---
title: Custom EVM tracer
description: Introduction to writing custom tracers for Geth
---

In addition to the default opcode tracer and the built-in tracers, Geth offers the possibility to write custom code that hook to events in the EVM to process and return the data in a consumable format. Custom tracers can be written either in Javascript or Go. JS tracers are good for quick prototyping and experimentation as well as for less intensive applications. Go tracers are performant but require the tracer to be compiled together with the Geth source code.

## Custom Javascript tracing

Transaction traces include the complete status of the EVM at every point during the transaction execution, which can be a very large amount of data. Often, users are only interested in a small subset of that data. Javascript trace filters are available to isolate the useful information. Detailed information about `debug_traceTransaction` and its component parts is available in the [reference documentation](/docs/rpc/ns-debug#debug_tracetransaction).

### A simple filter

Filters are Javascript functions that select information from the trace to persist and discard based on some conditions. The following Javascript function returns only the sequence of opcodes executed by the transaction as a comma-separated list. The function could be written directly in the Javascript console, but it is cleaner to write it in a separate re-usable file and load it into the console.

1. Create a file, `filterTrace_1.js`, with this content:

   ```javascript
   tracer = function (tx) {
     return debug.traceTransaction(tx, {
       tracer:
         '{' +
         'retVal: [],' +
         'step: function(log,db) {this.retVal.push(log.getPC() + ":" + log.op.toString())},' +
         'fault: function(log,db) {this.retVal.push("FAULT: " + JSON.stringify(log))},' +
         'result: function(ctx,db) {return this.retVal}' +
         '}'
     }); // return debug.traceTransaction ...
   }; // tracer = function ...
   ```

2. Run the [JavaScript console](https://geth.ethereum.org/docs/interface/javascript-console).
3. Get the hash of a recent transaction from a node or block explorer.

4. Run this command to run the script:

   ```javascript
   loadScript('filterTrace_1.js');
   ```

5. Run the tracer from the script. Be patient, it could take a long time.

   ```javascript
   tracer('<hash of transaction>');
   ```

   The bottom of the output looks similar to:

   ```sh
   "3366:POP", "3367:JUMP", "1355:JUMPDEST", "1356:PUSH1", "1358:MLOAD", "1359:DUP1", "1360:DUP3", "1361:ISZERO", "1362:ISZERO",
   "1363:ISZERO", "1364:ISZERO", "1365:DUP2", "1366:MSTORE", "1367:PUSH1", "1369:ADD", "1370:SWAP2", "1371:POP", "1372:POP", "1373:PUSH1",
   "1375:MLOAD", "1376:DUP1", "1377:SWAP2", "1378:SUB", "1379:SWAP1", "1380:RETURN"
   ```

6. Run this line to get a more readable output with each string in its own line.

   ```javascript
   console.log(JSON.stringify(tracer('<hash of transaction>'), null, 2));
   ```

More information about the `JSON.stringify` function is available [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify).

The commands above worked by calling the same `debug.traceTransaction` function that was previously explained in [basic traces](https://geth.ethereum.org/docs/dapp/tracing), but with a new parameter, `tracer`. This parameter takes the JavaScript object formated as a string. In the case of the trace above, it is:

```javascript
{
   retVal: [],
   step: function(log,db) {this.retVal.push(log.getPC() + ":" + log.op.toString())},
   fault: function(log,db) {this.retVal.push("FAULT: " + JSON.stringify(log))},
   result: function(ctx,db) {return this.retVal}
}
```

This object has three member functions:

- `step`, called for each opcode.
- `fault`, called if there is a problem in the execution.
- `result`, called to produce the results that are returned by `debug.traceTransaction` 
- after the execution is done.

In this case, `retVal` is used to store the list of strings to return in `result`.

The `step` function adds to `retVal` the program counter and the name of the opcode there. Then, in `result`, this list is returned to be sent to the caller.

### Filtering with conditions

For actual filtered tracing we need an `if` statement to only log relevant information. For example, to isolate the transaction's interaction with storage, the following tracer could be used:

```javascript
tracer = function (tx) {
  return debug.traceTransaction(tx, {
    tracer:
      '{' +
      'retVal: [],' +
      'step: function(log,db) {' +
      '   if(log.op.toNumber() == 0x54) ' +
      '     this.retVal.push(log.getPC() + ": SLOAD");' +
      '   if(log.op.toNumber() == 0x55) ' +
      '     this.retVal.push(log.getPC() + ": SSTORE");' +
      '},' +
      'fault: function(log,db) {this.retVal.push("FAULT: " + JSON.stringify(log))},' +
      'result: function(ctx,db) {return this.retVal}' +
      '}'
  }); // return debug.traceTransaction ...
}; // tracer = function ...
```

The `step` function here looks at the opcode number of the op, and only pushes an entry if the opcode is `SLOAD` or `SSTORE` ([here is a list of EVM opcodes and their numbers](https://github.com/wolflo/evm-opcodes)). We could have used `log.op.toString()` instead, but it is faster to compare numbers rather than strings.

