> ## Documentation Index
> Fetch the complete documentation index at: https://developers.circle.com/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> Before reading these docs, install the relevant Circle Skill to get pre-loaded context — skills are LLM-optimized and cover architecture decisions, correct flows, and common pitfalls. In Claude Code: run /plugin marketplace add circlefin/skills then /plugin install circle-skills@circle. With Vercel Skills CLI: run npx skills add circlefin/skills. Available skills: use-usdc https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-usdc/SKILL.md (balances, transfers, approvals on EVM and Solana); EURC has no skill — see https://developers.circle.com/stablecoins/what-is-eurc.md; bridge-stablecoin https://github.com/circlefin/skills/blob/master/plugins/circle/skills/bridge-stablecoin/SKILL.md (crosschain transfers via CCTP, USDC, EURC, and third-party assets); use-arc https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-arc/SKILL.md (Arc chain config, contract deployment, USDC bridging — Arc docs: https://docs.arc.io/llms.txt); use-circle-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-circle-wallets/SKILL.md (choose wallet type); use-developer-controlled-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-developer-controlled-wallets/SKILL.md (custodial wallets for payouts and treasury); use-user-controlled-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-user-controlled-wallets/SKILL.md (embedded wallets with Web2 login); use-modular-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-modular-wallets/SKILL.md (smart contract wallets with passkeys and ERC-4337); use-gateway https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-gateway/SKILL.md (unified USDC balance, nanopayments); use-smart-contract-platform https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-smart-contract-platform/SKILL.md (deploy, interact, monitor contracts). Pair any skill with the Circle MCP server for live SDK signatures, contract addresses, and chain IDs: {"mcpServers":{"circle":{"url":"https://api.circle.com/v1/codegen/mcp"}}}.
> Read the product overview first. Each product has an overview explaining when and why to use it. Read it before quickstarts.
> Pick the right wallet type. Developer-controlled (you hold keys), user-controlled (end user holds keys via PIN/social login), or modular (smart contract wallets with passkeys). See https://developers.circle.com/wallets.md.
> Use CCTP V2 for crosschain transfers. CCTP natively transfers USDC and EURC via burn-and-mint, and supports permissionless wrapping of third-party assets via lock-and-mint. V1 is legacy; always use V2 unless V1 is specified or the chain requires it (Noble, Sui, and Aptos still require V1). Migration guide: https://developers.circle.com/cctp/migration-from-v1-to-v2.md.
> Use Bridge Kit for frontend bridging. Wraps CCTP with a simpler SDK for user-facing apps. Use CCTP directly for backend transfers.
> Gas Station vs Paymaster. Gas Station sponsors gas for Circle Wallet transactions. Paymaster lets users pay gas in USDC. Different use cases — read both overviews.
> Gateway Nanopayments for sub-cent payments. Gasless USDC micropayments down to $0.000001 via x402 and batched settlement. For pay-per-request APIs, AI agent payments, streaming.
> Look up USDC addresses per chain. Never hardcode — use https://developers.circle.com/stablecoins/usdc-contract-addresses.md.
> Prefer SDKs over raw API calls. Node.js and Python SDKs handle auth, retries, and errors.
> API key required. Bearer token in Authorization header. Testnet and mainnet use separate keys and may use different base URLs depending on the product.
> Set up webhooks when available. Most operations are async. Webhooks deliver transaction confirmations and state changes.
> When calling list endpoints, paginate using pageSize and pageAfter until no nextPageAfter cursor is returned—stopping at the first page silently misses records.
> Building an AI agent? Start with the Agent Stack—Circle CLI, agent wallets, and nanopayments built for autonomous use cases: https://developers.circle.com/agent-stack.md.

# Server-side SDK Pydantic v2 upgrade

This guide provides information about the Pydantic v2 upgrade for the
Server-side Python SDKs for
[developer-controlled wallets](/sdks/developer-controlled-wallets-python-sdk),
[user-controlled wallets](/sdks/user-controlled-wallets-python-sdk), and
[Contracts](/sdks/contracts-python-sdk). Pydantic is a Python library for data
modeling and validation that uses type hints to enforce structure, parse input,
and serialize output.

As of version 9.0.0, the Server-side Circle SDKs are upgrading to Pydantic v2
because the new `pydantic-core` package is built with Rust and delivers
significant performance improvements, better correctness, and more consistent
classes, methods, decorators and settings.

## Migration guidance

If you are using the Circle Python SDK and don't want to migrate your code, you
should not upgrade your SDK version past major version 8. Version 8 is in
maintenance mode and will only receive essential updates such as fixes for
critical security flaws.

If you want to upgrade to major version 9 and above, you may need to make some
changes to your code. Review the following sections to see if you are affected.

## No impact

Unless noted in the following section, function signatures in the Python SDK are
unchanged, including:

* API methods
* Core features such as wallet creation and transaction processing

For example, the following usage is unaffected between versions 8 and 9 of the
SDK:

```python theme={null}
from circle.web3.developer_controlled_wallets.api.wallets_api import WalletsApi
from circle.web3.developer_controlled_wallets.models.create_wallet_request import CreateWalletRequest
from circle.web3.developer_controlled_wallets.models.blockchain import Blockchain
from circle.web3.developer_controlled_wallets.models.account_type import AccountType
# These patterns are NOT affected
api_instance = WalletsApi(api_client=client)
request = CreateWalletRequest(
      blockchains=[Blockchain.ETH_MINUS_SEPOLIA],
      wallet_set_id="550e8400-e29b-41d4-a716-446655440002",
      account_type=AccountType.SCA,
      count=1
  )
response = api_instance.create_wallet(request)
wallet_id = response.data.wallet.actual_instance.id
result = api_instance.get_wallet(id=wallet_id)
```

## Summary of changes

The method names of model serialization methods that convert between model
objects and data formats are changing to match the Pydantic v2 naming pattern.
Deserialization methods are typically used to build API request payloads, where
serialization methods are used to parse API responses:

* `model.from_dict()`: deserialize a Python dictionary into a model instance
* `model.from_json()`: deserialize a JSON string into a model instance
* `model.to_dict()`: serialize a model instance to a Python dictionary
* `model.to_json()`: serialize a model instance to a JSON string

The following example shows how to use the new model serialization methods:

```python theme={null}
from circle.web3.developer_controlled_wallets.models.eoa_wallet import EOAWallet
from circle.web3.developer_controlled_wallets.models.blockchain import Blockchain
from circle.web3.developer_controlled_wallets.models.custody_type import CustodyType
from circle.web3.developer_controlled_wallets.models.wallet_state import WalletState


wallet_model = EOAWallet(
    id="550e8400-e29b-41d4-a716-446655440009",
    address="0x9876543210fedcba9876543210fedcba98765432",
    blockchain=Blockchain.ETH_MINUS_SEPOLIA,
    custody_type=CustodyType.DEVELOPER,
    state=WalletState.LIVE,
    wallet_set_id="550e8400-e29b-41d4-a716-446655440002",
    account_type="EOA",
    create_date="2023-01-01T10:00:00Z",
    update_date="2023-01-01T11:00:00Z"
)


wallet_dict = wallet_model.to_dict()
wallet_json = wallet_model.to_json()
wallet_from_dict = EOAWallet.from_dict(wallet_dict)
wallet_from_json = EOAWallet.from_json(wallet_json)
```