Uniswap V3 Swap — Generalized Token Swaps on Base

Known Testnet Tokens (Base Sepolia)

For tokens not listed here, ask the user for the contract address, or use the contract-learner skill to look up a token by address.

Swap Workflow

Follow these steps in order. Each step is a separate cast call or cast calldata command. Do not skip steps — the agent must gather information before constructing the transaction.

Step 1: Resolve Token Addresses and Decimals

Determine the tokenIn and tokenOut contract addresses. If the user says "ETH", use the WETH address — Uniswap V3 only works with ERC-20 tokens, not native ETH.

For each token, confirm its decimals:

cast call <token-address> "decimals()(uint8)" --rpc-url https://sepolia.base.org

You need decimals to convert human-readable amounts (e.g. "0.001 ETH", "50 USDC") to raw integers:

• 0.001 ETH → 1000000000000000 (0.001 × 10^18)
• 50 USDC → 50000000 (50 × 10^6)

If you don't know a token's address, ask the user. Do not guess token addresses.

Step 2: Find a Pool

Uniswap V3 pools are keyed by (tokenA, tokenB, fee). The fee tiers are:

Query the factory for each fee tier until you find a pool (non-zero address):

# Try 500 first (most common for major pairs)
cast call <FACTORY> "getPool(address,address,uint24)(address)" <tokenIn> <tokenOut> 500 --rpc-url https://sepolia.base.org

# If zero, try 3000
cast call <FACTORY> "getPool(address,address,uint24)(address)" <tokenIn> <tokenOut> 3000 --rpc-url https://sepolia.base.org

# If zero, try 10000
cast call <FACTORY> "getPool(address,address,uint24)(address)" <tokenIn> <tokenOut> 10000 --rpc-url https://sepolia.base.org

# If zero, try 100
cast call <FACTORY> "getPool(address,address,uint24)(address)" <tokenIn> <tokenOut> 100 --rpc-url https://sepolia.base.org

A return value of 0x0000000000000000000000000000000000000000 means no pool exists at that fee tier. If no pool exists at any fee tier, tell the user there is no Uniswap V3 liquidity for this pair on this chain.

On testnets, liquidity may be very thin or nonexistent for many pairs. Common testnet pairs with liquidity: WETH/USDC.

Step 3: Check Pool Liquidity

Once you find a pool address, verify it has liquidity:

cast call <pool-address> "liquidity()(uint128)" --rpc-url https://sepolia.base.org

If liquidity is 0, the pool exists but has no liquidity — the swap will fail. Tell the user and try the next fee tier.

Step 4: Get a Quote

Use QuoterV2 to simulate the swap and get the expected output amount. This is a read-only call — no gas needed.

# For exact input (you know how much you're putting in):
cast call <QUOTER_V2> \
  "quoteExactInputSingle((address,address,uint256,uint24,uint160))(uint256,uint160,uint32,uint256)" \
  "(<tokenIn>,<tokenOut>,<amountIn>,<fee>,0)" \
  --rpc-url https://sepolia.base.org

The tuple parameter order is: (tokenIn, tokenOut, amountIn, fee, sqrtPriceLimitX96). Set sqrtPriceLimitX96 to 0 for no price limit.

The return values are: (amountOut, sqrtPriceX96After, initializedTicksCrossed, gasEstimate).

Convert amountOut to human-readable using the output token's decimals and show it to the user before executing. For example: "Swapping 0.001 WETH → expected ~2.34 USDC. Proceed?"

If the quote call reverts, the pool likely has insufficient liquidity for the requested amount. Try a smaller amount or a different fee tier.

Step 5: Check Balance and Allowance

Before executing, verify the wallet has enough of the input token:

# Check tokenIn balance
cast call <tokenIn> "balanceOf(address)(uint256)" <wallet-address> --rpc-url https://sepolia.base.org

# Check current allowance for the SwapRouter
cast call <tokenIn> "allowance(address,address)(uint256)" <wallet-address> <SWAP_ROUTER> --rpc-url https://sepolia.base.org

If the balance is insufficient, tell the user. If the allowance is less than amountIn, you'll need to approve in the same batch (see Step 6).

Step 6: Construct and Execute the Swap

Case A: TokenIn is an ERC-20 (already have the token)

If allowance is sufficient, just swap:

SWAP_DATA=$(cast calldata \
  "exactInputSingle((address,address,uint24,address,uint256,uint256,uint160))" \
  "(<tokenIn>,<tokenOut>,<fee>,<recipient>,<amountIn>,<amountOutMinimum>,0)")

keypo-wallet send --key <key-name> --to <SWAP_ROUTER> --data $SWAP_DATA

If allowance is insufficient, approve + swap in one batch:

APPROVE_DATA=$(cast calldata "approve(address,uint256)" <SWAP_ROUTER> <amountIn>)

SWAP_DATA=$(cast calldata \
  "exactInputSingle((address,address,uint24,address,uint256,uint256,uint160))" \
  "(<tokenIn>,<tokenOut>,<fee>,<recipient>,<amountIn>,<amountOutMinimum>,0)")

echo "[
  {\"to\": \"<tokenIn>\", \"value\": \"0\", \"data\": \"$APPROVE_DATA\"},
  {\"to\": \"<SWAP_ROUTER>\", \"value\": \"0\", \"data\": \"$SWAP_DATA\"}
]" | keypo-wallet batch --key <key-name> --calls -

