Why can't I decode an extrinsic created with the contracts tab in Polkadot JS apps?

Question

I am running the substrate-contracts-node and using the contracts UI to interact with it in Polkadot JS Apps. The contract call is failing when I try using the UI, although the same call works from a subxt test program.

Steps to reproduce:

1. Create smart contracts via the upload&deploy button.
2. Call it's method via the UI.

Since my contract failed with ContractTrapped, I went to the events tab to view the extrinsic and it's parameters. Using this I:

1. Reproduced the contract and called it from a test program using subxt.
2. I verified that it passed without problem.
3. I copied the extrinsic hash from the original failed extrinsic
4. I pasted it under the extrinsics tab to decode it. But it was unable to decode the extrinsic.

Why is the contract extrinsic undecodable? I'm suspecting the extrinsic created from the frontend is way different than hand rolling it via subxt.

Answer 1

You tried to decode a hash of the extrinsic, not the actual encoded extrinsic itself.

The "extrinsic hash" is a one-way blake256(<extrinsic data>) so it doesn't contain the decodable extrinsic data.

Answer 2

If submitted via subxt, if you view the specific block inside the apps UI, the extrinsic will be decoded - so you can compare one submitted in another blocks (via JS) to one submitted in a recent block (via subxt).

Since you copied the hash, I'm assuming you could see the extrinsic.

Alternatively, make a rpc query for that block and copy/paste the extrinsic response (hex string) into the UI to be decoded.