trace_call Developer Guide

This guide covers testing and development workflows for Forest's trace_call implementation. For API documentation and user-facing usage, see the trace_call 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 Geth style tracing (debug_traceCall with prestateTracer), while Forest uses Parity style tracing (trace_call with stateDiff). This makes Anvil useful for comparison testing — verifying that Forest produces semantically equivalent results in a different format.

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_traceCall with various tracers
No real tokens required

Starting Anvil

# Start Anvil with tracer to allow debug_traceCall 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...

Comparing Forest vs Anvil Responses

The same contract call can be tested against both nodes. The request data payloads are identical; only the method name and parameter ordering differ.

Forest (Parity-style trace_call):

curl -s -X POST "http://localhost:2345/rpc/v1" \
    -H "Content-Type: application/json" \
    -d '{
        "jsonrpc": "2.0",
        "id": 1,
        "method": "trace_call",
        "params": [
            {"from": "0x...", "to": "0x...", "data": "0x4018d9aa..."},
            ["stateDiff"],
            "latest"
        ]
    }'

Anvil (Geth style debug_traceCall):

curl -s -X POST "http://localhost:8545" \
    -H "Content-Type: application/json" \
    -d '{
        "jsonrpc": "2.0",
        "id": 1,
        "method": "debug_traceCall",
        "params": [
            {"from": "0x...", "to": "0x...", "data": "0x4018d9aa..."},
            "latest",
            {"tracer": "prestateTracer", "tracerConfig": {"diffMode": true}}
        ]
    }'

Integration Test Script

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

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

or

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

Test Categories

Trace Tests: Call hierarchy, subcalls, reverts, deep traces
Balance Diff Tests: ETH transfers, deposits
Storage Diff Tests: Single slot, multiple slots, value comparison

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​

Comparing Forest vs Anvil Responses​

Integration Test Script​

Test Categories​

Official Resources​