I am trying to create a genesis block on my mac for a local ethereum network but I am getting the error:

invalid genesis file: hex string has odd length

when i do it.

My code is the following:

  "nonce": "0x0000000000000042", 
  "timestamp": "0x0", 
  "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000", 
  "extraData": "0x0", 
  "gasLimit": "0x8000000", 
  "difficulty": "0x400", 
  "mixhash": "0x0000000000000000000000000000000000000000000000000000000000000000", 
  "coinbase": "0x3333333333333333333333333333333333333333", 
  "alloc": { }

The same problem happened to me with geth 1.6, with a genesis file similar to yours.

Last friday geth updated to 1.6 (https://github.com/ethereum/go-ethereum/releases/tag/v1.6.0).

In this release:

Genesis block JSON handling is stricter and safer. Notably, most JSON fields now require the "0x" prefix. (#3794)

I found this documentation page: https://github.com/ethereum/go-ethereum/wiki/Private-network

In this page there is an example of a genesis file that works on geth 1.6.

I think your genesis file works with previous versions.

  • Thanks Atrophy. I was also facing the same issue and after debugging for 2 days, I found this post and my problem got solved. Apr 19 '17 at 10:50

I had the same problem and found that the issue was:

"extraData": "0x0",

If you change it to:

"extraData": "0x00",

The problem goes away.

At this point though you'll have another issue because in the latest version of geth you need a config section. Add the following and you should be good to go.

"config": { }


I was using the same JSON file as above and getting the same "invalid hex" error. Per the suggestion above, I changed to the new JSON file per the link (see below). I can now successfully initialize the chain without getting the hex error. However, I am getting a different error when trying to start a mining node. I am wondering if it's because this includes some pre-funded accounts. Is there syntax available for the genesis JSON above, but with an empty file? (without any pre-funded accounts).

    "config": {
        "chainId": 15,
        "homesteadBlock": 0,
        "eip155Block": 0,
        "eip158Block": 0
    "difficulty": "200000000",
    "gasLimit": "2100000",
    "alloc": {
        "7df9a875a174b3bc565e6424a0050ebc1b2d1d82": { "balance": "300000" },
        "f41c74c9ae680c1aa78f42e5647a62f353b7bdde": { "balance": "400000" }
  • 1
    Please open a new post instead posting here
    – Badr Bellaj
    Apr 28 '17 at 14:22

As noted in another answer, this appears to be a change with the release of geth 1.6.0.

Since the genesis.json was downloaded instead of generated, I'm not sure if it's appropriate to edit the genesis.json.

I installed geth 1.5.9 and was able to use $ geth init genesis.json with the old format shown in the question.

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