agents@0.4.0

### Minor Changes

-   [#848](https://github.com/cloudflare/agents/pull/848) [`a167344`](https://github.com/cloudflare/agents/commit/a167344aab6960a51901886539c206a2c937bb1e) Thanks [@mattzcarey](https://github.com/mattzcarey)! - Upgrade MCP SDK to 1.26.0 to prevent cross-client response leakage. Updated examples for stateless MCP Servers create new `McpServer` instance per request instead of sharing a single instance. A guard is added in this version of the MCP SDK which will prevent connection to a Server instance that has already been connected to a transport. Developers will need to modify their code if they declare their `McpServer` instance as a global variable.

-   [#298](https://github.com/cloudflare/agents/pull/298) [`27f4e3e`](https://github.com/cloudflare/agents/commit/27f4e3ef4471f5c523a7e2f8a0ce548daa5738f5) Thanks [@jaredhanson](https://github.com/jaredhanson)! - Add `createMcpOAuthProvider` method to the `Agent` class, allowing subclasses to override the default OAuth provider used when connecting to MCP servers. This enables custom authentication strategies such as pre-registered client credentials or mTLS, beyond the built-in dynamic client registration.

-   [#610](https://github.com/cloudflare/agents/pull/610) [`f59f305`](https://github.com/cloudflare/agents/commit/f59f30533121e6e9fd41e9a2e22184d2fa9bdb1b) Thanks [@threepointone](https://github.com/threepointone)! - Deprecate `onStateUpdate` server-side hook in favor of `onStateChanged`
    -   `onStateChanged` is a drop-in rename of `onStateUpdate` (same signature, same behavior)
    -   `onStateUpdate` still works but emits a one-time console warning per class
    -   Throws if a class overrides both hooks simultaneously
    -   `validateStateChange` rejections now propagate a `CF_AGENT_STATE_ERROR` message back to the client

-   [#871](https://github.com/cloudflare/agents/pull/871) [`27f8f75`](https://github.com/cloudflare/agents/commit/27f8f755f04e23a71e7a0748c48a2e7ec25cede6) Thanks [@threepointone](https://github.com/threepointone)! - Migrate x402 MCP integration from legacy `x402` package to `@x402/core` and `@x402/evm` v2

    **Breaking changes for x402 users:**

    -   Peer dependencies changed: replace `x402` with `@x402/core` and `@x402/evm`
    -   `PaymentRequirements` type now uses v2 fields (e.g. `amount` instead of `maxAmountRequired`)
    -   `X402ClientConfig.account` type changed from `viem.Account` to `ClientEvmSigner` (structurally compatible with `privateKeyToAccount()`)

    **Migration guide:**

    1.  Update dependencies:

        ```bash
        npm uninstall x402
        npm install @x402/core @x402/evm
        ```

    2.  Update network identifiers — both legacy names and CAIP-2 format are accepted:

        ```typescript
        // Before
        {
          network: "base-sepolia";
        }
        // After (either works)
        {
          network: "base-sepolia";
        } // legacy name, auto-converted
        {
          network: "eip155:84532";
        } // CAIP-2 format (preferred)
        ```

    3.  If you access `PaymentRequirements` fields in callbacks, update to v2 field names (see `@x402/core` docs).

    4.  The `version` field on `X402Config` and `X402ClientConfig` is now deprecated and ignored — the protocol version is determined automatically.

    **Other changes:**

    -   `X402ClientConfig.network` is now optional — the client auto-selects from available payment requirements
    -   Server-side lazy initialization: facilitator connection is deferred until the first paid tool invocation
    -   Payment tokens support both v2 (`PAYMENT-SIGNATURE`) and v1 (`X-PAYMENT`) HTTP headers
    -   Added `normalizeNetwork` export for converting legacy network names to CAIP-2 format
    -   Re-exports `PaymentRequirements`, `PaymentRequired`, `Network`, `FacilitatorConfig`, and `ClientEvmSigner` from `agents/x402`

### Patch Changes

-   [#610](https://github.com/cloudflare/agents/pull/610) [`f59f305`](https://github.com/cloudflare/agents/commit/f59f30533121e6e9fd41e9a2e22184d2fa9bdb1b) Thanks [@threepointone](https://github.com/threepointone)! - Add readonly connections: restrict WebSocket clients from modifying agent state
    -   New hooks: `shouldConnectionBeReadonly`, `setConnectionReadonly`, `isConnectionReadonly`
    -   Blocks both client-side `setState()` and mutating `@callable()` methods for readonly connections
    -   Readonly flag stored in a namespaced connection attachment (`_cf_readonly`), surviving hibernation without extra SQL
    -   Connection state wrapping hides the internal flag from user code and preserves it across `connection.setState()` calls
    -   Client-side `onStateUpdateError` callback for handling rejected state updates

-   [#855](https://github.com/cloudflare/agents/pull/855) [`271a3cf`](https://github.com/cloudflare/agents/commit/271a3cffd769d646b1d6498f5676662ced94cf27) Thanks [@threepointone](https://github.com/threepointone)! - Fix `useAgent` and `AgentClient` crashing when using `basePath` routing.