Case B: Swapping from native ETH

Native ETH must be wrapped to WETH first. Then approve + swap:

DEPOSIT_DATA=$(cast calldata "deposit()")

APPROVE_DATA=$(cast calldata "approve(address,uint256)" <SWAP_ROUTER> <amountIn>)

SWAP_DATA=$(cast calldata \
  "exactInputSingle((address,address,uint24,address,uint256,uint256,uint160))" \
  "(<WETH>,<tokenOut>,<fee>,<recipient>,<amountIn>,<amountOutMinimum>,0)")

echo "[
  {\"to\": \"<WETH>\", \"value\": \"<amountIn>\", \"data\": \"$DEPOSIT_DATA\"},
  {\"to\": \"<WETH>\", \"value\": \"0\", \"data\": \"$APPROVE_DATA\"},
  {\"to\": \"<SWAP_ROUTER>\", \"value\": \"0\", \"data\": \"$SWAP_DATA\"}
]" | keypo-wallet batch --key <key-name> --calls -

The deposit() call wraps ETH → WETH. The value field on the deposit call must equal amountIn (in wei). All three calls execute atomically.

Case C: Swapping to native ETH

Swap tokenIn → WETH, then unwrap. Set the swap recipient to the wallet's own address, then unwrap:

SWAP_DATA=$(cast calldata \
  "exactInputSingle((address,address,uint24,address,uint256,uint256,uint160))" \
  "(<tokenIn>,<WETH>,<fee>,<wallet-address>,<amountIn>,<amountOutMinimum>,0)")

WITHDRAW_DATA=$(cast calldata "withdraw(uint256)" <expectedWethOut>)

echo "[
  {\"to\": \"<tokenIn>\", \"value\": \"0\", \"data\": \"$APPROVE_DATA\"},
  {\"to\": \"<SWAP_ROUTER>\", \"value\": \"0\", \"data\": \"$SWAP_DATA\"},
  {\"to\": \"<WETH>\", \"value\": \"0\", \"data\": \"$WITHDRAW_DATA\"}
]" | keypo-wallet batch --key <key-name> --calls -

Step 7: Verify the Result

After the transaction succeeds, check the output token balance:

keypo-wallet balance --key <key-name> --token <tokenOut>

Or for native ETH:

keypo-wallet balance --key <key-name>

exactInputSingle Parameters

The exactInputSingle function takes a tuple with these fields in order:

Note: The SwapRouter on Base Sepolia uses the V3 SwapRouter (not SwapRouter02). Its exactInputSingle does NOT include a deadline parameter in the tuple — the deadline is handled differently on this deployment. If you get encoding errors, check the router's ABI with cast interface.

Slippage Protection

Never set amountOutMinimum to 0 in production — this allows any output amount including near-zero (sandwich attack). Calculate a reasonable minimum:

# 3% slippage tolerance on testnet
python3 -c "
quoted = <amountOut-from-step-4>
min_out = int(quoted * 0.97)
print(min_out)
"

On testnet, 3-5% slippage is reasonable due to thin liquidity. On mainnet, 0.5-1% is typical.

Multi-Hop Swaps

If no direct pool exists for your pair but both tokens have pools with a common intermediary (usually WETH), you can route through multiple pools using exactInput:

# Example: TokenA → WETH → TokenB
# Path is encoded as: tokenA + fee1 + WETH + fee2 + tokenB (packed bytes)

# Encode the path
PATH=$(python3 -c "
tokenA = '<tokenA-address>'[2:]  # remove 0x
fee1 = '<fee1-hex>'  # e.g. '0001f4' for 500
weth = '<WETH-address>'[2:]
fee2 = '<fee2-hex>'
tokenB = '<tokenB-address>'[2:]
print('0x' + tokenA + fee1 + weth + fee2 + tokenB)
")

SWAP_DATA=$(cast calldata \
  "exactInput((bytes,address,uint256,uint256))" \
  "($PATH,<recipient>,<amountIn>,<amountOutMinimum>)")

Fee hex encoding: 100 → 000064, 500 → 0001f4, 3000 → 000bb8, 10000 → 002710.

Multi-hop is more complex. Only use it when no direct pool exists. Always try direct pools first.

Common Issues

"Pool not found" — No pool exists at any fee tier for this pair on this chain. On testnets, many pairs have no liquidity. Suggest the user try a different pair or check mainnet.

Quote reverts — The pool exists but doesn't have enough liquidity for the requested amount. Try a smaller amount.

Swap reverts with "STF" — "Safe Transfer From" failed. The approval didn't go through, or the wallet doesn't have enough tokens. Check allowance and balance.

Swap reverts with "TF" — "Transfer Failed" on output. Rare — usually means the output token has transfer restrictions.

Wrong decimals — Always call decimals() on both tokens. Getting this wrong sends the wrong amount (potentially 10^12x off).

"Too little received" — amountOutMinimum was set too high relative to actual pool price. This can happen if the quote is stale. Re-quote and use a slightly larger slippage tolerance.

Security Notes

• Always show the user the expected output amount and get confirmation before executing swaps.
• Never set amountOutMinimum to 0 except on testnets with trivial amounts.
• Verify token addresses — do not guess or hallucinate addresses. Use known addresses from this skill or ask the user.
• On testnets, pools may have extremely thin liquidity. Large swaps will have high slippage.
• The approve amount should match amountIn exactly, not type(uint256).max, to limit exposure if the router contract is compromised.