specs(exact): add TRON exact scheme specification

[boboliu-1010]

Description

Adds formal specification for the exact payment scheme on TRON, covering both assetTransferMethod variants defined by the EVM spec: permit2 and eip3009. Payload schemas are byte-identical to scheme_exact_evm.md — same JSON field names, same EIP-712 typehashes, same Permit2 Witness pattern. TRON-specific rules are limited to address conversion (Base58 in paymentRequirements, 0x hex inside signed payload) and on-chain SDK (TronWeb vs. viem).

Closes #2075.

• Adds specs/schemes/exact/scheme_exact_tron.md — full TRON exact spec (both paths).
• Updates specs/schemes/exact/scheme_exact.md — appends TRON to the scheme index and the critical-validation requirements list.

No SDK/runtime implementation changes. The TypeScript (@x402/tron) and Python (x402[tron]) implementations will follow in separate PRs after this spec is approved.

Design property: gasless client

TRON debits energy/bandwidth from the owner_address of the on-chain transaction. In this spec the facilitator is owner_address; the user signs only TIP-712 structured data. The user never needs to hold TRX, matching x402's gasless-client property on EVM.

Why TRON as an independent package

• Different address format: Base58 (T...) vs. 0x hex
• Different chain SDK: TronWeb vs. viem / ethers
• Different CAIP-2 namespace: tron: vs. eip155:
• No ERC-1271, ERC-6492, or Multicall3 support
• Follows the existing pattern: all 5 mechanism packages (evm, svm, avm, aptos, stellar) are independent.

Dual assetTransferMethod

This spec defines both paths already established by the EVM exact scheme:

1. permit2 — uses SUN.io's production Permit2 (source), byte-identical to Uniswap Permit2 adapted to TIP-712. Covers every TRC-20 (USDT, USDD, etc.). Mainnet deployment TTJxU3P8rHycAyFY4kVtGNfmnMH4ezcuM9 is TronScan-verified with 29,000+ live txs.
2. eip3009 — transferWithAuthorization signed via TIP-712. Requires the token to implement the ERC-3009 interface. BofAI will deploy an ERC-3009-compatible TRC-20 on Nile (and optionally mainnet) before PR2 opens, and add the address + source-verification link to the "Supported Tokens" appendix.

Why no TIP-3009 dependency

tronprotocol/tips does not currently contain a TIP-3009 document. This spec follows the USDC precedent — USDC shipped transferWithAuthorization on Ethereum before EIP-3009 reached Final status. The on-chain interface is well-established via EIP-3009 and the signing layer uses TIP-712 (Final). A formal TIP-3009 can be proposed to tronprotocol/tips in a separate thread without blocking this x402 work.

Permit2 compatibility with Uniswap Permit2

SUN.io Permit2 is a byte-identical fork of Uniswap Permit2:

• Same EIP-712 domain typehash and nameHash (keccak256("Permit2"), no version field)
• Same PermitTransferFrom / PermitBatchTransferFrom / PermitSingle / PermitBatch / AllowanceTransferDetails typehashes and struct layouts
• Same nonce bitmap (248-bit wordPos + 8-bit bitPos, via _useUnorderedNonce)
• DOMAIN_SEPARATOR uses block.chainid at full value (no truncation)

No contract modifications are required. An x402 exact.permit2 payload produces identical EIP-712 digests on EVM and TRON modulo chainId + verifyingContract.

TRON-specific constraints called out in the spec

• No approval sponsoring. TRC-20's approve() requires msg.sender to be the token owner, so a facilitator cannot sponsor the approve() call. The Permit2 fallback is two-layer: EIP-2612 permit → manual user approve.
• ecrecover-only signature verification. TRON has no ERC-1271, so contract-wallet signatures are not supported.
• No Multicall3. Diagnosis uses sequential triggerConstantContract calls.
• TIP-712 (Final) is the formal TRON standard referenced for signing semantics.

Tests

No code affected.

Checklist

• My commits are signed (required for merge)
• Spec follows the structure of scheme_exact_evm.md
• All error codes listed in the Appendix
• All supported networks enumerated with CAIP-2 + chain ID
• Both assetTransferMethod values documented with payload examples

[vercel]

@boboliu-1010 is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[boboliu-1010]

@x402-foundation/core gentle ping — this spec PR (companion to #2075) has been open without review since 2026-04-20. Whenever someone has bandwidth, would appreciate a triage pass or initial feedback.

Note: the failing Vercel check is due to deploy authorization scoped to the Coinbase team — not something I can resolve from this fork. The labeler workflow passed.

[ukstv]

@boboliu-1010 Apparently, Tron does not have a proper CAIP chain id. There are two PRs to CASA: ChainAgnostic/namespaces#170 and ChainAgnostic/namespaces#162, and looks like you're using custom "tron:nile" identifier.

[ukstv]

@boboliu-1010 FYI: the chain ids are borked in the PR. Should be:

Network	Hex	Numeric	Chain ID
Mainnet	0x2b6653dc	728126428	tron:0x2b6653dc
Nile	0xcd8690dc	3448148188	tron:0xcd8690dc

As for x402ExactPermit2Proxy contracts deployed:

Network	Chain ID	Address
Mainnet	tron:0x2b6653dc	TNtw4Wg6uQe4bqFywtcn5qagVZesSdYBSs
Nile	tron:0xcd8690dc	TTjbkCh8sC4gNTWG48iWNssrLBqZq2tiTy

[roger-gan]

@boboliu-1010 Apparently, Tron does not have a proper CAIP chain id. There are two PRs to CASA: ChainAgnostic/namespaces#170 and ChainAgnostic/namespaces#162, and looks like you're using custom "tron:nile" identifier.

Thanks @ukstv — agreed. I’ll update the spec&codes to avoid the custom tron:nile / tron:mainnet identifiers and use the TRON hex chain IDs instead.
BTW, we have also been working on a downstream implementation based on the community version of x402: https://github.com/BofAI/x402, This implementation currently adds support for both TRON and BSC. You mentioned that you have deployed the x402ExactPermit2Proxy contract on the TRON network. Are you also planning to support TRON-related development in x402?