccc.Script.fromKnownScript(client, ccc.KnownScript.XUdt, args)

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Checklist (UDT-specific)

• UDT change — udt.completeBy() called before completeInputsByCapacity for token transfers
• Supply scaling — totalSupply (and any amount) is the human-readable number (e.g., 100_000_000), which ccc.fixedPointFrom(totalSupply, decimals) converts to the on-chain integer by applying 10^decimals
• Tested on testnet first — for real issuance, run the full flow on ClientPublicTestnet and verify in an explorer before switching to ClientPublicMainnet
• symbol vs name — symbol holds the ticker/display identifier; name is supplementary description

Also check the checklists in ckb-ccc-fundamentals and ckb-ccc-transactions.