Auto Unwrap (Sidechains) and Polygon CDK: Developer Guide to Cross-Chain UX

Summary: This hands-on guide shows how to ship “one‑click” cross‑chain flows that unwrap gas tokens automatically, deliver canonical USDC, and top‑up destination gas — while your app runs on or integrates with Polygon CDK chains connected by the AggLayer. You’ll get concrete patterns, code snippets, and the latest standards (ERC‑7683, CCTP v2, EIP‑7702) that materially reduce user friction and failure rates. (polygon.technology)

TL;DR for decision‑makers

Shipping “auto unwrap” today = three things at once:
- Native gas tokens auto‑unwrapped (WETH→ETH, WMATIC→MATIC, etc.) on arrival. Uniswap’s Universal Router exposes WRAP_ETH (0x0b) and UNWRAP_WETH (0x0c) commands that can be chained into a single plan. (docs.uniswap.org)
- Canonical USDC, not bridged look‑alikes, delivered via Circle’s CCTP v2 “Fast Transfer,” with optional Hooks to execute actions post‑mint. CCTP v1 is being phased out; v2 is canonical with phase‑out of v1 starting July 31, 2026. (circle.com)
- “Gas on destination” so users aren’t stuck without native gas; available via LI.FI, Socket/Bungee Refuel, Skip’s “Gas on Receive,” and Wormhole Connect gas drop‑off. (docs.li.fi)
On Polygon, CDK is now multistack (Erigon or OP Stack) with native AggLayer interop; AggLayer v0.3 (June 23, 2025) adds an execution‑proof mode and opens the door to chain‑agnostic connectivity and sub‑10s cross‑chain UX. (polygon.technology)

Why cross‑chain UX still breaks (and how to fix it)

Friction usually comes from three predictable mismatches:

Users arrive with the wrong token standard

Example: They bridge to a sidechain and end up with WETH where the dapp expects ETH or needs a payable call. The fix: auto‑unwrap as part of a bundled plan using Uniswap Universal Router’s 0x0c UNWRAP_WETH at the end of your route. (docs.uniswap.org)

Users receive non‑canonical stables (USDC.e, axlUSDC, etc.)

This breaks downstream protocols expecting native USDC markets. The fix: deliver native USDC via CCTP v2 “Fast Transfer,” which cuts settlement from ~minutes to seconds and supports Hooks for post‑mint actions (swap, repay, deposit, payroll). (coindesk.com)

Users have no gas on the destination chain

This is the number‑one source of abandoned sessions. The fix: programmatically include “gas on destination” using integrators (LI.FI, Socket/Bungee Refuel, Skip, Wormhole Connect) so a small slice of value is converted into the destination’s native coin. (help.li.fi)

Where Polygon CDK + AggLayer change the calculus

CDK goes multistack:
- CDK OP Stack: familiar OP Stack semantics plus ZK enhancements, pessimistic proofs, and native AggLayer interop; no “7‑day delay” and no “superchain tax.” (polygon.technology)
- CDK Erigon: enterprise‑grade customization, native gas token support, zkRollup/Validium/Sovereign modes, and reported high throughput (e.g., 60+ MGas/s bursts). (agglayer.dev)
AggLayer v0.3 (June 23, 2025) expands the “pessimistic proof” with an execution‑proof mode so chains can prove their own state, and sets the stage for non‑CDK chains to join; Polygon PoS was slated to connect by end‑Q3 2025, with the path to sub‑10s cross‑chain transactions. (agglayer.dev)
Pessimistic proof, in practice:
- A ZK proof that ensures the shared, unified bridge cannot be drained by any single connected chain; the AggLayer continuously verifies that no chain’s withdrawals exceed deposits by building and verifying global/local exit trees. (polygon.technology)

What this means for your roadmap:

You can standardize cross‑chain UX patterns once, then inherit interop and shared liquidity across CDK chains via the AggLayer — instead of maintaining N bespoke bridges and token mappings. (agglayer.dev)

“Auto unwrap” broken down: what you actually ship

Native gas tokens

Always end your route with the correct “native” asset when the next call is payable or when the user has zero gas:
- ETH chains: UNWRAP_WETH (0x0c) to deliver ETH.
- Polygon PoS or CDK chain with native token X: unwrap to the chain’s configured native token (CDK supports custom native gas tokens; design your UI copy to reflect the chain’s symbol). (docs.uniswap.org)

Canonical stablecoins (USDC)

Route all stable transfers through CCTP v2; with Hooks you can atomically trigger post‑mint actions on the destination (e.g., deposit into a vault). CCTP v2 is the canonical path going forward; v1 is in legacy mode with deprecation beginning July 31, 2026. (circle.com)

Gas on destination

If wallet.balance(destGasToken) < threshold, add a “gas drop‑off” leg:
- Socket/Bungee Refuel: central service that sends you a $‑equivalent amount of dest gas token; easily toggled via widget/SDK. (docs.bungee.exchange)
- LI.FI: supports destination gas mechanisms in multi‑route flows. (help.li.fi)
- Wormhole Connect “gas dropoff” and Skip “Gas on Receive” cover multi‑runtime ecosystems. (wormhole.com)

