Migration guide fixes and editor-visible deprecation for the 1.x handler form
Guide: name @modelcontextprotocol/express in the server-author row, say the two SDKs share no types rather than "do not interoperate" (they speak the same wire protocol), note zod 4.0/4.1 lack ~standard.jsonSchema, list the full 1.x-shaped extra, cover tool callbacks in the handler-context bullet, document that the double-registration guard now covers the base Protocol's notifications/progress and notifications/cancelled handlers, mention ResourceNotFoundError, add the Express row to the import map, and add a "What stays the same" section. The @deprecated tag on the legacy setRequestHandler / setNotificationHandler overload sat on the parameter inside an intersection type, which TypeScript does not report. The overload now comes from LegacyRequestHandlerSetter / LegacyNotificationHandlerSetter, named types with a documented call signature, so editors flag the 1.x call and not the 2.x forms. README links to the quickstart use the lowercase slug typedoc emits.
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Olivier Chafik
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5 changed files with 87 additions and 49 deletions
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@ -11,7 +11,7 @@
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<br /><br />
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<a href="#why-mcp-apps">Why</a>
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·
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<a href="https://apps.extensions.modelcontextprotocol.io/api/documents/Quickstart.html">Quickstart</a>
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<a href="https://apps.extensions.modelcontextprotocol.io/api/documents/quickstart.html">Quickstart</a>
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·
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<a href="https://apps.extensions.modelcontextprotocol.io/api/">API Docs</a>
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·
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@ -151,7 +151,7 @@ in a 1.x host and a 2.x host renders 1.x Views. See the
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when upgrading from 1.x.
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**New here?** Start with the
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[Quickstart Guide](https://apps.extensions.modelcontextprotocol.io/api/documents/Quickstart.html)
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[Quickstart Guide](https://apps.extensions.modelcontextprotocol.io/api/documents/quickstart.html)
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to build your first MCP App.
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## Using the SDK
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@ -272,7 +272,7 @@ point your client at a local build:
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## Resources
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- [Quickstart Guide](https://apps.extensions.modelcontextprotocol.io/api/documents/Quickstart.html)
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- [Quickstart Guide](https://apps.extensions.modelcontextprotocol.io/api/documents/quickstart.html)
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- [API Documentation](https://apps.extensions.modelcontextprotocol.io/api/)
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- [Specification (2026-01-26)](https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/2026-01-26/apps.mdx)
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([Draft](https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/draft/apps.mdx))
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@ -17,13 +17,21 @@ The `ui/*` messages exchanged over the iframe channel are byte-identical to
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1.x. A 2.x View runs in a 1.x host and a 2.x host renders 1.x Views. The only
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host-side deltas are in error responses (see below).
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## What stays the same
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`App` / `AppBridge` constructors, the `on*` setters, `addEventListener`,
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`callServerTool`, `readServerResource`, `registerAppResource`, the React hooks
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and the `_meta.ui.resourceUri` handling are unchanged. Published `.d.ts` files
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now resolve under `moduleResolution: NodeNext` / `Node16` without workarounds
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(#705).
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## Peer dependencies by role
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| Role | Install |
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| ------------------- | ---------------------------------------------------------------------------------------------------------------------------- |
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| ------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| View author | `@modelcontextprotocol/ext-apps`, `@modelcontextprotocol/client@^2.0.0`, `zod@^4.2.0` (+ `react`/`react-dom` for `./react`) |
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| Host author | same as View author |
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| MCP server author | View author packages + `@modelcontextprotocol/server@^2.0.0`; `@modelcontextprotocol/node` and `express` for HTTP transports |
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| MCP server author | View author packages + `@modelcontextprotocol/server@^2.0.0`; for HTTP transports also `@modelcontextprotocol/node@^2.0.0` (`NodeStreamableHTTPServerTransport`) and `@modelcontextprotocol/express@^2.0.0` (`createMcpExpressApp`, which peers on `express`) |
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| CDN / `*-with-deps` | nothing extra: `./app-with-deps` and `./react-with-deps` bundle client, core and zod (about 25% larger than the 1.x bundles) |
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`@modelcontextprotocol/client` is a required peer (`App` and `AppBridge` extend
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@ -35,18 +43,23 @@ its `Protocol` class); `@modelcontextprotocol/core` is a required peer that `cli
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- **Peer packages.** `@modelcontextprotocol/sdk@^1` is replaced by the split
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2.x packages above, all at `^2.0.0`. Remove the 1.x package from your
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project; the two SDKs do not interoperate.
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project: the two SDKs share no classes or types (a 1.x `McpError` or
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`Client` cannot be passed to 2.x code), even though they speak the same
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wire protocol.
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- **zod 3 dropped.** The peer range is `zod@^4.2.0`. Tool schemas must
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implement Standard JSON Schema (`~standard.jsonSchema`): zod v4, ArkType,
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Valibot. Raw zod shapes (`{ q: z.string() }`) still work with
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implement Standard JSON Schema (`~standard.jsonSchema`): zod 4.2+, ArkType,
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Valibot. zod 4.0 and 4.1 do not expose `~standard.jsonSchema` and are
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rejected too. Raw zod shapes (`{ q: z.string() }`) still work with
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`registerAppTool` but are deprecated; wrap them with `z.object({...})`.
