Task 29: Grat Cannot Decode Which Function and Arguments a Transaction Actually Called

[grantfox-oss]

Every Soroban transaction begins with a Host Function Invocation, which is the exact command instructing the VM to execute a specific function on a specific contract with a specific set of arguments. In the raw XDR, this invocation is encoded as an untyped array of raw bytes (SCVec). Without decoding this invocation, developers cannot verify if they accidentally passed the wrong variables to their contract. Currently, Grat has no logic to map the raw argument array against the contract's actual function signature, meaning it cannot confidently tell the user what function they actually called or what parameters they supplied.

Issues/what to fix

A new analytical module named FunctionCallDecoder must be created. This module takes the raw XDR HostFunction object as input. First, it must extract the target function_name (a Symbol) and the raw args (SCVec). It must then utilize the SpecParser to retrieve the contract's ContractFunction metadata that matches the function_name. Once matched, the decoder must utilize a zip algorithm to iterate over the function's expected parameter list alongside the raw XDR arguments array simultaneously. For each pair, it must utilize the SCVal to JSON Converter to parse the argument, attaching the expected parameter name to the output. If the number of provided arguments does not match the expected parameters, the decoder must intercept the arity mismatch and throw a highly descriptive GratError::ArityMismatch.

Files to create / modify

Create: crates/core/src/decode/function_call_decoder.rs

Expected result

Grat will be able to reconstruct the exact function call that initiated the transaction. Developers will be presented with a beautiful, fully labeled summary of their invocation (e.g., transfer(from: "G...", to: "G...", amount: 1000)), making it incredibly easy to spot logical errors or typos in their input payloads.

Contributor telegram group

https://t.me/+sII7WPhll2liMGNk

[egwujiohaifesinachiperpetual-max]

Hello boss, Please assign this issue to me. i checked it out and im confident i will solve it
my eta is 4 hours

[JesseJohn7]

Hi, I'd like to work on this issue. I've gone through the description and acceptance criteria and I'm confident I can deliver it. I'll make sure every acceptance criterion is met and submit a clean, tested PR. Please assign it to me.

[Lspnjr1]

Hello maintainers,

I’d like to work on this issue and would appreciate being assigned if it’s available. I’ve reviewed the details and am confident I can implement the changes, test them thoroughly, and submit a pull request once complete. I expect to finish the issue and open a PR within 2–3 days.

Thank you.

[grantfox-oss]

🦊 GrantFox — @JesseJohn7 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #342)
2. Your PR will be reviewed by the Toolbox-Lab maintainers

Good luck! Track your progress on GrantFox.

[only1rezie]

My Implementation Plan — Decode the Actual Soroban Function

Goal- Add a deterministic decoder that reconstructs the exact contract function and labeled arguments that a Soroban transaction invoked, instead of showing only raw XDR bytes.

1.Create FunctionCallDecoder

Add crates/core/src/decode/function_call_decoder.rs to decode Soroban HostFunction invocations into a structured, human-readable function call.

2.Extract Invocation Details

Parse the raw HostFunction to extract the function_name (Symbol) and raw argument list (SCVec).
Resolve Function Signature
Use SpecParser to find the matching ContractFunction metadata based on the extracted function name.

3.Validate Arity

Compare the number of provided arguments with the function's expected parameters. If they differ, return a descriptive GratError::ArityMismatch.

4.Decode and Label Arguments

Zip the expected parameter list with the raw arguments, decode each argument using the existing SCVal-to-JSON converter, and attach the corresponding parameter name.

5.Return a Decoded Function Call

Produce a structured representation of the invocation (e.g., transfer(from: "...", to: "...", amount: 1000)) for use in the CLI/UI, making incorrect or misplaced arguments immediately visible.