Standards that matter in 2026

ERC‑7683 Cross‑Chain Intents
- A shared order and settlement interface so your intents can be filled by a universal network of fillers across chains. This reduces relayer fragmentation and improves route competition. (eips.ethereum.org)
CCTP v2 (Fast Transfer + Hooks)
- Faster‑than‑finality USDC transfers plus programmable post‑mint actions; now the canonical CCTP variant. (coindesk.com)
EIP‑7702 (Pectra, mainnet May 7, 2025)
- Lets EOAs temporarily delegate to smart‑wallet code and batch steps like approvals, route encoding, and cross‑chain intent submission — without forcing users to migrate addresses. Treat the new delegation flow with care and clear UX warnings. (coindesk.com)

Reference architecture: One‑click cross‑chain swap, auto‑unwrap, and deposit (CDK chain as destination)

Flow overview:

User signs a 7683 intent in their EOA; wallet uses EIP‑7702 to delegate execution to a hardened wallet contract for the session, bundling approvals and the “open” event emission. (eip.info)
Solver picks the best route:
- Origin: swap into USDC if needed.
- Cross‑chain: CCTP v2 Fast Transfer to destination chain.
- Destination:
  - Optional: “gas drop‑off” based on balance threshold.
  - Uniswap UR sub‑plan: swap into WETH, then UNWRAP_WETH to deliver native ETH for a payable call, or unwrap to the CDK chain’s native token. (coindesk.com)
Execute a destination payload to deposit into the protocol (vault, lending market, game wallet), all in one user click. (docs.bungee.exchange)

Key components:

AggLayer connectivity is native for CDK chains, so cross‑chain accounting and safety are handled via pessimistic proofs under the hood. (docs.polygon.technology)

Practical snippets you can adapt

Universal Router: ending a plan with an auto‑unwrap

// Final step on destination after swaps: unwrap WETH to ETH for a payable deposit
// Command 0x0c = UNWRAP_WETH(recipient, amountMin)
bytes memory commands = abi.encodePacked(
  /* ... prior swap commands ... */,
  bytes1(0x0c) // UNWRAP_WETH
);

bytes[] memory inputs = new bytes[](N);
inputs[last] = abi.encode(recipient, MIN_ETH_AFTER_UNWRAP);

// router.execute(commands, inputs, block.timestamp + 300);

The WRAP_ETH (0x0b) and UNWRAP_WETH (0x0c) commands are part of Uniswap’s Universal Router v2. Chain them with swaps to guarantee the right native asset for the next step. (docs.uniswap.org)

ERC‑7683 intent skeleton (TypeScript, off‑chain order)

// Minimalized 7683-style order fields (names simplified for clarity)
const order = {
  originChainId: 8453,        // Base
  destChainId: 1101,          // Polygon zkEVM / CDK chain example
  initiator: user.address,    // EOA that signs
  inputToken: USDC_BASE,
  inputAmount: parseUnits("100", 6),
  outputToken: NATIVE_DEST,   // “auto unwrap” on destination
  minOutputAmount: parseUnits("0.029", 18),
  deadline: Math.floor(Date.now()/1000) + 600,
  receiver: user.address,
  settlementContract: ORIGIN_SETTLER,
  fillerNetworkFee: parseUnits("0.2", 6),
};
// Sign with Permit2 witness so approval + order are 1 signature

Why: standardized order structs and settlement interfaces let different solvers/fillers interoperate. Pair with Permit2 “witness” signing to cut approvals. (eips.ethereum.org)

CCTP v2 “Fast Transfer” with Hooks (conceptual)

{
  "source_chain": "ethereum",
  "dest_chain": "base",
  "amount": "100000000",          // 100 USDC
  "recipient": "0xRecipient",
  "transfer_type": "FAST",
  "hook": {
    "target": "0xVault",
    "calldata": "0xabcdef...",     // ABI-encoded deposit(uint256,address)
    "gas_limit": "300000"
  }
}

Fast transfers settle in seconds; Hooks let you execute a deposit immediately after mint on the destination. (coindesk.com)

Gas on destination toggle (Socket/Bungee widget)

<Bridge
  provider={Provider}
  API_KEY={process.env.SOCKET_API_KEY}
  enabledRefuel={true}
/>

Refuel sends a small amount of the destination chain’s native gas token alongside your route, removing the “I can’t click anything” dead‑end. (docs.bungee.exchange)

CDK specifics you should decide up front

Which stack?
- Choose CDK Erigon for maximum control (custom native token, fine‑grained gas metering, mode flexibility) and enterprise‑grade throughput; choose CDK OP Stack for OP‑stack familiarity plus ZK‑backed interop and fast finality via pessimistic proofs. (agglayer.dev)
Rollup mode:
- Sovereign (AggLayer‑optimized fees), Validium (lower DA costs), or zkRollup (full on‑chain security). Validate against your compliance and data‑availability needs. (agglayer.dev)
AggLayer proofing:
- Understand how pessimistic proofs protect the unified bridge and your users even if another connected chain is compromised. Bake into your risk docs. (polygon.technology)
Native token UX:
- If you pick a custom native gas token on your CDK chain, you must implement auto‑unwrap mapping in your router plans and clearly label symbols in your UI. (agglayer.dev)

