Backtrace

Prerequisites

Backtrace feature relies on debug information provided by Scarb. To generate the necessary debug information, you need to have:

  1. Scarb version 2.8.0 or higher
  2. Scarb.toml file with the following Cairo compiler configuration:

📝 Note

If you are using scarb nightly-2025-03-27 there is a way to improve backtrace for panic in contracts if you set panic-backtrace to true in Scarb.toml

[profile.dev.cairo]
unstable-add-statements-code-locations-debug-info = true
unstable-add-statements-functions-debug-info = true
panic-backtrace = true # only for scarb nightly-2025-03-27

📝 Note

That unstable-add-statements-code-locations-debug-info = true and unstable-add-statements-functions-debug-info = true will slow down the compilation and cause it to use more system memory. It will also make the compilation artifacts larger. So it is only recommended to add these flags when you need their functionality.

Usage

When a contract call fails, the error message alone may not always provide enough information to identify the root cause of the issue. To aid in debugging, snforge offers a feature that can generate a backtrace of the execution.

If your contract fails and a backtrace can be generated, snforge will prompt you to run the operation again with the SNFORGE_BACKTRACE=1 environment variable (if it’s not already configured). For example, you may see failure data like this:

$ snforge test
Output: 
Failure data:
    0x417373657274206661696c6564 ('Assert failed')
note: run with `SNFORGE_BACKTRACE=1` environment variable to display a backtrace

To enable backtraces, simply set the SNFORGE_BACKTRACE=1 environment variable and rerun the operation.

When enabled, the backtrace will display the call tree of the execution, including the specific line numbers in the contracts where the errors occurred. Here's an example of what you might see:

$ SNFORGE_BACKTRACE=1 snforge test
Output: 
"Failure data:
    0x417373657274206661696c6564 ('Assert failed')
error occurred in contract 'InnerContract'
stack backtrace:
   0: (inlined) core::array::ArrayImpl::append
       at [..]array.cairo:135:9
   1: core::array_inline_macro
       at [..]lib.cairo:364:11
   2: (inlined) core::Felt252PartialEq::eq
       at [..]lib.cairo:231:9
   3: (inlined) backtrace_panic::InnerContract::inner_call
       at [..]traits.cairo:442:10
   4: (inlined) backtrace_panic::InnerContract::InnerContract::inner
       at [..]lib.cairo:40:16
   5: backtrace_panic::InnerContract::__wrapper__InnerContract__inner
       at [..]lib.cairo:35:13

error occurred in contract 'OuterContract'
stack backtrace:
   0: (inlined) backtrace_panic::IInnerContractDispatcherImpl::inner
       at [..]lib.cairo:22:1
   1: (inlined) backtrace_panic::OuterContract::OuterContract::outer
       at [..]lib.cairo:17:13
   2: backtrace_panic::OuterContract::__wrapper__OuterContract__outer
       at [..]lib.cairo:15:9"