debug_traceTransaction Developer Guide

This guide covers testing and development workflows for Forest's debug_traceTransaction implementation. For API documentation and user-facing usage, see the debug_traceTransaction API guide.

Tracer Contract

The Tracer.sol contract provides various functions to test different tracing scenarios.

Storage Layout

Slot	Variable	Description
0	`x`	Initialized to 42
1	`balances`	Mapping base slot
2	`storageTestA`	Starts empty (for add tests)
3	`storageTestB`	Starts empty
4	`storageTestC`	Starts empty
5	`dynamicArray`	Array length slot

Function Reference

Basic Operations

Function	Selector	Description
`setX(uint256)`	`0x4018d9aa`	Write to slot 0
`deposit()`	`0xd0e30db0`	Receive ETH, update mapping
`withdraw(uint256)`	`0x2e1a7d4d`	Send ETH from contract
`doRevert()`	`0xafc874d2`	Always reverts

Call Tracing

Function	Selector	Description
`callSelf(uint256)`	`0xa1a88595`	Single nested CALL
`delegateSelf(uint256)`	`0x8f5e07b8`	`DELEGATECALL` trace
`complexTrace()`	`0x6659ab96`	Multiple nested calls
`deepTrace(uint256)`	`0x0f3a17b8`	Recursive N-level deep

Storage Diff Testing

Function	Selector	Description
`storageAdd(uint256)`	`0x55cb64b4`	Add to empty slot 2
`storageChange(uint256)`	`0x7c8f6e57`	Modify existing slot
`storageDelete()`	`0xd92846a3`	Set slot to zero
`storageMultiple(uint256,uint256,uint256)`	`0x310af204`	Change slots 2,3,4

Generating Function Selectors

Use cast from Foundry to generate function selectors:

# Get selector for a function
cast sig "setX(uint256)"
# Output: 0x4018d9aa

# Encode full calldata
cast calldata "setX(uint256)" 123
# Output: 0x4018d9aa000000000000000000000000000000000000000000000000000000000000007b

Deployed Contracts

Pre-deployed Tracer contracts for quick testing:

Network	Contract Address
Calibnet	`0x73a43475aa2ccb14246613708b399f4b2ba546c7`
Mainnet	`0x9BB686Ba6a50D1CF670a98f522a59555d4977fb2`

Comparison Testing with Anvil

Anvil uses the same Geth style tracing as debug_traceTransaction, making it ideal for direct comparison testing — verifying that Forest produces identical or semantically equivalent results.

Prerequisites

Foundry installed (forge, cast commands)
A running Forest node and Anvil instance

What is Anvil?

Anvil is a local Ethereum development node included with Foundry. It provides:

Instant block mining
Pre-funded test accounts (10 accounts with 10,000 ETH each)
Support for debug_traceTransaction with various tracers
No real tokens required

Starting Anvil

# Start Anvil with tracer to allow debug_traceTransaction API's
anvil --tracing

Anvil RPC endpoint: http://localhost:8545

Deploying Contract on Anvil

forge create src/tool/subcommands/api_cmd/contracts/tracer/Tracer.sol:Tracer \
    --rpc-url http://localhost:8545 \
    --broadcast \
    --private-key <ANVIL_OUTPUT_PRIVATE_KEY>

# Output:
# Deployer: 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266
# Deployed to: 0x5FbDB2315678afecb367f032d93F642f64180aa3
# Transaction hash: 0x...

Sending Test Transactions on Anvil

Unlike trace_call (which simulates calls), debug_traceTransaction traces mined transactions. You must first send transactions to get transaction hashes:

# Set variables
export ANVIL_RPC="http://localhost:8545"
export ANVIL_KEY="0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80"  # Anvil default key
export ANVIL_CONTRACT="0x5FbDB2315678afecb367f032d93F642f64180aa3"  # Deployed address

# Send transactions
cast send $ANVIL_CONTRACT "setX(uint256)" 123 \
    --rpc-url $ANVIL_RPC --private-key $ANVIL_KEY

cast send $ANVIL_CONTRACT "deposit()" \
    --value 1ether --rpc-url $ANVIL_RPC --private-key $ANVIL_KEY

cast send $ANVIL_CONTRACT "storageAdd(uint256)" 100 \
    --rpc-url $ANVIL_RPC --private-key $ANVIL_KEY

cast send $ANVIL_CONTRACT "callSelf(uint256)" 456 \
    --rpc-url $ANVIL_RPC --private-key $ANVIL_KEY

cast send $ANVIL_CONTRACT "storageMultiple(uint256,uint256,uint256)" 10 20 30 \
    --rpc-url $ANVIL_RPC --private-key $ANVIL_KEY

Save the transaction hashes from the output for use in the tracing examples below.