Emerging best practices (what we see working)

Prefer CCTP v2 for stablecoin legs and plan your v1 deprecation migration now; set internal SLAs based on “seconds‑level” settlement for time‑sensitive flows. (circle.com)
Make “gas on destination” the default:
- Detect balances and silently add gas top‑ups for first‑time users unless they opt out. Diversify providers for redundancy (LI.FI + Bungee + Wormhole). (docs.li.fi)
Standardize on ERC‑7683:
- You’ll get a larger solver/filler pool and lower costs over time. It’s becoming the lingua franca for cross‑chain order intent and settlement. (eips.ethereum.org)
Use EIP‑7702 for sessions and batching — carefully:
- Wallets should present high‑friction warnings for delegation; educate users on revocation and scope. Monitor ecosystem risk reports. (coindesk.com)
Instrument your cross‑chain funnel:
- Track “bridge success but destination call revert” cases; LI.FI notes this edge case and you should show friendly recovery paths when it happens. (help.li.fi)

Deep dive: the pessimistic proof in plain English

Every chain connected to AggLayer maintains a local exit tree of withdrawals.
AggLayer builds a global exit tree and proves — in zero knowledge — that no chain’s withdrawals exceed deposits. If any chain would go negative, the proof won’t verify, blocking settlement and isolating the fault. That’s how the unified bridge avoids contagion. (polygon.technology)
As of June 23, 2025 (v0.3), a new execution‑proof mode allows chains to prove their own state, paving fast interop and chain‑agnostic connections (Polygon PoS connection announced for end‑Q3 2025). (agglayer.dev)

Implementation checklist

Token flows
- Map canonical assets per route; no USDC.e where native USDC exists.
- End plans with WRAP/UNWRAP as needed for payable calls. (docs.uniswap.org)
Cross‑chain settlement
- Prefer CCTP v2 Fast Transfer with Hooks for USDC legs. (coindesk.com)
- For non‑USDC assets, choose bridge providers that support destination gas drop‑off. (wormhole.com)
Intents and execution
- Emit 7683 “open” events; support standard settlement interfaces.
- Offer EIP‑7702 sessions for one‑click flows; expose revoke controls in‑app. (eips.ethereum.org)
CDK choices
- Pick stack (Erigon vs OP Stack) and rollup mode (Sovereign/Validium/zkRollup).
- Document AggLayer guarantees and failure modes in your risk registers. (agglayer.dev)
Telemetry and support
- Alert when “bridge succeeded, destination revert” occurs; surface a retry or claim interface. (help.li.fi)

Common pitfalls we debug for clients

Confusing token symbols at destination (e.g., wETH labeled as ETH in some UIs); always verify the unwrap happened before a payable call. Use balance checks in your UR plan to fail safely. (github.com)
Returning bridged tokens when a destination payload reverts (expected behavior in many integrators); your UI must explain status and next steps. (help.li.fi)
Forgetting to provision gas for first‑time users; default to enabling gas drop‑off or Refuel. (docs.bungee.exchange)
Shipping non‑canonical stablecoins; your downstream protocol may reject or apply worse rates. Route through CCTP v2. (coindesk.com)

What “good” looks like in 2026

1 signature, 1 click, 1 minute:
- The user picks an outcome; your app emits a 7683 intent, funds bridge in the background, delivers native USDC via CCTP v2, drops gas on destination, auto‑unwraps for the next step, and executes a destination payload — with the AggLayer providing safety for the shared bridge. (eips.ethereum.org)
Operates across any CDK chain configuration (Erigon or OP Stack) you choose, with native AggLayer interop and an execution‑proof path as v0.3+ rolls out. (polygon.technology)

Brief in‑depth: configuring a CDK chain for seamless “auto unwrap”

If you launch a CDK chain with a custom native token (e.g., GASX):
- Publish a token map so routers know that WETH→ETH is not sufficient; they must swap into WGASX and unwrap to GASX for payable calls.
- Expose a tiny faucet for developers but rely on “gas on destination” for production UX.
- Validate that Uniswap UR and your preferred aggregator support the chain’s token conventions in their command planners. CDK Erigon supports custom native tokens out‑of‑the‑box; document this for partners. (agglayer.dev)

The bottom line

“Auto unwrap” is a design pattern, not a single feature: deliver native gas tokens, canonical stables, and destination gas — every time. On Polygon, CDK’s multistack approach and the AggLayer’s pessimistic proofs let you standardize these patterns once and scale them across many chains, while ERC‑7683, CCTP v2, and EIP‑7702 compress user steps into a single, comprehensible click. If you’re planning a chain or app launch in 2026, bake these into your MVP; they’re now table stakes. (polygon.technology)