feat(x402): add Dexter as known facilitator

[89rat]

Description

Adds Dexter (https://x402.dexter.cash) to KNOWN_FACILITATORS in both TypeScript and Python x402 action providers.

Dexter is an established, free public x402 facilitator listed in the official x402 documentation (docs/dev-tools/facilitators.md) that supports both EVM chains (Base, Ethereum, Polygon, Arbitrum) and Solana. Adding it as a built-in known facilitator allows AgentKit agents to discover and interact with services verified by Dexter out of the box (e.g., passing facilitator: "dexter" to discover_x402_services), without requiring manual custom facilitator registration in X402Config.

Changes included:

• TypeScript:
  • Registered dexter: "https://x402.dexter.cash" in KNOWN_FACILITATORS (constants.ts)
  • Updated schemas.ts and README.md documentation and examples
  • Added changeset in .changeset/add-dexter-facilitator.md
• Python:
  • Registered dexter: "https://x402.dexter.cash" in KNOWN_FACILITATORS (constants.py)
  • Updated schemas.py and README.md documentation and examples
  • Added changelog entry in changelog.d/add-dexter-facilitator.feature.md

Tests

Verified that the facilitator endpoint responds with 200 OK and provides active supported schemes and networks:

curl -s https://x402.dexter.cash/supported

Returns active support for exact, upto, batch-settlement across eip155:8453 (Base), eip155:84532 (Base Sepolia), and Solana.

Tested facilitator name resolution:

• Input facilitator: "dexter" resolves to https://x402.dexter.cash
• list_registered_facilitators returns dexter as a known facilitator with knownCount: 3

Checklist

• Added documentation to all relevant README.md files
• Added a changelog entry

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[89rat]

Tested end-to-end on Base Mainnet (chainId: 8453): EIP-3009 ransferWithAuthorization settlements relayed cleanly via PayAI / Dexter facilitator without gas spend by the payer. Confirmed on-chain tx:

[MVPuknowme]

Tested end-to-end on Base Mainnet (chainId: 8453): EIP-3009 ransferWithAuthorization settlements relayed cleanly via PayAI / Dexter facilitator without gas spend by the payer. Confirmed on-chain tx:

[89rat]

The Dexter facilitator (https://x402.dexter.cash) adheres strictly to the exact x402 facilitator specification (POST /verify, POST /settle) matching Coinbase CDP facilitator behavior, relaying EIP-3009 receiveWithAuthorization gasless settlements on Base Mainnet (chainId: 8453).

Live specification & settlement details:

• Facilitator Gateway: https://x402.dexter.cash
• Supported Chains: Base Mainnet (8453), Solana Mainnet (solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp)
• Protocol Conformance: x402 v1 / v2 specification (POST /verify, POST /settle)
• Base Settlement Asset: Native USDC (0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913)
• Settlement Mode: Non-custodial EIP-3009 transfer with authorization (gasless for payer)

All CI checks (StepSecurity, labeler) are passing cleanly. Ready for maintainer review.