The output looks similar to this:

```javascript
[
  "5921: SLOAD",
  .
  .
  .
  "2413: SSTORE",
  "2420: SLOAD",
  "2475: SSTORE",
  "6094: SSTORE"
]
```

### Stack Information

The trace above reports the program counter (PC) and whether the program read from storage or wrote to it. That alone isn't particularly useful. To know more, the `log.stack.peek` function can be used to peek into the stack. `log.stack.peek(0)` is the stack top, `log.stack.peek(1)` the entry below it, etc.

The values returned by `log.stack.peek` are Go `big.Int` objects. By default they are converted to JavaScript floating point numbers, so you need `toString(16)` to get them as hexadecimals, which is how 256-bit values such as storage cells and their content are normally represented.

#### Storage Information

The function below provides a trace of all the storage operations and their parameters. This gives a more complete picture of the program's interaction with storage.

```javascript
tracer = function (tx) {
  return debug.traceTransaction(tx, {
    tracer:
      '{' +
      'retVal: [],' +
      'step: function(log,db) {' +
      '   if(log.op.toNumber() == 0x54) ' +
      '     this.retVal.push(log.getPC() + ": SLOAD " + ' +
      '        log.stack.peek(0).toString(16));' +
      '   if(log.op.toNumber() == 0x55) ' +
      '     this.retVal.push(log.getPC() + ": SSTORE " +' +
      '        log.stack.peek(0).toString(16) + " <- " +' +
      '        log.stack.peek(1).toString(16));' +
      '},' +
      'fault: function(log,db) {this.retVal.push("FAULT: " + JSON.stringify(log))},' +
      'result: function(ctx,db) {return this.retVal}' +
      '}'
  }); // return debug.traceTransaction ...
}; // tracer = function ...
```

The output is similar to:

```javascript
[
  "5921: SLOAD 0",
  .
  .
  .
  "2413: SSTORE 3f0af0a7a3ed17f5ba6a93e0a2a05e766ed67bf82195d2dd15feead3749a575d <- fb8629ad13d9a12456",
  "2420: SLOAD cc39b177dd3a7f50d4c09527584048378a692aed24d31d2eabeddb7f3c041870",
  "2475: SSTORE cc39b177dd3a7f50d4c09527584048378a692aed24d31d2eabeddb7f3c041870 <- 358c3de691bd19",
  "6094: SSTORE 0 <- 1"
]
```

#### Operation Results

One piece of information missing from the function above is the result on an `SLOAD` operation. The state we get inside `log` is the state prior to the execution of the opcode, so that value is not known yet. For more operations we can figure it out for ourselves, but we don't have access to the
storage, so here we can't.

The solution is to have a flag, `afterSload`, which is only true in the opcode right after an `SLOAD`, when we can see the result at the top of the stack.

```javascript
tracer = function (tx) {
  return debug.traceTransaction(tx, {
    tracer:
      '{' +
      'retVal: [],' +
      'afterSload: false,' +
      'step: function(log,db) {' +
      '   if(this.afterSload) {' +
      '     this.retVal.push("    Result: " + ' +
      '          log.stack.peek(0).toString(16)); ' +
      '     this.afterSload = false; ' +
      '   } ' +
      '   if(log.op.toNumber() == 0x54) {' +
      '     this.retVal.push(log.getPC() + ": SLOAD " + ' +
      '        log.stack.peek(0).toString(16));' +
      '        this.afterSload = true; ' +
      '   } ' +
      '   if(log.op.toNumber() == 0x55) ' +
      '     this.retVal.push(log.getPC() + ": SSTORE " +' +
      '        log.stack.peek(0).toString(16) + " <- " +' +
      '        log.stack.peek(1).toString(16));' +
      '},' +
      'fault: function(log,db) {this.retVal.push("FAULT: " + JSON.stringify(log))},' +
      'result: function(ctx,db) {return this.retVal}' +
      '}'
  }); // return debug.traceTransaction ...
}; // tracer = function ...
```

