EIP-7201 Decoder

Decode namespaced storage from its root slot and source-defined struct layout.

# EIP-7201 Decoder

Decode namespaced storage from its root slot and source-defined struct layout.

**EIP-7201 Decoder** inspects namespaced storage. It pairs a root slot with the struct layout from source code, so you can inspect each variable's relative position, type, and mapping path.

## Use the Decoder

The Decoder appears below Storage Layout.

| Area              | Description                                                                    |
| ----------------- | ------------------------------------------------------------------------------ |
| Suggestions       | Detected `root slot → struct` pairs. Select one to add it to the layout below. |
| Base Slot         | Select or enter the namespace's root slot.                                     |
| Storage Structure | Select a namespace struct detected in the source code.                         |
| Add               | Adds the slot and struct pair to the storage layout below.                     |

## Example

> Interactive content: EIP-7201 storage walkthrough. Open the online documentation to view it.

<span className="text-text-secondary block text-right text-xs">
  This example uses a local namespaced layout. It does not send requests.
</span>

1. Select `root slot → example.token` under **Suggestions** to add the layout below.
2. After it is added, the suggestion shows **Added** and cannot be selected again. If there are no suggestions, select or enter a root slot under **Base Slot**, select a struct under **Storage Structure**, then select **Add**.
3. Select the type for `totalSupply` or `balances` to inspect its position and type relative to the root slot.
4. Mappings and dynamic arrays also need a key or index before you can locate the actual data position.

## What is EIP-7201?

EIP-7201 defines a pattern for namespaced storage: a contract places related variables in a `struct`, then assigns that struct an independent root slot. Variables inside the struct still follow the normal Solidity storage rules.

Source code commonly marks a namespace on the struct:

<div className="not-prose border-border-secondary my-4 rounded-lg border p-4 font-mono text-sm">
  /// @custom:storage-location erc7201:example.token

  <br />

  struct TokenStorage \{

  <br />

      uint256 totalSupply;

  <br />

      mapping(address => uint256) balances;

  <br />

  }
</div>

`erc7201` calculates the root slot from the namespace ID with `keccak256(keccak256(id) - 1) & ~0xff`. This gives different modules independent storage areas, reducing the risk of slot collisions during upgrades or module composition. See the full specification in [ERC-7201](https://eips.ethereum.org/EIPS/eip-7201).

It is different from a proxy: a proxy determines which code runs; EIP-7201 determines where a module's state is stored. They can be used together or separately.

## Notes

* A namespace does not use an ordinary slot `0` layout. Confirm its root slot before interpreting variables inside it.
* The namespace ID should remain stable. Changing it points to a different storage area.
* `@custom:storage-location` marks the layout; the contract must still read and write at that root slot itself.

## Next steps

* [Proxy Storage](/docs/address/proxy-storage)
* [Storage Layout](/docs/address/storage-layout)

EIP-7201 Decoder inspects namespaced storage. It pairs a root slot with the struct layout from source code, so you can inspect each variable's relative position, type, and mapping path.

Use the Decoder

The Decoder appears below Storage Layout.

Area	Description
Suggestions	Detected `root slot → struct` pairs. Select one to add it to the layout below.
Base Slot	Select or enter the namespace's root slot.
Storage Structure	Select a namespace struct detected in the source code.
Add	Adds the slot and struct pair to the storage layout below.

Example

This example uses a local namespaced layout. It does not send requests.

Select root slot → example.token under Suggestions to add the layout below.
After it is added, the suggestion shows Added and cannot be selected again. If there are no suggestions, select or enter a root slot under Base Slot, select a struct under Storage Structure, then select Add.
Select the type for totalSupply or balances to inspect its position and type relative to the root slot.
Mappings and dynamic arrays also need a key or index before you can locate the actual data position.

What is EIP-7201?

EIP-7201 defines a pattern for namespaced storage: a contract places related variables in a struct, then assigns that struct an independent root slot. Variables inside the struct still follow the normal Solidity storage rules.

Source code commonly marks a namespace on the struct:

/// @custom:storage-location erc7201:example.token

struct TokenStorage {

uint256 totalSupply;

mapping(address => uint256) balances;

}

erc7201 calculates the root slot from the namespace ID with keccak256(keccak256(id) - 1) & ~0xff. This gives different modules independent storage areas, reducing the risk of slot collisions during upgrades or module composition. See the full specification in ERC-7201.

It is different from a proxy: a proxy determines which code runs; EIP-7201 determines where a module's state is stored. They can be used together or separately.

Notes

A namespace does not use an ordinary slot 0 layout. Confirm its root slot before interpreting variables inside it.
The namespace ID should remain stable. Changing it points to a different storage area.
@custom:storage-location marks the layout; the contract must still read and write at that root slot itself.

Next steps

Proxy Storage

Load implementation source and layout while continuing to read state from the proxy address.

Virtual TestNets Overview

Create a private chain from live state and experiment without affecting mainnet.

On this page

Use the Decoder

What is EIP-7201?