Simulate and verify before you sign

Two layers: SwapChan simulates an EVM transaction online; blocktools decodes and checks any transaction offline against what you expect.

1. Simulate on SwapChan. Open Simulate or have your agent call the MCP tool simulate_swap: a dry run of a quote or raw tx showing balance changes, approvals and the revert reason. Before every signature, verify_tx adds a guard and a plain-English summary for the human.

2. Decode it with blocktools. tx-decode reads one transaction on stdin (hex/base64 raw tx, PSBT, EVM RLP):

    tx-decode --text < signed.json
   

   See Decode a transaction before you sign or broadcast it.

3. Check it against what you expect. tx-verify checks a transaction against an expected destination and maximum fee (example from recipes/goal-verify-and-offline.md):

    ./blocktools run 'tx-verify --expect-to 1Q2TWHE3GMdB6BZKafqwxXtWAWgFt5Jvm3 --max-fee 10000 < /in/block170.hex'
   

   In that example the check correctly fails (input_amounts_unknown): a raw transaction has no input amounts, so the fee is unknown. A PSBT from tx-build carries them, so the check can pass. An optional policy file caps fees and destinations before signing.

4. A human signs. tx-sign signs ONE transaction after a human reads its summary and types a code on /dev/tty; tx-broadcast asks for a second code. On SwapChan your own wallet signs; SwapChan never holds keys.

Sources: blocktools recipes/goal-verify-and-offline.md, recipes/decode-a-transaction.md, docs/getting-started.md, man pages for tx-decode / tx-verify / tx-sign; SwapChan /simulate and the agent tool list.