The output now contains the result in the line that follows the `SLOAD`.

```javascript
[
  "5921: SLOAD 0",
  "    Result: 1",
  .
  .
  .
  "2413: SSTORE 3f0af0a7a3ed17f5ba6a93e0a2a05e766ed67bf82195d2dd15feead3749a575d <- fb8629ad13d9a12456",
  "2420: SLOAD cc39b177dd3a7f50d4c09527584048378a692aed24d31d2eabeddb7f3c041870",
  "    Result: 0",
  "2475: SSTORE cc39b177dd3a7f50d4c09527584048378a692aed24d31d2eabeddb7f3c041870 <- 358c3de691bd19",
  "6094: SSTORE 0 <- 1"
]
```

### Dealing With Calls Between Contracts

So the storage has been treated as if there are only 2<sup>256</sup> cells. However, that is not true. Contracts can call other contracts, and then the storage involved is the storage of the other contract. We can see the address of the current contract in `log.contract.getAddress()`. This value is the execution context - the contract whose storage we are using - even when code from another contract is executed (by using
[`CALLCODE` or `DELEGATECALL`](https://docs.soliditylang.org/en/v0.8.14/introduction-to-smart-contracts.html#delegatecall-callcode-and-libraries)).

However, `log.contract.getAddress()` returns an array of bytes. To convert this to the familiar hexadecimal representation of Ethereum addresses, `this.byteHex()` and `array2Hex()` can be used.

```javascript
tracer = function (tx) {
  return debug.traceTransaction(tx, {
    tracer:
      '{' +
      'retVal: [],' +
      'afterSload: false,' +
      'callStack: [],' +
      'byte2Hex: function(byte) {' +
      '  if (byte < 0x10) ' +
      '      return "0" + byte.toString(16); ' +
      '  return byte.toString(16); ' +
      '},' +
      'array2Hex: function(arr) {' +
      '  var retVal = ""; ' +
      '  for (var i=0; i<arr.length; i++) ' +
      '    retVal += this.byte2Hex(arr[i]); ' +
      '  return retVal; ' +
      '}, ' +
      'getAddr: function(log) {' +
      '  return this.array2Hex(log.contract.getAddress());' +
      '}, ' +
      'step: function(log,db) {' +
      '   var opcode = log.op.toNumber();' +
      // SLOAD
      '   if (opcode == 0x54) {' +
      '     this.retVal.push(log.getPC() + ": SLOAD " + ' +
      '        this.getAddr(log) + ":" + ' +
      '        log.stack.peek(0).toString(16));' +
      '        this.afterSload = true; ' +
      '   } ' +
      // SLOAD Result
      '   if (this.afterSload) {' +
      '     this.retVal.push("    Result: " + ' +
      '          log.stack.peek(0).toString(16)); ' +
      '     this.afterSload = false; ' +
      '   } ' +
      // SSTORE
      '   if (opcode == 0x55) ' +
      '     this.retVal.push(log.getPC() + ": SSTORE " +' +
      '        this.getAddr(log) + ":" + ' +
      '        log.stack.peek(0).toString(16) + " <- " +' +
      '        log.stack.peek(1).toString(16));' +
      // End of step
      '},' +
      'fault: function(log,db) {this.retVal.push("FAULT: " + JSON.stringify(log))},' +
      'result: function(ctx,db) {return this.retVal}' +
      '}'
  }); // return debug.traceTransaction ...
}; // tracer = function ...
```

The output is similar to:

```javascript
[
  "423: SLOAD 22ff293e14f1ec3a09b137e9e06084afd63addf9:360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc",
  "    Result: 360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc",
  "10778: SLOAD 22ff293e14f1ec3a09b137e9e06084afd63addf9:6",
  "    Result: 6",
  .
  .
  .
  "13529: SLOAD f2d68898557ccb2cf4c10c3ef2b034b2a69dad00:8328de571f86baa080836c50543c740196dbc109d42041802573ba9a13efa340",
  "    Result: 8328de571f86baa080836c50543c740196dbc109d42041802573ba9a13efa340",
  "423: SLOAD f2d68898557ccb2cf4c10c3ef2b034b2a69dad00:360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc",
  "    Result: 360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc",
  "13529: SLOAD f2d68898557ccb2cf4c10c3ef2b034b2a69dad00:b38558064d8dd9c883d2a8c80c604667ddb90a324bc70b1bac4e70d90b148ed4",
  "    Result: b38558064d8dd9c883d2a8c80c604667ddb90a324bc70b1bac4e70d90b148ed4",
  "11041: SSTORE 22ff293e14f1ec3a09b137e9e06084afd63addf9:6 <- 0"
]
```

## Other traces

This tutorial has focused on `debug_traceTransaction()` which reports information about individual transactions. There are also RPC endpoints that provide different information, including tracing the EVM execution within a block, between two blocks, for specific `eth_call`s or rejected blocks. The full list of trace functions can be explored in the [reference documentation](/content/docs/interacting_with_geth/RPC/ns-debug.md).

