Proposal: add `verifying-contracts-on-basescan` skill

[xam-dev-ux]

Proposal

Add a new skill verifying-contracts-on-basescan covering contract verification on Base Mainnet and Sepolia.

Why this keeps breaking in practice

The most common failure mode — for both developers and AI coding agents — is using the deprecated Basescan V1 API. V1 was fully deactivated on August 15, 2025, and the error it returns is:

You are using a deprecated V1 endpoint, switch to Etherscan API V2
using https://docs.etherscan.io/v2-migration

Stale docs, LLM training data, and blog posts keep surfacing api.basescan.org/api and per-chain Basescan keys. The correct path is the unified Etherscan V2 endpoint (https://api.etherscan.io/v2/api) with chainid=8453 / 84532 and a single Etherscan key. A dedicated skill is the cleanest fix: one authoritative reference that agents can be pointed to.

This is already tested — not just proposed

I verified the full flow on Base Sepolia before opening this issue:

• Deployed SimpleStorage with a constructor argument
• Verified via Etherscan V2 (forge verify-contract --verifier-url "https://api.etherscan.io/v2/api?chainid=84532"): 0xA6E4e2977dD3470Fa3E9683B98DC30B888755D0a → Pass - Verified
• Verified the same contract via Sourcify (--verifier sourcify, no API key): 0xF0E6DF13b99525f9144D82f23b9DF860BAB4EE1e → exact_match
• Confirmed the V1 endpoint returns exactly the error above
• Confirmed Foundry's already verified. Skipping verification behavior (graceful, not an error)

The skill content is already drafted and matches the structure and tone of existing skills.

Scope (small, focused)

• V1 → V2 migration table with the exact error message
• Foundry forge verify-contract (Mainnet/Sepolia, constructor args, --watch, linked libraries)
• foundry.toml V2 config replacing the deprecated per-chain basescan URLs
• Hardhat @nomicfoundation/hardhat-verify — apiKey as single string + customChains pointing to V2
• Manual UI verification on basescan.org (single file vs standard JSON input)
• Sourcify as a decentralized alternative (no API key)
• Common errors table with causes and fixes
• Cross-reference to deploying-contracts-on-base for deploy+verify flow — no duplication

Out of scope

Deployment itself — that lives in deploying-contracts-on-base.

On CONTRIBUTING.md

I noticed it currently says contributions are limited to the Base core team. Happy to work within whatever process you prefer — if you'd rather take the content and merge it internally that's fine too, just say the word. cc @youssefea @soheimam