Feedback: `BlockNotFoundError` in Step 5 (Prove the withdrawal on L1)

[opfocus]

While following the ETH bridge tutorial with Viem (OP Sepolia → Sepolia), I ran into a BlockNotFoundError at Step 5 – Prove the withdrawal on the L1, inside buildProveWithdrawal.

The error happens because the output returned in Step 4 – Wait for withdrawal prove is incorrect.
Specifically, the l2BlockNumber value looks like a timestamp (e.g. 1768953611) rather than a valid L2 block number, which later causes getBlock to fail.

After some investigation, this output seems to come from an invalid dispute game created by:

• EOA: 0xbAA4165C68C1A02b435E85575505A494aDc98B93
• DisputeGameFactoryProxy: 0x05F9613aDB30026FFd634f38e5C4dFd30a197Fa1

This issue currently affects the tutorial example when following the documented steps.
This address is repeatedly doing the same thing. I don't know if this indicates a problem; it looks like a test or a prank.

[opfocus]

This contamination affects the tutorial examples on Sepolia quite broadly. In particular, it often occurs when a withdrawal is initiated on L2 and the waitToProve action is executed immediately afterward. At that point, the corresponding state root has not yet been published to L1, but the contaminated output happens to satisfy the condition that l2BlockNumber is greater than the block number of the withdrawal transaction.

A temporary workaround could be the following:
I haven’t investigated yet how viem handles this case internally. However, the Optimism SDK does include a validation process to ensure the output root is valid.

1. Go to
   https://sepolia.etherscan.io/address/0x05f9613adb30026ffd634f38e5c4dfd30a197fa1#readProxyContract

2. Read gameCount to get count.

3. Read gameAtIndex with _index = count - 1. From the output, you can obtain:

   gameType_   uint32  : 0
   timestamp_  uint64  : 1769354772
   proxy_      address: 0x80B4C8dD207585a902a7D959e33256149E82EEDB
   

4. Using the proxy_ contract, retrieve the following data:

   • Call l2BlockNumber()
     (note: this value must be greater than the block number of the withdrawal transaction)
   • Call rootClaim()

   This yields data like:

   {
     l2BlockNumber: 38776023n,
     outputIndex: 64076n,        // the index used above
     outputRoot: '0x0c8d599b846b6488753e0bf4bb1077888dae112dda0e586dd8de52202c58d8b9',
     timestamp: 1769354772n
   }

5. Override the unavailable output obtained from waitToProve with the values above:

   output.l2BlockNumber = 38776023n
   output.outputIndex  = 64076n
   output.outputRoot   = '0x0c8d599b846b6488753e0bf4bb1077888dae112dda0e586dd8de52202c58d8b9'
   output.timestamp    = 1769354772n

6. Then try running the buildProveWithdrawal action again.

[opfocus]

In the Optimism SDK packages there is a validation step for the output root:

isValidOutputRoot(option.rootClaim, option.l2BlockNumber)

However, in viem’s Withdrawals guide I did not see any equivalent validation of the output root after retrieving the L2 output. Additionally, looking at viem’s actions such as getGame, getGames, and getL2Output, none of these appear to perform any output-root validation either.