-   [#868](https://github.com/cloudflare/agents/pull/868) [`b3e2dc1`](https://github.com/cloudflare/agents/commit/b3e2dc1c389b0d874eee5407099d8c20fe684b8b) Thanks [@threepointone](https://github.com/threepointone)! - Fix MCP OAuth callback URL leaking instance name

    Add `callbackPath` option to `addMcpServer` to prevent instance name leakage in MCP OAuth callback URLs. When `sendIdentityOnConnect` is `false`, `callbackPath` is now required — the default callback URL would expose the instance name, undermining the security intent. Also fixes callback request detection to match via the `state` parameter instead of a loose `/callback` URL substring check, enabling custom callback paths.

-   [#872](https://github.com/cloudflare/agents/pull/872) [`de71f9e`](https://github.com/cloudflare/agents/commit/de71f9ecfae019061651716cb7d2a350a4283ada) Thanks [@threepointone](https://github.com/threepointone)! - update dependencies

-   [`8893fbe`](https://github.com/cloudflare/agents/commit/8893fbef32bea9581dd308d5b6d9c066e135feef) Thanks [@threepointone](https://github.com/threepointone)! - partykit releases

    ## partyserver

    ### `0.1.3` (Feb 8, 2026)

    -   [#319](https://github.com/cloudflare/partykit/pull/319) — Add `configurable: true` to the `state`, `setState`, `serializeAttachment`, and `deserializeAttachment` property descriptors on connection objects. This allows downstream consumers (like the Cloudflare Agents SDK) to redefine these properties with `Object.defineProperty` for namespacing or wrapping internal state storage. Default behavior is unchanged.

    ### `0.1.4` (Feb 9, 2026)

    -   [#320](https://github.com/cloudflare/partykit/pull/320) — **Add CORS support to `routePartykitRequest`**. Pass `cors: true` for permissive defaults or `cors: { ...headers }` for custom CORS headers. Preflight (OPTIONS) requests are handled automatically for matched routes, and CORS headers are appended to all non-WebSocket responses — including responses returned by `onBeforeRequest`.
    -   [#260](https://github.com/cloudflare/partykit/pull/260) — Remove redundant initialize code as `setName` takes care of it, along with the nested `blockConcurrencyWhile` call.

    * * *

    ## partysocket

    ### `1.1.12` (Feb 8, 2026)

    -   [#317](https://github.com/cloudflare/partykit/pull/317) — Fix `PartySocket.reconnect()` crashing when using `basePath` without `room`. The reconnect guard now accepts either `room` or `basePath` as sufficient context to construct a connection URL.
    -   [#319](https://github.com/cloudflare/partykit/pull/319) — Throw a clear error when constructing a `PartySocket` without `room` or `basePath` (and without `startClosed: true`), instead of silently connecting to a malformed URL containing `"undefined"` as the room name.

    ### `1.1.13` (Feb 9, 2026)

    -   [#322](https://github.com/cloudflare/partykit/pull/322) — Fix `reconnect()` not working after `maxRetries` has been exhausted. The `_connectLock` was not released when the max retries early return was hit in `_connect()`, preventing any subsequent `reconnect()` call from initiating a new connection.

-   [#869](https://github.com/cloudflare/agents/pull/869) [`fc17506`](https://github.com/cloudflare/agents/commit/fc17506a1d6fb8f6b7fed56be98ab1729d338c2c) Thanks [@threepointone](https://github.com/threepointone)! - Remove `room`/`party` workaround for `basePath` routing now that partysocket handles reconnect without requiring `room` to be set.

-   [#873](https://github.com/cloudflare/agents/pull/873) [`d0579fa`](https://github.com/cloudflare/agents/commit/d0579fa13a60e47395a2dde199be3197299b8668) Thanks [@threepointone](https://github.com/threepointone)! - Remove CORS wrapping from `routeAgentRequest` and delegate to partyserver's native CORS support. The `cors` option is now passed directly through to `routePartykitRequest`, which handles preflight and response headers automatically since partyserver 0.1.4.

-   [#865](https://github.com/cloudflare/agents/pull/865) [`c3211d0`](https://github.com/cloudflare/agents/commit/c3211d0b0cc36aa294c15569ae650d3afeab9926) Thanks [@threepointone](https://github.com/threepointone)! - update dependencies

-   Updated dependencies \[[`21a7977`](https://github.com/cloudflare/agents/commit/21a79778f5150aecd890f55a164d397f70db681e), [`3de98a3`](https://github.com/cloudflare/agents/commit/3de98a398d55aeca51c7b845ed4c5d6051887d6d), [`c3211d0`](https://github.com/cloudflare/agents/commit/c3211d0b0cc36aa294c15569ae650d3afeab9926)]:
    -   @cloudflare/codemode@0.0.7
    -   @cloudflare/ai-chat@0.0.7