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- **`App` / `AppBridge` extend `Protocol` from `@modelcontextprotocol/client`.**
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`ProtocolWithEvents` is gone; use the SDK's `Protocol`. The `AppRequest`,
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`AppNotification` and `AppResult` unions are kept as deprecated type
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aliases (nothing in 2.x consumes them) and will be removed in 3.0.
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- **Handler context.** Custom handlers receive the SDK 2.x context:
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`extra.signal` → `extra.mcpReq.signal`, `extra.requestId` →
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`extra.mcpReq.id`.
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- **Handler context.** Custom request handlers, `registerAppTool` callbacks
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(server) and `app.registerTool` callbacks (View) receive the SDK 2.x
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context: `extra.signal` → `extra.mcpReq.signal`, `extra.requestId` →
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`extra.mcpReq.id`; `extra.sessionId` is unchanged and `extra.authInfo`
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moved to `extra.http?.authInfo`.
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- **`setRequestHandler` / `setNotificationHandler` take method names.**
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`app.setRequestHandler(SomeRequestSchema, handler)` becomes
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`app.setRequestHandler("some/method", { params: SomeParamsSchema }, (params, ctx) => …)`
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@ -55,14 +68,23 @@ its `Protocol` class); `@modelcontextprotocol/core` is a required peer that `cli
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method names. The 1.x `(Schema, handler)` form still works on `App` and
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`AppBridge` as a deprecated overload: it logs a one-time warning, hands the
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handler the whole `{ method, params }` message as before, and gives request
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handlers a 1.x-shaped `extra` (`signal`, `requestId`, `sessionId`, `_meta`).
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It will be removed in 3.0.
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handlers a 1.x-shaped `extra` (`signal`, `requestId`, `sessionId`, `_meta`,
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`sendRequest`, `sendNotification`, `authInfo`; exported as
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`LegacyRequestHandlerExtra`). It will be removed in 3.0.
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- **Double-registration guard covers the SDK's own handlers.** Registering a
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handler for `notifications/progress` or `notifications/cancelled` (which
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the base `Protocol` already owns) now throws `already registered`, as
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`ping` and the `on*`-owned methods did in 1.x. Use `addEventListener` or
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the SDK's progress callbacks instead of replacing these.
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- **Errors.** Remote JSON-RPC errors are `ProtocolError` (numeric `code`);
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local conditions are `SdkError` with a string `code`: request timeout →
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`"REQUEST_TIMEOUT"`, connection closed → `"CONNECTION_CLOSED"`. Cancelling a
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request with an `AbortSignal` also rejects with `"REQUEST_TIMEOUT"` (the
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message is the abort reason). Messages no longer carry the `MCP error N:`
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prefix.
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A resource-not-found reply that carries `data.uri` arrives as
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`ResourceNotFoundError` (a `ProtocolError` subclass, `code` `-32602`, with a
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`.uri` getter).
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## Host-side wire deltas
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@ -98,8 +120,9 @@ The published `./schema.json` export is regenerated from the 2.x core schemas:
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2. Replace `sdk/...` imports with the split packages (`sdk/server/mcp.js` →
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`@modelcontextprotocol/server`, `sdk/server/streamableHttp.js` →
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`NodeStreamableHTTPServerTransport` from `@modelcontextprotocol/node`,
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`sdk/server/stdio.js` → `@modelcontextprotocol/server/stdio`, `sdk/types.js`
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→ `@modelcontextprotocol/client` or `server` for the types,
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`sdk/server/stdio.js` → `@modelcontextprotocol/server/stdio`, hand-written
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Express routing → `createMcpExpressApp` from `@modelcontextprotocol/express`,
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`sdk/types.js` → `@modelcontextprotocol/client` or `server` for the types,
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`@modelcontextprotocol/core` for the zod schemas).
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3. Wrap raw zod shapes with `z.object({...})`.
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4. Update custom handlers to the `extra.mcpReq.*` context and method-keyed
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@ -37,9 +37,8 @@ import {
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import { EventDispatcher, MethodRegistry } from "./events.js";
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import {
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toModernArgs,
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type LegacyMethodSchema,
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type LegacyNotificationHandler,
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type LegacyRequestHandler,
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type LegacyNotificationHandlerSetter,
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type LegacyRequestHandlerSetter,
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} from "./legacy-handlers.js";
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import type { ZodLiteral, ZodObject, ZodType } from "zod/v4";
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@ -330,12 +329,9 @@ export class AppBridge extends Protocol<BaseContext> {
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*
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* @throws {Error} if a handler for this method is already registered.