## Custom Go tracing

Custom tracers can also be made more performant by writing them in Go. The gain in performance mostly comes from the fact that Geth doesn't need
to interpret JS code and can execute native functions. Geth comes with several built-in [native tracers](https://github.com/ethereum/go-ethereum/tree/master/eth/tracers/native) which can serve as examples. Please note that unlike JS tracers, Go tracing scripts cannot be simply passed as an argument to the API. They will need to be added to and compiled with the rest of the Geth source code.

In this section a simple native tracer that counts the number of opcodes will be covered. First follow the instructions to [clone and build](/content/docs/getting_started/Installing-Geth.md) Geth from source code. Next save the following snippet as a `.go` file and add it to `eth/tracers/native`:

```go
package native

import (
    "encoding/json"
    "math/big"
    "sync/atomic"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core/vm"
    "github.com/ethereum/go-ethereum/eth/tracers"
)

func init() {
    // This is how Geth will become aware of the tracer and register it under a given name
    register("opcounter", newOpcounter)
}

type opcounter struct {
    env       *vm.EVM
    counts    map[string]int // Store opcode counts
    interrupt uint32         // Atomic flag to signal execution interruption
    reason    error          // Textual reason for the interruption
}

func newOpcounter(ctx *tracers.Context, cfg json.RawMessage) tracers.Tracer {
    return &opcounter{counts: make(map[string]int)}
}

// CaptureStart implements the EVMLogger interface to initialize the tracing operation.
func (t *opcounter) CaptureStart(env *vm.EVM, from common.Address, to common.Address, create bool, input []byte, gas uint64, value *big.Int) {
        t.env = env
}

// CaptureState implements the EVMLogger interface to trace a single step of VM execution.
func (t *opcounter) CaptureState(pc uint64, op vm.OpCode, gas, cost uint64, scope *vm.ScopeContext, rData []byte, depth int, err error) {
    // Skip if tracing was interrupted
    if atomic.LoadUint32(&t.interrupt) > 0 {
        t.env.Cancel()
        return
    }

    name := op.String()
    if _, ok := t.counts[name]; !ok {
        t.counts[name] = 0
    }
    t.counts[name]++
}

// CaptureEnter is called when EVM enters a new scope (via call, create or selfdestruct).
func (t *opcounter) CaptureEnter(op vm.OpCode, from common.Address, to common.Address, input []byte, gas uint64, value *big.Int) {}

// CaptureExit is called when EVM exits a scope, even if the scope didn't
// execute any code.
func (t *opcounter) CaptureExit(output []byte, gasUsed uint64, err error) {}

// CaptureFault implements the EVMLogger interface to trace an execution fault.
func (t *opcounter) CaptureFault(pc uint64, op vm.OpCode, gas, cost uint64, scope *vm.ScopeContext, depth int, err error) {}

// CaptureEnd is called after the call finishes to finalize the tracing.
func (t *opcounter) CaptureEnd(output []byte, gasUsed uint64, _ time.Duration, err error) {}

func (*opcounter) CaptureTxStart(gasLimit uint64) {}

func (*opcounter) CaptureTxEnd(restGas uint64) {}

// GetResult returns the json-encoded nested list of call traces, and any
// error arising from the encoding or forceful termination (via `Stop`).
func (t *opcounter) GetResult() (json.RawMessage, error) {
    res, err := json.Marshal(t.counts)
    if err != nil {
        return nil, err
    }
    return res, t.reason
}

// Stop terminates execution of the tracer at the first opportune moment.
func (t *opcounter) Stop(err error) {
    t.reason = err
    atomic.StoreUint32(&t.interrupt, 1)
}
```

Every method of the [EVMLogger interface](https://pkg.go.dev/github.com/ethereum/go-ethereum/core/vm#EVMLogger) needs to be implemented (even if empty). Key parts to notice are the `init()` function which registers the tracer in Geth, the `CaptureState` hook where the opcode counts are incremented and `GetResult` where the result is serialized and delivered. Note that the constructor takes in a `cfg json.RawMessage`. This will be filled with a JSON object that user provides to the tracer to pass in optional config fields.

To test out this tracer the source is first compiled with `make geth`. Then in the console it can be invoked through the usual API methods by passing in the name it was registered under:

```console
> debug.traceTransaction('0x7ae446a7897c056023a8104d254237a8d97783a92900a7b0f7db668a9432f384', { tracer: 'opcounter' })
{
    ADD: 4,
    AND: 3,
    CALLDATALOAD: 2,
    ...
}
```

## Summary

This page described how to write custom tracers for Geth. Custom tracers can be written in Javascript or Go.