> For the complete documentation index, see [llms.txt](https://docs.megaeth.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.megaeth.com/mega-evme/configuration/chain-and-spec.md).

# Chain and Spec

These options control which MegaETH spec and chain ID the EVM uses during execution. They are available in the `run` and `tx` commands. The `replay` command auto-detects the spec from the chain ID and block timestamp (see [replay](/mega-evme/commands/replay.md#spec-auto-detection)).

## Options

| Flag              | Default | Aliases     | Description         |
| ----------------- | ------- | ----------- | ------------------- |
| `--spec <SPEC>`   | `Rex6`  | —           | MegaETH spec to use |
| `--chain-id <ID>` | `6342`  | `--chainid` | Chain ID            |

## Available Specs

Spec names are case-sensitive.

| Name          | Description                                                                                                                   |
| ------------- | ----------------------------------------------------------------------------------------------------------------------------- |
| `Equivalence` | Optimism Isthmus compatibility mode                                                                                           |
| `MiniRex`     | Initial MegaETH execution model with multidimensional gas                                                                     |
| `Rex`         | Revised storage gas economics and gas forwarding                                                                              |
| `Rex1`        | Compute gas limit reset fix                                                                                                   |
| `Rex2`        | SELFDESTRUCT restored (EIP-6780), KeylessDeploy system contract                                                               |
| `Rex3`        | SLOAD-based oracle detention, increased oracle gas limit                                                                      |
| `Rex4`        | Per-call-frame resource budgets, relative gas detention, storage gas stipend                                                  |
| `Rex5`        | SequencerRegistry, dynamic system address (Oracle v2.0.0), storage-gas-stipend separated allowance, resource-accounting fixes |
| `Rex6`        | Unified gas-metering order, consolidated EIP-7702 accounting, system-tx metering exemption (**unstable**)                     |

## Examples

```bash
# Use MiniRex spec
mega-evme run 0x600160005260... --spec MiniRex

# Use Equivalence mode (Optimism Isthmus compatible)
mega-evme tx --spec Equivalence --receiver 0x1234...

# Custom chain ID
mega-evme run 0x600160005260... --chain-id 1
```


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