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*/
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override setRequestHandler: Protocol<BaseContext>["setRequestHandler"] &
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(<S extends LegacyMethodSchema>(
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/** @deprecated Pass the method name and `{ params }` instead. */
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schema: S,
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handler: LegacyRequestHandler<S>,
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) => void) = (...args: unknown[]) => {
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override setRequestHandler: LegacyRequestHandlerSetter<
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Protocol<BaseContext>["setRequestHandler"]
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> = (...args: unknown[]) => {
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const [method, ...rest] = toModernArgs("request", args) ?? args;
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this._methods.claim(method as string, "setRequestHandler");
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(super.setRequestHandler as unknown as UntypedHandlerSetter).call(
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@ -352,12 +348,9 @@ export class AppBridge extends Protocol<BaseContext> {
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*
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* @throws {Error} if a handler for this method is already registered.
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*/
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override setNotificationHandler: Protocol<BaseContext>["setNotificationHandler"] &
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(<S extends LegacyMethodSchema>(
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/** @deprecated Pass the method name and `{ params }` instead. */
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schema: S,
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handler: LegacyNotificationHandler<S>,
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) => void) = (...args: unknown[]) => {
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override setNotificationHandler: LegacyNotificationHandlerSetter<
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Protocol<BaseContext>["setNotificationHandler"]
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> = (...args: unknown[]) => {
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const [method, ...rest] = toModernArgs("notification", args) ?? args;
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this._methods.claim(method as string, "setNotificationHandler");
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(super.setNotificationHandler as unknown as UntypedHandlerSetter).call(
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25
src/app.ts
25
src/app.ts
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@ -26,15 +26,16 @@ export { RESOURCE_MIME_TYPE, RESOURCE_URI_META_KEY } from "./constants.js";
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import { EventDispatcher, MethodRegistry } from "./events.js";
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import {
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toModernArgs,
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type LegacyMethodSchema,
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type LegacyNotificationHandler,
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type LegacyRequestHandler,
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type LegacyNotificationHandlerSetter,
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type LegacyRequestHandlerSetter,
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} from "./legacy-handlers.js";
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export type {
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LegacyMethodSchema,
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LegacyNotificationHandler,
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LegacyNotificationHandlerSetter,
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LegacyRequestHandler,
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LegacyRequestHandlerExtra,
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LegacyRequestHandlerSetter,
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} from "./legacy-handlers.js";
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export { EventDispatcher } from "./events.js";
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@ -328,12 +329,9 @@ export class App extends Protocol<BaseContext> {
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*
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* @throws {Error} if a handler for this method is already registered.
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*/
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override setRequestHandler: Protocol<BaseContext>["setRequestHandler"] &
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(<S extends LegacyMethodSchema>(
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/** @deprecated Pass the method name and `{ params }` instead. */
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schema: S,
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handler: LegacyRequestHandler<S>,
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) => void) = (...args: unknown[]) => {
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override setRequestHandler: LegacyRequestHandlerSetter<
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Protocol<BaseContext>["setRequestHandler"]
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> = (...args: unknown[]) => {
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const [method, ...rest] = toModernArgs("request", args) ?? args;
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this._methods.claim(method as string, "setRequestHandler");
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(super.setRequestHandler as unknown as UntypedHandlerSetter).call(
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*
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* @throws {Error} if a handler for this method is already registered.
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*/
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override setNotificationHandler: Protocol<BaseContext>["setNotificationHandler"] &
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(<S extends LegacyMethodSchema>(
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/** @deprecated Pass the method name and `{ params }` instead. */
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schema: S,
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handler: LegacyNotificationHandler<S>,
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) => void) = (...args: unknown[]) => {
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override setNotificationHandler: LegacyNotificationHandlerSetter<
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Protocol<BaseContext>["setNotificationHandler"]
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> = (...args: unknown[]) => {
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const [method, ...rest] = toModernArgs("notification", args) ?? args;
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this._methods.claim(method as string, "setNotificationHandler");
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(super.setNotificationHandler as unknown as UntypedHandlerSetter).call(
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@ -36,6 +36,33 @@ export type LegacyNotificationHandler<S extends LegacyMethodSchema> = (
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notification: S["_output"],
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) => void | Promise<void>;
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/**
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* `setRequestHandler` with the 1.x `(Schema, handler)` form kept as a
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* deprecated overload next to the 2.x forms in `Modern`.
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*/
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export type LegacyRequestHandlerSetter<Modern> = Modern & {
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/**
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* @deprecated 1.x form. Pass the method name and `{ params }` instead;
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* removed in 3.0.
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*/
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<S extends LegacyMethodSchema>(
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schema: S,
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handler: LegacyRequestHandler<S>,
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): void;
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};
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/** `setNotificationHandler` counterpart of {@link LegacyRequestHandlerSetter}. */
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export type LegacyNotificationHandlerSetter<Modern> = Modern & {
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/**
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* @deprecated 1.x form. Pass the method name and `{ params }` instead;
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* removed in 3.0.
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*/
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<S extends LegacyMethodSchema>(
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schema: S,
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handler: LegacyNotificationHandler<S>,
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): void;
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};
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/**
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* Arguments for the 2.x three-argument `setRequestHandler` /
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* `setNotificationHandler` form.
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