Why miner.start Returns Null in Geth Dev Mode and How to Fix It

Earlier versions of Geth in --dev mode started mining automatically when miner.start() was called. After a Geth update the call returns null and mining only occurs after a transaction is submitted.

Background

In developer mode Geth creates an ephemeral proof‑of‑authority network with a pre‑funded account. The original workflow was:

geth --dev
miner.start()

and blocks were produced continuously.

Cause

Starting with recent Geth releases a new flag --dev.period was added. Its definition is:

--dev.period <seconds>   Block period in developer mode (0 = mine only when a transaction is pending) (default: 0)

The default value 0 disables automatic mining, so miner.start() appears to do nothing and returns null.

Solution

Launch the node with the original --dev flag.

Add a non‑zero period, e.g. --dev.period 1 (any positive integer works).

Restart the node and invoke miner.start() again.

Example command: geth --dev --dev.period 1 After the restart the node mines continuously; the block number increases in the logs and via eth.blockNumber regardless of the return value of miner.start().

Notes

The --dev.period value must be greater than zero; setting it to 0 reproduces the issue.

This flag is documented in the Geth command‑line reference but is often overlooked in community solutions.

Conclusion

Adjusting --dev.period to a non‑zero value restores the expected mining behavior in Geth’s developer mode.