Comparing Forest vs Anvil Responses

Both Forest and Anvil use the same debug_traceTransaction method and tracer format, so responses can be compared directly (unlike trace_call vs debug_traceCall which use different formats).

Forest:

curl -s -X POST "http://localhost:2345/rpc/v1" \
    -H "Content-Type: application/json" \
    -d '{
        "jsonrpc": "2.0",
        "id": 1,
        "method": "debug_traceTransaction",
        "params": [
            "'$TX_HASH'",
            {"tracer": "prestateTracer", "tracerConfig": {"diffMode": true}}
        ]
    }'

Anvil:

curl -s -X POST "http://localhost:8545" \
    -H "Content-Type: application/json" \
    -d '{
        "jsonrpc": "2.0",
        "id": 1,
        "method": "debug_traceTransaction",
        "params": [
            "'$TX_HASH'",
            {"tracer": "prestateTracer", "tracerConfig": {"diffMode": true}}
        ]
    }'

Expected Differences Between Forest and Anvil

When comparing debug_traceTransaction output from Forest and Anvil, expect these Filecoin-specific differences:

Aspect	Forest (Filecoin)	Anvil (Ethereum)
Extra addresses	Includes `0xff00...` Filecoin ID addresses	Only EVM addresses
Coinbase	Not included (gas at protocol level)	Included as `0x0000...0000`
Nonce format	Hex string (e.g., `"0x1e"`)	Integer (e.g., `30`)
Balance format	Hex string (e.g., `"0xde0b6b3a7640000"`)	Hex string (same)
Storage key format	Full 32-byte padded hex	Full 32-byte padded hex (same)

Test Scenarios

When testing debug_traceTransaction, cover these categories:

1. prestateTracer Tests

Test Case	Function	What to Verify
Simple storage write	`setX(123)`	Pre-state shows old value, post-state shows new value
ETH deposit	`deposit()` with value	Balance changes in pre/post for sender and contract
Storage add (empty → value)	`storageAdd(100)`	Storage slot absent in pre, present in post (diff mode)
Storage change	`storageChange(200)`	Storage slot values differ between pre and post
Multiple storage writes	`storageMultiple(10,20,30)`	Multiple storage slots change in single transaction
Default mode (no diff)	`setX(123)`	Only pre-state returned, no post object

2. callTracer Tests

Test Case	Function	What to Verify
Simple call	`setX(123)`	Single top-level CALL frame
Nested call	`callSelf(456)`	Parent CALL with child CALL in `calls` array
Delegate call	`delegateSelf(789)`	DELEGATECALL type in call frame
Deep recursive	`deepTrace(3)`	N-level nested CALL hierarchy
Complex mixed	`complexTrace()`	Mix of CALL, DELEGATECALL, STATICCALL
Revert at depth	`failAtDepth(3,1)`	Error field populated in failing frame

3. flatCallTracer Tests

Test Case	Function	What to Verify
Simple call	`setX(123)`	Single flat trace entry
Nested call	`callSelf(456)`	Two entries with correct `traceAddress` and `subtraces`
Deep recursive	`deepTrace(3)`	Flat list with incrementing `traceAddress` depth

Integration Test Script

An automated test script is available to compare Forest's debug_traceTransaction with Anvil:

# Run the test (requires Forest and Anvil running)
./scripts/tests/debug_trace_transaction_test.sh

# Deploy contract on Anvil first, forest and anvil node should already be running
./scripts/tests/debug_trace_transaction_test.sh --deploy

Test Categories

Prestate Tests: Pre/post state snapshots, default mode, diff mode
Call Trace Tests: Call hierarchy, nested calls, reverts
Storage Tests: Single slot, multiple slots, add/change/delete patterns

Tracer Contract​

Storage Layout​

Function Reference​

Basic Operations​

Call Tracing​

Storage Diff Testing​

Generating Function Selectors​

Deployed Contracts​

Comparison Testing with Anvil​

Prerequisites​

What is Anvil?​

Starting Anvil​

Deploying Contract on Anvil​

Sending Test Transactions on Anvil​

Comparing Forest vs Anvil Responses​

Expected Differences Between Forest and Anvil​

Test Scenarios​

1. prestateTracer Tests​

2. callTracer Tests​

3. flatCallTracer Tests​

Integration Test Script​

Test Categories​

Official Resources​