Add declare-from docs (#3729)

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Closes #3587

**Stack**:
- #3729 ⬅
- #3727
- #3726

## Introduced changes

<!-- A brief description of the changes -->

-

## Checklist

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- [x] Linked relevant issue
- [x] Updated relevant documentation
- [ ] Added relevant tests
- [x] Performed self-review of the code
- [x] Added changes to `CHANGELOG.md`

Author: franciszekjob

CHANGELOG.md

@@ -18,6 +18,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Possibility to use `#[available_gas]` with unnamed argument. Use named arguments instead, e.g. `#[available_gas(l2_gas: 5)]`.
 - The deprecated command `snforge init`. Use `snforge new` to initialize new `Forge` projects
 
+### Cast
+
+#### Added
+
+`sncast declare-from` command to declare a contract by fetching it from a different Starknet instance
+
 ## [0.49.0] - 2025-09-03
 
 ### Forge

docs/src/SUMMARY.md

@@ -134,6 +134,7 @@
         * [delete](appendix/sncast/account/delete.md)
         * [list](appendix/sncast/account/list.md)
     * [declare](appendix/sncast/declare.md)
+    * [declare-from](appendix/sncast/declare_from.md)
     * [deploy](appendix/sncast/deploy.md)
     * [invoke](appendix/sncast/invoke.md)
     * [call](appendix/sncast/call.md)

docs/src/appendix/sncast.md

@@ -7,6 +7,7 @@
     * [deploy](./sncast/account/deploy.md)
     * [delete](./sncast/account/delete.md)
 * [declare](./sncast/declare.md)
+* [declare-from](./sncast/declare_from.md)
 * [deploy](./sncast/deploy.md)
 * [invoke](./sncast/invoke.md)
 * [call](./sncast/call.md)

docs/src/appendix/sncast/declare_from.md

@@ -0,0 +1,96 @@
+# `declare`
+Declare a contract by fetching it from a different Starknet instance.
+
+## Required Common Arguments — Passed By CLI or Specified in `snfoundry.toml`
+
+* [`account`](./common.md#--account--a-account_name)
+
+## `--class-hash, -c <CLASS_HASH>`
+Required.
+
+Class hash of contract declared on a different network.
+
+## `--url, -u <RPC_URL>`
+Optional.
+
+Starknet RPC node url address.
+
+Overrides url from `snfoundry.toml`.
+
+## `--network <NETWORK>`
+Optional.
+
+Use predefined network with public provider
+
+Possible values: `mainnet`, `sepolia`.
+
+## `--source-url, -u <RPC_URL>`
+Optional.
+
+Starknet RPC node url address of the source network where the contract is already declared.
+
+## `--source-network <NETWORK>`
+Optional.
+
+Use predefined network with public provider where the contract is already declared.
+
+Possible values: `mainnet`, `sepolia`.
+
+## `--max-fee, -m <MAX_FEE>`
+Optional.
+
+Maximum fee for the `declare` denoted in FRI. Must be greater than zero. If provided, it is not possible to use any of the following fee related flags: `--l1-gas`, `--l1-data-price`, `--l2-gas`, `--l2-gas-price`, `--l1-data-gas`, `--l1-data-gas-price`.
+
+## `--l1-gas <L1_GAS>`
+Optional.
+
+Maximum L1 gas for the `declare` transaction. When not used, defaults to auto-estimation.
+
+## ` --l1-gas-price <l1_gas_price>`
+Optional.
+
+Maximum L1 gas unit price for the `declare` transaction. When not used, defaults to auto-estimation.
+
+## `--l2-gas <L2_GAS>`
+Optional.
+
+Maximum L2 gas for the `declare` transaction. When not used, defaults to auto-estimation.
+
+## `--l2-gas-price <L2_GAS_PRICE>`
+Optional.
+
+Maximum L2 gas unit price for the `declare` transaction. When not used, defaults to auto-estimation.
+
+## `--l1-data-gas <L1_DATA_GAS>`
+Optional.
+
+Maximum L1 data gas for the `declare` transaction. When not used, defaults to auto-estimation.
+
+## `--l1-data-gas-price <l1_data_gas_price>`
+Optional.
+
+Maximum L1 data gas unit price for the `declare` transaction. When not used, defaults to auto-estimation.
+
+## `--tip <TIP>`
+Optional.
+Conflicts with: [`--estimate-tip`](#--estimate-tip-estimate_tip)
+
+Tip for the transaction. Tip for the transaction. When not provided, defaults to 0 unless [`--estimate-tip`](#--estimate-tip-estimate_tip) is used.
+
+## `--estimate-tip <ESTIMATE_TIP>`
+Optional.
+Conflicts with: [`--tip`](#--tip-tip)
+
+If passed, an estimated tip will be added to pay for the transaction. The tip is estimated based on the current network conditions and added to the transaction fee.
+
+## `--nonce, -n <NONCE>`
+Optional.
+
+Nonce for transaction. If not provided, nonce will be set automatically.
+
+## `--block-id, -b <BLOCK_ID>`
+Optional.
+
+Block identifier on which class of declared contract should be fetched.
+Possible values: `pending`, `latest`, block hash (0x prefixed string), and block number (u64).
+`pending` is used as a default value.

docs/src/starknet/declare.md

@@ -53,4 +53,39 @@ transaction: https://starkscan.co/search/[..]
 > 💡 **Info**
 > Max fee will be automatically computed if `--max-fee <MAX_FEE>` is not passed.
 
+# Declaring a Contract by Fetching It From a Different Starknet Instance
+
+In some cases, you may need to declare a contract that was already compiled elsewhere and reuse the exact same class hash across multiple networks.
+This is especially important for some contracts, e.g. Universal Deployer Contract (UDC), which must preserve the same class hash across mainnet, testnets, and appchains.
+
+Compiling a contract locally with a different Cairo compiler version may result in a different class hash.
+
+To avoid this, you can use the `declare-from` which allows you to declare a contract by providing its class hash and the source network where it is already declared.
+
+## Example
+
+Let's consider a basic contract, which is already declared on Sepolia network.
+
+To declare it on another network, e.g. local devnet, run:
+
+```shell
+$ sncast --account my_account \
+    declare-from \
+    --class-hash 0x283a4f96ee7de15894d9205a93db7cec648562cfe90db14cb018c039e895e78 \
+    --source-network sepolia \
+    --url http://127.0.0.1:5055/rpc
+```
+
+<details>
+<summary>Output:</summary>
+
+```shell
+Success: Declaration completed
+
+Class Hash:       0x283a4f96ee7de15894d9205a93db7cec648562cfe90db14cb018c039e895e78
+Transaction Hash: 0x[..]
+```
+</details>
+<br>
+